REXECD(8) BSD System Manager’s Manual REXECD(8)


rexecd — remote execution server


rexecd [−i]


The rexecd utility is the server for the rexec(3) routine. The server provides remote execution facilities with authentication based on user names and passwords.

The rexecd utility listens for service requests at the port indicated in the ’’exec’’ service specification; see services(5). When a service request is received the following protocol is initiated:


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


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.


A NUL terminated user name of at most 16 characters is retrieved on the initial socket.


A NUL terminated, unencrypted password of at most 16 characters is retrieved on the initial socket.


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.


The rexecd utility 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.


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.


The rexecd utility will not allow root logins unless the −i option is given on the command line (typically in /etc/inetd.conf). It will also disallow access for users listed in /etc/ftpusers, or users with no passwords, which were all serious security holes. The entire concept of rexec/rexecd is a major security hole and an example of how not to do things. The rexecd utility is disabled by default in /etc/inetd.conf.


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(1) command to the home directory failed.

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.




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


The rexecd utility appeared in 4.2BSD.

BSD September 23, 1994 BSD