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In computing, syslog is a widely used 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.

Computer system designers can 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 and routers) and message receivers across multiple platforms use the syslog standard. Because of this, system designers can use syslog to integrate log data from different types of systems in a central repository.

In the syslog standard, each message is labeled with a facility code and assigned a severity label. The facility code indicates which of the following software types generated the message: auth, authpriv, daemon, cron, ftp, lpr, kern, mail, news, syslog, user, uucp, or local0 ... local7. The severity designations, from most to least severe, are: Emergency, Alert, Critical, Error, Warning, Notice, Info, and Debug.

Implementations of syslog exist for many operating systems. Specific configuration may permit directing messages to various devices (e.g., console), files (e.g., /var/log/), or remote syslog servers. Most implementations provide a command line utility, often called logger, that can send messages to the log. Some implementations permit filtering and display of syslog messages.

In 2009, the Internet Engineering Task Force (IETF) standardized syslog in RFC 5424.


Syslog was developed in the 1980s by Eric Allman as part of the Sendmail project, and was initially used solely for Sendmail. It proved so valuable that other applications began using it as well. Syslog has since become the standard logging solution on Unix and Unix-like systems; there have also been a variety of syslog implementations on other operating systems and is commonly found in network devices such as routers.

Syslog 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 made obsolete by subsequent additions in RFC 5424.[1]

At different points in time, various companies have attempted patent claims on syslog.[2][3] This had little effect on the use and standardization of the protocol.


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 SOX, 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. Companies calling themselves Managed Security Service Providers attempt to apply analytics techniques (and sometimes artificial intelligence algorithms) to detect patterns and alert customers to problems.

Facility levels[edit]

A facility level is used to specify what type of program is logging the message. This lets the configuration file specify that messages from different facilities will be handled differently.[4] The list of facilities available:[5] (defined by RFC 3164)

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)

The mapping between Facility Number and Keyword is not uniform over different operating systems and different syslog implementations.[6]
For cron either 9 or 15 or both may be used.
The confusion is even greater regarding auth/authpriv. 4 and 10 are most common but 13 and 14 may also be used.

Severity levels[edit]

RFC 5424 defines eight severity 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 secondary 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 from bottom to top is: "Do I Notice When Evenings Come Around Early".

Format of a Syslog packet[edit]

The full format of a Syslog message seen on the wire has three distinct parts:


The total length of the packet cannot exceed 1,024 bytes, and there is no minimum length.


The PRI part is a number that is enclosed in angle brackets. This represents both the Facility and Severity of the message. This number is an eight bit number. The first 3 least significant bits represent the Severity of the message (with 3 bits you can represent 8 different Severities) and the other 5 bits represent the Facility of the message. You can use the Facility and the Severity values to apply certain filters on the events in the Syslog Daemon.

Calculating Priority Value[edit]

The Priority value is calculated by first multiplying the Facility number by 8 and then adding the numerical value of the Severity. For example, a kernel message (Facility=0) with a Severity of Emergency (Severity=0) would have a Priority value of 0. Also, a "local use 4" message (Facility=20) with a Severity of Notice (Severity=5) would have a Priority value of 165. In the PRI part of a Syslog message, these values would be placed between the angle brackets as <0> and <165> respectively.

Calculating Facility and Severity Values from a Priority Value[edit]

This is a calculation derived from the previous one. To get the Facility number implied in a given Priority value, divide the Priority number by 8. The whole number part is the Facility. To get the Severity, multiply the Facility by 8 and subtract that number from the Priority.

For example:

Priority = 191

191/8 = 23.875

Facility = 23

Severity = 191 - (23 * 8 ) = 7

Work backword to check the formula: 23*8 = 184 + 7 = 191

Another Method:

To get the Facility number from a given priority value, divide priority by 8. The whole number is the Facility. Then to get Severity, take Priority mod 8.

For example:

Priority = 191

191 / 8 = 23.875

Facility = 23

Severity = 191 mod 8 = 7


The HEADER part contains the following:

  • Timestamp -- the date and time at which the message was generated. This is picked up from the sending system's system time which might differ from the receiving system's system time
  • Hostname or IP address of the device.


The MSG part will fill the remainder of the Syslog packet. This will usually contain some additional information of the process that generated the message, and then the text of the message. The MSG part has two fields:

  • TAG field
  • CONTENT field

The value in the TAG field will be the name of the program or process that generated the message. The CONTENT contains the details of the message.


The UDP-based Syslog protocol is unreliable. Unlike TCP-based transmission of messages, UDP does not guarantee you the delivery of the messages. They may either be dropped through network congestion, or they may be maliciously intercepted and discarded. The Syslog protocol does not ensure ordered delivery of packets.

Since each process, application and operating system was written independently, there is little uniformity to the content of syslog messages. For this reason, no assumption is made upon the formatting or contents of the messages. The protocol is simply designed to transport these messages.

The receiver of a Syslog packet may not be able to authenticate that the message was indeed sent from the reported sender. A misconfigured machine may send syslog messages to a Syslog daemon representing itself as another machine. The administrative staff may become confused because the status of the supposed sender of the messages may not be accurately reflected in the received messages. Another problem associated with authentication is that an attacker may start sending fake messages indicating a problem on some machine. This may get the attention of the system administrators who will spend their time investigating the alleged problem. During this time, the attacker may be able to compromise a different machine, or a different process on the same machine. An attacker may record a set of messages that indicate normal activity of a machine. At a later time, that attacker may remove that machine from the network and replay the syslog messages to the daemon.


Syslog is a client/server protocol:[7] a logging application transmits a text message to the syslog receiver. The receiver is commonly called syslogd, syslog daemon or syslog server. Syslog messages may be sent via the User Datagram Protocol (UDP) or the Transmission Control Protocol (TCP).[8] The data is sent in cleartext; although not part of the syslog protocol itself, an SSL wrapper may be used to provide for a layer of encryption through SSL/TLS. Syslog uses the port number 514.

The original specification in RFC 3164 did not specify many protocol aspects, such as the maximum message size and the character encoding for the message text. RFC 5424 added many details. Among others, implementations must support a minimum message size of at least 480 octets, and should support 2048 octets; messages should be encoded as UTF-8.

Internet standards[edit]

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:[9]

See also[edit]


  1. ^ Gerhards R. "RFC 5424". The Syslog Protocol. 
  2. ^ "LXer: Patent jeopardizes IETF syslog standard". 
  3. ^ "IETF IPR disclosure on HUAWEI's patent claims". 
  4. ^ "Syslog Facility". Retrieved 22 November 2012. 
  5. ^ "Syslog Facilities". Retrieved 15 February 2012. 
  6. ^ "The Ins and Outs of System Logging Using Syslog". 
  7. ^ RFC 3164, The BSD syslog Protocol
  8. ^ RFC 3195, Reliable Delivery for syslog
  9. ^ "Security Issues in Network Event Logging (syslog)". IETF. 

External links[edit]