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23 rshd: Remote shell server

The rshd server is the server for the rcmd routine and, consequently, for the rsh (see rsh invocation) 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. When a service request is received the following protocol is initiated:

  1. The server checks the client’s source port. If the port is not in the range 512–1023, the server aborts the connection. However, this condition is not applied for Kerberized service.
  2. The server reads characters from the socket up to a NUL (‘\0’) byte. The resultant string is interpreted as an ASCII number, base 10.
  3. 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.
  4. The server checks the client’s source address and requests the corresponding host name. 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.
  5. 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.
  6. 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.
  7. 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.
  8. Rshd then validates the user using ruserok, which uses the file /etc/hosts.equiv and the .rhosts file found in the user’s home directory. The -l option prevents ruserok from doing any validation based on the user’s .rhosts file, unless the user is the superuser.
  9. If the file /etc/nologin exists and the user is not the superuser, the connection is closed.
  10. 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.
  11. 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.
  12. The -L option causes all successful accesses to be logged to syslogd (see syslogd invocation) as ‘auth.info’ messages.

See ruserok in The GNU C Library Reference Manual, for details.

23.1 Invoking

The options are as follows:

-a
--verify-hostname

Ask hostname for verification.

-k
--kerberos

Use Kerberos authentication.

-l
--no-rhosts

Ignore .rhosts file.

-L
--log-sessions

Log successful logins.

-n
--no-keepalive

Do not set SO_KEEPALIVE.

-S name
--servername=name

Set Kerberos server name, overriding canonical hostname.

-v
--vacuous

Fail any call asking for non-Kerberos authentication.

-r
--reverse-required

Demand that the client’s IP address be resolvable as a host name.

Should rshd have been built with PAM support, it reads any setting specified for a service named either ‘rsh’ or ‘krsh’, the latter name for clients seeking Kerberised authentication.

23.2 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 command to the home directory failed.

Permission denied

The authentication procedure described above failed, or address resolution was insufficient.

Can't make pipe.

The pipe needed for the stderr, wasn’t created.

Can't fork; try again.

A fork 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.

The authentication procedure used here assumes the integrity of each client machine and the connecting medium. This is insecure, but is useful in an “open” environment.


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