Debugging sendmail problems
There are many tools available for debugging problems with
sendmail. For the most part, standard types of configurations
shouldn't provide any problems. However, for complex configurations
requiring the rewriting of address parsing rules, problems may arise
and debugging may be necessary.
Verbose Delivery
First of all, the standard mail utility, mail (or
Mail, or mailx) can be invoked with the -v flag
to turn on verbose delivery. Verbose delivery shows the steps that
sendmail takes in delivering the mail message. For example, the
following is a transcript from an unsuccessful mail connection:
fard@uwsg.indiana.edu... Connecting to herbie.ucs.indiana.edu. via smtp... 220 herbie.ucs.indiana.edu ESMTP Sendmail 8.7.5/8.6.12 ready at Thu, 18 Jul 1996 09:16:24 -0500 (EST) >>> EHLO outland.uwsg.indiana.edu 250-herbie.ucs.indiana.edu Hello outland.uwsg.indiana.edu [129.79.17.234], pleased to meet you 250-EXPN 250-8BITMIME 250-SIZE 250-DSN 250-VERB 250-ONEX 250 HELP >>> MAIL From:<brier@outland.uwsg.indiana.edu> SIZE=28 250 <brier@outland.uwsg.indiana.edu>... Sender ok >>> RCPT To:<fard@uwsg.indiana.edu> 550 <fard@uwsg.indiana.edu>... User unknown fard@uwsg.indiana.edu... User unknown >>> RSET 250 Reset state /home/brier/dead.letter... Saved message in /home/brier/dead.letter /home/brier/dead.letter... Closing connection to herbie.ucs.indiana.edu. >>> QUIT 221 herbie.ucs.indiana.edu closing connection
This transcript provides a lot of information about what happened
with this mail message. The first line shows that the message was
intended for the address fard@uwsg.indiana.edu, but that the
SMTP connection was made to the machine herbie.ucs.indiana.edu.
A little investigation with nslookup shows that
uwsg.indiana.edu is a mail exchange record in the Domain Name
System (DNS) for the machine herbie.ucs.indiana.edu, so all
mail sent to uwsg.indiana.edu is actually delivered to
herbie.ucs.indiana.edu.
Most of the remaining lines show the SMTP conversation between the
two hosts. Much of this conversation is standard, but at line 15
there is an error: 550 <fard@uwsg.indiana.edu>... User
unknown. Here is our problem. There is no user fard on
herbie.ucs.indiana.edu, and so the mail was not delivered.
Invoking sendmail In Verify
Mode
One useful command line option of sendmail is -bv.
This option puts sendmail into verify mode; it verifies
addresses only, it does not deliver them. This is a good method for
validating users or mailing lists.
For example, the following invocation:
$ /usr/lib/sendmail -bv brier brier... deliverable: mailer local, user brier
shows us that the address brier is a valid address and
will be delivered locally. The next invocation:
$ /usr/lib/sendmail -bv ombuds ombuds... deliverable: mailer smtp, host indiana.edu, user ombuds@indiana.edu
shows that the address ombuds is also a valid address, but
instead of being delivered locally, it will be delivered to
ombuds@indiana.edu.
Invoking sendmail In Test Mode
sendmail has a test mode which only performs and displays
address rewriting, it does not actually deliver any mail. This test
mode is an excellent resource for researching what sendmail
does to an address during it's rewriting stage, and where mail sent to
that address ends up being transported. Test mode can also be used to
test changes made to a configuration file, especially changes to the
address rewriting rules.
Test mode is invoked with the -bt option to
sendmail. Since it is used often with experimental .cf
files, it is often invoked in conjunction with the -C option
which allows specification of an alternate .cf file. For
example:
/usr/lib/sendmail -bt -Ciu-standalone.cf
would invoke sendmail in test mode, using the configuration
file iu-standalone.cf in the current working directory, rather
than the default /etc/sendmail.cf.
Using sendmail in test mode requires some knowledge about
how sendmail rewrites addresses to allow them to be delivered.
Briefly, the .cf file contains a number of rules for
rewriting addresses. These rules are gathered up into rulesets
which perform some task. So, a ruleset is a collection of rules
responsible for a specific purpose in the address rewriting process,
for example, canonifying names. These sendmail rulesets are
numbered (0-99 in V8). The details of rewriting rules are a bit
outside the scope of this course. For more information, check out the
optional reference, Open Computing "Hands-On"
Tutorial: January 1994,
in a Punctuation Factory by Bryan Costales, or
Bryan's book,
also called "the bat book".
In test mode, sendmail requires the user to specify an
address to rewrite, and which rulesets through which that address
should be sent. Without knowing a whole lot about rulesets and a mail
message's flow through them, it is a little hard to know which
rulesets to use. For general purposes, rulesets 3 and 0 give good
information. Both of these rulesets are run on every message that goes
through sendmail (ruleset 3 is the very first one, and ruleset
0 is the very last). For example, here is a sample transcript of a
sendmail in test mode:
$ /usr/lib/sendmail -bt ADDRESS TEST MODE (ruleset 3 NOT automatically invoked) Enter <ruleset> <address> > 3,0 uwsg@uwsg rewrite: ruleset 3 input: uwsg @ uwsg rewrite: ruleset 96 input: uwsg < @ uwsg > rewrite: ruleset 96 returns: uwsg < @ herbie . ucs . indiana . edu . > rewrite: ruleset 3 returns: uwsg < @ herbie . ucs . indiana . edu . > rewrite: ruleset 0 input: uwsg < @ herbie . ucs . indiana . edu . > rewrite: ruleset 98 input: uwsg < @ herbie . ucs . indiana . edu . > rewrite: ruleset 98 returns: uwsg < @ herbie . ucs . indiana . edu . > rewrite: ruleset 95 input: < > uwsg < @ herbie . ucs . indiana . edu . > rewrite: ruleset 95 returns: uwsg < @ herbie . ucs . indiana . edu . > rewrite: ruleset 0 returns: $# smtp $@ herbie . ucs . indiana . edu . $: uwsg < @ herbie . ucs . indiana . edu . >
Ruleset 3 canonifies the address, invoking ruleset 96 to do part of
it's work. First it puts angle brackets around the host part of the
address (this notation is for sendmail's internal use, they are
stripped off before sending), then it calls ruleset 96 to look up the
hostname and convert it, if needed, to a fully qualified domain name.
Note that ruleset 96 also returns the true name of the host. In this
case, internally, uwsg became uwsg.indiana.edu, which
was discovered to be an MX record and became the true hostname,
herbie.ucs.indiana.edu.
Ruleset 0 is in charge of doing final parsing of addresses. In this
example, ruleset 0 calls ruleset 98 (which is empty by default, so
does nothing), then calls ruleset 95, which resolves addresses to the
triple that sendmail expects as a final result. This triple
consists of the mailer to use (in this case smtp as denoted
by $#), the address to which the message should be
transported (in this case herbie.ucs.indiana.edu as denoted by
$@, and the address to put on the message (in this case
uwsg < @ herbie . ucs . indiana . edu . > as denoted by
$:).
| /canon host | Canonify host name. |
|---|---|
| /mx host | Look up MX records. |
| /map mapname key | Do Map lookup. |
| /parse address | Parse address. |
| /try mailer address | Externalize address. |
| /tryflags [HE][SR] | Tweak try. |
| .Dx value | Define macro. |
| .Cc value | Extend class. |
| =Sruleset | Print ruleset. |
| =M | Print mailers. |
| $m | Dump macro. |
| $=c | Dump class. |
| -ddebugflags | Set debug flags. |
To summarize, with this sendmail configuration, a mail sent
to uwsg@uwsg will have it's address rewritten to
uwsg@herbie.ucs.indiana.edu and will be delivered to
herbie.ucs.indiana.edu by the smtp mailer.
Beginning with version 8.7, sendmail supports a set of
extensions to test mode for making certain debugging operations
easier. These options are summarized in Table 1 (also available as
sendmail Command Line
Debugging Options
| -d21.2 | Shows rewriting of addresses. |
|---|---|
| -d21.12 | Shows more detailed rewriting of addresses. |
| -d60 | Shows map lookups. |
| -d11 | Shows mailer invocations |
| -d8.7 | Shows DNS chat. |
| -d27.4 | Shows alias, forward, and include traffic. |
| -d28.4 | Shows user database traffic. |
| -d12 | Shows remote name editing. |
| -d0.1 | Shows compile flags and system identity. |
| -d0.4 | Shows proving for local names. |
| -d0.10 | Shows OS-dependent compile flags. |
sendmail has a -d flag for debugging options. For
essential information on sendmail's command line debugging
options, see
Installation and Operation Guide - Debugging.
The information reported by these debugging options is often very
low-level and may not always be helpful without the source at hand.
However, Table 2 (a plain text version is
available) shows a list of some of the more useful debugging flags.
The debugging options are most often used in tandem with the
-v option which turns on verbose delivery for sendmail
(the same as it does for mail).
Terms used: SMTP, mail
exchange, DNS, sendmail, sendmail.cf.