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Messages with the text “error” inside the text part of the message shall be written to a specific file. They shall not be written to any other file or be processed in any other way.

Things to think about

The configuration given here should be placed on top of the rsyslog.conf file.

Config Statements

:msg, contains, "error" /var/log/error.log

& ~

# Log anything (except mail) of level info or higher.

# Don't log private authentication messages!

*.info;mail.none;authpriv.none;cron.none      /var/log/messages

# 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                                       *

# Save news errors of level crit and higher in a special file.

uucp,news.crit                                /var/log/spooler

# Save boot messages also to boot.log

local7.*                                      /var/log/boot.log

How it works

The configuration uses a property-based filter to see if the string “error is contained” inside the MSG part of the syslog message. If so, the message is written to /var/log/error.log. The next line then discards all messages that have been written. Thus, no additional rules will be applied to the message. As such, it will not be written to /var/log/other.log.

Note the difference to this invalid sequence:

*.* /var/log/other.log

:msg, contains, "error" /var/log/error.log

& ~

Here everything is first written to /var/log/other.log and only then the message content is checked. In the later case, the message with “error” in them will be written to both files.

Important

Sequence of configuration statements is very important. Invalid sequence of otherwise perfectly legal configuration statements can lead to totally wrong results.

In this recipe, we forward messages from one system to another one. Typical use cases are:

• the local system does not store any messages (e.g. has not sufficient space to do so)
• there is a (e.g. legal) requirement to consolidate all logs on a single system
• the server may run some advanced alerting rules, and needs to have a full picture or network activity to work well
• you want to get the logs to a different system in a different security domain (to prevent attackers from hiding their tracks)
• and many more …

In our case, we forward all messages to the remote system. Note that by applying different filters, you may only forward select entries to the remote system. Also note that you can include as many forwarding actions as you like. For example, if you need to have a backup central server, you can simply forward to both of them, using two different forwarding actions.

To learn how to configure the remote server, see recipe Receiving Messages from a Remote System.

Config Statements

# this is the simplest forwarding action:
*.* action(type="omfwd" target="192.0.2.1" port="10514" protocol="tcp")
# it is equivalent to the following obsolete legacy format line:
*.* @@192.0.2.1:10514 # do NOT use this any longer!
# Note: if the remote system is unreachable, processing will
# block here and discard messages after a while

# so a better use is
*.*  action(type="omfwd" target="192.0.2.2" port="10514" protocol="tcp"
            action.resumeRetryCount="100"
            queue.type="linkedList" queue.size="10000")
# this will de-couple the sending from the other logging actions,
# and prevent delays when the remote system is not reachable. Also,
# it will try to connect 100 times before it discards messages as
# undeliverable.
# the rest below is more or less a plain vanilla rsyslog.conf as 
# many distros ship it - it's more for your reference...
# Log anything (except mail) of level info or higher.
# Don't log private authentication messages!
*.info;mail.none;authpriv.none;cron.none      /var/log/messages
# 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                                       :omusrmsg:*
# Save news errors of level crit and higher in a special file.
uucp,news.crit                                /var/log/spooler
# Save boot messages also to boot.log
local7.*                                      /var/log/boot.log

Things to think about

You need to select the protocol best suitable for your use case. If in doubt, TCP is a decent choice. This recipe uses TCP for that reason.

TCP forwarding is a build-in capability and always present. As such, no plugin needs to be loaded. The target can be specified by DNS name or IP address. Use IP addresses for most robust operations. If you use a DNS name and name resolution fails, forwarding may be disabled for some time. DNS resolution typically fails on the DNS server itself during system startup.

In this example, we forward to port 10514. We could as well remove the port=”…” parameter from the configuration, which would result in the default port being used. However, you need to specify the port address on the server in any case. So it is strongly advised to use an explicit port number to make sure that client and server configuration match each other (if they used different ports, the message transfer would not work.