LDP
/
old-www
1
1
Fork 0
Code Issues Pull Requests Releases Wiki Activity
old-www/LDP/LG/issue31/tag_maildns.html

356 lines
13 KiB
HTML
Raw Permalink Blame History

<!--startcut ======================================================= -->
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<html>
<head>
<TITLE>The Answer Guy 31: 'sendmail' requires DNS ... won't use /etc/hosts</TITLE>
</head>
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#A000A0"
ALINK="#FF0000">
<!--endcut ========================================================= -->
<H4>"Linux Gazette...<I>making Linux just a little more fun!</I>"
</H4>
<P> <hr> <P>
<!-- =============================================================== -->
<H1 align="center"><A NAME="answer">
<img src="../gx/dennis/qbubble.gif" alt="" border="0" align="middle">
<a href="./index.html">The Answer Guy</a>
<img src="../gx/dennis/bbubble.gif" alt="" border="0" align="middle">
</A></H1> <BR>
<H4 align="center">By James T. Dennis,
<a href="mailto:linux-questions-only@ssc.com">linux-questions-only@ssc.com</a><BR>
Starshine Technical Services,
<A HREF="http://www.starshine.org/">http://www.starshine.org/</A> </H4>
<p><hr><p>
<H3><img src="../gx/dennis/qbub.gif" alt="(?)" width="50" height="28"
align="left" border="0">'<tt>sendmail</tt>' requires DNS ...
won't use /etc/hosts</H3>
<p><strong>From Carlos Javier Castro Pe\qa on Fri, 03 Jul 1998
Hi,
<br><br>
I have been looking for an answer for my question for a long time,
but I could find no useful solution. I am building a Linux LAN and I
cannot send mail with <tt>sendmail</tt> ! I do not have a DNS server. I have
found a lot of people with the same problem and no answer.
<br><br>
I am using Red Hat 5 with the version of sendmail distributed by
them. My kernel 2.0.33 and networking enabled. The mails don't get
delivered because a 'hostname lookup failure'. The manual of Red Hat
5 says how to configure <tt>sendmail</tt>, but that does not work if you
don't have a DNS server. I was also suggested to change the
<tt>/etc/nsswitch.conf</tt> file, but it didn't work. Can you help me,
please?
<br><br>
Regards, javier
</strong></p>
<blockquote><img src="../gx/dennis/bbub.gif" alt="(!)" width="50" height="28"
align="left" border="0">
I understand your problem and feel your pain.
<br><br>
The problem is that '<tt>sendmail</tt>' is conforming to the
SMTP standards --- which require that it look up the
MX record for any host prior to sending mail to it.
<br><br>
This sort of record (MX) can only be served over DNS (there
isn't any way that I know of to mark an <tt>/etc/hosts</tt> entry as an
"MX" record) (There might be some weird NIS or NIS+ way to do it
-- but I don't know much about those protocols).
<br><br>
I've worked around that here at my house (I don't run DNS
internally --- I just point to a caching DNS server on my
gateway router (a Linux box doing masquerading for
<strong>external</strong> references).
<br><br>
Internally I just use a mailertable that relays all mail from my
systems to the mail router (which then spools them out over
uucp). That mailertable refers to the gateway system using its
IP address (which forces <tt>sendmail</tt> to skip the DNS MX query). On
another system I just use uucp over TCP (for outgoing mail) and
POP (for incoming).
<br><br>
It would be much easier for me to set up DNS and leave it at
that -- but I can be incredibly stubborn sometimes and my main
mail feed is over uucp in any event --- so it's just as easy to
use that internally.
<br><br>
The reason modifying your <tt>/etc/nsswitch.conf</tt> (or
<tt>/etc/host.conf</tt>
--- as it's usually done under the Linux resolvers) doesn't work
for <tt>sendmail</tt> --- but does work for normal programs using other
protocols is that other protocols are normally only interested
in address and reverse address records (using the
<tt>gethostbyname()</tt> and
<tt>gethostbyadd()</tt> library calls). '<tt>sendmail</tt>'
is looking for MX records since it is common to require special
routing for mail.
<br><br>
As I said --- the easiest solution is to create a local DNS
domain or subdomain for yourself. For example I'd create the
<tt>lan.starshine.org</tt> domain and name my local systems things like
<tt>antares.lan.starshine.org</tt> etc. This would allow me to set up an
"authoritive" set of records on my internal LAN and still defer
to my ISP for the virtually hosted <tt>www.starshine.org</tt>
<tt>ftp.starshine.org</tt>, and <tt>mail.starshine.org</tt> (I'd
list myself as an
"unregistered secondary" to my ISP's nameservers for the
<tt>starshine.org</tt> zone).
<br><br>
(I could also just copy his records into my own db and
essentially <strong>lie</strong> to my system about being
authoritative for the
whole zone. This would break any time my ISP changed any of my
publicly accessible address records --- but that would probably
only be a minor issue. It would only affect my systems while
they were resolving my virtual hosts. My ISP would never refer
to my name servers as authorities or secondaries --- and I could
use IPFW (now IPChains) to prevent any such requests from
getting through to my internal nameserver in any event).
<br><br>
The point is that SMTP (the mail transport protocol to which
sendmail defaults) relies on DNS --- and it is not trivial to
get SMTP working with DNS disabled. You can use a different
transport protocol (such as I do with uucp) or you can configure
your systems to use direct IP addresses rather than names
in their configuration files. This last option isn't so
bad if your plan is to set up masquerading and null clients
(a sort of sendmail client that forward <strong>all</strong> of its mail
to a hub, even the local addressed items -- and lets the
hub deal with it).
<br><br>
Naturally I'm glossing over the details here. I have the
<a href="http://www.ora.com/">O'Reilly</a>
'sendmail' and 'DNS & BIND' books here at my side
--- and I have the notes from a week long seminar I took
on these topics (Robert Harker ---
<A HREF="http://www.harker.com">http://www.harker.com</A>)
--- which are also about 1000 pages. It is basically
impossible to comprehensively explain DNS and <tt>sendmail</tt> in
this column, or on the newsgroups and mailing lists.
<br><br>
I hope though that this helps. If you like I'll post
(sanitized) copies of my <tt>sendmail.mc</tt> files and my
uucp configuration files to show a couple of examples of
how I do it --- with the warning that my configuration is
uniquely atypical!
</blockquote>
<p><hr width="40%"></p>
<H3><img src="../gx/dennis/qbub.gif" alt="(?)" width="50" height="28"
align="left" border="0">More on:
'sendmail' requires DNS ... won't use /etc/hosts</H3>
<p><strong>From Carlos Javier Castro Pe\qa on Thu, 09 Jul 1998
<br>Also posted in Newsgroups:</strong>
<a href="news:comp.unix.questions">comp.unix.questions</a>
</strong></p>
<p><strong>
Dennis,
</strong></p>
<p><strong>
Thank you for your answer. Maybe the most important thing is that
you let me know that I cannot use sendmail for my purpouses wihthout
a DNS server. If I knew that from the beginning, I would have spared
a lot of time trying different configurations. If you don't mind,
I'll post your answer in some Linux mailing lists, because a lot of
people has the same question, and there is no usefull answer.
</strong></p>
<blockquote><img src="../gx/dennis/bbub.gif" alt="(!)" width="50" height="28"
align="left" border="0">
Like the rest of the Linux Gazette all of my articles
are covered by the LDP GPL (the variation of the GNU
General Public License that is applied to the Linux
Documentation Project). Please feel free to post, copy,
modify, publish, sell, spindle, mutilate, cite it to
your heart's content.
</blockquote>
<blockquote>
I hope it helps. I don't get into the newsgroups as much
as I'd like (and I spend a bit more of my time in the
<a href="news:comp.text.tex">comp.text.tex</a> and
<a href="news:comp.unix.security">comp.unix.security</a> and
<a href="news:comp.unix.admin">comp.unix.admin</a>
groups when I do make it out there).
</blockquote>
<blockquote>
Over the years I've always drifted from one newsgroup
and mailing list to another (except for the security stuff
--- I always stay up on that, though mostly as a "lurker").
For awhile I was the most frequent poster on the
<a href="news:comp.lang.awk">comp.lang.awk</a>
(formerly the alt.lang.awk) newsgroup. About 5 years ago
I was in the top 5 or 10 posted on
<a href="http://www.compuserve.com/">Compuserve</a>'s "UNIXFORUM"
</blockquote>
<blockquote>
I start by reading and lurking. When I've read for a few
weeks (and usually done some experimentation and other research)
I start to recognize the common questions, and start to piece
things together. After about a month I start answering questions.
For a few months I'll answer questions in the NG or ML. Eventually,
I move on.
</blockquote>
<blockquote>
That turns out to have been good experience for answering
questions in LG --- since the run the gamut of Unix, Linux,
and PC related questions.
</blockquote>
<p><strong><img src="../gx/dennis/qbub.gif" alt="(?)" width="50" height="28"
align="left" border="0">
<font color="navy"><em> You can use a different
transport protocol (such as I do with uucp) or you can configure
your systems to use direct IP addresses rather than names
in their configuration files. This last option isn't so
bad if your plan is to set up masquerading and null clients
(a sort of sendmail client that forward <em>all</em> of its mail
to a hub, even the local addressed items -- and lets the
hub deal with it).
</em></font></strong></p>
<p><strong>
Is it possible to make that sendmail uses IP addresses. I need to
forward the mail of the Linux host, to the Linux gateway. The users
of the Linux host can download the mail from the gateway with POP
and IMAP.
</strong></p>
<blockquote><img src="../gx/dennis/bbub.gif" alt="(!)" width="50" height="28"
align="left" border="0">
I think the one time I did this was with a slight variant
of the "<tt>clientproto.mc</tt>" file that ships with
'<tt>sendmail</tt>'
</blockquote>
<blockquote>
It looks like this:
</blockquote>
<pre>
divert(-1)
divert(0)dnl
VERSIONID(`@(#)clientproto.mc 8.7 (Berkeley) 3/23/96')
OSTYPE(linux)
FEATURE(nullclient, `[192.168.1.1]')
</pre>
<blockquote>
... replacing the IP address with the one for your
smart hub or gateway. You use the m4 package (also
include with all recent versions of sendmail) to generate
a cf file from this using a command like:
</blockquote>
<blockquote><blockquote><code>
m4 ../m4/cf.m4 betel.mc &gt; /etc/sendmail.cf
</code></blockquote></blockquote>
<blockquote>
(after making backups of your existing cf file, of course).
</blockquote>
<blockquote>
The important thing is that is will define a line like:
</blockquote>
<pre>
DM[192.168.64.1.1]
</pre>
<blockquote>
... which should prevent '<tt>sendmail</tt>' from using DNS to
do any resolution.
</blockquote>
<blockquote>
Try that and see if it works. I really can't set up a test
environment for that at the moment. If that doesn't work, try
changing the address to an arbitrary name, adding the
"mailertable" <tt>FEATURE</tt> and creating a small mailertable that
points the name at the IP address using the syntax:
</blockquote>
<blockquote><blockquote><code>
myhub: smtp:[192.168.1.1]
</code></blockquote></blockquote>
<blockquote>
Please let me know how it goes.
</blockquote>
<!--================================================================-->
<P> <hr> <P>
<H5 align="center"><a href="http://www.linuxgazette.com/copying.html"
>Copyright &copy;</a> 1998, James T. Dennis <BR>
Published in <I>Linux Gazette</I> Issue 31 August 1998</H5>
<P> <hr> <P>
<!--================================================================-->
<table width="98%"><tr valign="center" align="center">
<td rowspan="3"><A HREF="./lg_answer31.html"><IMG
SRC="../gx/dennis/answernew.gif"
ALT="[ Answer Guy Index ]"></A></td>
<td><A HREF="tag_backup.html">backup</A></td>
<td><A HREF="tag_uidgid.html">uidgid</A></td>
<td><A HREF="tag_connect.html">connect</A></td>
<td><A HREF="tag_95slow.html">95slow</A></td>
<td><A HREF="tag_badblock.html">badblock</A></td>
<td><A HREF="tag_trident.html">trident</A></td>
<td><A HREF="tag_sound.html">sound</A></td>
</tr><tr valign="center" align="center">
<td><A HREF="tag_kernel.html">kernel</A></td>
<td><A HREF="tag_solprint.html">solprint</A></td>
<td><A HREF="tag_idescsi.html">idescsi</A></td>
<td><A HREF="tag_distrib.html">distrib</A></td>
<td><A HREF="tag_modem.html">modem</A></td>
<td><A HREF="tag_NDS.html">NDS</a></td>
<td><A HREF="tag_rpm.html">rpm</A></td>
</tr><tr valign="center" align="center">
<td><A HREF="tag_guy.html">guy</A></td>
<td><A HREF="tag_maildns.html">maildns</A></td>
<td><A HREF="tag_memleak.html">memleak</a></td>
<td><A HREF="tag_multihead.html">multihead</A></td>
<td><A HREF="tag_cdr.html">cdr</A></td>
</tr></table>
<P> <hr> <P>
<!--================================================================-->
<A HREF="./index.html"><IMG SRC="../gx/indexnew.gif"
ALT="[ Table Of Contents ]"></A>
<A HREF="../index.html"><IMG SRC="../gx/homenew.gif"
ALT="[ Front Page ]"></A>
<A HREF="lg_bytes31.html"><IMG SRC="../gx/back2.gif"
ALT="[ Previous Section ]"></A>
<A HREF="./searls.html"><IMG SRC="../gx/fwd.gif"
ALT="[ Next Section ]"></A>
<!--startcut ======================================================= -->
</body>
</html>
<!--endcut ========================================================= -->
Reference in New Issue View Git Blame Copy Permalink