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UNIX Remote-Shell Services

Three protocols form the basis for UNIX remote-shell services:

Exec—Remote command execution; enables a user on the client system to execute a command on the remote system. The first command from client (rcmd) to server (rshd) uses well-known TCP port 512. A second TCP connection can be opened at the request of rcmd. The client port number for the second connection is sent to the server as an ASCII string.

Login—Better known as rlogin; uses well-known TCP port 513. For details, see RFC 1282. No special firewall processing is required.

Shell—Remote command execution; enables a user on the client system to execute a command on the remote system. The first command from client (rcmd) to server (rshd) uses well-known TCP port 514. A second TCP connection can be opened at the request of rcmd. The client port number for the second connection is sent to the server as an ASCII string.

Support of stateful firewall services requires that you configure the Exec ALG on TCP port 512, the Login ALG on TCP port 513, and the Shell ALG on TCP port 514. NAT remote-shell services require that any dynamic source port assigned be within the port range 512 to 1023. If you configure a NAT pool, this port range is reserved exclusively for remote shell applications.

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