Recording the Priority of Syslog Messages¶

Written by Rainer Gerhards (2007-06-18)

Abstract¶

The so-called priority (PRI) is very important in syslog messages, because almost all filtering in syslog.conf is based on it. However, many syslogds (including the Linux stock sysklogd) do not provide a way to record that value. In this article, I’ll give a brief overview of how PRI can be written to a log file.

Background¶

The PRI value is a combination of so-called severity and facility. The facility indicates where the message originated from (e.g. kernel, mail subsystem) while the severity provides a glimpse of how important the message might be (e.g. error or informational). Be careful with these values: they are in no way consistent across applications (especially severity). However, they still form the basis of most filtering in syslog.conf. For example, the directive (aka “selector line)” historically looked like mail.* /var/log/mail.log. Rainerscript expresses the same idea more explicitly via the prifilt() function:

if prifilt("mail.*") then {
    action(type="omfile" file="/var/log/mail.log")
}

Messages with the mail facility are stored to /var/log/mail.log, no matter which severity indicator they have (that is telling us the asterisk). If you set up complex conditions, it can be annoying to find out which PRI value a specific syslog message has. Most stock syslogds do not provide any way to record them.

How is it done?¶

With rsyslog, PRI recording is simple. All you need is the correct template. Even if you do not use rsyslog on a regular basis, it might be a handy tool for finding out the priority.

Rsyslog provides a flexible system to specify the output formats. Modern Rainerscript configurations prefer list templates: each template is composed of property() and constant() statements that are expanded by the property replacer before the final text is emitted. A template with the traditional syslog format looks as follows:

template(
    name="TraditionalFormat"
    type="list"
) {
    property(name="timegenerated" dateFormat="rfc3164")
    constant(value=" ")
    property(name="hostname")
    constant(value=" ")
    property(name="syslogtag")
    property(name="msg" dropLastLf="on")
    constant(value="\n")
}

The Templates documentation walks through the available template types and modifiers in more detail.

Each property() statement pulls in a message property (for example, msg or hostname) and can apply property replacer options such as dropLastLf. The constant() statements inject literal text such as spaces or newline characters. Rsyslog concatenates the list entries in the order they are defined.

Thankfully, rsyslog provides message properties for the priority. These are called “PRI”, “syslogfacility” and “syslogpriority” (case is important!). They are numerical values. Starting with rsyslog 1.13.4, there is also a property “pri-text”, which contains the priority in friendly text format (e.g. “local0.err<133>”). For the rest of this article, I assume that you run version 1.13.4 or higher.

Recording the priority is now a simple matter of adding the respective field to the template. It now looks like this:

template(
    name="TraditionalFormatWithPRI"
    type="list"
) {
    property(name="pri-text")
    constant(value=": ")
    property(name="timegenerated" dateFormat="rfc3164")
    constant(value=" ")
    property(name="hostname")
    constant(value=" ")
    property(name="syslogtag")
    property(name="msg" dropLastLf="on")
    constant(value="\n")
}

Now we have the right template - but how to write it to a file? You probably have a line like this in your syslog.conf:

action(
    type="omfile"
    file="/var/log/messages.log"
    template="TraditionalFormatWithPRI"
)

That’s all you need to do. There is one common pitfall: you need to define the template before you use it in an action. Otherwise, you will receive an error.

Once you have applied the changes, you need to restart rsyslogd. It will then pick the new configuration.

What if I do not want rsyslogd to be the standard syslogd?¶

If you do not want to switch to rsyslog, you can still use it as a setup aid. A little bit of configuration is required.

Download, make and install rsyslog.
Copy your existing configuration to a test file (for example, /etc/rsyslog-pri.conf).
Add the template and action described above to it; select the file that should use it.
Stop your regular syslog daemon for the time being.
Run rsyslogd (you may even do this interactively by calling it with the -n additional option from a shell).
Stop rsyslogd (press ctrl-c when running interactively).
Restart your regular syslogd.

That’s it - you can now review the priorities.

Some Sample Data¶

Below is some sample data created with the template specified above. Note the priority recording at the start of each line.

kern.info<6>: Jun 15 18:10:38 host kernel: PCI: Sharing IRQ 11 with 00:04.0
kern.info<6>: Jun 15 18:10:38 host kernel: PCI: Sharing IRQ 11 with 01:00.0
kern.warn<4>: Jun 15 18:10:38 host kernel: Yenta IRQ list 06b8, PCI irq11
kern.warn<4>: Jun 15 18:10:38 host kernel: Socket status: 30000006
kern.warn<4>: Jun 15 18:10:38 host kernel: Yenta IRQ list 06b8, PCI irq11
kern.warn<4>: Jun 15 18:10:38 host kernel: Socket status: 30000010
kern.info<6>: Jun 15 18:10:38 host kernel: cs: IO port probe 0x0c00-0x0cff: clean.
kern.info<6>: Jun 15 18:10:38 host kernel: cs: IO port probe 0x0100-0x04ff: excluding 0x100-0x107 0x378-0x37f 0x4d0-0x4d7
kern.info<6>: Jun 15 18:10:38 host kernel: cs: IO port probe 0x0a00-0x0aff: clean.
local7.notice<189>: Jun 15 18:17:24 host dd: 1+0 records out
local7.notice<189>: Jun 15 18:17:24 host random: Saving random seed: succeeded
local7.notice<189>: Jun 15 18:17:25 host portmap: portmap shutdown succeeded
local7.notice<189>: Jun 15 18:17:25 host network: Shutting down interface eth1: succeeded
local7.notice<189>: Jun 15 18:17:25 host network: Shutting down loopback interface: succeeded
local7.notice<189>: Jun 15 18:17:25 host pcmcia: Shutting down PCMCIA services: cardmgr
user.notice<13>: Jun 15 18:17:25 host /etc/hotplug/net.agent: NET unregister event not supported
local7.notice<189>: Jun 15 18:17:27 host pcmcia: modules.
local7.notice<189>: Jun 15 18:17:29 host rc: Stopping pcmcia: succeeded
local7.notice<189>: Jun 15 18:17:30 host rc: Starting killall: succeeded
syslog.info<46>: Jun 15 18:17:33 host [origin software="rsyslogd" swVersion="1.13.3" x-pid="2464"] exiting on signal 15.
syslog.info<46>: Jun 18 10:55:47 host [origin software="rsyslogd" swVersion="1.13.3" x-pid="2367"][x-configInfo udpReception="Yes" udpPort="514" tcpReception="Yes" tcpPort="1470"] restart
user.notice<13>: Jun 18 10:55:50 host rger: test
syslog.info<46>: Jun 18 10:55:52 host [origin software="rsyslogd" swVersion="1.13.3" x-pid="2367"] exiting on signal 2.``

Support: rsyslog Assistant | GitHub Discussions | GitHub Issues: rsyslog source project

Contributing: Source & docs: rsyslog source project