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telnetd: Telnet servertelnetd [option]…
Specify what mode to use for authentication. Allowed values are: ‘none’, ‘other’, ‘user’, ‘valid’, and ‘off’.
Set the debugging level. The argument is a comma separated list of these categories: ‘options’, ‘report’, ‘netdata’, ‘ptydata’, ‘auth’, and ‘encr’. All these may be used in the form ‘name[=level]’. Omission of ‘level’ implies the maximal possible debugging level for that particular category.
There is one additional category ‘tcp’, which does not take an additional level indicator, but is instead equivalent to setting the socket option ‘SO_DEBUG’ for debugging the complete traffic.
The output is written to the file /tmp/telnet.debug, and any new data is incrementally added as time passes.
Set program to be executed instead of /bin/login.
Do not print host information before login has been completed.
Set line mode. An empty argument will force line read mode at all times. The only recognised value is otherwise ‘nokludge’.
Disable TCP keep-alives.
Set principal name for the server, to be used in Kerberos authentication. The value principal can be set to provide full specification like ‘srv.local@REALM’ and ‘tnt/localhost@REALM’, where the first uses the standard prefix ‘host/’. Or principal can override default settings in part only, like ‘srv.local’, ‘tnt/srv.local’, or ‘@REALM’.
Refuse connections from addresses that cannot be mapped back into a symbolic name. A client is accepted only if the IP address can be resolved as a host name, and the same name is resolvable to addresses among which the clients’s address is included.
Disable the use of the given authentication type. Use this option multiple times if more than one type is to be disabled. Standard choices are ‘null’, ‘kerberos_v4’, and ‘kerberos_v5’.
The server telnetd contains a built-in execution string
which invokes login with arguments suitable for the
operating system at hand. This preset choice corresponds to the
standard use case of the service. For specialized purposes
this implementation also offers a command line option -E,
or --exec-login, to override the built-in execution of
login, thus allowing almost any choice of handler.
A custom execution string could look like
telnetd -h -E '/usr/local/sbin/avrop %t %U'
The execution string must as its first part provide an absolute
path to an executable file. After that may follow arbitrary
additional arguments.
For this latter part, telnetd offers some replacement
tokens that dynamically are replaced by content. All are of
the form %<var>, where ‘<var>’ is a single letter
from the following collection of selectors.
A valid letter is called variable.
The mark conditional, appearing below, indicates that the
corresponding variable is conditionally assigned a value.
%a ¶Returns ‘ok’ whenever authentication is complete. conditional
%d ¶Produces a time and date string.
%h ¶Gives the remote host name in canonical form.
%l ¶States the local host name, also in canonical form.
%L ¶Returns the path of the pseudo terminal assigned to the client.
%t ¶Gives the terminal device stripped of the leading ‘/dev/’.
%T ¶States the terminal type, like ‘xterm’. conditional
%u ¶Provides the authenticated user name. conditional
%U ¶Returns the user name passed as an environment variable USER
by the remote client software. The value is empty, should the
environment not provide a value.
In addition, a conditional construct is able to take one action in case a variable has an assigned value, and optionally to take another action in the opposite case. The contruct is
%?<var>{true-stmt}[{false-stmt}]
The braces are here mandatory, while the brackets enclose the optional else-clause and are not included in actual use. The initial, motivating example, could thus be expanded to read
telnetd -h -E '/usr/local/sbin/avrop %t %?a{%u krb5}{%U}'
In case authentication was completed as user ‘sigge’, the execution string would resolve to
/usr/local/sbin/avrop pts/1 sigge krb5
In all other cases the result would be
/usr/local/sbin/avrop pts/1 $USER
where $USER is the value of the corresponding environment
variable and could possibly be empty.
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