rshd —
remote shell server
The rshd server is the server for the
  rcmd(3) routine and, consequently,
  for the rsh(1) program. The server
  provides remote execution facilities with authentication based on privileged
  port numbers from trusted hosts.
The rshd server listens for service
    requests at the port indicated in the “cmd” service
    specification; see
    services(5). When a service
    request is received the following protocol is initiated:
  - The server checks the client's source port. If the port is not in the
      range 512-1023, the server aborts the connection.
- The server reads characters from the socket up to a null
      (‘\0’) byte. The resultant string is interpreted as an ASCII
      number, base 10.
- If the number received in step 2 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. The source port of this second
      connection is also in the range 512-1023.
- The server checks the client's source address and requests the
      corresponding host name (see
      getnameinfo(3),
      hosts(5), and
      named(8)). If the hostname
      cannot be determined, the dot-notation representation of the host address
      is used. If the hostname is in the same domain as the server (according to
      the last two components of the domain name), or if the
      -aoption is given, the addresses for the hostname
      are requested, verifying that the name and address correspond. If address
      verification fails, the connection is aborted with the message
      “Host address mismatch.”
- A null terminated user name of at most 16 characters is retrieved on the
      initial socket. This user name is interpreted as the user identity on the
      client's machine.
- A null terminated user name of at most 16 characters is retrieved on the
      initial socket. This user name is interpreted as a user identity to use on
      the server's machine.
- A null 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.
- rshdthen validates the user using
      ruserok(3), which uses the
      file /etc/hosts.equiv and the
      .rhosts file found in the user's home directory.
      The- -loption prevents
      ruserok(3) from doing any
      validation based on the user's
      “.rhosts” file, unless the user is
      the superuser.
- If the file /etc/nologin exists and the user is
      not the superuser, the connection is closed.
- A null 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 rshd.
Transport-level keepalive messages are enabled unless the
    -n option is present. The use of keepalive messages
    allows sessions to be timed out if the client crashes or becomes
    unreachable.
The -L option causes all successful
    accesses to be logged to
    syslogd(8) as
    auth.info messages.
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 10
  above upon successful completion of all the steps prior to the execution of
  the login shell).
  - Locuser too long.
- The name of the user on the client's machine is longer than 16
    characters.
- Ruser too long.
- The name of the user on the remote machine 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.
- Remote directory.
- The chdir(2) to the home
      directory failed.
- Permission denied.
- The authentication procedure described above failed.
- Can't make pipe.
- The pipe needed for the stderr, wasn't created.
- Can't fork; 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.
The authentication procedure used here assumes the integrity of every machine
  and every network that can reach the rshd/rlogind ports on the server. This is
  insecure, but is useful in an “open” environment.
  sshd(8) or a Kerberized version of
  this server are much more secure.A facility to allow all data exchanges to be encrypted should be
    present.
A more extensible protocol (such as Telnet) should be used.
rshd intentionally rejects accesses from
    IPv4 mapped address on top of AF_INET6 socket, since
    IPv4 mapped address complicates host-address based authentication. If you
    would like to accept connections from IPv4 peers, you will need to run
    rshd on top of an AF_INET
    socket, not an AF_INET6 socket.