Linux Logging Basics

Operating system logs provide a wealth of diagnostic information about your computers, and Linux is no exception. Everything from kernel events to user actions is logged by Linux, allowing you to see almost any action performed on your servers. In this guide, we’ll explain what Linux logs are, where they’re located, and how to interpret them.

Linux System Logs

Linux has a special directory for storing logs called /var/log. This directory contains logs from the OS itself, services, and various applications running on the system. Here’s what this directory looks like on a typical Ubuntu system.

Some of the most important Linux system logs include:

/var/log/syslog and /var/log/messages store all global system activity data, including startup messages. Debian-based systems like Ubuntu store this in /var/log/syslog, while Red Hat-based systems like RHEL or CentOS use /var/log/messages.
/var/log/auth.log and /var/log/secure store all security-related events such as logins, root user actions, and output from pluggable authentication modules (PAM). Ubuntu and Debian use /var/log/auth.log, while Red Hat and CentOS use /var/log/secure.
/var/log/kern.log stores kernel events, errors, and warning logs, which are particularly helpful for troubleshooting custom kernels.
/var/log/cron stores information about scheduled tasks (cron jobs). Use this data to verify your cron jobs are running successfully.

Some applications also write log files to this directory. For example, the Apache web server writes logs to the /var/log/apache2 directory (on Debian), while MySQL writes logs to the /var/log/mysql directory. Some applications also log via Syslog, which we’ll explain in the next section.

What’s Syslog?

Syslog is a standard for creating and transmitting logs. The word “syslog” can refer to any of the following:

The syslog service receives and processes syslog messages and listens for events by creating a socket located at /dev/log, which applications can write to. It can write messages to a local file or forward messages to a remote server. There are different syslog implementations, including rsyslogd and syslog-ng.
The Syslog protocol (RFC 5424) is a transport protocol specifying how to transmit logs over a network. It’s also a data format defining how messages are structured. By default, it uses port 514 for plaintext messages and port 6514 for encrypted messages.
A syslog message is any log formatted in the syslog message format and consists of a standardized header and message containing the log’s contents.

Since Syslog can forward messages to remote servers, it’s often used to forward system logs to log management solutions such as SolarWinds® Loggly® and SolarWinds Papertrail^™.

RFC 3164 vs. RFC 5424

While RFC 5424 is the current Syslog protocol, it’s not the only standard you’ll see in the wild. RFC 3164 (a.k.a. “BSD syslog” or “old syslog”) is an older syslog format still used by many devices. In practice, admins are likely to see syslog messages that use both RFC 3164 and RFC 5424 formatting.

Syslog Format and Fields

Syslog messages contain a standardized header with several fields. These include the timestamp, the name of the application that generated the event, the location in the system where the message originated, and its priority. You can change this format in your syslog implementation’s configuration file, but using the standard format makes it easier to parse, analyze, and route syslog events.

Here’s an example log message using the default format. It’s from the SSH daemon (sshd), which controls remote logins to the system. This message describes a failed login attempt:

Jun 4 22:14:15 server1 sshd[41458] : Failed password for root from 10.0.2.2 port 22 ssh2

Priority

The priority field or pri for short (“<34>” in the example) tells you how urgent or severe the event is. It’s a combination of two numerical fields: the facility and the severity. The facility specifies the type of process that created the event, ranging from 0 for kernel messages to 23 for local applications. The severity ranges from 0 – 7, with 0 indicating an emergency and 7 indicating a debug event.

Pri can be output in two ways. The first is as a single number, prival, which is calculated as the facility field value multiplied by eight; then, the result is added to the severity field value: (facility)(8) + (severity). The second is pri-text, which will output in the string format “facility.severity”. The latter format is often easier to read and search but takes up more storage space.

Timestamp

The timestamp field (“2019-06-05T22:14:15.003Z” in the above example) indicates the time and date the message was generated on the system sending the message. The example timestamp breaks down like this:

“2019-06-05” is the year, month, and day.
“T” is a required element of the timestamp field, separating the date and the time.
“22:14:15.003” is the 24-hour format of the time, including the number of milliseconds (003).
“Z” indicates UTC time. Instead of Z, the example could have included an offset, such as -08:00, which indicates that the time is offset from UTC by eight hours.

Hostname

The hostname field (“server1” in the example) indicates the name of the host or system that originally sent the message.

App-name

The app-name field (“sshd:auth in the example) indicates the name of the application that sent the message.

Logging with systemd

Many Linux distributions ship with systemd—a process and service manager. Systemd implements its own logging service called journald, which can replace or complement Syslog. Journald logs in a significantly different manner than systemd, which is why it has its own section in the Ultimate Guide to Logging. You can learn more about logging via systemd in the Systemd Logging section.

Additional Resources

How to View and Configure Linux Logs on Ubuntu and CentOS (DigitalOcean)

Last updated: 2022

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