20 rshd: Remote shell server
The rshd server is the server for the rcmd
routine
and, consequently, for the rsh (see rsh invocation)
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. 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 NUL (‘\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. 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
, which uses the file
/etc/hosts.equiv and the .rhosts file found in the
user's home directory. The -l option prevents ruserok
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 (see syslogd invocation) as ‘auth.info’
messages.
See ruserok,
for details.
20.1 Invoking
The options are as follows:
- -a
- --verify-hostname
- Ask hostname for verification.
- -l
- --no-rhosts
- Ignore .rhosts file.
- -L name
- --local-domain=name
- Set local domain name.
- -n
- --no-keepalive
- Do not set SO_KEEPALIVE.
- -k
- --kerberos
- Use kerberos IV authentication.
- -x
- --encrypt
- Turns on DES encryption for all data passed via the rshd
session. This may impact response time and CPU utilization, but
provides increased security.
- -D[level]
- --debug[=level]
- Set debug level, not implemented.
20.2 Diagnostics
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 command 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 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 each
client machine and the connecting medium. This is insecure, but is
useful in an “open” environment.