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<TITLE>The Answer Guy 50: Redirecting stdin into telnet</TITLE>
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<H4>"The Linux Gazette...<I>making Linux just a little more fun!</I>"</H4>
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<H1><A NAME="answer">
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<H4>By James T. Dennis,
<a href="mailto:linux-questions-only@ssc.com">linux-questions-only@ssc.com</a><BR>
LinuxCare,
<A HREF="http://www.linuxcare.com/">http://www.linuxcare.com/</A>
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<H3 align="left"><img src="../../gx/dennis/qbubble.gif"
height="50" width="60" alt="(?) " border="0"
>Redirecting stdin into telnet</H3>
<p><strong>From Christopher Smith on Tue, 18 Jan 2000
</strong></p>
<!-- ::
Redirecting stdin into telnet
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
:: -->
<P><STRONG>
I'm running <A HREF="http://www.redhat.com/">Red Hat</A> 6.0, and I'm
trying to redirect my telnet
stdin from a file. Whenever I use either a pipe or <tt>&lt;</tt> input, the
client connects, informs me that the <tt>escape is ^]</tt>, and says <tt>the
remote server has closed the connection</tt>. What's wrong?
</STRONG></P>
<P><STRONG>
Example: $ <tt>telnet csmith &lt; input.txt</tt>
</STRONG></P>
<P><STRONG>
Christopher Smith
</STRONG></P>
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
HEIGHT="28" WIDTH="50" BORDER="0"
>
I've seen examples of telnet scripting being done this
way. However, I think it probably depends on the
version of telnet that you're using and there is a
better way.
</BLOCKQUOTE>
<BLOCKQUOTE>
For example when I do an strace on my copy of
telnet I find a number of system calls like:
</BLOCKQUOTE>
<blockquote><pre>ioctl(0, SNDCTL_TMR_STOP, {B134 -opost -isig -icanon echo ...}) = -1 ENOTTY (Inappropriate ioctl for device)
</pre></blockquote>
<BLOCKQUOTE>
Note that result: <tt>ENOTTY</tt> which is kernel speak for
"Error, this file handle doesn't refer to a terminal
device (TTY)"
</BLOCKQUOTE>
<BLOCKQUOTE>
This suggests that telnet is detecting that its
input file handle is not a terminal device, and is
bailing at that point.
<ul><li> (It would be nice if it
gave an error message a unique error code and if
they listed this constraint in their man pages).</ul>
</BLOCKQUOTE>
<BLOCKQUOTE>
Anyway, simple redirection is not the way to
run interactive programs like telnet under scripted
control. You probably want to use the 'expect'
scripting language instead.
</BLOCKQUOTE>
<BLOCKQUOTE>
For example, here's a simple 'expect' script to
control telnet sessions:
</BLOCKQUOTE>
<blockquote><pre>#!/usr/bin/expect -f
# Sample telnet automation
## call with autotel host username password
set host [lindex $argv 0]
set rcfile [open ~/.autotel/$host r ]
gets $rcfile user
gets $rcfile pass
spawn telnet "$host"
expect "login:"
send "$user\r"
expect "word:"
send "$pass\r"
interact
</pre></blockquote>
<BLOCKQUOTE>
This appears in my book (<a href="http://www.amazon.com/exec/obidos/ASIN/1562059343/linuxsystemadmin/104-3347499-9099625"
>Linux System Administration</a>,
published by New Riders Publishing). You call it like:
</BLOCKQUOTE>
<BLOCKQUOTE><BlockQuote><Code>
autotel foo
</Code></BlockQuote></BLOCKQUOTE>
<BLOCKQUOTE>
... and it will automatically look for a .autotel/foo
file under you home directory, read a pair of lines
for the user name and password. Then it uses them to
login to the host (whose name must match the
<tt>.autotel/</tt> filename (*) ).
</BLOCKQUOTE>
<BLOCKQUOTE><ul><li>
(Actually the version of this on my own
system read three lines from the autotel
files: a location, and the username/password
pair).
</UL></BLOCKQUOTE>
<BLOCKQUOTE>
As I say this is a very simple 'expect' script. It
shows the most basic features of the language:
you can do all of the normal things like open
files, read from and write to them, do basic string
handling and arithmetic, etc. But the features of
'expect' for which it is renouned include the ability
to '<tt>spawn</tt>' interactive processes on psuedo-terminals
(ptys), and to '<tt>expect</tt>' certain patterns from the
them, and '<tt>send</tt>' strings back to them.
</BLOCKQUOTE>
<BLOCKQUOTE>
Of course 'expect' has support for the common conditional
and looping structures, and the '<tt>expect</tt>' function supports
timeouts values. So you can write arbitrary handling for
errors and situations that don't match your intended
dialog.
</BLOCKQUOTE>
<BLOCKQUOTE>
There's also the '<tt>interact</tt>' function which allows
your script to turn the keyboard back over to you,
so that the script then passively relays your input
and the program's output. Actually the '<tt>interact</tt>'
function is even more powerful than than since it
can also monitor the communications and dynamically
"catch" certain key sequences, expanding them into
macros, or using them as triggers to execute your
own defined procedures or to break out of the
interaction and complete other parts of the script.
</BLOCKQUOTE>
<BLOCKQUOTE>
So, I'd check out the 'expect' programming language
and see if that will help. Here's an untested script
that would do something like what you wanted:
</BLOCKQUOTE>
<blockquote><pre>#!/usr/bin/expect -f
# Sample telnet "feeder"
set host [lindex $argv 0]
set infilename [lindex $argv 1]
set infile [open $infilename r ]
spawn telnet "$host"
while [ gets $infile line ] {
sleep 1
send -- $line
}
</pre></blockquote>
<BLOCKQUOTE>
(This would just blindly feed lines into a telnet
session from your input file. It's pretty stupid,
but you'd call it with something like:
</BLOCKQUOTE>
<BLOCKQUOTE><BlockQuote><Code>
telfeeder host.domain.not ./filename
</Code></BlockQuote></BLOCKQUOTE>
<BLOCKQUOTE>
... note the lack of redirection here).
</BLOCKQUOTE>
<BLOCKQUOTE>
Note that it's also possible to use C-Kermit
(the kermit communications package from Columbia
University <A HREF="http://www.columbia.edu/kermit"
>http://www.columbia.edu/kermit</A>) to automate
and script a communications session. I wrote an
article about this for SysAdmin Magazine
(<A HREF="http://www.samag.com/archive/0607/index.shtml"
>http://www.samag.com/archive/0607/index.shtml</A>) back
in 1997. The key point of that article (which is
not online, though I could probably re-publish myself
now) is that Kermit is not just a file transfer protocol
and it is not limited simply to modem and serial
communications, it has support for telnet and rlogin
protocols.
</BLOCKQUOTE>
<BLOCKQUOTE>
In fact Kermit as improved since my article:
</BLOCKQUOTE>
<BLOCKQUOTE><ul><li>
it now supports Kerberos authentication and encryption
<li>
it has internal SSL support
<li>
it can use some other program like ssh for its
communications channel.
<li>
it can now operate as an alternative to FTP
using providing a number of advantages over the
somewhat decrepit FTP protocol.
<li>
the license has been clarified and liberalized
a bit, to allow re-distribution with free OS
packages (like Linux, *BSD, etc) etc.
</UL></BLOCKQUOTE>
<BLOCKQUOTE>
(Some of these were features I recommended to Frank
de la Cruz, the father of Kermit, during my research and
in subsequent e-mail conversations. However, I'm by no
means the only person to make these sorts of suggestions and
I couldn't claim any credit for their development).
</BLOCKQUOTE>
<BLOCKQUOTE>
One other alternatives to 'expect' that comes to mind is the
<tt>Expect.pm</tt>, PERL module which add 'expect' features to the
PERL 5 language. (The old PERL 4 had a <tt>comm.pl</tt> library that
offered some of these features as well).
</BLOCKQUOTE>
<BLOCKQUOTE>
I'm sure there are others.
</BLOCKQUOTE>
<BLOCKQUOTE>
Hopefully this will help you solve the real problem that
you're bumping into. Obviously, solving the problem goes
way beyond just answering the question you asked.
</BLOCKQUOTE>
<!-- sig -->
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<P> <hr> <P>
<H5 align="center"><a href="http://www.linuxgazette.com/copying.html"
>Copyright &copy;</a> 2000, James T. Dennis
<BR>Published in <I>The Linux Gazette</I> Issue 50 February 2000</H5>
<H6 ALIGN="center">HTML transformation by
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