In computing, syslog is a standard for message logging. It permits separation of the software that generates messages, the system that stores them, and the software that reports and analyzes them. Each message is labeled with a facility code, indicating the software type generating the message, and assigned a severity label.
Computer system designers may use syslog for system management and security auditing as well as general informational, analysis, and debugging messages. A wide variety of devices, such as printers, routers, and message receivers across many platforms use the syslog standard. This permits the consolidation of logging data from different types of systems in a central repository. Implementations of syslog exist for many operating systems.
Syslog was developed in the 1980s by Eric Allman as part of the Sendmail project. It was readily adopted by other applications and has since become the standard logging solution on Unix-like systems. A variety of implementations also exist on other operating systems and it is commonly found in network devices, such as routers.
Syslog originally functioned as a de facto standard, without any authoritative published specification, and many implementations existed, some of which were incompatible. The Internet Engineering Task Force documented the status quo in RFC 3164. It was standardized by RFC 5424.
Syslog message components
The information provided by the originator of a syslog message includes the facility code and the severity level. The syslog software adds information to the information header before passing the entry to the syslog receiver. Such components include an originator process ID, a timestamp, and the hostname or IP address of the device.
A facility code is used to specify the type of program that is logging the message. Messages with different facilities may be handled differently. The list of facilities available is defined by RFC 3164:
|5||syslog||messages generated internally by syslogd|
|6||lpr||line printer subsystem|
|7||news||network news subsystem|
|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)|
The mapping between facility code and keyword is not uniform between operating systems and different syslog implementations.
The list of severities is also defined by RFC 3164:
|0||Emergency||emerg||System is unusable||This level should not be used by applications.|
|1||Alert||alert||Should be corrected immediately||Loss of the primary ISP connection.|
|2||Critical||crit||Critical conditions||A failure in the system's primary application.|
|3||Error||err||Error conditions||An application has exceeded its file storage limit and attempts to write are failing.|
|4||Warning||warning||May indicate that an error will occur if action is not taken.||A non-root file system has only 2GB remaining.|
|5||Notice||notice||Events that are unusual, but not error conditions.|
|6||Informational||info||Normal operational messages that require no action.||An application has started, paused or ended successfully.|
|7||Debug||debug||Information useful to developers for debugging the application.|
The meaning of severity levels other than Emergency and Debug are relative to the application. For example, if the purpose of the system is to process transactions to update customer account balance information, an error in the final step should be assigned Alert level. However, an error occurring in an attempt to display the ZIP code of the customer may be assigned Error or even Warning level.
The server process which handles the message (syslogd) usually includes all lower levels. That is, if messages are separated by individual severity, a Warning level entry will also be included in Notice, Info and Debug processing.
The message component has these fields: TAG, which should be the name of the program or process that generated the message, and CONTENT which contains the details of the message. The content field should be encoded in a UTF-8 character set and octet values in the traditional ASCII control character range should be avoided.
The messages may be directed to various destinations, tuned by facility and severity, including console, files, remote syslog servers, or relays.
Most implementations provide a command line utility, often called logger, as well as a link library, to send messages to the log. Some implementations include reporting programs for filtering and displaying of syslog messages.
When operating over a network, syslog implements a client-server application structure where the server listens on a well-known or registered port for protocol requests from clients. Historically the most common Transport Layer protocol for network logging has been User Datagram Protocol (UDP), with the server listening on port 514. As UDP lacks congestion control mechanisms, support for Transport Layer Security is required to implement and also recommended for general use on Transmission Control Protocol port 6514.
Since each process, application and operating system, was written independently, there is little uniformity to the content of the log message. For this reason, no assumption is made about its formatting or contents.
The network protocol is simplex communication with no means to acknowledge the delivery to the originator.
Various groups are working on draft standards detailing the use of syslog for more than just network and security event logging, such as its proposed application within the health care environment.
Regulations, such as the Sarbanes-Oxley Act, PCI DSS, HIPAA, and many others, are requiring organizations to implement comprehensive security measures, which often include collecting and analyzing logs from many different sources. Syslog has proven to be an effective format to consolidate logs, as there are many open-source and proprietary tools for reporting and analysis. Converters exist from Windows Event Log as well as other log formats to syslog.
An emerging area of managed security services is the collection and analysis of syslog records for organizations. Managed Security Service Providers attempt to apply analytics techniques and artificial intelligence algorithms to detect patterns and alert customers to problems.
Internet standard documents
The Syslog protocol is defined by Request for Comments (RFC) documents published by the Internet Engineering Task Force (Internet standards). The following is a list of RFCs that define the syslog protocol:
- RFC 3164 The BSD syslog Protocol (obsoleted by RFC 5424)
- RFC 3195 Reliable Delivery for syslog
- RFC 5424 The Syslog Protocol
- RFC 5425 TLS Transport Mapping for Syslog
- RFC 5426 Transmission of Syslog Messages over UDP
- RFC 5427 Textual Conventions for Syslog Management
- RFC 5848 Signed Syslog Messages
- RFC 6012 Datagram Transport Layer Security (DTLS) Transport Mapping for Syslog
- RFC 6587 Transmission of Syslog Messages over TCP
- Gerhards R, RFC 5424 The Syslog Protocol
- "LXer: Patent jeopardizes IETF syslog standard".
- "IETF IPR disclosure on HUAWEI's patent claims".
- "Syslog Facility". Retrieved 22 November 2012.
- "Syslog Facilities". Retrieved 15 February 2012.
- "The Ins and Outs of System Logging Using Syslog".
- "RFC 5424 - The Syslog Protocol".
- "RFC 5425 - TLS Transport Mapping for Syslog".
- "Security Issues in Network Event Logging (syslog)". IETF.
- Internet Engineering Task Force: Datatracker: syslog Working Group (concluded)
- SANS Institute: "The Ins and Outs of System Logging Using Syslog" (white paper)
- National Institute of Standards and Technology: "Guide to Computer Security Log Management" (Special Publication 800-92) (white paper)
- Network Management Software: "Understanding Syslog: Servers, Messages & Security"
- MonitorWare: All about Syslog
- Syslog Servers for Windows & Linux