The rshd server is the server for the rcmd() routine and, consequently, for the rsh() 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:

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The server checks the client's source port. If the port is not in the range 512-1023, the server aborts the connection.

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The server reads characters from the socket up to a null (`\0') byte. The resultant string is interpreted as an ASCII number, base 10.

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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 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.

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The server checks the client's source address and requests the corresponding host name (see gethostbyaddr(3) , hosts() 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.''

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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 machine.

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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 machine.

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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.

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Rshd then validates the user using PAM , which uses the file /etc/hosts.equiv and the .rhosts file found in the user's home directory. Due to the way the protocol was designed, it is not possible to exchange information other than those specified here for authentication. Thus PAM authentication modules such as pam_unix should not be used with this service. Also note that the design of the .rhosts system is COMPLETELY INSECURE except on a carefully firewalled private network. Under all other circumstances, rshd should be disabled entirely.

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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.

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 9 above upon successful completion of all the steps prior to the execution of the login shell).

Sy Locuser too long.

The name of the user on the client's machine is longer than 16 characters.

Sy Ruser too long.

The name of the user on the remote machine is longer than 16 characters.

Sy Command too long .

The command line passed exceeds the size of the argument list (as configured into the system).

Sy Remote directory.

The () command to the home directory failed.

Sy Permission denied.

The authentication procedure described above failed, or the user requested did not exist. (These conditions are intentionally conflated.)

Sy Can't make pipe.

The pipe needed for the wasn't created.

Sy Can't fork; try again.

A () by the server failed.

Sy <shellname>: ...

The user's login shell could not be started. This message is returned on the connection associated with the 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.

A facility to allow all data exchanges to be encrypted should be present.

A more extensible protocol (such as Telnet) should be used.