NAME
dhcpd.conf - dhcpd configuration file
DESCRIPTION
The dhcpd.conf file contains configuration information for
dhcpd, the
Internet Systems Consortium DHCP Server.
The dhcpd.conf file is a free-form ASCII text file. It is parsed by the
recursive-descent parser built into dhcpd. The file may contain extra tabs and
newlines for formatting purposes. Keywords in the file are case-insensitive.
Comments may be placed anywhere within the file (except within quotes).
Comments begin with the # character and end at the end of the line.
The file essentially consists of a list of statements. Statements fall into two
broad categories - parameters and declarations.
Parameter statements either say how to do something (e.g., how long a lease to
offer), whether to do something (e.g., should dhcpd provide addresses to
unknown clients), or what parameters to provide to the client (e.g., use
gateway 220.177.244.7).
Declarations are used to describe the topology of the network, to describe
clients on the network, to provide addresses that can be assigned to clients,
or to apply a group of parameters to a group of declarations. In any group of
parameters and declarations, all parameters must be specified before any
declarations which depend on those parameters may be specified.
Declarations about network topology include the
shared-network and the
subnet declarations. If clients on a subnet are to be assigned
addresses dynamically, a
range declaration must appear within the
subnet declaration. For clients with statically assigned addresses, or
for installations where only known clients will be served, each such client
must have a
host declaration. If parameters are to be applied to a
group of declarations which are not related strictly on a per-subnet basis,
the
group declaration can be used.
For every subnet which will be served, and for every subnet to which the dhcp
server is connected, there must be one
subnet declaration, which tells
dhcpd how to recognize that an address is on that subnet. A
subnet
declaration is required for each subnet even if no addresses will be
dynamically allocated on that subnet.
Some installations have physical networks on which more than one IP subnet
operates. For example, if there is a site-wide requirement that 8-bit subnet
masks be used, but a department with a single physical ethernet network
expands to the point where it has more than 254 nodes, it may be necessary to
run two 8-bit subnets on the same ethernet until such time as a new physical
network can be added. In this case, the
subnet declarations for these
two networks must be enclosed in a
shared-network declaration.
Note that even when the
shared-network declaration is absent, an empty
one is created by the server to contain the
subnet (and any scoped
parameters included in the
subnet). For practical purposes, this means
that "stateless" DHCP clients, which are not tied to addresses (and
therefore subnets) will receive the same configuration as stateful ones.
Some sites may have departments which have clients on more than one subnet, but
it may be desirable to offer those clients a uniform set of parameters which
are different than what would be offered to clients from other departments on
the same subnet. For clients which will be declared explicitly with
host declarations, these declarations can be enclosed in a
group
declaration along with the parameters which are common to that department. For
clients whose addresses will be dynamically assigned, class declarations and
conditional declarations may be used to group parameter assignments based on
information the client sends.
When a client is to be booted, its boot parameters are determined by consulting
that client's
host declaration (if any), and then consulting any
class declarations matching the client, followed by the
pool,
subnet and
shared-network declarations for the IP address
assigned to the client. Each of these declarations itself appears within a
lexical scope, and all declarations at less specific lexical scopes are also
consulted for client option declarations. Scopes are never considered twice,
and if parameters are declared in more than one scope, the parameter declared
in the most specific scope is the one that is used.
When dhcpd tries to find a
host declaration for a client, it first looks
for a
host declaration which has a
fixed-address declaration
that lists an IP address that is valid for the subnet or shared network on
which the client is booting. If it doesn't find any such entry, it tries to
find an entry which has no
fixed-address declaration.
EXAMPLES
A typical dhcpd.conf file will look something like this:
global parameters...
subnet 204.254.239.0 netmask 255.255.255.224 {
subnet-specific parameters...
range 204.254.239.10 204.254.239.30;
}
subnet 204.254.239.32 netmask 255.255.255.224 {
subnet-specific parameters...
range 204.254.239.42 204.254.239.62;
}
subnet 204.254.239.64 netmask 255.255.255.224 {
subnet-specific parameters...
range 204.254.239.74 204.254.239.94;
}
group {
group-specific parameters...
host zappo.test.isc.org {
host-specific parameters...
}
host beppo.test.isc.org {
host-specific parameters...
}
host harpo.test.isc.org {
host-specific parameters...
}
}
Figure 1
Notice that at the beginning of the file, there's a place for global parameters.
These might be things like the organization's domain name, the addresses of
the name servers (if they are common to the entire organization), and so on.
So, for example:
option domain-name "isc.org";
option domain-name-servers ns1.isc.org, ns2.isc.org;
Figure 2
As you can see in Figure 2, you can specify host addresses in parameters using
their domain names rather than their numeric IP addresses. If a given hostname
resolves to more than one IP address (for example, if that host has two
ethernet interfaces), then where possible, both addresses are supplied to the
client.
The most obvious reason for having subnet-specific parameters as shown in Figure
1 is that each subnet, of necessity, has its own router. So for the first
subnet, for example, there should be something like:
option routers 204.254.239.1;
Note that the address here is specified numerically. This is not required - if
you have a different domain name for each interface on your router, it's
perfectly legitimate to use the domain name for that interface instead of the
numeric address. However, in many cases there may be only one domain name for
all of a router's IP addresses, and it would not be appropriate to use that
name here.
In Figure 1 there is also a
group statement, which provides common
parameters for a set of three hosts - zappo, beppo and harpo. As you can see,
these hosts are all in the test.isc.org domain, so it might make sense for a
group-specific parameter to override the domain name supplied to these hosts:
option domain-name "test.isc.org";
Also, given the domain they're in, these are probably test machines. If we
wanted to test the DHCP leasing mechanism, we might set the lease timeout
somewhat shorter than the default:
max-lease-time 120;
default-lease-time 120;
You may have noticed that while some parameters start with the
option
keyword, some do not. Parameters starting with the
option keyword
correspond to actual DHCP options, while parameters that do not start with the
option keyword either control the behavior of the DHCP server (e.g., how long
a lease dhcpd will give out), or specify client parameters that are not
optional in the DHCP protocol (for example, server-name and filename).
In Figure 1, each host had
host-specific parameters. These could include
such things as the
hostname option, the name of a file to upload (the
filename parameter) and the address of the server from which to upload
the file (the
next-server parameter). In general, any parameter can
appear anywhere that parameters are allowed, and will be applied according to
the scope in which the parameter appears.
Imagine that you have a site with a lot of NCD X-Terminals. These terminals come
in a variety of models, and you want to specify the boot files for each model.
One way to do this would be to have host declarations for each server and
group them by model:
group {
filename "Xncd19r";
next-server ncd-booter;
host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }
}
group {
filename "Xncd19c";
next-server ncd-booter;
host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }
}
group {
filename "XncdHMX";
next-server ncd-booter;
host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
host ncd4 { hardware ethernet 0:c0:c3:91:a7:8; }
host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f; }
}
ADDRESS POOLS
The
pool and
pool6 declarations can be used to specify a pool of
addresses that will be treated differently than another pool of addresses,
even on the same network segment or subnet. For example, you may want to
provide a large set of addresses that can be assigned to DHCP clients that are
registered to your DHCP server, while providing a smaller set of addresses,
possibly with short lease times, that are available for unknown clients. If
you have a firewall, you may be able to arrange for addresses from one pool to
be allowed access to the Internet, while addresses in another pool are not,
thus encouraging users to register their DHCP clients. To do this, you would
set up a pair of pool declarations:
subnet 10.0.0.0 netmask 255.255.255.0 {
option routers 10.0.0.254;
# Unknown clients get this pool.
pool {
option domain-name-servers bogus.example.com;
max-lease-time 300;
range 10.0.0.200 10.0.0.253;
allow unknown-clients;
}
# Known clients get this pool.
pool {
option domain-name-servers ns1.example.com, ns2.example.com;
max-lease-time 28800;
range 10.0.0.5 10.0.0.199;
deny unknown-clients;
}
}
It is also possible to set up entirely different subnets for known and unknown
clients - address pools exist at the level of shared networks, so address
ranges within pool declarations can be on different subnets.
As you can see in the preceding example, pools can have permit lists that
control which clients are allowed access to the pool and which aren't. Each
entry in a pool's permit list is introduced with the
allow or
deny keyword. If a pool has a permit list, then only those clients that
match specific entries on the permit list will be eligible to be assigned
addresses from the pool. If a pool has a deny list, then only those clients
that do not match any entries on the deny list will be eligible. If both
permit and deny lists exist for a pool, then only clients that match the
permit list and do not match the deny list will be allowed access.
The
pool6 declaration is similar to the
pool declaration.
Currently it is only allowed within a
subnet6 declaration, and may not
be included directly in a shared network declaration. In addition to the
range6 statement it allows the
prefix6 statement to be included.
You may include
range6 statements for both NA and TA and
prefixy6 statements in a single
pool6 statement.
DYNAMIC ADDRESS ALLOCATION
Address allocation is actually only done when a client is in the INIT state and
has sent a DHCPDISCOVER message. If the client thinks it has a valid lease and
sends a DHCPREQUEST to initiate or renew that lease, the server has only three
choices - it can ignore the DHCPREQUEST, send a DHCPNAK to tell the client it
should stop using the address, or send a DHCPACK, telling the client to go
ahead and use the address for a while.
If the server finds the address the client is requesting, and that address is
available to the client, the server will send a DHCPACK. If the address is no
longer available, or the client isn't permitted to have it, the server will
send a DHCPNAK. If the server knows nothing about the address, it will remain
silent, unless the address is incorrect for the network segment to which the
client has been attached and the server is authoritative for that network
segment, in which case the server will send a DHCPNAK even though it doesn't
know about the address.
There may be a host declaration matching the client's identification. If that
host declaration contains a fixed-address declaration that lists an IP address
that is valid for the network segment to which the client is connected. In
this case, the DHCP server will never do dynamic address allocation. In this
case, the client is
required to take the address specified in the host
declaration. If the client sends a DHCPREQUEST for some other address, the
server will respond with a DHCPNAK.
When the DHCP server allocates a new address for a client (remember, this only
happens if the client has sent a DHCPDISCOVER), it first looks to see if the
client already has a valid lease on an IP address, or if there is an old IP
address the client had before that hasn't yet been reassigned. In that case,
the server will take that address and check it to see if the client is still
permitted to use it. If the client is no longer permitted to use it, the lease
is freed if the server thought it was still in use - the fact that the client
has sent a DHCPDISCOVER proves to the server that the client is no longer
using the lease.
If no existing lease is found, or if the client is forbidden to receive the
existing lease, then the server will look in the list of address pools for the
network segment to which the client is attached for a lease that is not in use
and that the client is permitted to have. It looks through each pool
declaration in sequence (all
range declarations that appear outside of
pool declarations are grouped into a single pool with no permit list). If the
permit list for the pool allows the client to be allocated an address from
that pool, the pool is examined to see if there is an address available. If
so, then the client is tentatively assigned that address. Otherwise, the next
pool is tested. If no addresses are found that can be assigned to the client,
no response is sent to the client.
If an address is found that the client is permitted to have, and that has never
been assigned to any client before, the address is immediately allocated to
the client. If the address is available for allocation but has been previously
assigned to a different client, the server will keep looking in hopes of
finding an address that has never before been assigned to a client.
The DHCP server generates the list of available IP addresses from a hash table.
This means that the addresses are not sorted in any particular order, and so
it is not possible to predict the order in which the DHCP server will allocate
IP addresses. Users of previous versions of the ISC DHCP server may have
become accustomed to the DHCP server allocating IP addresses in ascending
order, but this is no longer possible, and there is no way to configure this
behavior with version 3 of the ISC DHCP server.
IP ADDRESS CONFLICT PREVENTION
The DHCP server checks IP addresses to see if they are in use before allocating
them to clients. It does this by sending an ICMP Echo request message to the
IP address being allocated. If no ICMP Echo reply is received within a second,
the address is assumed to be free. This is only done for leases that have been
specified in range statements, and only when the lease is thought by the DHCP
server to be free - i.e., the DHCP server or its failover peer has not listed
the lease as in use.
If a response is received to an ICMP Echo request, the DHCP server assumes that
there is a configuration error - the IP address is in use by some host on the
network that is not a DHCP client. It marks the address as abandoned, and will
not assign it to clients.
If a DHCP client tries to get an IP address, but none are available, but there
are abandoned IP addresses, then the DHCP server will attempt to reclaim an
abandoned IP address. It marks one IP address as free, and then does the same
ICMP Echo request check described previously. If there is no answer to the
ICMP Echo request, the address is assigned to the client.
The DHCP server does not cycle through abandoned IP addresses if the first IP
address it tries to reclaim is free. Rather, when the next DHCPDISCOVER comes
in from the client, it will attempt a new allocation using the same method
described here, and will typically try a new IP address.
DHCP FAILOVER
This version of the ISC DHCP server supports the DHCP failover protocol as
documented in draft-ietf-dhc-failover-12.txt. This is not a final protocol
document, and we have not done interoperability testing with other vendors'
implementations of this protocol, so you must not assume that this
implementation conforms to the standard. If you wish to use the failover
protocol, make sure that both failover peers are running the same version of
the ISC DHCP server.
The failover protocol allows two DHCP servers (and no more than two) to share a
common address pool. Each server will have about half of the available IP
addresses in the pool at any given time for allocation. If one server fails,
the other server will continue to renew leases out of the pool, and will
allocate new addresses out of the roughly half of available addresses that it
had when communications with the other server were lost.
It is possible during a prolonged failure to tell the remaining server that the
other server is down, in which case the remaining server will (over time)
reclaim all the addresses the other server had available for allocation, and
begin to reuse them. This is called putting the server into the PARTNER-DOWN
state.
You can put the server into the PARTNER-DOWN state either by using the
omshell (1) command or by stopping the server, editing the last
failover state declaration in the lease file, and restarting the server. If
you use this last method, change the "my state" line to:
failover peer name state {
my state partner-down;.
peer state state at date;
}
It is only required to change "my state" as shown above.
When the other server comes back online, it should automatically detect that it
has been offline and request a complete update from the server that was
running in the PARTNER-DOWN state, and then both servers will resume
processing together.
It is possible to get into a dangerous situation: if you put one server into the
PARTNER-DOWN state, and then *that* server goes down, and the other server
comes back up, the other server will not know that the first server was in the
PARTNER-DOWN state, and may issue addresses previously issued by the other
server to different clients, resulting in IP address conflicts. Before putting
a server into PARTNER-DOWN state, therefore, make
sure that the other
server will not restart automatically.
The failover protocol defines a primary server role and a secondary server role.
There are some differences in how primaries and secondaries act, but most of
the differences simply have to do with providing a way for each peer to behave
in the opposite way from the other. So one server must be configured as
primary, and the other must be configured as secondary, and it doesn't matter
too much which one is which.
FAILOVER STARTUP
When a server starts that has not previously communicated with its failover
peer, it must establish communications with its failover peer and synchronize
with it before it can serve clients. This can happen either because you have
just configured your DHCP servers to perform failover for the first time, or
because one of your failover servers has failed catastrophically and lost its
database.
The initial recovery process is designed to ensure that when one failover peer
loses its database and then resynchronizes, any leases that the failed server
gave out before it failed will be honored. When the failed server starts up,
it notices that it has no saved failover state, and attempts to contact its
peer.
When it has established contact, it asks the peer for a complete copy its peer's
lease database. The peer then sends its complete database, and sends a message
indicating that it is done. The failed server then waits until MCLT has
passed, and once MCLT has passed both servers make the transition back into
normal operation. This waiting period ensures that any leases the failed
server may have given out while out of contact with its partner will have
expired.
While the failed server is recovering, its partner remains in the partner-down
state, which means that it is serving all clients. The failed server provides
no service at all to DHCP clients until it has made the transition into normal
operation.
In the case where both servers detect that they have never before communicated
with their partner, they both come up in this recovery state and follow the
procedure we have just described. In this case, no service will be provided to
DHCP clients until MCLT has expired.
CONFIGURING FAILOVER
In order to configure failover, you need to write a peer declaration that
configures the failover protocol, and you need to write peer references in
each pool declaration for which you want to do failover. You do not have to do
failover for all pools on a given network segment. You must not tell one
server it's doing failover on a particular address pool and tell the other it
is not. You must not have any common address pools on which you are not doing
failover. A pool declaration that utilizes failover would look like this:
pool {
failover peer "foo";
pool specific parameters
};
The server currently does very little sanity checking, so if you configure it
wrong, it will just fail in odd ways. I would recommend therefore that you
either do failover or don't do failover, but don't do any mixed pools. Also,
use the same master configuration file for both servers, and have a separate
file that contains the peer declaration and includes the master file. This
will help you to avoid configuration mismatches. As our implementation
evolves, this will become less of a problem. A basic sample dhcpd.conf file
for a primary server might look like this:
failover peer "foo" {
primary;
address anthrax.rc.vix.com;
port 519;
peer address trantor.rc.vix.com;
peer port 520;
max-response-delay 60;
max-unacked-updates 10;
mclt 3600;
split 128;
load balance max seconds 3;
}
include "/etc/dhcpd.master";
The statements in the peer declaration are as follows:
The
primary and
secondary statements
[
primary |
secondary ]
;
This determines whether the server is primary or secondary, as described earlier
under DHCP FAILOVER.
The
address statement
address address;
The
address statement declares the IP address or DNS name on which the
server should listen for connections from its failover peer, and also the
value to use for the DHCP Failover Protocol server identifier. Because this
value is used as an identifier, it may not be omitted.
The
peer address statement
peer address address;
The
peer address statement declares the IP address or DNS name to which
the server should connect to reach its failover peer for failover
messages.
The
port statement
port port-number;
The
port statement declares the TCP port on which the server should
listen for connections from its failover peer. This statement may be omitted,
in which case the IANA assigned port number 647 will be used by default.
The
peer port statement
peer port port-number;
The
peer port statement declares the TCP port to which the server should
connect to reach its failover peer for failover messages. This statement may
be omitted, in which case the IANA assigned port number 647 will be used by
default.
The
max-response-delay statement
max-response-delay seconds;
The
max-response-delay statement tells the DHCP server how many seconds
may pass without receiving a message from its failover peer before it assumes
that connection has failed. This number should be small enough that a
transient network failure that breaks the connection will not result in the
servers being out of communication for a long time, but large enough that the
server isn't constantly making and breaking connections. This parameter must
be specified.
The
max-unacked-updates statement
max-unacked-updates count;
The
max-unacked-updates statement tells the remote DHCP server how many
BNDUPD messages it can send before it receives a BNDACK from the local system.
We don't have enough operational experience to say what a good value for this
is, but 10 seems to work. This parameter must be specified.
The
mclt statement
mclt seconds;
The
mclt statement defines the Maximum Client Lead Time. It must be
specified on the primary, and may not be specified on the secondary. This is
the length of time for which a lease may be renewed by either failover peer
without contacting the other. The longer you set this, the longer it will take
for the running server to recover IP addresses after moving into PARTNER-DOWN
state. The shorter you set it, the more load your servers will experience when
they are not communicating. A value of something like 3600 is probably
reasonable, but again bear in mind that we have no real operational experience
with this.
The
split statement
split bits;
The split statement specifies the split between the primary and secondary for
the purposes of load balancing. Whenever a client makes a DHCP request, the
DHCP server runs a hash on the client identification, resulting in value from
0 to 255. This is used as an index into a 256 bit field. If the bit at that
index is set, the primary is responsible. If the bit at that index is not set,
the secondary is responsible. The
split value determines how many of
the leading bits are set to one. So, in practice, higher split values will
cause the primary to serve more clients than the secondary. Lower split
values, the converse. Legal values are between 0 and 256 inclusive, of which
the most reasonable is 128. Note that a value of 0 makes the secondary
responsible for all clients and a value of 256 makes the primary responsible
for all clients.
The
hba statement
hba colon-separated-hex-list;
The hba statement specifies the split between the primary and secondary as a
bitmap rather than a cutoff, which theoretically allows for finer-grained
control. In practice, there is probably no need for such fine-grained control,
however. An example hba statement:
hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00;
This is equivalent to a
split 128; statement, and identical. The
following two examples are also equivalent to a
split of 128, but are
not identical:
hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:
aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa;
hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:
55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55;
They are equivalent, because half the bits are set to 0, half are set to 1 (0xa
and 0x5 are 1010 and 0101 binary respectively) and consequently this would
roughly divide the clients equally between the servers. They are not
identical, because the actual peers this would load balance to each server are
different for each example.
You must only have
split or
hba defined, never both. For most
cases, the fine-grained control that
hba offers isn't necessary, and
split should be used.
The
load balance max seconds statement
load balance max seconds seconds;
This statement allows you to configure a cutoff after which load balancing is
disabled. The cutoff is based on the number of seconds since the client sent
its first DHCPDISCOVER or DHCPREQUEST message, and only works with clients
that correctly implement the
secs field - fortunately most clients do.
We recommend setting this to something like 3 or 5. The effect of this is that
if one of the failover peers gets into a state where it is responding to
failover messages but not responding to some client requests, the other
failover peer will take over its client load automatically as the clients
retry.
The
auto-partner-down statement
auto-partner-down seconds;
This statement instructs the server to initiate a timed delay upon entering the
communications-interrupted state (any situation of being out-of-contact with
the remote failover peer). At the conclusion of the timer, the server will
automatically enter the partner-down state. This permits the server to
allocate leases from the partner's free lease pool after an STOS+MCLT timer
expires, which can be dangerous if the partner is in fact operating at the
time (the two servers will give conflicting bindings).
Think very carefully before enabling this feature. The partner-down and
communications-interrupted states are intentionally segregated because there
do exist situations where a failover server can fail to communicate with its
peer, but still has the ability to receive and reply to requests from DHCP
clients. In general, this feature should only be used in those deployments
where the failover servers are directly connected to one another, such as by a
dedicated hardwired link ("a heartbeat cable").
A zero value disables the auto-partner-down feature (also the default), and any
positive value indicates the time in seconds to wait before automatically
entering partner-down.
The Failover pool balance statements.
max-lease-misbalance percentage;
max-lease-ownership percentage;
min-balance seconds;
max-balance seconds;
This version of the DHCP Server evaluates pool balance on a schedule, rather
than on demand as leases are allocated. The latter approach proved to be
slightly klunky when pool misbalanced reach total saturation — when any
server ran out of leases to assign, it also lost its ability to notice it had
run dry.
In order to understand pool balance, some elements of its operation first need
to be defined. First, there are ´free´ and ´backup´
leases. Both of these are referred to as ´free state leases´.
´free´ and ´backup´ are ´the free states´ for
the purpose of this document. The difference is that only the primary may
allocate from ´free´ leases unless under special circumstances, and
only the secondary may allocate ´backup´ leases.
When pool balance is performed, the only plausible expectation is to provide a
50/50 split of the free state leases between the two servers. This is because
no one can predict which server will fail, regardless of the relative load
placed upon the two servers, so giving each server half the leases gives both
servers the same amount of ´failure endurance´. Therefore, there is
no way to configure any different behaviour, outside of some very small
windows we will describe shortly.
The first thing calculated on any pool balance run is a value referred to as
´lts´, or "Leases To Send". This, simply, is the
difference in the count of free and backup leases, divided by two. For the
secondary, it is the difference in the backup and free leases, divided by two.
The resulting value is signed: if it is positive, the local server is expected
to hand out leases to retain a 50/50 balance. If it is negative, the remote
server would need to send leases to balance the pool. Once the lts value
reaches zero, the pool is perfectly balanced (give or take one lease in the
case of an odd number of total free state leases).
The current approach is still something of a hybrid of the old approach, marked
by the presence of the
max-lease-misbalance statement. This parameter
configures what used to be a 10% fixed value in previous versions: if lts is
less than free+backup *
max-lease-misbalance percent, then the server
will skip balancing a given pool (it won't bother moving any leases, even if
some leases "should" be moved). The meaning of this value is also
somewhat overloaded, however, in that it also governs the estimation of when
to attempt to balance the pool (which may then also be skipped over). The
oldest leases in the free and backup states are examined. The time they have
resided in their respective queues is used as an estimate to indicate how much
time it is probable it would take before the leases at the top of the list
would be consumed (and thus, how long it would take to use all leases in that
state). This percentage is directly multiplied by this time, and fit into the
schedule if it falls within the
min-balance and
max-balance
configured values. The scheduled pool check time is only moved in a downwards
direction, it is never increased. Lastly, if the lts is more than double this
number in the negative direction, the local server will ´panic´ and
transmit a Failover protocol POOLREQ message, in the hopes that the remote
system will be woken up into action.
Once the lts value exceeds the
max-lease-misbalance percentage of total
free state leases as described above, leases are moved to the remote server.
This is done in two passes.
In the first pass, only leases whose most recent bound client would have been
served by the remote server - according to the Load Balance Algorithm (see
above
split and
hba configuration statements) - are given away
to the peer. This first pass will happily continue to give away leases,
decrementing the lts value by one for each, until the lts value has reached
the negative of the total number of leases multiplied by the
max-lease-ownership percentage. So it is through this value that you
can permit a small misbalance of the lease pools - for the purpose of giving
the peer more than a 50/50 share of leases in the hopes that their clients
might some day return and be allocated by the peer (operating normally). This
process is referred to as ´MAC Address Affinity´, but this is
somewhat misnamed: it applies equally to DHCP Client Identifier options. Note
also that affinity is applied to leases when they enter the state
´free´ from ´expired´ or ´released´. In this
case also, leases will not be moved from free to backup if the secondary
already has more than its share.
The second pass is only entered into if the first pass fails to reduce the lts
underneath the total number of free state leases multiplied by the
max-lease-ownership percentage. In this pass, the oldest leases are
given over to the peer without second thought about the Load Balance
Algorithm, and this continues until the lts falls under this value. In this
way, the local server will also happily keep a small percentage of the leases
that would normally load balance to itself.
So, the
max-lease-misbalance value acts as a behavioural gate. Smaller
values will cause more leases to transition states to balance the pools over
time, higher values will decrease the amount of change (but may lead to pool
starvation if there's a run on leases).
The
max-lease-ownership value permits a small (percentage) skew in the
lease balance of a percentage of the total number of free state leases.
Finally, the
min-balance and
max-balance make certain that a
scheduled rebalance event happens within a reasonable timeframe (not to be
thrown off by, for example, a 7 year old free lease).
Plausible values for the percentages lie between 0 and 100, inclusive, but
values over 50 are indistinguishable from one another (once lts exceeds 50% of
the free state leases, one server must therefore have 100% of the leases in
its respective free state). It is recommended to select a
max-lease-ownership value that is lower than the value selected for the
max-lease-misbalance value.
max-lease-ownership defaults to 10,
and
max-lease-misbalance defaults to 15.
Plausible values for the
min-balance and
max-balance times also
range from 0 to (2^32)-1 (or the limit of your local time_t value), but
default to values 60 and 3600 respectively (to place balance events between 1
minute and 1 hour).
CLIENT CLASSING
Clients can be separated into classes, and treated differently depending on what
class they are in. This separation can be done either with a conditional
statement, or with a match statement within the class declaration. It is
possible to specify a limit on the total number of clients within a particular
class or subclass that may hold leases at one time, and it is possible to
specify automatic subclassing based on the contents of the client packet.
Classing support for DHCPv6 clients was addded in 4.3.0. It follows the same
rules as for DHCPv4 except that support for billing classes has not been added
yet.
To add clients to classes based on conditional evaluation, you can specify a
matching expression in the class statement:
class "ras-clients" {
match if substring (option dhcp-client-identifier, 1, 3) = "RAS";
}
Note that whether you use matching expressions or add statements (or both) to
classify clients, you must always write a class declaration for any class that
you use. If there will be no match statement and no in-scope statements for a
class, the declaration should look like this:
class "ras-clients" {
}
SUBCLASSES
In addition to classes, it is possible to declare subclasses. A subclass is a
class with the same name as a regular class, but with a specific submatch
expression which is hashed for quick matching. This is essentially a speed
hack - the main difference between five classes with match expressions and one
class with five subclasses is that it will be quicker to find the subclasses.
Subclasses work as follows:
class "allocation-class-1" {
match pick-first-value (option dhcp-client-identifier, hardware);
}
class "allocation-class-2" {
match pick-first-value (option dhcp-client-identifier, hardware);
}
subclass "allocation-class-1" 1:8:0:2b:4c:39:ad;
subclass "allocation-class-2" 1:8:0:2b:a9:cc:e3;
subclass "allocation-class-1" 1:0:0:c4:aa:29:44;
subnet 10.0.0.0 netmask 255.255.255.0 {
pool {
allow members of "allocation-class-1";
range 10.0.0.11 10.0.0.50;
}
pool {
allow members of "allocation-class-2";
range 10.0.0.51 10.0.0.100;
}
}
The data following the class name in the subclass declaration is a constant
value to use in matching the match expression for the class. When class
matching is done, the server will evaluate the match expression and then look
the result up in the hash table. If it finds a match, the client is considered
a member of both the class and the subclass.
Subclasses can be declared with or without scope. In the above example, the sole
purpose of the subclass is to allow some clients access to one address pool,
while other clients are given access to the other pool, so these subclasses
are declared without scopes. If part of the purpose of the subclass were to
define different parameter values for some clients, you might want to declare
some subclasses with scopes.
In the above example, if you had a single client that needed some configuration
parameters, while most didn't, you might write the following subclass
declaration for that client:
subclass "allocation-class-2" 1:08:00:2b:a1:11:31 {
option root-path "samsara:/var/diskless/alphapc";
filename "/tftpboot/netbsd.alphapc-diskless";
}
In this example, we've used subclassing as a way to control address allocation
on a per-client basis. However, it's also possible to use subclassing in ways
that are not specific to clients - for example, to use the value of the
vendor-class-identifier option to determine what values to send in the
vendor-encapsulated-options option. An example of this is shown under the
VENDOR ENCAPSULATED OPTIONS head in the
dhcp-options(5) manual page.
PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION
You may specify a limit to the number of clients in a class that can be assigned
leases. The effect of this will be to make it difficult for a new client in a
class to get an address. Once a class with such a limit has reached its limit,
the only way a new client in that class can get a lease is for an existing
client to relinquish its lease, either by letting it expire, or by sending a
DHCPRELEASE packet. Classes with lease limits are specified as follows:
class "limited-1" {
lease limit 4;
}
This will produce a class in which a maximum of four members may hold a lease at
one time.
SPAWNING CLASSES
It is possible to declare a
spawning class. A spawning class is a class
that automatically produces subclasses based on what the client sends. The
reason that spawning classes were created was to make it possible to create
lease-limited classes on the fly. The envisioned application is a cable-modem
environment where the ISP wishes to provide clients at a particular site with
more than one IP address, but does not wish to provide such clients with their
own subnet, nor give them an unlimited number of IP addresses from the network
segment to which they are connected.
Many cable modem head-end systems can be configured to add a Relay Agent
Information option to DHCP packets when relaying them to the DHCP server.
These systems typically add a circuit ID or remote ID option that uniquely
identifies the customer site. To take advantage of this, you can write a class
declaration as follows:
class "customer" {
spawn with option agent.circuit-id;
lease limit 4;
}
Now whenever a request comes in from a customer site, the circuit ID option will
be checked against the class´s hash table. If a subclass is found that
matches the circuit ID, the client will be classified in that subclass and
treated accordingly. If no subclass is found matching the circuit ID, a new
one will be created and logged in the
dhcpd.leases file, and the client
will be classified in this new class. Once the client has been classified, it
will be treated according to the rules of the class, including, in this case,
being subject to the per-site limit of four leases.
The use of the subclass spawning mechanism is not restricted to relay agent
options - this particular example is given only because it is a fairly
straightforward one.
COMBINING MATCH, MATCH IF AND SPAWN WITH
In some cases, it may be useful to use one expression to assign a client to a
particular class, and a second expression to put it into a subclass of that
class. This can be done by combining the
match if and
spawn
with statements, or the
match if and
match statements. For
example:
class "jr-cable-modems" {
match if option dhcp-vendor-identifier = "jrcm";
spawn with option agent.circuit-id;
lease limit 4;
}
class "dv-dsl-modems" {
match if option dhcp-vendor-identifier = "dvdsl";
spawn with option agent.circuit-id;
lease limit 16;
}
This allows you to have two classes that both have the same
spawn
with expression without getting the clients in the two classes confused
with each other.
DYNAMIC DNS UPDATES
The DHCP server has the ability to dynamically update the Domain Name System.
Within the configuration files, you can define how you want the Domain Name
System to be updated. These updates are RFC 2136 compliant so any DNS server
supporting RFC 2136 should be able to accept updates from the DHCP server.
There are two DNS schemes implemented. The interim option is based on draft
revisions of the DDNS documents while the standard option is based on the RFCs
for DHCP-DNS interaction and DHCIDs. A third option, ad-hoc, was deprecated
and has now been removed from the code base. The DHCP server must be
configured to use one of the two currently-supported methods, or not to do DNS
updates.
New installations should use the standard option. Older installations may want
to continue using the interim option for backwards compatibility with the DNS
database until the database can be updated. This can be done with the
ddns-update-style configuration parameter.
THE DNS UPDATE SCHEME
the interim and standard DNS update schemes operate mostly according to work
from the IETF. The interim version was based on the drafts in progress at the
time while the standard is based on the completed RFCs. The standard RFCs are:
RFC 4701 (updated by RF5494)
RFC 4702
RFC 4703
And the corresponding drafts were:
draft-ietf-dnsext-dhcid-rr-??.txt
draft-ietf-dhc-fqdn-option-??.txt
draft-ietf-dhc-ddns-resolution-??.txt
The basic framework for the two schemes is similar with the main material
difference being that a DHCID RR is used in the standard version while the
interim versions uses a TXT RR. The format of the TXT record bears a
resemblance to the DHCID RR but it is not equivalent (MD5 vs SHA2, field
length differences etc).
In these two schemes the DHCP server does not necessarily always update both the
A and the PTR records. The FQDN option includes a flag which, when sent by the
client, indicates that the client wishes to update its own A record. In that
case, the server can be configured either to honor the client´s
intentions or ignore them. This is done with the statement
allow
client-updates; or the statement
ignore client-updates;. By
default, client updates are allowed.
If the server is configured to allow client updates, then if the client sends a
fully-qualified domain name in the FQDN option, the server will use that name
the client sent in the FQDN option to update the PTR record. For example, let
us say that the client is a visitor from the "radish.org" domain,
whose hostname is "jschmoe". The server is for the
"example.org" domain. The DHCP client indicates in the FQDN option
that its FQDN is "jschmoe.radish.org.". It also indicates that it
wants to update its own A record. The DHCP server therefore does not attempt
to set up an A record for the client, but does set up a PTR record for the IP
address that it assigns the client, pointing at jschmoe.radish.org. Once the
DHCP client has an IP address, it can update its own A record, assuming that
the "radish.org" DNS server will allow it to do so.
If the server is configured not to allow client updates, or if the client
doesn´t want to do its own update, the server will simply choose a name
for the client. By default, the server will choose from the following three
values:
1.
fqdn option (if present)
2. hostname option (if present)
3. Configured hostname option (if defined).
If these defaults for choosing the host name are not appropriate you can write
your own statement to set the ddns-hostname variable as you wish. If none of
the above are found the server will use the host declaration name (if one) and
use-host-decl-names is on.
It will use its own domain name for the client. It will then update both the A
and PTR record, using the name that it chose for the client. If the client
sends a fully-qualified domain name in the
fqdn option, the server uses
only the leftmost part of the domain name - in the example above,
"jschmoe" instead of "jschmoe.radish.org".
Further, if the
ignore client-updates; directive is used, then the server
will in addition send a response in the DHCP packet, using the FQDN Option,
that implies to the client that it should perform its own updates if it
chooses to do so. With
deny client-updates;, a response is sent which
indicates the client may not perform updates.
Both the standard and interim options also include a method to allow more than
one DHCP server to update the DNS database without accidentally deleting A
records that shouldn´t be deleted nor failing to add A records that
should be added. For the standard option the method works as follows:
When the DHCP server issues a client a new lease, it creates a text string that
is an SHA hash over the DHCP client´s identification (see RFCs 4701 &
4702 for details). The update attempts to add an A record with the name the
server chose and a DHCID record containing the hashed identifier string
(hashid). If this update succeeds, the server is done.
If the update fails because the A record already exists, then the DHCP server
attempts to add the A record with the prerequisite that there must be a DHCID
record in the same name as the new A record, and that DHCID record´s
contents must be equal to hashid. If this update succeeds, then the client has
its A record and PTR record. If it fails, then the name the client has been
assigned (or requested) is in use, and can´t be used by the client. At
this point the DHCP server gives up trying to do a DNS update for the client
until the client chooses a new name.
The server also does not update very aggressively. Because each DNS update
involves a round trip to the DNS server, there is a cost associated with doing
updates even if they do not actually modify the DNS database. So the DHCP
server tracks whether or not it has updated the record in the past (this
information is stored on the lease) and does not attempt to update records
that it thinks it has already updated.
This can lead to cases where the DHCP server adds a record, and then the record
is deleted through some other mechanism, but the server never again updates
the DNS because it thinks the data is already there. In this case the data can
be removed from the lease through operator intervention, and once this has
been done, the DNS will be updated the next time the client renews.
The interim DNS update scheme was written before the RFCs were finalized and
does not quite follow them. The RFCs call for a new DHCID RRtype while he
interim DNS update scheme uses a TXT record. In addition the ddns-resolution
draft called for the DHCP server to put a DHCID RR on the PTR record, but the
interim update method does not do this. In the final RFC this
requirement was relaxed such that a server may add a DHCID RR to the PTR
record.
DYNAMIC DNS UPDATE SECURITY
When you set your DNS server up to allow updates from the DHCP server, you may
be exposing it to unauthorized updates. To avoid this, you should use TSIG
signatures - a method of cryptographically signing updates using a shared
secret key. As long as you protect the secrecy of this key, your updates
should also be secure. Note, however, that the DHCP protocol itself provides
no security, and that clients can therefore provide information to the DHCP
server which the DHCP server will then use in its updates, with the
constraints described previously.
The DNS server must be configured to allow updates for any zone that the DHCP
server will be updating. For example, let us say that clients in the
sneedville.edu domain will be assigned addresses on the 10.10.17.0/24 subnet.
In that case, you will need a key declaration for the TSIG key you will be
using, and also two zone declarations - one for the zone containing A records
that will be updates and one for the zone containing PTR records - for ISC
BIND, something like this:
key DHCP_UPDATER {
algorithm HMAC-MD5.SIG-ALG.REG.INT;
secret pRP5FapFoJ95JEL06sv4PQ==;
};
zone "example.org" {
type master;
file "example.org.db";
allow-update { key DHCP_UPDATER; };
};
zone "17.10.10.in-addr.arpa" {
type master;
file "10.10.17.db";
allow-update { key DHCP_UPDATER; };
};
You will also have to configure your DHCP server to do updates to these zones.
To do so, you need to add something like this to your dhcpd.conf file:
key DHCP_UPDATER {
algorithm HMAC-MD5.SIG-ALG.REG.INT;
secret pRP5FapFoJ95JEL06sv4PQ==;
};
zone EXAMPLE.ORG. {
primary 127.0.0.1;
key DHCP_UPDATER;
}
zone 17.127.10.in-addr.arpa. {
primary 127.0.0.1;
key DHCP_UPDATER;
}
The
primary statement specifies the IP address of the name server whose
zone information is to be updated. In addition to the
primary statement
there are also the
primary6 ,
secondary and
secondary6
statements. The
primary6 statement specifies an IPv6 address for the
name server. The secondaries provide for additional addresses for name servers
to be used if the primary does not respond. The number of name servers the
DDNS code will attempt to use before giving up is limited and is currently set
to three.
Note that the zone declarations have to correspond to authority records in your
name server - in the above example, there must be an SOA record for
"example.org." and for "17.10.10.in-addr.arpa.". For
example, if there were a subdomain "foo.example.org" with no
separate SOA, you could not write a zone declaration for
"foo.example.org." Also keep in mind that zone names in your DHCP
configuration should end in a "."; this is the preferred syntax. If
you do not end your zone name in a ".", the DHCP server will figure
it out. Also note that in the DHCP configuration, zone names are not
encapsulated in quotes where there are in the DNS configuration.
You should choose your own secret key, of course. The ISC BIND 9 distribution
comes with a program for generating secret keys called dnssec-keygen. If you
are using BIND 9´s dnssec-keygen, the above key would be created as
follows:
dnssec-keygen -a HMAC-MD5 -b 128 -n USER DHCP_UPDATER
The key name, algorithm, and secret must match that being used by the DNS
server. The DHCP server currently supports the following algorithms:
HMAC-MD5
HMAC-SHA1
HMAC-SHA224
HMAC-SHA256
HMAC-SHA384
HMAC-SHA512
You may wish to enable logging of DNS updates on your DNS server. To do so, you
might write a logging statement like the following:
logging {
channel update_debug {
file "/var/log/update-debug.log";
severity debug 3;
print-category yes;
print-severity yes;
print-time yes;
};
channel security_info {
file "/var/log/named-auth.info";
severity info;
print-category yes;
print-severity yes;
print-time yes;
};
category update { update_debug; };
category security { security_info; };
};
You must create the /var/log/named-auth.info and /var/log/update-debug.log files
before starting the name server. For more information on configuring ISC BIND,
consult the documentation that accompanies it.
REFERENCE: EVENTS
There are three kinds of events that can happen regarding a lease, and it is
possible to declare statements that occur when any of these events happen.
These events are the commit event, when the server has made a commitment of a
certain lease to a client, the release event, when the client has released the
server from its commitment, and the expiry event, when the commitment expires.
To declare a set of statements to execute when an event happens, you must use
the
on statement, followed by the name of the event, followed by a
series of statements to execute when the event happens, enclosed in braces.
REFERENCE: DECLARATIONS
The include statement
include "filename";
The
include statement is used to read in a named file, and process the
contents of that file as though it were entered in place of the include
statement.
The shared-network statement
shared-network name {
[ parameters ]
[ declarations ]
}
The
shared-network statement is used to inform the DHCP server that some
IP subnets actually share the same physical network. Any subnets in a shared
network should be declared within a
shared-network statement.
Parameters specified in the
shared-network statement will be used when
booting clients on those subnets unless parameters provided at the subnet or
host level override them. If any subnet in a shared network has addresses
available for dynamic allocation, those addresses are collected into a common
pool for that shared network and assigned to clients as needed. There is no
way to distinguish on which subnet of a shared network a client should boot.
Name should be the name of the shared network. This name is used when
printing debugging messages, so it should be descriptive for the shared
network. The name may have the syntax of a valid domain name (although it will
never be used as such), or it may be any arbitrary name, enclosed in quotes.
The subnet statement
subnet subnet-number netmask netmask {
[ parameters ]
[ declarations ]
}
The
subnet statement is used to provide dhcpd with enough information to
tell whether or not an IP address is on that subnet. It may also be used to
provide subnet-specific parameters and to specify what addresses may be
dynamically allocated to clients booting on that subnet. Such addresses are
specified using the
range declaration.
The
subnet-number should be an IP address or domain name which resolves
to the subnet number of the subnet being described. The
netmask should
be an IP address or domain name which resolves to the subnet mask of the
subnet being described. The subnet number, together with the netmask, are
sufficient to determine whether any given IP address is on the specified
subnet.
Although a netmask must be given with every subnet declaration, it is
recommended that if there is any variance in subnet masks at a site, a
subnet-mask option statement be used in each subnet declaration to set the
desired subnet mask, since any subnet-mask option statement will override the
subnet mask declared in the subnet statement.
The subnet6 statement
subnet6 subnet6-number {
[ parameters ]
[ declarations ]
}
The
subnet6 statement is used to provide dhcpd with enough information to
tell whether or not an IPv6 address is on that subnet6. It may also be used to
provide subnet-specific parameters and to specify what addresses may be
dynamically allocated to clients booting on that subnet.
The
subnet6-number should be an IPv6 network identifier, specified as
ip6-address/bits.
The range statement
range [ dynamic-bootp ] low-address [ high-address];
For any subnet on which addresses will be assigned dynamically, there must be at
least one
range statement. The range statement gives the lowest and
highest IP addresses in a range. All IP addresses in the range should be in
the subnet in which the
range statement is declared. The
dynamic-bootp flag may be specified if addresses in the specified range
may be dynamically assigned to BOOTP clients as well as DHCP clients. When
specifying a single address,
high-address can be omitted.
The range6 statement
range6 low-address high-address;
range6 subnet6-number;
range6 subnet6-number temporary;
range6 address temporary;
For any IPv6 subnet6 on which addresses will be assigned dynamically, there must
be at least one
range6 statement. The
range6 statement can
either be the lowest and highest IPv6 addresses in a
range6, or use
CIDR notation, specified as ip6-address/bits. All IP addresses in the
range6 should be in the subnet6 in which the
range6 statement is
declared.
The
temporary variant makes the prefix (by default on 64 bits) available
for temporary (RFC 4941) addresses. A new address per prefix in the shared
network is computed at each request with an IA_TA option. Release and Confirm
ignores temporary addresses.
Any IPv6 addresses given to hosts with
fixed-address6 are excluded from
the
range6, as are IPv6 addresses on the server itself.
The prefix6 statement
prefix6 low-address high-address / bits;
The
prefix6 is the
range6 equivalent for Prefix Delegation (RFC
3633). Prefixes of
bits length are assigned between
low-address
and
high-address.
Any IPv6 prefixes given to static entries (hosts) with
fixed-prefix6 are
excluded from the
prefix6.
This statement is currently global but it should have a shared-network scope.
The host statement
host hostname {
[ parameters ]
[ declarations ]
}
The
host declaration provides a way for the DHCP server to identify a
DHCP or BOOTP client. This allows the server to provide configuration
information including fixed addresses or, in DHCPv6, fixed prefixes for a
specific client.
If it is desirable to be able to boot a DHCP or BOOTP client on more than one
subnet with fixed v4 addresses, more than one address may be specified in the
fixed-address declaration, or more than one
host statement may
be specified matching the same client.
The
fixed-address6 delcaration is used for v6 addresses. At this time it
only works with a single address. For multiple addresses specify multiple
host statements.
If client-specific boot parameters must change based on the network to which the
client is attached, then multiple
host declarations should be used. The
host declarations will only match a client if one of their
fixed-address statements is viable on the subnet (or shared network)
where the client is attached. Conversely, for a
host declaration to
match a client being allocated a dynamic address, it must not have any
fixed-address statements. You may therefore need a mixture of
host declarations for any given client...some having
fixed-address statements, others without.
hostname should be a name identifying the host. If a
hostname
option is not specified for the host,
hostname is used.
Host declarations are matched to actual DHCP or BOOTP clients by matching
the dhcp-client-identifier option specified in the
host declaration to
the one supplied by the client, or, if the
host declaration or the
client does not provide a dhcp-client-identifier option, by matching the
hardware parameter in the
host declaration to the network
hardware address supplied by the client. BOOTP clients do not normally provide
a
dhcp-client-identifier, so the hardware address must be used for all
clients that may boot using the BOOTP protocol.
DHCPv6 servers can use the
host-identifier option parameter in the
host declaration, and specify any option with a fixed value to identify
hosts.
Please be aware that
only the
dhcp-client-identifier option and
the hardware address can be used to match a host declaration, or the
host-identifier option parameter for DHCPv6 servers. For example, it is
not possible to match a host declaration to a
host-name option. This is
because the host-name option cannot be guaranteed to be unique for any given
client, whereas both the hardware address and
dhcp-client-identifier
option are at least theoretically guaranteed to be unique to a given client.
The group statement
group {
[ parameters ]
[ declarations ]
}
The group statement is used simply to apply one or more parameters to a group of
declarations. It can be used to group hosts, shared networks, subnets, or even
other groups.
REFERENCE: ALLOW AND DENY
The
allow and
deny statements can be used to control the response
of the DHCP server to various sorts of requests. The allow and deny keywords
actually have different meanings depending on the context. In a pool context,
these keywords can be used to set up access lists for address allocation
pools. In other contexts, the keywords simply control general server behavior
with respect to clients based on scope. In a non-pool context, the
ignore keyword can be used in place of the
deny keyword to
prevent logging of denied requests.
ALLOW DENY AND IGNORE IN SCOPE
The following usages of allow and deny will work in any scope, although it is
not recommended that they be used in pool declarations.
The unknown-clients keyword
allow unknown-clients;
deny unknown-clients;
ignore unknown-clients;
The
unknown-clients flag is used to tell dhcpd whether or not to
dynamically assign addresses to unknown clients. Dynamic address assignment to
unknown clients is
allowed by default. An unknown client is simply a
client that has no host declaration.
The use of this option is now
deprecated. If you are trying to restrict
access on your network to known clients, you should use
deny
unknown-clients; inside of your address pool, as described under the
heading ALLOW AND DENY WITHIN POOL DECLARATIONS.
The bootp keyword
allow bootp;
deny bootp;
ignore bootp;
The
bootp flag is used to tell dhcpd whether or not to respond to bootp
queries. Bootp queries are
allowed by default.
The booting keyword
allow booting;
deny booting;
ignore booting;
The
booting flag is used to tell dhcpd whether or not to respond to
queries from a particular client. This keyword only has meaning when it
appears in a host declaration. By default, booting is
allowed, but if
it is disabled for a particular client, then that client will not be able to
get an address from the DHCP server.
The duplicates keyword
allow duplicates;
deny duplicates;
Host declarations can match client messages based on the DHCP Client Identifier
option or based on the client's network hardware type and MAC address. If the
MAC address is used, the host declaration will match any client with that MAC
address - even clients with different client identifiers. This doesn't
normally happen, but is possible when one computer has more than one operating
system installed on it - for example, Microsoft Windows and NetBSD or Linux.
The
duplicates flag tells the DHCP server that if a request is received
from a client that matches the MAC address of a host declaration, any other
leases matching that MAC address should be discarded by the server, even if
the UID is not the same. This is a violation of the DHCP protocol, but can
prevent clients whose client identifiers change regularly from holding many
leases at the same time. By default, duplicates are
allowed.
The declines keyword
allow declines;
deny declines;
ignore declines;
The DHCPDECLINE message is used by DHCP clients to indicate that the lease the
server has offered is not valid. When the server receives a DHCPDECLINE for a
particular address, it normally abandons that address, assuming that some
unauthorized system is using it. Unfortunately, a malicious or buggy client
can, using DHCPDECLINE messages, completely exhaust the DHCP server's
allocation pool. The server will eventually reclaim these leases, but not
while the client is running through the pool. This may cause serious thrashing
in the DNS, and it will also cause the DHCP server to forget old DHCP client
address allocations.
The
declines flag tells the DHCP server whether or not to honor
DHCPDECLINE messages. If it is set to
deny or
ignore in a
particular scope, the DHCP server will not respond to DHCPDECLINE messages.
The
declines flag is only supported by DHCPv4 servers. Given the large
IPv6 address space and the internal limits imposed by the server's address
generation mechanism we don't think it is necessary for DHCPv6 servers at this
time.
Currently, abandoned IPv6 addresses are reclaimed in one of two ways:
a) Client renews a specific address:
If a client using a given DUID submits a DHCP REQUEST containing
the last address abandoned by that DUID, the address will be
reassigned to that client.
b) Upon the second restart following an address abandonment. When
an address is abandoned it is both recorded as such in the lease
file and retained as abandoned in server memory until the server
is restarted. Upon restart, the server will process the lease file
and all addresses whose last known state is abandoned will be
retained as such in memory but not rewritten to the lease file.
This means that a subsequent restart of the server will not see the
abandoned addresses in the lease file and therefore have no record
of them as abandoned in memory and as such perceive them as free
for assignment.
The total number addresses in a pool, available for a given DUID value, is
internally limited by the server's address generation mechanism. If through
mistaken configuration, multiple clients are using the same DUID they will
competing for the same addresses causing the server to reach this internal
limit rather quickly. The internal limit isolates this type of activity such
that address range is not exhausted for other DUID values. The appearance of
the following error log, can be an indication of this condition:
"Best match for DUID <XX> is an abandoned address, This may be a
result of multiple clients attempting to use this DUID"
where <XX> is an actual DUID value depicted as colon separated
string of bytes in hexadecimal values.
The client-updates keyword
allow client-updates;
deny client-updates;
The
client-updates flag tells the DHCP server whether or not to honor the
client's intention to do its own update of its A record. This is only relevant
when doing
interim DNS updates. See the documentation under the heading
THE INTERIM DNS UPDATE SCHEME for details.
The leasequery keyword
allow leasequery;
deny leasequery;
The
leasequery flag tells the DHCP server whether or not to answer
DHCPLEASEQUERY packets. The answer to a DHCPLEASEQUERY packet includes
information about a specific lease, such as when it was issued and when it
will expire. By default, the server will not respond to these packets.
ALLOW AND DENY WITHIN POOL DECLARATIONS
The uses of the allow and deny keywords shown in the previous section work
pretty much the same way whether the client is sending a DHCPDISCOVER or a
DHCPREQUEST message - an address will be allocated to the client (either the
old address it's requesting, or a new address) and then that address will be
tested to see if it's okay to let the client have it. If the client requested
it, and it's not okay, the server will send a DHCPNAK message. Otherwise, the
server will simply not respond to the client. If it is okay to give the
address to the client, the server will send a DHCPACK message.
The primary motivation behind pool declarations is to have address allocation
pools whose allocation policies are different. A client may be denied access
to one pool, but allowed access to another pool on the same network segment.
In order for this to work, access control has to be done during address
allocation, not after address allocation is done.
When a DHCPREQUEST message is processed, address allocation simply consists of
looking up the address the client is requesting and seeing if it's still
available for the client. If it is, then the DHCP server checks both the
address pool permit lists and the relevant in-scope allow and deny statements
to see if it's okay to give the lease to the client. In the case of a
DHCPDISCOVER message, the allocation process is done as described previously
in the ADDRESS ALLOCATION section.
When declaring permit lists for address allocation pools, the following syntaxes
are recognized following the allow or deny keywords:
known-clients;
If specified, this statement either allows or prevents allocation from this pool
to any client that has a host declaration (i.e., is known). A client is known
if it has a host declaration in
any scope, not just the current scope.
unknown-clients;
If specified, this statement either allows or prevents allocation from this pool
to any client that has no host declaration (i.e., is not known).
members of "class
";
If specified, this statement either allows or prevents allocation from this pool
to any client that is a member of the named class.
dynamic bootp clients;
If specified, this statement either allows or prevents allocation from this pool
to any bootp client.
authenticated clients;
If specified, this statement either allows or prevents allocation from this pool
to any client that has been authenticated using the DHCP authentication
protocol. This is not yet supported.
unauthenticated clients;
If specified, this statement either allows or prevents allocation from this pool
to any client that has not been authenticated using the DHCP authentication
protocol. This is not yet supported.
all clients;
If specified, this statement either allows or prevents allocation from this pool
to all clients. This can be used when you want to write a pool declaration for
some reason, but hold it in reserve, or when you want to renumber your network
quickly, and thus want the server to force all clients that have been
allocated addresses from this pool to obtain new addresses immediately when
they next renew.
after time;
If specified, this statement either allows or prevents allocation from this pool
after a given date. This can be used when you want to move clients from one
pool to another. The server adjusts the regular lease time so that the latest
expiry time is at the given time+min-lease-time. A short min-lease-time
enforces a step change, whereas a longer min-lease-time allows for a gradual
change.
time is either second since epoch, or a UTC time string e.g. 4
2007/08/24 09:14:32 or a string with time zone offset in seconds e.g. 4
2007/08/24 11:14:32 -7200
REFERENCE: PARAMETERS
The
adaptive-lease-time-threshold statement
adaptive-lease-time-threshold percentage;
When the number of allocated leases within a pool rises above the
percentage given in this statement, the DHCP server decreases the lease
length for new clients within this pool to
min-lease-time seconds.
Clients renewing an already valid (long) leases get at least the remaining
time from the current lease. Since the leases expire faster, the server may
either recover more quickly or avoid pool exhaustion entirely. Once the number
of allocated leases drop below the threshold, the server reverts back to
normal lease times. Valid percentages are between 1 and 99.
The
always-broadcast statement
always-broadcast flag;
The DHCP and BOOTP protocols both require DHCP and BOOTP clients to set the
broadcast bit in the flags field of the BOOTP message header. Unfortunately,
some DHCP and BOOTP clients do not do this, and therefore may not receive
responses from the DHCP server. The DHCP server can be made to always
broadcast its responses to clients by setting this flag to ´on´ for
the relevant scope; relevant scopes would be inside a conditional statement,
as a parameter for a class, or as a parameter for a host declaration. To avoid
creating excess broadcast traffic on your network, we recommend that you
restrict the use of this option to as few clients as possible. For example,
the Microsoft DHCP client is known not to have this problem, as are the
OpenTransport and ISC DHCP clients.
The
always-reply-rfc1048 statement
always-reply-rfc1048 flag;
Some BOOTP clients expect RFC1048-style responses, but do not follow RFC1048
when sending their requests. You can tell that a client is having this problem
if it is not getting the options you have configured for it and if you see in
the server log the message "(non-rfc1048)" printed with each
BOOTREQUEST that is logged.
If you want to send rfc1048 options to such a client, you can set the
always-reply-rfc1048 option in that client's host declaration, and the
DHCP server will respond with an RFC-1048-style vendor options field. This
flag can be set in any scope, and will affect all clients covered by that
scope.
The
authoritative statement
authoritative;
not authoritative;
The DHCP server will normally assume that the configuration information about a
given network segment is not known to be correct and is not authoritative.
This is so that if a naive user installs a DHCP server not fully understanding
how to configure it, it does not send spurious DHCPNAK messages to clients
that have obtained addresses from a legitimate DHCP server on the network.
Network administrators setting up authoritative DHCP servers for their networks
should always write
authoritative; at the top of their configuration
file to indicate that the DHCP server
should send DHCPNAK messages to
misconfigured clients. If this is not done, clients will be unable to get a
correct IP address after changing subnets until their old lease has expired,
which could take quite a long time.
Usually, writing
authoritative; at the top level of the file should be
sufficient. However, if a DHCP server is to be set up so that it is aware of
some networks for which it is authoritative and some networks for which it is
not, it may be more appropriate to declare authority on a per-network-segment
basis.
Note that the most specific scope for which the concept of authority makes any
sense is the physical network segment - either a shared-network statement or a
subnet statement that is not contained within a shared-network statement. It
is not meaningful to specify that the server is authoritative for some subnets
within a shared network, but not authoritative for others, nor is it
meaningful to specify that the server is authoritative for some host
declarations and not others.
The
boot-unknown-clients statement
boot-unknown-clients flag;
If the
boot-unknown-clients statement is present and has a value of
false or
off, then clients for which there is no
host
declaration will not be allowed to obtain IP addresses. If this statement is
not present or has a value of
true or
on, then clients without
host declarations will be allowed to obtain IP addresses, as long as those
addresses are not restricted by
allow and
deny statements within
their
pool declarations.
The
db-time-format statement
db-time-format [ default | local ] ;
The DHCP server software outputs several timestamps when writing leases to
persistent storage. This configuration parameter selects one of two output
formats. The
default format prints the day, date, and time in UTC,
while the
local format prints the system seconds-since-epoch, and
helpfully provides the day and time in the system timezone in a comment. The
time formats are described in detail in the dhcpd.leases(5) manpage.
The
ddns-hostname statement
ddns-hostname name;
The
name parameter should be the hostname that will be used in setting up
the client's A and PTR records. If no
ddns-hostname is specified in
scope, then the server will derive the hostname automatically, using an
algorithm that varies for each of the different update methods.
The
ddns-domainname statement
ddns-domainname name;
The
name parameter should be the domain name that will be appended to the
client's hostname to form a fully-qualified domain-name (FQDN).
The dns-local-address4 and dns-local-address6 statements
ddns-local-address4 address;
ddns-local-address6 address;
The
address parameter should be the local IPv4 or IPv6 address the server
should use as the from address when sending DDNS update requests.
The
ddns-rev-domainname statement
ddns-rev-domainname name;
The
name parameter should be the domain name that will be appended to the
client's reversed IP address to produce a name for use in the client's PTR
record. By default, this is "in-addr.arpa.", but the default can be
overridden here.
The reversed IP address to which this domain name is appended is always the IP
address of the client, in dotted quad notation, reversed - for example, if the
IP address assigned to the client is 10.17.92.74, then the reversed IP address
is 74.92.17.10. So a client with that IP address would, by default, be given a
PTR record of 10.17.92.74.in-addr.arpa.
The
ddns-update-style parameter
ddns-update-style style;
The
style parameter must be one of
standard,
interim or
none. The
ddns-update-style statement is only meaningful in the
outer scope - it is evaluated once after reading the dhcpd.conf file, rather
than each time a client is assigned an IP address, so there is no way to use
different DNS update styles for different clients. The default is
none.
The ddns-updates statement
ddns-updates flag;
The
ddns-updates parameter controls whether or not the server will
attempt to do a DNS update when a lease is confirmed. Set this to
off
if the server should not attempt to do updates within a certain scope. The
ddns-updates parameter is on by default. To disable DNS updates in all
scopes, it is preferable to use the
ddns-update-style statement,
setting the style to
none.
The
default-lease-time statement
default-lease-time time;
Time should be the length in seconds that will be assigned to a lease if
the client requesting the lease does not ask for a specific expiration time.
This is used for both DHCPv4 and DHCPv6 leases (it is also known as the
"valid lifetime" in DHCPv6). The default is 43200 seconds.
The
delayed-ack and
max-ack-delay statements
delayed-ack count;
max-ack-delay microseconds;
Count should be an integer value from zero to 2^16-1, and defaults to 28.
The count represents how many DHCPv4 replies maximum will be queued pending
transmission until after a database commit event. If this number is reached, a
database commit event (commonly resulting in fsync() and representing a
performance penalty) will be made, and the reply packets will be transmitted
in a batch afterwards. This preserves the RFC2131 direction that "stable
storage" be updated prior to replying to clients. Should the DHCPv4
sockets "go dry" (select() returns immediately with no read
sockets), the commit is made and any queued packets are transmitted.
Similarly,
microseconds indicates how many microseconds are permitted to
pass inbetween queuing a packet pending an fsync, and performing the fsync.
Valid values range from 0 to 2^32-1, and defaults to 250,000 (1/4 of a
second).
Please note that as delayed-ack is currently experimental, the delayed-ack
feature is not compiled in by default, but must be enabled at compile time
with ´./configure --enable-delayed-ack´.
The
dhcp-cache-threshold statement
dhcp-cache-threshold percentage;
The
dhcp-cache-threshold statement takes one integer parameter with
allowed values between 0 and 100. The default value is 25 (25% of the lease
time). This parameter expresses the percentage of the total lease time,
measured from the beginning, during which a client's attempt to renew its
lease will result in getting the already assigned lease, rather than an
extended lease.
Clients that attempt renewal frequently can cause the server to update and write
the database frequently resulting in a performance impact on the server. The
dhcp-cache-threshold statement instructs the DHCP server to avoid
updating leases too frequently thus avoiding this behavior. Instead the server
assigns the same lease (i.e. reuses it) with no modifications except for CLTT
(Client Last Transmission Time) which does not require disk operations. This
feature applies to IPv4 only.
When an existing lease is matched to a renewing client, it will be reused if all
of the following conditions are true:
1. The dhcp-cache-threshold is larger than zero
2. The current lease is active
3. The percentage of the lease time that has elapsed is less than
dhcp-cache-threshold
4. The client information provided in the renewal does not alter
any of the following:
a. DNS information and DNS updates are enabled
b. Billing class to which the lease is associated
The
do-forward-updates statement
do-forward-updates flag;
The
do-forward-updates statement instructs the DHCP server as to whether
it should attempt to update a DHCP client´s A record when the client
acquires or renews a lease. This statement has no effect unless DNS updates
are enabled. Forward updates are enabled by default. If this statement is used
to disable forward updates, the DHCP server will never attempt to update the
client´s A record, and will only ever attempt to update the client´s
PTR record if the client supplies an FQDN that should be placed in the PTR
record using the
fqdn option. If forward updates are enabled, the DHCP
server will still honor the setting of the
client-updates flag.
The
dont-use-fsync statement
dont-use-fsync flag;
The
dont-use-fsync statement instructs the DHCP server if it should call
fsync() when writing leases to the lease file. By default and if the flag is
set to false the server
will call fsync(). Suppressing the call to
fsync() may increase the performance of the server but it also adds a risk
that a lease will not be properly written to the disk after it has been issued
to a client and before the server stops. This can lead to duplicate leases
being issued to different clients. Using this option is
not
recommended.
The
dynamic-bootp-lease-cutoff statement
dynamic-bootp-lease-cutoff date;
The
dynamic-bootp-lease-cutoff statement sets the ending time for all
leases assigned dynamically to BOOTP clients. Because BOOTP clients do not
have any way of renewing leases, and don't know that their leases could
expire, by default dhcpd assigns infinite leases to all BOOTP clients.
However, it may make sense in some situations to set a cutoff date for all
BOOTP leases - for example, the end of a school term, or the time at night
when a facility is closed and all machines are required to be powered off.
Date should be the date on which all assigned BOOTP leases will end. The
date is specified in the form:
W YYYY/MM/DD HH:MM:SS
W is the day of the week expressed as a number from zero (Sunday) to six
(Saturday). YYYY is the year, including the century. MM is the month expressed
as a number from 1 to 12. DD is the day of the month, counting from 1. HH is
the hour, from zero to 23. MM is the minute and SS is the second. The time is
always in Coordinated Universal Time (UTC), not local time.
The
dynamic-bootp-lease-length statement
dynamic-bootp-lease-length length;
The
dynamic-bootp-lease-length statement is used to set the length of
leases dynamically assigned to BOOTP clients. At some sites, it may be
possible to assume that a lease is no longer in use if its holder has not used
BOOTP or DHCP to get its address within a certain time period. The period is
specified in
length as a number of seconds. If a client reboots using
BOOTP during the timeout period, the lease duration is reset to
length,
so a BOOTP client that boots frequently enough will never lose its lease.
Needless to say, this parameter should be adjusted with extreme caution.
The
echo-client-id statement
echo-client-id flag;
The
echo-client-id statement is used to enable or disable RFC 6842
compliant behavior. If the echo-client-id statement is present and has a value
of true or on, and a DHCP DISCOVER or REQUEST is received which contains the
client identifier option (Option code 61), the server will copy the option
into its response (DHCP ACK or NAK) per RFC 6842. In other words if the client
sends the option it will receive it back. By default, this flag is off and
client identifiers will not echoed back to the client.
The
filename statement
filename "filename";
The
filename statement can be used to specify the name of the initial
boot file which is to be loaded by a client. The
filename should be a
filename recognizable to whatever file transfer protocol the client can be
expected to use to load the file.
The
fixed-address declaration
fixed-address address [, address ... ];
The
fixed-address declaration is used to assign one or more fixed IP
addresses to a client. It should only appear in a
host declaration. If
more than one address is supplied, then when the client boots, it will be
assigned the address that corresponds to the network on which it is booting.
If none of the addresses in the
fixed-address statement are valid for
the network to which the client is connected, that client will not match the
host declaration containing that
fixed-address declaration. Each
address in the
fixed-address declaration should be either an IP
address or a domain name that resolves to one or more IP addresses.
The
fixed-address6 declaration
fixed-address6 ip6-address ;
The
fixed-address6 declaration is used to assign a fixed IPv6 addresses
to a client. It should only appear in a
host declaration.
The
fixed-prefix6 declaration
fixed-prefix6 low-address / bits;
The
fixed-prefix6 declaration is used to assign a fixed IPv6 prefix to a
client. It should only appear in a
host declaration, but multiple
fixed-prefix6 statements may appear in a single
host
declaration.
The
low-address specifies the start of the prefix and the
bits
specifies the size of the prefix in bits.
If there are multiple prefixes for a given host entry the server will choose one
that matches the requested prefix size or, if none match, the first one.
If there are multiple
host delcarations the server will try to choose a
declaration where the
fixed-address6 matches the client's subnet. If
none match it will choose one that doesn't have a
fixed-address6
statement.
Note Well: Unlike the fixed address the fixed prefix does not need to match a
subnet in order to be served. This allows you to provide a prefix to a client
that is outside of the subnet on which the client makes the request to the the
server.
The
get-lease-hostnames statement
get-lease-hostnames flag;
The
get-lease-hostnames statement is used to tell dhcpd whether or not to
look up the domain name corresponding to the IP address of each address in the
lease pool and use that address for the DHCP
hostname option. If
flag is true, then this lookup is done for all addresses in the current
scope. By default, or if
flag is false, no lookups are done.
The
hardware statement
hardware hardware-type hardware-address;
In order for a BOOTP client to be recognized, its network hardware address must
be declared using a
hardware clause in the
host statement.
hardware-type must be the name of a physical hardware interface type.
Currently, only the
ethernet and
token-ring types are
recognized, although support for a
fddi hardware type (and others)
would also be desirable. The
hardware-address should be a set of
hexadecimal octets (numbers from 0 through ff) separated by colons. The
hardware statement may also be used for DHCP clients.
The
host-identifier option statement
host-identifier option option-name option-data;
or
host-identifier v6relopt number option-name option-data;
This identifies a DHCPv6 client in a
host statement.
option-name
is any option, and
option-data is the value for the option that the
client will send. The
option-data must be a constant value. In the
v6relopts case the additional number is the relay to examine for the specified
option name and value. The values are the same as for the v6relay option. 0 is
a no-op, 1 is the relay closest to the client, 2 the next one in and so on.
Values that are larger than the maximum number of relays (currently 32)
indicate the relay closest to the server independent of number.
The
ignore-client-uids statement
ignore-client-uids flag;
If the
ignore-client-uids statement is present and has a value of
true or
on, the UID for clients will not be recorded. If this
statement is not present or has a value of
false or
off, then
client UIDs will be recorded.
The
infinite-is-reserved statement
infinite-is-reserved flag;
ISC DHCP now supports ´reserved´ leases. See the section on RESERVED
LEASES below. If this
flag is on, the server will automatically reserve
leases allocated to clients which requested an infinite (0xffffffff)
lease-time.
The default is off.
The
lease-file-name statement
lease-file-name name;
Name should be the name of the DHCP server's lease file. By default, this
is DBDIR/dhcpd.leases. This statement
must appear in the outer scope of
the configuration file - if it appears in some other scope, it will have no
effect. Furthermore, it has no effect if overridden by the
-lf flag or
the
PATH_DHCPD_DB environment variable.
The
limit-addrs-per-ia statement
limit-addrs-per-ia number;
By default, the DHCPv6 server will limit clients to one IAADDR per IA option,
meaning one address. If you wish to permit clients to hang onto multiple
addresses at a time, configure a larger
number here.
Note that there is no present method to configure the server to forcibly
configure the client with one IP address per each subnet on a shared network.
This is left to future work.
The
dhcpv6-lease-file-name statement
dhcpv6-lease-file-name name;
Name is the name of the lease file to use if and only if the server is
running in DHCPv6 mode. By default, this is DBDIR/dhcpd6.leases. This
statement, like
lease-file-name, must appear in the outer scope
of the configuration file. It has no effect if overridden by the
-lf
flag or the
PATH_DHCPD6_DB environment variable. If
dhcpv6-lease-file-name is not specified, but
lease-file-name is,
the latter value will be used.
The
local-port statement
local-port port;
This statement causes the DHCP server to listen for DHCP requests on the UDP
port specified in
port, rather than on port 67.
The
local-address statement
local-address address;
This statement causes the DHCP server to listen for DHCP requests sent to the
specified
address, rather than requests sent to all addresses. Since
serving directly attached DHCP clients implies that the server must respond to
requests sent to the all-ones IP address, this option cannot be used if
clients are on directly attached networks; it is only realistically useful for
a server whose only clients are reached via unicasts, such as via DHCP relay
agents.
Note: This statement is only effective if the server was compiled using the
USE_SOCKETS #define statement, which is default on a small number of operating
systems, and must be explicitly chosen at compile-time for all others. You can
be sure if your server is compiled with USE_SOCKETS if you see lines of this
format at startup:
Listening on Socket/eth0
Note also that since this bind()s all DHCP sockets to the specified address,
that only one address may be supported in a daemon at a given time.
The
log-facility statement
log-facility facility;
This statement causes the DHCP server to do all of its logging on the specified
log facility once the dhcpd.conf file has been read. By default the DHCP
server logs to the daemon facility. Possible log facilities include auth,
authpriv, cron, daemon, ftp, kern, lpr, mail, mark, news, ntp, security,
syslog, user, uucp, and local0 through local7. Not all of these facilities are
available on all systems, and there may be other facilities available on other
systems.
In addition to setting this value, you may need to modify your
syslog.conf file to configure logging of the DHCP server. For example,
you might add a line like this:
local7.debug /var/log/dhcpd.log
The syntax of the
syslog.conf file may be different on some operating
systems - consult the
syslog.conf manual page to be sure. To get syslog
to start logging to the new file, you must first create the file with correct
ownership and permissions (usually, the same owner and permissions of your
/var/log/messages or /usr/adm/messages file should be fine) and send a SIGHUP
to syslogd. Some systems support log rollover using a shell script or program
called newsyslog or logrotate, and you may be able to configure this as well
so that your log file doesn't grow uncontrollably.
Because the
log-facility setting is controlled by the dhcpd.conf file,
log messages printed while parsing the dhcpd.conf file or before parsing it
are logged to the default log facility. To prevent this, see the README file
included with this distribution, which describes BUG: where is that mentioned
in README? how to change the default log facility. When this parameter is
used, the DHCP server prints its startup message a second time after parsing
the configuration file, so that the log will be as complete as possible.
The
log-threshold-high and
log-threshold-low statements
log-threshold-high percentage;
log-threshold-low percentage;
The
log-threshold-low and
log-threshold-high statements are used
to control when a message is output about pool usage. The value for both of
them is the percentage of the pool in use. If the high threshold is 0 or has
not been specified, no messages will be produced. If a high threshold is
given, a message is output once the pool usage passes that level. After that,
no more messages will be output until the pool usage falls below the low
threshold. If the low threshold is not given, it default to a value of zero.
A special case occurs when the low threshold is set to be higer than the high
threshold. In this case, a message will be generated each time a lease is
acknowledged when the pool usage is above the high threshold.
Note that threshold logging will be automatically disabled for shared subnets
whose total number of addresses is larger than (2^64)-1. The server will emit
a log statement at startup when threshold logging is disabled as shown below:
"Threshold logging disabled for shared subnet of ranges:
<addresses>"
This is likely to have no practical runtime effect as CPUs are unlikely to
support a server actually reaching such a large number of leases.
The
max-lease-time statement
max-lease-time time;
Time should be the maximum length in seconds that will be assigned to a
lease. If not defined, the default maximum lease time is 86400. The only
exception to this is that Dynamic BOOTP lease lengths, which are not specified
by the client, are not limited by this maximum.
The
min-lease-time statement
min-lease-time time;
Time should be the minimum length in seconds that will be assigned to a
lease. The default is the minimum of 300 seconds or
max-lease-time.
The
min-secs statement
min-secs seconds;
Seconds should be the minimum number of seconds since a client began
trying to acquire a new lease before the DHCP server will respond to its
request. The number of seconds is based on what the client reports, and the
maximum value that the client can report is 255 seconds. Generally, setting
this to one will result in the DHCP server not responding to the client's
first request, but always responding to its second request.
This can be used to set up a secondary DHCP server which never offers an address
to a client until the primary server has been given a chance to do so. If the
primary server is down, the client will bind to the secondary server, but
otherwise clients should always bind to the primary. Note that this does not,
by itself, permit a primary server and a secondary server to share a pool of
dynamically-allocatable addresses.
The
next-server statement
next-server server-name;
The
next-server statement is used to specify the host address of the
server from which the initial boot file (specified in the
filename
statement) is to be loaded.
Server-name should be a numeric IP address
or a domain name.
The
omapi-port statement
omapi-port port;
The
omapi-port statement causes the DHCP server to listen for OMAPI
connections on the specified port. This statement is required to enable the
OMAPI protocol, which is used to examine and modify the state of the DHCP
server as it is running.
The
one-lease-per-client statement
one-lease-per-client flag;
If this flag is enabled, whenever a client sends a DHCPREQUEST for a particular
lease, the server will automatically free any other leases the client holds.
This presumes that when the client sends a DHCPREQUEST, it has forgotten any
lease not mentioned in the DHCPREQUEST - i.e., the client has only a single
network interface
and it does not remember leases it's holding on
networks to which it is not currently attached. Neither of these assumptions
are guaranteed or provable, so we urge caution in the use of this
statement.
The
pid-file-name statement
pid-file-name name;
Name should be the name of the DHCP server's process ID file. This is the
file in which the DHCP server's process ID is stored when the server starts.
By default, this is RUNDIR/dhcpd.pid. Like the
lease-file-name
statement, this statement must appear in the outer scope of the configuration
file. It has no effect if overridden by the
-pf flag or the
PATH_DHCPD_PID environment variable.
The
dhcpv6-pid-file-name statement
dhcpv6-pid-file-name name;
Name is the name of the pid file to use if and only if the server is
running in DHCPv6 mode. By default, this is DBDIR/dhcpd6.pid. This statement,
like
pid-file-name, must appear in the outer scope of the
configuration file. It has no effect if overridden by the
-pf flag or
the
PATH_DHCPD6_PID environment variable. If
dhcpv6-pid-file-name is not specified, but
pid-file-name is, the
latter value will be used.
The
ping-check statement
ping-check flag;
When the DHCP server is considering dynamically allocating an IP address to a
client, it first sends an ICMP Echo request (a
ping) to the address
being assigned. It waits for a second, and if no ICMP Echo response has been
heard, it assigns the address. If a response
is heard, the lease is
abandoned, and the server does not respond to the client.
This
ping check introduces a default one-second delay in responding to
DHCPDISCOVER messages, which can be a problem for some clients. The default
delay of one second may be configured using the ping-timeout parameter. The
ping-check configuration parameter can be used to control checking - if its
value is false, no ping check is done.
The
ping-timeout statement
ping-timeout seconds;
If the DHCP server determined it should send an ICMP echo request (a
ping) because the ping-check statement is true, ping-timeout allows you
to configure how many seconds the DHCP server should wait for an ICMP Echo
response to be heard, if no ICMP Echo response has been received before the
timeout expires, it assigns the address. If a response
is heard, the
lease is abandoned, and the server does not respond to the client. If no value
is set, ping-timeout defaults to 1 second.
The
preferred-lifetime statement
preferred-lifetime seconds;
IPv6 addresses have ´valid´ and ´preferred´ lifetimes. The
valid lifetime determines at what point at lease might be said to have
expired, and is no longer useable. A preferred lifetime is an advisory
condition to help applications move off of the address and onto currently
valid addresses (should there still be any open TCP sockets or similar).
The preferred lifetime defaults to the renew+rebind timers, or 3/4 the default
lease time if none were specified.
The
prefix-length-mode statement
prefix-length-mode mode;
According to RFC 3633, DHCPv6 clients may specify preferences when soliciting
prefixes by including an IA_PD Prefix option within the IA_PD option. Among
the preferences that may be conveyed is the "prefix-length". When
non-zero it indicates a client's desired length for offered prefixes. The RFC
states that servers "MAY choose to use the information...to select
prefix(es)" but does not specify any particular rules for doing so. The
prefix-length-mode statement can be used to set the prefix selection rules
employed by the server, when clients send a non-zero prefix-length value. The
mode parameter must be one of
ignore,
prefer,
exact,
minimum, or
maximum where:
1. ignore - The requested length is ignored. The server will offer the first
available prefix.
2. prefer - The server will offer the first available prefix with the same
length as the requested length. If none are found then it will offer the first
available prefix of any length.
3. exact - The server will offer the first available prefix with the same length
as the requested length. If none are found, it will return a status indicating
no prefixes available. This is the default behavior.
4. minimum - The server will offer the first available prefix with the same
length as the requested length. If none are found, it will return the first
available prefix whose length is greater than (e.g. longer than), the
requested value. If none of those are found, it will return a status
indicating no prefixes available. For example, if client requests a length of
/60, and the server has available prefixes of lengths /56 and /64, it will
offer prefix of length /64.
5. maximum - The server will offer the first available prefix with the same
length as the requested length. If none are found, it will return the first
available prefix whose length is less than (e.g. shorter than), the requested
value. If none of those are found, it will return a status indicating no
prefixes available. For example, if client requests a length of /60, and the
server has available prefixes of lengths /56 and /64, it will offer a prefix
of length /56.
In general "first available" is determined by the order in which pools
are defined in the server's configuration. For example, if a subnet is defined
with three prefix pools A,B, and C:
subnet 3000::/64 {
# pool A
pool6 {
:
}
# pool B
pool6 {
:
}
# pool C
pool6 {
:
}
}
then the pools will be checked in the order A, B, C. For modes
prefer,
minimum, and
maximum this may mean checking the pools in that
order twice. A first pass through is made looking for an available prefix of
exactly the preferred length. If none are found, then a second pass is
performed starting with pool A but with appropriately adjusted length
criteria.
The
remote-port statement
remote-port port;
This statement causes the DHCP server to transmit DHCP responses to DHCP clients
upon the UDP port specified in
port, rather than on port 68. In the
event that the UDP response is transmitted to a DHCP Relay, the server
generally uses the
local-port configuration value. Should the DHCP
Relay happen to be addressed as 127.0.0.1, however, the DHCP Server transmits
its response to the
remote-port configuration value. This is generally
only useful for testing purposes, and this configuration value should
generally not be used.
The
server-identifier statement
server-identifier hostname;
The server-identifier statement can be used to define the value that is sent in
the DHCP Server Identifier option for a given scope. The value specified
must be an IP address for the DHCP server, and must be reachable by all
clients served by a particular scope.
The use of the server-identifier statement is not recommended - the only reason
to use it is to force a value other than the default value to be sent on
occasions where the default value would be incorrect. The default value is the
first IP address associated with the physical network interface on which the
request arrived.
The usual case where the
server-identifier statement needs to be sent is
when a physical interface has more than one IP address, and the one being sent
by default isn't appropriate for some or all clients served by that interface.
Another common case is when an alias is defined for the purpose of having a
consistent IP address for the DHCP server, and it is desired that the clients
use this IP address when contacting the server.
Supplying a value for the dhcp-server-identifier option is equivalent to using
the server-identifier statement.
The
server-id-check statement
server-id-check flag;
The server-id-check statement is used to control whether or not a server,
participating in failover, verifies that the value of the
dhcp-server-identifier option in received DHCP REQUESTs match the server's id
before processing the request. Server id checking is disabled by default.
Setting this flag enables id checking and thereafter the server will only
process requests that match. Note the flag setting should be consistent
between failover partners.
Unless overridden by use of the server-identifier statement, the value the
server uses as its id will be the first IP address associated with the
physical network interface on which the request arrived.
In order to reduce runtime overhead the server only checks for a server id
option in the global and subnet scopes. Complicated configurations may result
in different server ids for this check and when the server id for a reply
packet is determined, which would prohibit the server from responding.
The primary use for this option is when a client broadcasts a request but
requires that the response come from a specific failover peer. An example of
this would be when a client reboots while its lease is still active - in this
case both servers will normally respond. Most of the time the client won't
check the server id and can use either of the responses. However if the client
does check the server id it may reject the response if it came from the wrong
peer. If the timing is such that the "wrong" peer responds first
most of the time the client may not get an address for some time.
Care should be taken before enabling this option.
The
server-duid statement
server-duid LLT [ hardware-type timestamp
hardware-address ] ;
server-duid EN enterprise-number enterprise-identifier
;
server-duid LL [ hardware-type hardware-address ]
;
The server-duid statement configures the server DUID. You may pick either LLT
(link local address plus time), EN (enterprise), or LL (link local).
If you choose LLT or LL, you may specify the exact contents of the DUID.
Otherwise the server will generate a DUID of the specified type.
If you choose EN, you must include the enterprise number and the
enterprise-identifier.
If there is a server-duid statement in the lease file it will take precedence
over the server-duid statement from the config file and a dhcp6.server-id
option in the config file will override both.
The default server-duid type is LLT.
The
server-name statement
server-name name ;
The
server-name statement can be used to inform the client of the name of
the server from which it is booting.
Name should be the name that will
be provided to the client.
The
site-option-space statement
site-option-space name ;
The
site-option-space statement can be used to determine from what option
space site-local options will be taken. This can be used in much the same way
as the
vendor-option-space statement. Site-local options in DHCP are
those options whose numeric codes are greater than 224. These options are
intended for site-specific uses, but are frequently used by vendors of
embedded hardware that contains DHCP clients. Because site-specific options
are allocated on an ad hoc basis, it is quite possible that one vendor's DHCP
client might use the same option code that another vendor's client uses, for
different purposes. The
site-option-space option can be used to assign
a different set of site-specific options for each such vendor, using
conditional evaluation (see
dhcp-eval (5) for details).
The
stash-agent-options statement
stash-agent-options flag;
If the
stash-agent-options parameter is true for a given client, the
server will record the relay agent information options sent during the
client's initial DHCPREQUEST message when the client was in the SELECTING
state and behave as if those options are included in all subsequent
DHCPREQUEST messages sent in the RENEWING state. This works around a problem
with relay agent information options, which is that they usually not appear in
DHCPREQUEST messages sent by the client in the RENEWING state, because such
messages are unicast directly to the server and not sent through a relay
agent.
The
update-conflict-detection statement
update-conflict-detection flag;
If the
update-conflict-detection parameter is true, the server will
perform standard DHCID multiple-client, one-name conflict detection. If the
parameter has been set false, the server will skip this check and instead
simply tear down any previous bindings to install the new binding without
question. The default is true.
The
update-optimization statement
update-optimization flag;
If the
update-optimization parameter is false for a given client, the
server will attempt a DNS update for that client each time the client renews
its lease, rather than only attempting an update when it appears to be
necessary. This will allow the DNS to heal from database inconsistencies more
easily, but the cost is that the DHCP server must do many more DNS updates. We
recommend leaving this option enabled, which is the default. This option only
affects the behavior of the interim DNS update scheme, and has no effect on
the ad-hoc DNS update scheme. If this parameter is not specified, or is true,
the DHCP server will only update when the client information changes, the
client gets a different lease, or the client's lease expires.
The
update-static-leases statement
update-static-leases flag;
The
update-static-leases flag, if enabled, causes the DHCP server to do
DNS updates for clients even if those clients are being assigned their IP
address using a
fixed-address statement - that is, the client is being
given a static assignment. This can only work with the
interim DNS
update scheme. It is not recommended because the DHCP server has no way to
tell that the update has been done, and therefore will not delete the record
when it is not in use. Also, the server must attempt the update each time the
client renews its lease, which could have a significant performance impact in
environments that place heavy demands on the DHCP server.
The
use-host-decl-names statement
use-host-decl-names flag;
If the
use-host-decl-names parameter is true in a given scope, then for
every host declaration within that scope, the name provided for the host
declaration will be supplied to the client as its hostname. So, for example,
group {
use-host-decl-names on;
host joe {
hardware ethernet 08:00:2b:4c:29:32;
fixed-address joe.fugue.com;
}
}
is equivalent to
host joe {
hardware ethernet 08:00:2b:4c:29:32;
fixed-address joe.fugue.com;
option host-name "joe";
}
Additionally, enabling use-host-decl-names instructs the server to use the host
declaration name in the the forward DNS name, if no other values are
available. This value selection process is discussed in more detail under DNS
updates.
An
option host-name statement within a host declaration will override the
use of the name in the host declaration.
It should be noted here that most DHCP clients completely ignore the host-name
option sent by the DHCP server, and there is no way to configure them not to
do this. So you generally have a choice of either not having any hostname to
client IP address mapping that the client will recognize, or doing DNS
updates. It is beyond the scope of this document to describe how to make this
determination.
The
use-lease-addr-for-default-route statement
use-lease-addr-for-default-route flag;
If the
use-lease-addr-for-default-route parameter is true in a given
scope, then instead of sending the value specified in the routers option (or
sending no value at all), the IP address of the lease being assigned is sent
to the client. This supposedly causes Win95 machines to ARP for all IP
addresses, which can be helpful if your router is configured for proxy ARP.
The use of this feature is not recommended, because it won't work for many
DHCP clients.
The
vendor-option-space statement
vendor-option-space string;
The
vendor-option-space parameter determines from what option space
vendor options are taken. The use of this configuration parameter is
illustrated in the
dhcp-options(5) manual page, in the
VENDOR
ENCAPSULATED OPTIONS section.
SETTING PARAMETER VALUES USING EXPRESSIONS
Sometimes it's helpful to be able to set the value of a DHCP server parameter
based on some value that the client has sent. To do this, you can use
expression evaluation. The
dhcp-eval(5) manual page describes how to
write expressions. To assign the result of an evaluation to an option, define
the option as follows:
my-parameter = expression ;
For example:
ddns-hostname = binary-to-ascii (16, 8, "-",
substring (hardware, 1, 6));
RESERVED LEASES
It's often useful to allocate a single address to a single client, in
approximate perpetuity. Host statements with
fixed-address clauses
exist to a certain extent to serve this purpose, but because host statements
are intended to approximate ´static configuration´, they suffer from
not being referenced in a littany of other Server Services, such as dynamic
DNS, failover, ´on events´ and so forth.
If a standard dynamic lease, as from any range statement, is marked
´reserved´, then the server will only allocate this lease to the
client it is identified by (be that by client identifier or hardware address).
In practice, this means that the lease follows the normal state engine, enters
ACTIVE state when the client is bound to it, expires, or is released, and any
events or services that would normally be supplied during these events are
processed normally, as with any other dynamic lease. The only difference is
that failover servers treat reserved leases as special when they enter the
FREE or BACKUP states - each server applies the lease into the state it may
allocate from - and the leases are not placed on the queue for allocation to
other clients. Instead they may only be ´found´ by client identity.
The result is that the lease is only offered to the returning client.
Care should probably be taken to ensure that the client only has one lease
within a given subnet that it is identified by.
Leases may be set ´reserved´ either through OMAPI, or through the
´infinite-is-reserved´ configuration option (if this is applicable
to your environment and mixture of clients).
It should also be noted that leases marked ´reserved´ are effectively
treated the same as leases marked ´bootp´.
REFERENCE: OPTION STATEMENTS
DHCP option statements are documented in the
dhcp-options(5) manual page.
REFERENCE: EXPRESSIONS
Expressions used in DHCP option statements and elsewhere are documented in the
dhcp-eval(5) manual page.
SEE ALSO
dhcpd(8), dhcpd.leases(5), dhcp-options(5), dhcp-eval(5), RFC2132, RFC2131.
AUTHOR
dhcpd.conf(5) is maintained by ISC. Information about Internet Systems
Consortium can be found at
https://www.isc.org.