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21 rexecd: server for rexec

rexecd is the server for the rexec routine. The server provides remote execution facilities with authentication based on user names and passwords. It passes error messages and notices to the syslog facility ‘LOG_DAEMON’.

rexecd [option]…

rexecd listens for service requests at the port indicated in the ‘exec’ service specification. When a service request is received the following protocol is initiated:

  1. The server reads characters from the socket up to a NUL (‘\0’) byte. The resultant string is interpreted as an ASCII number, base 10.
  2. If the number received in step 1 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.
  3. A NUL terminated user name of at most 16 characters is retrieved on the initial socket.
  4. A NUL terminated, unencrypted password of at most 16 characters is retrieved on the initial socket.
  5. A NUL 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.
  6. rexecd then validates the user as is done at login time and, if the authentication was successful, changes to the user’s home directory, and establishes the user and group protections of the user. If any of these steps fail the connection is aborted with a diagnostic message returned.
  7. A NUL 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 rexecd.

21.1 Invoking

The only option is as follows:


Raise logging level for this service; use more than once for increased verbosity. The syslog facility in use is ‘LOG_DAEMON’.

Should rexecd have been built with PAM support, it reads any setting specified for a service named ‘rexec’.

21.2 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 7 above upon successful completion of all the steps prior to the command execution).

username too long

The name is longer than 16 characters.

password too long

The password 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.

Password incorrect.

The wrong password was supplied.

No remote directory.

The chdir command to the home directory failed.

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 ...

Note, that indicating ‘Login incorrect’ as opposed to ‘Password incorrect’ is a security breach which allows people to probe a system for users with null passwords.

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