NAME
rshd —
remote shell server
SYNOPSIS
DESCRIPTION
The
rshd server is the server for the
rcmd(3) routine and, consequently,
for the
rsh(1) program. The server
provides remote execution facilities with authentication based on privileged
port numbers from trusted hosts.
The
rshd server listens for service requests at the port
indicated in the “cmd” service specification; see
services(5). When a service
request is received the following protocol is initiated:
- The server checks the client's source port. If the port is
not in the range 512-1023, the server aborts the connection.
- The server reads characters from the socket up to a null
(‘\0’) byte. The resultant string is interpreted as an ASCII
number, base 10.
- If the number received in step 2 is non-zero, it is
interpreted as the port number of a secondary stream to be used for the
stderr. A second connection is then created to the
specified port on the client's machine. The source port of this second
connection is also in the range 512-1023.
- The server checks the client's source address and requests
the corresponding host name (see
getnameinfo(3),
hosts(5), and
named(8)). If the hostname
cannot be determined, the dot-notation representation of the host address
is used. If the hostname is in the same domain as the server (according to
the last two components of the domain name), or if the
-a option is given, the addresses for the hostname are
requested, verifying that the name and address correspond. If address
verification fails, the connection is aborted with the message “Host
address mismatch.”
- A null terminated user name of at most 16 characters is
retrieved on the initial socket. This user name is interpreted as the user
identity on the client's machine.
- A null terminated user name of at most 16 characters is
retrieved on the initial socket. This user name is interpreted as a user
identity to use on the server's machine.
- A null terminated command to be passed to a shell is
retrieved on the initial socket. The length of the command is limited by
the upper bound on the size of the system's argument list.
- rshd then validates the user using
ruserok(3), which uses the
file /etc/hosts.equiv and the .rhosts
file found in the user's home directory. The -l option
prevents ruserok(3) from
doing any validation based on the user's
“.rhosts” file, unless the user is the
superuser.
- If the file /etc/nologin exists and the
user is not the superuser, the connection is closed.
- A null byte is returned on the initial socket and the
command line is passed to the normal login shell of the user. The shell
inherits the network connections established by
rshd.
Transport-level keepalive messages are enabled unless the
-n
option is present. The use of keepalive messages allows sessions to be timed
out if the client crashes or becomes unreachable.
The
-L option causes all successful accesses to be logged to
syslogd(8) as
auth.info
messages.
DIAGNOSTICS
Except for the last one listed below, all diagnostic messages are returned on
the initial socket, after which any network connections are closed. An error
is indicated by a leading byte with a value of 1 (0 is returned in step 10
above upon successful completion of all the steps prior to the execution of
the login shell).
-
-
- Locuser too
long.
- The name of the user on the client's machine is longer than
16 characters.
-
-
- Ruser too
long.
- The name of the user on the remote machine is longer than
16 characters.
-
-
- Command too
long.
- The command line passed exceeds the size of the argument
list (as configured into the system).
-
-
- Login
incorrect.
- No password file entry for the user name existed.
-
-
- Remote
directory.
- The chdir(2)
to the home directory failed.
-
-
- Permission
denied.
- The authentication procedure described above failed.
-
-
- Can't make
pipe.
- The pipe needed for the stderr, wasn't
created.
-
-
- Can't fork; try
again.
- A fork(2) by
the server failed.
-
-
- <shellname>:
...
- The user's login shell could not be started. This message
is returned on the connection associated with the
stderr, and is not preceded by a flag byte.
SEE ALSO
rsh(1),
ssh(1),
rcmd(3),
ruserok(3),
hosts_access(5),
login.conf(5),
sshd(8)
BUGS
The authentication procedure used here assumes the integrity of every machine
and every network that can reach the rshd/rlogind ports on the server. This is
insecure, but is useful in an “open” environment.
sshd(8) or a Kerberized version of
this server are much more secure.
A facility to allow all data exchanges to be encrypted should be present.
A more extensible protocol (such as Telnet) should be used.
rshd intentionally rejects accesses from IPv4 mapped address
on top of
AF_INET6
socket, since IPv4 mapped address
complicates host-address based authentication. If you would like to accept
connections from IPv4 peers, you will need to run
rshd on
top of an
AF_INET
socket, not an
AF_INET6
socket.