Sunday, November 23, 2008

syslog-ng 3.0 and SNMP traps

Last time I've written about how syslog-ng is able to change message contents. I thought it'd be useful to give you a more practical example, instead of a generic description.

It is quite common to convert SNMP traps to syslog messages. The easiest implementation is to run snmptrapd and have it create a log message based on the trap. There's a small issue though: snmptrapd uses the UNIX syslog() API, and as such it is not able to propagate the originating host of the SNMP trap to the hostname portion of the syslog message. This means that all traps are logged as messages coming from the host running snmptrapd, and the hostname information is part of the message payload.

Of course it'd be much easier to process syslog messages, if this were not the case.

A solution would be to patch snmptrapd to send complete syslog frames, but that would require changing snmptrapd source. The alternative is to use the new parse and rewrite features of syslog-ng 3.0.

First, you need to filter snmptrapd messages:

filter f_snmptrapd { program("snmptrapd"); };

Then we'd need to grab the first field of the message payload, where snmptrapd is configured to put it:

rewrite r_snmptrapd {
subst("^([^ ]+) (.*)$ ", "${2}");
set("${1}" value("HOST"));
};


Both rewrite expression kinds are demonstrated here:
  • subst() has two arguments: the first is a regexp to search for, the second is a template to be substituted if there's a match
  • set() has a single argument: a template to be used as the new value
Rewrite rules operate by the contents of the $MESSAGE value by default, which holds the message payload. Of course this can be changed by specifying the value() option. The notion 'value' in syslog-ng 3.0 refers to a name-value pair, in syslog-ng 3.0 every message is composed of a set of name-value pairs. The names of standard values match the name of the corresponding macro, but without the '$' sign.

Please NOTE that the new value is a template which makes it possible to use macros such as $HOST or $MESSAGE defined by syslog-ng.

Now let's wire the complete configuration together:

filter f_snmptrapd { program("snmptrapd"); };

rewrite r_snmptrapd {
subst("^([^ ]+) (.*)$ ", "${2}");
set("${1}" value("HOST"));
};

log {
source(s_all);
filter(f_snmptrapd);
rewrite(r_snmptrapd);
destination(d_all);
flags(final);
};

log {
source(s_all);
destination(d_all);
flags(final);
};

Of course this is only an example of the power of what syslog-ng is now capable of doing. Please let me know if you can think of other uses.

The current 3.0 branch of syslog-ng has not been released yet, it is available in the git repository at git.balabit.hu, and also as nightly snapshots.

I'd be grateful for any kind of feedback you might have, please post it either as comments on this blog, or to the mailing list.

Saturday, November 08, 2008

syslog-ng statistics

For a long time I meant to give the "log statistics" feature of syslog-ng an overhaul, and finally with the advent of syslog-ng 3.0, this was done.

I'm not sure all of you know, but even earlier syslog-ng versions (2.1 and 2.0) did collect some per-source and per-destination statistics. These were reported periodically in the system log. The problem with this approach that it didn't really scale: with a large configuration the statistics message could become kilobytes long, and parsing this information from a file possibly several gigabytes in size is daunting.

syslog-ng 3.0 has two important changes in this area: it adds several new kinds of counters (like per-host counters), and a UNIX domain socket where you can query the current status of these counters.

As counters certainly have an overhead, you can now control how much statistics you want to gather. The new stats_level() option has three levels for now:
  1. stats_level(0) is basically the same as earlier syslog-ng versions, per-source and per-destination statistics are kept here. This is the default.
  2. stats_level(1) adds new counters without a big overhead, that is it adds counters for TCP connections, but does not keep per-host counters
  3. stats_level(2) adds counters that can have a measurable performance impact, it adds for example per-host (as in $HOST) counters and also keeps track of the time the last message was received from a given host. These counters usually require an hash table lookup in the fastpath.
Once you have the counters, you can still use the venerable "log statistics" message, by setting the stats_freq() option which defaults to 10 minutes, just like in earlier versions.

However if you don't want to dig the logs produced by syslog-ng, you can also use the new UNIX domain socket at /var/run/syslog-ng/syslog-ng.ctl (the path might depend on the compilation options).

If you connect to this socket using netcat (some netcat versions do support UNIX domain sockets), and you send a "STATS" command to it, you get the list of counters.

There are no proper, command line clients for the UNIX domain channel yet, but if you have some scripting ability, you can start gather statistics easily, without the hassles of parsing log files, right after installing a syslog-ng 3.0 snapshot. :)