Rsyslog

rsyslog is an alternative logger to syslog-ng and offers many benefits over syslog-ng. rsyslog is also the default logger for the latest versions of Red Hat Enterprise Linux, as well as many other Linux distributions. Many of rsyslog's benefits over syslog-ng can be found here.

Installation

install the rsyslog package which is available in the official repositories.

Configure Hostname

Rsyslog uses the glibc routine gethostname() or gethostbyname() to determine the hostname of the local machine. The gethostname() or gethostbyname() routine check the contents of /etc/hosts for the fully qualified domain name (FQDN) if you are not using BIND or NIS.

You can check what the local machine's currently configured FQDN is by running hostname --fqdn. The output of hostname --short will be used by rsyslog when writing log messages. If you want to have full hostnames in logs, you need to add $PreserveFQDN on to the beginning of the file (before using any directive that write to files). This is because, rsyslog reads config file and applies it on-the-go and then reads the later lines.

The /etc/hosts file contains a number of lines that map FQDNs to IP addresses and that map aliases to FQDNs. See the example /etc/hosts file below:

/etc/hosts

#<ip-address>	<hostname.domain.org>	<hostname>
#<ip-address>      <actual FQDN>                       <aliases>
127.0.0.1	localhost.localdomain somehost.localdomain	localhost somehost
::1		        localhost.localdomain somehost.localdomain	localhost somehost

localhost.localdomain is the first item following the IP address, so gethostbyname() function will return localhost.localdomain as the local machine's FQDN. Then /var/log/messages file will use localhost as hostname.

To use somehost as the hostname. Move somehost.localdomain to the first item:

/etc/hosts

#<ip-address>	<hostname.domain.org>	                        <hostname>
#<ip-address>      <actual FQDN>                                              <aliases>
127.0.0.1	somehost.localdomain localhost.localdomain	localhost somehost
::1		        somehost.localdomain localhost.localdomain 	localhost somehost

Activation

After installing rsyslog, systemd will need to know about the service file packaged with rsyslog:

 # systemctl daemon-reload

Disable your old logger, (usually syslog-ng), then enable and start the new service:

# systemctl {disable|stop} syslog-ng
# systemctl {enable|start} rsyslog

Configuration

Log output can be fine tuned in /etc/rsyslog.conf. The daemon uses Facility levels (see below) to determine what gets put where. For example:

/etc/rsyslog.conf

# The authpriv file has restricted access.
authpriv.*                                              /var/log/secure

States that all messages falling under the authpriv facility are logged to /var/log/secure.

Another example, which would be similar to syslog-ngs behavior for the old auth.log:

/etc/rsyslog.conf

auth.*                                                  -/var/log/auth

Facility Levels

Note: The mapping between Facility Number and Keyword is not uniform over different operating systems and different syslog implementations. Use the keyword where possible, until it is determined which numbers are used by Arch.

Facility Number	Keyword	Facility Description
0	kern	kernel messages
1	user	user-level messages
2	mail	mail system
3	daemon	system daemons
4	auth	security/authorization messages
5	syslog	messages generated internally by syslogd
6	lpr	line printer subsystem
7	news	network news subsystem
8	uucp	UUCP subsystem
9		clock daemon
10	authpriv	security/authorization messages
11	ftp	FTP daemon
12	-	NTP subsystem
13	-	log audit
14	-	log alert
15	cron	clock daemon
16	local0	local use 0 (local0)
17	local1	local use 1 (local1)
18	local2	local use 2 (local2)
19	local3	local use 3 (local3)
20	local4	local use 4 (local4)
21	local5	local use 5 (local5)
22	local6	local use 6 (local6)
23	local7	local use 7 (local7)

Security Levels

As defined in RFC 5424, there are eight security levels:

Code	Severity	Keyword	Description	General Description
0	Emergency	emerg (panic)	System is unusable.	A "panic" condition usually affecting multiple apps/servers/sites. At this level it would usually notify all tech staff on call.
1	Alert	alert	Action must be taken immediately.	Should be corrected immediately, therefore notify staff who can fix the problem. An example would be the loss of a primary ISP connection.
2	Critical	crit	Critical conditions.	Should be corrected immediately, but indicates failure in a primary system, an example is a loss of a backup ISP connection.
3	Error	err (error)	Error conditions.	Non-urgent failures, these should be relayed to developers or admins; each item must be resolved within a given time.
4	Warning	warning (warn)	Warning conditions.	Warning messages, not an error, but indication that an error will occur if action is not taken, e.g. file system 85% full - each item must be resolved within a given time.
5	Notice	notice	Normal but significant condition.	Events that are unusual but not error conditions - might be summarized in an email to developers or admins to spot potential problems - no immediate action required.
6	Informational	info	Informational messages.	Normal operational messages - may be harvested for reporting, measuring throughput, etc. - no action required.
7	Debug	debug	Debug-level messages.	Info useful to developers for debugging the application, not useful during operations.

A common mnemonic used to remember the syslog levels in reverse order: "Do I Notice When Evenings Come Around Early".

See also: http://www.rsyslog.com/doc/manual.html

Examples

journald with rsyslog for kernel messages

Since the syslog component of systemd, journald, does not flush its logs to disk during normal operation, these logs will be gone when the machine is shut down abnormally (power loss, kernel lock-ups, ...). In the case of kernel lock-ups, it is pretty important to have some kernel logs for debugging. Until journald gains a configuration option for flushing kernel logs, rsyslog can be used in conjunction with journald.

Summary of requirements:

• journald must still get all log messages.
• rsyslog must only log kernel messages, all other logs are handled by journald.
• Kernel logs must be logged separatedly to /var/log/kernel.log.
• Use systemd to start the service.

Installation and configuration steps:

1. Install rsyslog.
2. Edit /etc/logrotate.d/rsyslog and add /var/log/kernel.log to the list of logs. Without this modification, the kernel log would grow indefinitely.
3. Edit /etc/rsyslog.conf and comment everything except for $ModLoad imklog. I also kept $ModLoad immark to have a heart-beat logged.
4. Add the next line to the same configuration file:

   kern.*     /var/log/kernel.log;RSYSLOG_TraditionalFileFormat

   The kern.* part catches all messages originating from the kernel. ;RSYSLOG_TraditionalFileFormat is used here to use a less verbose date format. By default, a date format like 2013-03-09T19:29:33.103897+01:00 is used. Since the kernel log contains a precision already (printk time) and the actual log time is irrelevant, I prefer something like Mar 9 19:29:13.

5. Since rsyslog should operate completely separated from systemd, remove the option that shares a socket with systemd:

   sed 's/^Sockets=/#&/' /usr/lib/systemd/system/rsyslog.service | sudo tee /etc/systemd/system/rsyslog.service

6. Next, make rsyslog start on boot and start it for this session:

   sudo systemctl enable rsyslog.service

   sudo systemctl start rsyslog.service

Note: rsyslog reads from /proc/kmsg. This means that subsequent reads from that file (either the user or a syslog daemon) will not read "old" logs from that file anymore. journald is not affected as it reads from /dev/kmsg which allows multiple readers.