Multiple Rulesets in rsyslog

Starting with version 4.5.0 and 5.1.1, rsyslog supports multiple rulesets within a single configuration. This is especially useful for routing the recpetion of remote messages to a set of specific rules. Note that the input module must support binding to non-standard rulesets, so the functionality may not be available with all inputs.

In this document, I am using imtcp, an input module that supports binding to non-standard rulesets since rsyslog started to support them.

What is a Ruleset?

If you have worked with (r)syslog.conf, you know that it is made up of what I call rules (others tend to call them selectors, a sysklogd term). Each rule consist of a filter and one or more actions to be carried out when the filter evaluates to true. A filter may be as simple as a traditional syslog priority based filter (like "*.*" or "mail.info" or a as complex as a script-like expression. Details on that are covered in the config file documentation. After the filter come action specifiers, and an action is something that does something to a message, e.g. write it to a file or forward it to a remote logging server.

A traditional configuration file is made up of one or more of these rules. When a new message arrives, its processing starts with the first rule (in order of appearance in rsyslog.conf) and continues for each rule until either all rules have been processed or a so-called "e;discard" action happens, in which case processing stops and the message is thrown away (what also happens after the last rule has been processed).

The multi-ruleset support now permits to specify more than one such rule sequence. You can think of a traditional config file just as a single default rule set, which is automatically bound to each of the inputs. This is even what actually happens. When rsyslog.conf is processed, the config file parser looks for the directive

$RuleSet <name>

Where name is any name the user likes (but must not start with "RSYSLOG_", which is the name space reserved for rsyslog use). If it finds this directive, it begins a new rule set (if the name was not yet know) or switches to an already-existing one (if the name was known). All rules defined between this $RuleSet directive and the next one are appended to the named ruleset. Note that the reserved name "RSYSLOG_DefaultRuleset" is used to specify rsyslogd's default ruleset. You can use that name whereever you can use a ruleset name, including when binding an input to it.

Inside a ruleset, messages are processed as described above: they start with the first rule and rules are processed in the order of appearance of the configuration file until either there are no more rules or the discard action is executed. Note that with multiple rulesets no longer all rsyslog.conf rules are executed but only those that are contained within the specific ruleset.

Inputs must explicitely bind to rulesets. If they don't do, the default ruleset is bound.

This brings up the next question:

What does "To bind to a Ruleset" mean?

This term is used in the same sense as "to bind an IP address to an interface": it means that a specific input, or part of an input (like a tcp listener) will use a specific ruleset to "pass its messages to". So when a new message arrives, it will be processed via the bound ruleset. Rule from all other rulesets are irrelevant and will never be processed.

This makes multiple rulesets very handy to process local and remote message via seperate means: bind the respective receivers to different rule sets, and you do not need to seperate the messages by any other method.

Binding to rulesets is input-specifc. For imtcp, this is done via the

$InputTCPServerBindRuleset <name>

directive. Note that "name"e; must be the name of a ruleset that is already defined at the time the bind directive is given. There are many ways to make sure this happens, but I personally think that it is best to define all rule sets at the top of rsyslog.conf and define the inputs at the bottom. This kind of reverses the traditional recommended ordering, but seems to be a really useful and straightforward way of doing things.

Can I use a different Ruleset as the default?

This is possible by using the

$DefaultRuleset <name>

Directive. Please note, however, that this directive is actually global: that is, it does not modify the ruleset to which the next input is bound but rather provides a system-wide default rule set for those inputs that did not explicitly bind to one. As such, the directive can not be used as a work-around to bind inputs to non-default rulesets that do not support ruleset binding.

Examples

Split local and remote logging

Let's say you have a pretty standard system that logs its local messages to the usual bunch of files that are specified in the default rsyslog.conf. As an example, your rsyslog.conf might look like this:

# ... module loading ...
# The authpriv file has restricted access.
authpriv.*  /var/log/secure
# Log all the mail messages in one place.
mail.*      /var/log/maillog
# Log cron stuff
cron.*      /var/log/cron
# Everybody gets emergency messages
*.emerg     *
... more ...

Now, you want to add receive messages from a remote system and log these to a special file, but you do not want to have these messages written to the files specified above. The traditional approach is to add a rule in front of all others that filters on the message, processes it and then discards it:

# ... module loading ...
# process remote messages
:fromhost-ip, isequal, "192.0.2.1"    /var/log/remotefile
& ~
# only messages not from 192.0.21 make it past this point

# The authpriv file has restricted access.
authpriv.*                            /var/log/secure
# Log all the mail messages in one place.
mail.*                                /var/log/maillog
# Log cron stuff
cron.*                                /var/log/cron
# Everybody gets emergency messages
*.emerg                               *
... more ...

Note the tilde character, which is the discard action!. Also note that we assume that 192.0.2.1 is the sole remote sender (to keep it simple).

With multiple rulesets, we can simply define a dedicated ruleset for the remote reception case and bind it to the receiver. This may be written as follows:

# ... module loading ...
# process remote messages
# define new ruleset and add rules to it:
$RuleSet remote
*.*           /var/log/remotefile
# only messages not from 192.0.21 make it past this point

# bind ruleset to tcp listener
$InputTCPServerBindRuleset remote
# and activate it:
$InputTCPServerRun 10514

# switch back to the default ruleset:
$RuleSet RSYSLOG_DefaultRuleset
# The authpriv file has restricted access.
authpriv.*    /var/log/secure
# Log all the mail messages in one place.
mail.*        /var/log/maillog
# Log cron stuff
cron.*        /var/log/cron
# Everybody gets emergency messages
*.emerg       *
... more ...

Here, we need to switch back to the default ruleset after we have defined our custom one. This is why I recommend a different ordering, which I find more intuitive. The sample below has it, and it leads to the same results:

# ... module loading ...
# at first, this is a copy of the unmodified rsyslog.conf
# The authpriv file has restricted access.
authpriv.*    /var/log/secure
# Log all the mail messages in one place.
mail.*        /var/log/maillog
# Log cron stuff
cron.*        /var/log/cron
# Everybody gets emergency messages
*.emerg       *
... more ...
# end of the "regular" rsyslog.conf. Now come the new definitions:

# process remote messages
# define new ruleset and add rules to it:
$RuleSet remote
*.*           /var/log/remotefile

# bind ruleset to tcp listener
$InputTCPServerBindRuleset remote
# and activate it:
$InputTCPServerRun 10514

Here, we do not switch back to the default ruleset, because this is not needed as it is completely defined when we begin the "remote" ruleset.

Now look at the examples and compare them to the single-ruleset solution. You will notice that we do not need a real filter in the multi-ruleset case: we can simply use "*.*" as all messages now means all messages that are being processed by this rule set and all of them come in via the TCP receiver! This is what makes using multiple rulesets so much easier.

Split local and remote logging for three different ports

This example is almost like the first one, but it extends it a little bit. While it is very similar, I hope it is different enough to provide a useful example why you may want to have more than two rulesets.

Again, we would like to use the "regular" log files for local logging, only. But this time we set up three syslog/tcp listeners, each one listening to a different port (in this example 10514, 10515, and 10516). Logs received from these receivers shall go into different files. Also, logs received from 10516 (and only from that port!) with "mail.*" priority, shall be written into a specif file and not be written to 10516's general log file.

This is the config:

# ... module loading ...
# at first, this is a copy of the unmodified rsyslog.conf
# The authpriv file has restricted access.
authpriv.* /var/log/secure
# Log all the mail messages in one place.
mail.*  /var/log/maillog
# Log cron stuff
cron.*  /var/log/cron
# Everybody gets emergency messages
*.emerg       *
... more ...
# end of the "regular" rsyslog.conf. Now come the new definitions:

# process remote messages

#define rulesets first
$RuleSet remote10514
*.*     /var/log/remote10514

$RuleSet remote10515
*.*     /var/log/remote10515

$RuleSet remote10516
mail.*	/var/log/mail10516
&       ~
# note that the discard-action will prevent this messag from 
# being written to the remote10516 file - as usual...
*.*     /var/log/remote10516

# and now define listners bound to the relevant ruleset
$InputTCPServerBindRuleset remote10514
$InputTCPServerRun 10514

$InputTCPServerBindRuleset remote10515
$InputTCPServerRun 10515

$InputTCPServerBindRuleset remote10516
$InputTCPServerRun 10516

Note that the "mail.*" rule inside the "remote10516"e; ruleset does not affect processing inside any other rule set, including the default rule set.

Performance

No rule processing can be faster than not processing a rule at all. As such, it is useful for a high performance system to identify disjunct actions and try to split these off to different rule sets. In the example section, we had a case where three different tcp listeners need to write to three different files. This is a perfect example of where multiple rule sets are easier to use and offer more performance. The performance is better simply because there is no need to check the reception service - instead messages are automatically pushed to the right rule set and can be processed by very simple rules (maybe even with "*.*"-filters, the fastest ones available).

In the long term, multiple rule sets will probably lay the foundation for even better optimizations. So it is not a bad idea to get aquainted with them.

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