22 rexecd: server for
rexecd is the server for the
Rexec 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. 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
- 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
- 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.
- 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
- 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 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.