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
-a
option 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.
rshd
then 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 -l
option 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.