285 lines
12 KiB
HTML
285 lines
12 KiB
HTML
<!--startcut ======================================================= -->
|
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
|
|
<html>
|
|
<head>
|
|
<META NAME="generator" CONTENT="lgazmail v1.3D.j">
|
|
<TITLE>The Answer Guy 54: Can't telnet to Linux server</TITLE>
|
|
</HEAD><BODY BGCOLOR="#FFFFFF" TEXT="#000000"
|
|
LINK="#3366FF" VLINK="#A000A0">
|
|
<!-- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: -->
|
|
<P> <hr>
|
|
<!-- *** BEGIN navbar *** :::::::::::::::::::::::::::::::::::::::::::::::: -->
|
|
<p align="center">
|
|
<A HREF="../lg_bytes54.html"><IMG ALT="[ Prev ]"
|
|
SRC="../../gx/navbar/prev.jpg"
|
|
WIDTH="16" HEIGHT="45" BORDER="0" ALIGN="bottom"></A>
|
|
<IMG ALT="" SRC="../../gx/navbar/left.jpg"
|
|
WIDTH="14" HEIGHT="45" BORDER="0" ALIGN="bottom" >
|
|
<A HREF="../index.html"><IMG ALT="[ Table of Contents ]"
|
|
SRC="../../gx/navbar/toc.jpg"
|
|
WIDTH="220" HEIGHT="45" BORDER="0" ALIGN="bottom" ></A>
|
|
<A HREF="../../index.html"><IMG ALT="[ Front Page ]"
|
|
SRC="../../gx/navbar/frontpage.jpg"
|
|
WIDTH="137" HEIGHT="45" BORDER="0" ALIGN="bottom"></A>
|
|
<A HREF="../../faq/index.html"><IMG ALT="[ FAQ ]"
|
|
SRC="../../gx/navbar/faq.jpg"
|
|
WIDTH="62" HEIGHT="45" BORDER="0" ALIGN="bottom"></A>
|
|
<IMG ALT="" SRC="../../gx/navbar/right.jpg"
|
|
WIDTH="15" HEIGHT="45" ALIGN="bottom" >
|
|
<A HREF="../lg_tips54.html"><IMG ALT="[ Next ]"
|
|
SRC="../../gx/navbar/next.jpg"
|
|
WIDTH="15" HEIGHT="45" BORDER="0" ALIGN="bottom" ></A>
|
|
<!-- *** END navbar *** :::::::::::::::::::::::::::::::::::::::::::::::::: -->
|
|
</p>
|
|
<P> <hr> <P>
|
|
<!-- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: -->
|
|
<center>
|
|
<H1><A NAME="answer">
|
|
<img src="../../gx/dennis/qbubble.gif" alt="(?)"
|
|
border="0" align="middle">
|
|
<font color="#B03060">The Answer Guy</font>
|
|
<img src="../../gx/dennis/bbubble.gif" alt="(!)"
|
|
border="0" align="middle">
|
|
</A></H1>
|
|
<BR>
|
|
<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>
|
|
</H4>
|
|
</center>
|
|
|
|
<p><hr><p>
|
|
<!-- endcut ======================================================= -->
|
|
<!-- begin 3 -->
|
|
<H3 align="left"><img src="../../gx/dennis/qbubble.gif"
|
|
height="50" width="60" alt="(?) " border="0"
|
|
>Can't telnet to Linux server</H3>
|
|
|
|
|
|
<p><strong>From kd on Mon, 01 May 2000
|
|
</strong></p>
|
|
<!-- ::
|
|
Can't telnet to Linux server
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
:: -->
|
|
<P><STRONG>
|
|
I recently installed Suse Linux on a machine to be a server, but I
|
|
cannot telnet to the linux server from my other machines. can you help?
|
|
</STRONG></P>
|
|
<P><STRONG>
|
|
~kelly
|
|
</STRONG></P>
|
|
<BLOCKQUOTE><IMG SRC="../../gx/dennis/bbub.gif" ALT="(!)"
|
|
HEIGHT="28" WIDTH="50" BORDER="0"
|
|
>
|
|
Short answer: Probably TCP Wrappers and the old "double reverse lookup
|
|
problem." Try adding an entry in <TT>/etc/hosts</TT> to refer back to your
|
|
client(s) and make sure that your <TT>/etc/nsswitch.conf</TT> and <TT>/etc/hosts.conf</TT>
|
|
are configured to honor "files" over DNS and NIS.
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
You could have been a bit more vague. You could have
|
|
left out the word "telnet"
|
|
<IMG SRC="../../gx/dennis/smily.gif" ALT=";)"
|
|
height="24" width="20" align="middle">?
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
When asking people technical support questions you have to ask:
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE><BLOCKQuote>
|
|
How many possible causes are there to this problem?
|
|
How many of them have I eliminated?
|
|
How have I eliminated them?
|
|
Can I eliminate some more?
|
|
What is the error message I'm getting (if any)?
|
|
What was I expecting?
|
|
What happened that didn't match that/those expection(s)?
|
|
</BLOCKQuote></BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
For example: Can you ping the server from the client system?
|
|
(That eliminates many IP addressing, routing, firewall and
|
|
packet filtering questions). Can you telnet from that client to
|
|
any other server? (That eliminates most of the questions that
|
|
relate to proper client software/system configuration and function).
|
|
Can I access any other service on this client? (Web server, file
|
|
or print services, etc.)
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
Then you ask: What did I expect to happen when I telnetted to that
|
|
system? I'd expect to get a set of responses something like:
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE><BLOCKQuote>
|
|
Trying 123.45.67.89
|
|
Connected to myserver.mydomain.not
|
|
Escape character is '^]'.
|
|
<A HREF="http://www.debian.org/">Debian</A> GNU/Linux 2.2 myserver.mydomain.not
|
|
myserver login:
|
|
</BLOCKQuote></BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
So, what did you get. Did you see the "Trying" line? That would
|
|
mean that the telnet DNS or other hostname lookup returned something.
|
|
Did the IP address in the trying line match that of your new server?
|
|
That would mean that your DNS is correct! Did you get the "connected
|
|
to" line? That suggests that the routing is correct. Did it just
|
|
sit there for a long time? How long? What if you wait for five or
|
|
ten minutes? Does it eventually connect?
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
It sounds like you have the old "double reverse DNS" problem. You
|
|
are probably using DNS and you probably don't have proper reverse
|
|
DNS (PTR) records for you client system(s). Do a search in the
|
|
Linux Gazette archives for several discussions on this.
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
When you are getting free software and free support, it's important
|
|
to do your homework. I typically will put about 10 hours into
|
|
trying to solve a problem before I'll write up a question to post
|
|
to the newsgroups, mailing lists, authors/maintainers, etc.
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
Of course I can understand part of the problem you might be
|
|
facing. It sounds like you have little or no Linux experience,
|
|
or at least little or no experience in setting up Linux networking.
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
You probaby don't know all of the elements that go into
|
|
"telnetting into your server." Here's the basic rundown:
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE><BLOCKQuote>
|
|
You have to have a client (telnet command). That has to be on a
|
|
system with TCP/IP installed, configured and working. It must
|
|
have an IP address and a route to your server.
|
|
</BLOCKQuote></BLOCKQUOTE>
|
|
<BLOCKQUOTE><BLOCKQuote>
|
|
You have to have a server (in.telnetd). It would normally be
|
|
launched on demand by a dispatch program (inetd) which would be
|
|
reading configuration out of a configuration file (<TT>/etc/inetd.conf</TT>).
|
|
</BLOCKQuote></BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
On Linux systems the <TT>/etc/inetd.conf</TT> is usually configured to run
|
|
most programs under an access control and logging utility called
|
|
"TCP Wrappers" (<TT>/usr/sbin/tcpd</TT>). That utility refers to a couple of
|
|
configuration files (<TT>/etc/hosts.allow</TT>, and <TT>/etc/hosts.deny</TT>) and it
|
|
does some "paranoid" consistency checking to try and ensure that the
|
|
client "is who he claims to be." The specifics of this paranoid
|
|
checking are referred to as a "double reverse DNS lookup."
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
This requires that the client system's IP address somehow be
|
|
registered in some sort of naming service that the server is configured
|
|
to query. The easiest of these in most cases is to simply add the
|
|
appropriate IP address (and some arbitrary name) int the <TT>/etc/hosts</TT>
|
|
file. A better way is to add an appropriate PTR record to your
|
|
DNS zone.
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
Linux uses a modular name services resolution system. Newer
|
|
versions of Linux use the <TT>/etc/nsswitch.conf</TT> files to control the
|
|
list of name services that are used for each name space (users/accounts,
|
|
groups, hosts and networks, services, mail aliases, file server maps,
|
|
etc). In most cases you wouldn't have to modify the nsswitch.conf
|
|
to make it look at the <TT>/etc/hosts</TT> file. In other cases you might.
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
In previous months I've gone into greater detail about how to
|
|
troubleshoot problems in accessing TCP services on Linux systems.
|
|
Look for references to tcpdump and strace to find out more.
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
(Summary: You can replace the entry in <TT>/etc/inetd.conf</TT> with a
|
|
wrapper script that runs 'strace' on the program, thus logging
|
|
what the program is trying to do in great detail. You can also
|
|
run 'tcpdump' on any machine on the local LAN segment, seeing
|
|
the traffic between your client and server in great detail).
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
Unfortunately these tools are rather advanced, very powerful
|
|
and coresponding difficult to use effectively. (You can
|
|
probably get the information from them pretty easily <TT>--</TT> the
|
|
problem is to configure them to provide just the info you
|
|
need and in parsing and understanding what they tell you).
|
|
</BLOCKQUOTE>
|
|
<BLOCKQUOTE>
|
|
Hopefully I've guessed correctly on what you problem is.
|
|
Otherwise search through my back issues and the FAQ and
|
|
do lots of troubleshooting. Ask a more detailed question.
|
|
</BLOCKQUOTE>
|
|
|
|
<!-- sig -->
|
|
|
|
|
|
<!-- end 3 -->
|
|
<!--startcut ======================================================= -->
|
|
<P> <hr> </p>
|
|
<H5 align="center"><a href="http://www.linuxgazette.com/copying.html"
|
|
>Copyright ©</a> 2000, James T. Dennis
|
|
<BR>Published in <I>The Linux Gazette</I> Issue 54 June 2000</H5>
|
|
<H6 ALIGN="center">HTML transformation by
|
|
<A HREF="mailto:star@tuxtops.com">Heather Stern</a> of
|
|
Tuxtops, Inc.,
|
|
<A HREF="http://www.tuxtops.com/">http://www.tuxtops.com/</A>
|
|
</H6>
|
|
<P> <hr>
|
|
<!-- begin tagnav ::::::::::::::::::::::::::::::::::::::::::::::::::-->
|
|
<p align="center">
|
|
<IMG ALT="" SRC="../../gx/navbar/left.jpg"
|
|
WIDTH="14" HEIGHT="45" BORDER="0" ALIGN="middle" border="0">
|
|
<A HREF="../lg_answer54.html"
|
|
><IMG SRC="../../gx/dennis/answertoc.jpg" align="middle"
|
|
ALT="[ Answer Guy Current Index ]" border="0"></A>
|
|
<A HREF="../lg_answer54.html#greeting"><img align="middle"
|
|
src="../../gx/dennis/smily.gif" alt="greetings" border="0"></A>
|
|
<A HREF="1.html">1</A>
|
|
<A HREF="2.html">2</A>
|
|
<A HREF="3.html">3</A>
|
|
<A HREF="4.html">4</A>
|
|
<A HREF="5.html">5</A>
|
|
<A HREF="6.html">6</A>
|
|
<A HREF="7.html">7</A>
|
|
<A HREF="8.html">8</A>
|
|
<A HREF="9.html">9</A>
|
|
<A HREF="10.html">10</A>
|
|
<A HREF="11.html">11</A>
|
|
<A HREF="12.html">12</A>
|
|
<A HREF="13.html">13</A>
|
|
<A HREF="14.html">14</A>
|
|
<A HREF="15.html">15</A>
|
|
<A HREF="16.html">16</A>
|
|
<A HREF="17.html">17</A>
|
|
<A HREF="18.html">18</A>
|
|
<A HREF="../../tag/kb.html"
|
|
><IMG SRC="../../gx/dennis/answerpast.jpg" align="middle"
|
|
ALT="[ Index of Past Answers ]" border="0"></A></td>
|
|
<IMG ALT="" SRC="../../gx/navbar/right.jpg" align="middle"
|
|
WIDTH="14" HEIGHT="45" BORDER="0">
|
|
</p>
|
|
<!-- end tagnav ::::::::::::::::::::::::::::::::::::::::::::::::::::-->
|
|
<P> <hr>
|
|
<!-- *** BEGIN navbar *** :::::::::::::::::::::::::::::::::::::::::::::::: -->
|
|
<p align="center">
|
|
<A HREF="../lg_bytes54.html"><IMG ALT="[ Prev ]"
|
|
SRC="../../gx/navbar/prev.jpg"
|
|
WIDTH="16" HEIGHT="45" BORDER="0" ALIGN="bottom"></A>
|
|
<IMG ALT="" SRC="../../gx/navbar/left.jpg"
|
|
WIDTH="14" HEIGHT="45" BORDER="0" ALIGN="bottom" >
|
|
<A HREF="../index.html"><IMG ALT="[ Table of Contents ]"
|
|
SRC="../../gx/navbar/toc.jpg"
|
|
WIDTH="220" HEIGHT="45" BORDER="0" ALIGN="bottom" ></A>
|
|
<A HREF="../../index.html"><IMG ALT="[ Front Page ]"
|
|
SRC="../../gx/navbar/frontpage.jpg"
|
|
WIDTH="137" HEIGHT="45" BORDER="0" ALIGN="bottom"></A>
|
|
<A HREF="../../faq/index.html"><IMG ALT="[ FAQ ]"
|
|
SRC="../../gx/navbar/faq.jpg"
|
|
WIDTH="62" HEIGHT="45" BORDER="0" ALIGN="bottom"></A>
|
|
<IMG ALT="" SRC="../../gx/navbar/right.jpg"
|
|
WIDTH="15" HEIGHT="45" ALIGN="bottom" >
|
|
<A HREF="../lg_tips54.html"><IMG ALT="[ Next ]"
|
|
SRC="../../gx/navbar/next.jpg"
|
|
WIDTH="15" HEIGHT="45" BORDER="0" ALIGN="bottom" ></A>
|
|
<!-- *** END navbar *** :::::::::::::::::::::::::::::::::::::::::::::::::: -->
|
|
</p>
|
|
<!-- ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: -->
|
|
</BODY></HTML>
|
|
<!--endcut ========================================================= -->
|