rexecd —
remote execution server
rexecd is the server for the
  rexec(3) routine. The server
  provides remote execution facilities with authentication based on user names
  and passwords.
rexecd 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.
- rexecdthen 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.
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(2) 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.
Therexecd command appeared in
  4.2BSD.
Indicating ``Login incorrect'' as opposed to ``Password incorrect'' is a
  security breach which allows people to probe a system for users with null
  passwords.
A facility to allow all data and password exchanges to be
    encrypted should be present.
As the passwords exchanged by the client and rexecd are
  not encrypted, it is strongly recommended that this service
  is not enabled.