154 lines
5.0 KiB
Plaintext
154 lines
5.0 KiB
Plaintext
1 - Purpose of this document
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============================
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This document describes how to debug parts of the Postfix mail
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system, either by making the software log a lot of detail to the
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syslog daemon, or by running some daemon processes under control
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of an interactive debugger.
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2 - Verbose logging for specific SMTP connections
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=================================================
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In /etc/postfix/main.cf, list the remote site name or address in
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the "debug_peer_list" parameter. For example, in order to make the
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software log a lot of information to the syslog daemon for connections
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from or to the loopback interface:
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debug_peer_list = 127.0.0.1
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You can specify one or more hosts, domains, addresses or net/masks.
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2b - Record the SMTP connection with a sniffer
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==============================================
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This example uses tcpdump. In order to record a conversation you
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need to specify a large enough buffer or else you will miss some
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or all of the packet payload.
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tcpdump -w /file/name -s 2000 host hostname and port 25
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Run this for a while, stop with Ctrl-C when done. To view the data
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use a binary viewer, or use my tcpdumpx utility that is available
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from ftp://ftp.porcupine.org/pub/debugging.
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3 - Making Postfix daemon programs more verbose
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===============================================
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Append one or more -v options to selected daemon definitions in
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/etc/postfix/master.cf and type "postfix reload". This will cause
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a lot of activity to be logged to the syslog daemon.
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4 - Manually tracing a Postfix daemon process
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=============================================
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Some systems allow you to inspect a running process with a system
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call tracer. For example:
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# trace -p process-id
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# strace -p process-id
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# truss -p process-id
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# ktrace -p process-id
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See your system documentation for details.
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Tracing a running process can give valuable information about what
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a process is attempting to do. This is as much information as you
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can get without running an interactive debugger program, as described
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in a later section.
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5 - Automatically tracing a Postfix daemon process
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==================================================
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Postfix can attach a call tracer whenever a daemon process starts.
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Append a -D option to the suspect command in /etc/postfix/master.cf,
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for example:
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smtp inet n - n - - smtpd -D
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Edit the debugger_command definition in /etc/postfix/main.cf so
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that it invokes the call tracer of your choice, for example:
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debugger_command =
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PATH=/bin:/usr/bin:/usr/local/bin
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(truss -p $process_id 2>&1 | logger -p mail.info) & sleep 5
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Instead of truss use trace or strace.
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Type "postfix reload" and watch the logfile.
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6 - Running daemon programs under an interactive debugger
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=========================================================
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Append a -D option to the suspect command in /etc/postfix/master.cf,
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for example:
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smtp inet n - n - - smtpd -D
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Edit the debugger_command definition in /etc/postfix/main.cf so
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that it invokes the debugger of your choice, for example:
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debugger_command =
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PATH=/usr/bin:/usr/X11R6/bin
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xxgdb $daemon_directory/$process_name $process_id & sleep 5
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If you use xxgdb, be sure that gdb is in the command search path.
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Export XAUTHORITY so that X access control works, for example:
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% setenv XAUTHORITY ~/.Xauthority
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Stop and start the Postfix system.
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Whenever the suspect daemon process is started, a debugger window
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pops up and you can watch in detail what happens.
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7 - Unreasonable behavior
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=========================
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Sometimes the behavior exhibit by Postfix just does not match the
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source code. Why can a program deviate from the instructions given
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by its author? There are two possibilities.
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1 - The compiler has messed up.
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2 - The hardware has messed up.
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In both cases, the program being executed is not the program that
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was supposed to be executed, so anything can happen.
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There is a third possibility:
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3 - Bugs in system software (kernel or libraries).
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Hardware-related failures happen erratically, and they usually do
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not reproduce after power cycling and rebooting the system. There's
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little I can do about bad hardware. Be sure to use hardware that
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at the very least can detect memory errors. Otherwise, Postfix will
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just be a sitting duck waiting to be hit by a bit error. Critical
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systems deserve real hardware.
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When a compiler messes up, the problem can be reproduced whenever
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the resulting program is run. Compiler errors are most likely to
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happen in the code optimizer. If a problem is reproducible across
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power cycles and system reboots, it can be worthwhile to rebuild
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Postfix with optimization disabled, and to see if optimization
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makes a difference.
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In order to compile Postfix with optimizations turned off:
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% make tidy
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% make makefiles OPT=
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This produces a set of Makefiles that do not request compiler
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optimization.
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Once the makefiles are set up, build the software:
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% make
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% su
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# make install
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And see if the problem reproduces. If the problem goes away, talk
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to your vendor.
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