HomeDocsIETF standardsRFCRFC 3100 — 3199
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Network Working Group
Request for Comments: 3164
Category: Informational
C. Lonvick
Cisco Systems
August 2001

The BSD syslog Protocol

Status of this Memo

This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.

Copyright Notice

Copyright © The Internet Society (2001). All Rights Reserved.

Abstract

This document describes the observed behavior of the syslog protocol. This protocol has been used for the transmission of event notification messages across networks for many years. While this protocol was originally developed on the University of California Berkeley Software Distribution (BSD) TCP/IP system implementations, its value to operations and management has led it to be ported to many other operating systems as well as being embedded into many other networked devices.

Table of Contents

   1. Introduction....................................................2
   1.1 Events and Generated Messages..................................3
   1.2 Operations of the Message Receivers............................5
   2. Transport Layer Protocol........................................5
   3. Definitions and Architecture....................................5
   4. Packet Format and Contents......................................7
   4.1 syslog Message Parts...........................................8
   4.1.1 PRI Part.....................................................8
   4.1.2 HEADER Part of a syslog Packet..............................10
   4.1.3 MSG Part of a syslog Packet.................................11
   4.2 Original syslog Packets Generated by a Device.................12
   4.3 Relayed syslog Packets........................................12
   4.3.1 Valid PRI and TIMESTAMP.....................................13
   4.3.2 Valid PRI but no TIMESTAMP or invalid TIMESTAMP.............13
   4.3.3 No PRI or Unidentifiable PRI................................14
   5. Conventions....................................................14
   5.1 Dates and Times...............................................15
   5.2 Domain Name and Address.......................................15
   5.3 Originating Process Information...............................15
   5.4 Examples......................................................16
   6. Security Considerations........................................18
   6.1 Packet Parameters.............................................19
   6.2 Message Authenticity..........................................19
   6.2.1 Authentication Problems.....................................19
   6.2.2 Message Forgery.............................................20
   6.3 Sequenced Delivery............................................20
   6.3.1 Single Source to a Destination..............................20
   6.3.2 Multiple Sources to a Destination...........................21
   6.3.3 Multiple Sources to Multiple Destinations...................21
   6.3.4 Replaying...................................................22
   6.4 Reliable Delivery.............................................22
   6.5 Message Integrity.............................................22
   6.6 Message Observation...........................................22
   6.7 Message Prioritization and Differentiation....................23
   6.8 Misconfiguration..............................................24
   6.9 Forwarding Loop...............................................24
   6.10 Load Considerations..........................................25
   7. IANA Considerations............................................25
   8. Conclusion and Other Efforts...................................25
   Acknowledgements..................................................26
   References........................................................27
   Author's Address..................................................28
   Full Copyright Statement..........................................29

1. Introduction

Since the beginning, life has relied upon the transmission of messages. For the self-aware organic unit, these messages can relay many different things. The messages may signal danger, the presence of food or the other necessities of life, and many other things. In many cases, these messages are informative to other units and require no acknowledgement. As people interacted and created processes, this same principle was applied to societal communications. As an example, severe weather warnings may be delivered through any number of channels - a siren blowing, warnings delivered over television and radio stations, and even through the use of flags on ships. The expectation is that people hearing or seeing these warnings would realize their significance and take appropriate action. In most cases, no responding acknowledgement of receipt of the warning is required or even desired. Along these same lines, operating systems, processes and applications were written to send messages of their own status, or messages to indicate that certain events had occurred. These event messages generally had local significance to the machine operators. As the operating systems, processes and applications grew ever more complex, systems were devised to categorize and log these diverse messages and allow the operations staff to more quickly differentiate the notifications of problems from simple status messages. The syslog process was one such system that has been widely accepted in many operating systems. Flexibility was designed into this process so the operations staff have the ability to configure the destination of messages sent from the processes running on the device. In one dimension, the events that were received by the syslog process could be logged to different files and also displayed on the console of the device. In another dimension, the syslog process could be configured to forward the messages across a network to the syslog process on another machine. The syslog process had to be built network-aware for some modicum of scalability since it was known that the operators of multiple systems would not have the time to access each system to review the messages logged there. The syslog process running on the remote devices could therefore be configured to either add the message to a file, or to subsequently forward it to another machine.

In its most simplistic terms, the syslog protocol provides a transport to allow a machine to send event notification messages across IP networks to event message collectors - also known as syslog servers. Since each process, application and operating system was written somewhat 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 event messages. In all cases, there is one device that originates the message. The syslog process on that machine may send the message to a collector. No acknowledgement of the receipt is made.

One of the fundamental tenets of the syslog protocol and process is its simplicity. No stringent coordination is required between the transmitters and the receivers. Indeed, the transmission of syslog messages may be started on a device without a receiver being configured, or even actually physically present. Conversely, many devices will most likely be able to receive messages without explicit configuration or definitions. This simplicity has greatly aided the acceptance and deployment of syslog.

1.1 Events and Generated Messages

The writers of the operating systems, processes and applications have had total control over the circumstances that would generate any message. In some cases, messages are generated to give status. These can be either at a certain period of time, or at some other interval such as the invocation or exit of a program. In other cases, the messages may be generated due to a set of conditions being met. In those cases, either a status message or a message containing an alarm of some type may be generated. It was considered that the writers of 

   the operating systems, processes and applications would quantify
   their messages into one of several broad categories.  These broad
   categories generally consist of the facility that generated them,
   along with an indication of the severity of the message.  This was so
   that the operations staff could selectively filter the messages and
   be presented with the more important and time sensitive notifications
   quickly, while also having the ability to place status or informative
   messages in a file for later perusal.   Other options for displaying
   or storing messages have been seen to exist as well.

Devices MUST be configured with rules for displaying and/or forwarding the event messages. The rules that have been seen are generally very flexible. An administrator may want to have all messages stored locally as well as to have all messages of a high severity forwarded to another device. They may find it appropriate to also have messages from a particular facility sent to some or all of the users of the device and displayed on the system console. However the administrator decides to configure the disposition of the event messages, the process of having them sent to a syslog collector generally consists of deciding which facility messages and which severity levels will be forwarded, and then defining the remote receiver. For example, an administrator may want all messages that are generated by the mail facility to be forwarded to one particular event message collector. Then the administrator may want to have all kernel generated messages sent to a different syslog receiver while, at the same time, having the critically severe messages from the kernel also sent to a third receiver. It may also be appropriate to have those messages displayed on the system console as well as being mailed to some appropriate people, while at the same time, being sent to a file on the local disk of the device. Conversely, it may be appropriate to have messages from a locally defined process only displayed on the console but not saved or forwarded from the device. In any event, the rules for this will have to be generated on the device. Since the administrators will then know which types of messages will be received on the collectors, they should then make appropriate rules on those syslog servers as well.

The contents of a message have also been at the discretion of its creator. It has been considered to be good form to write the messages so that they are informative to the person who may be reading them. It has also been considered good practice to include a timestamp and some indication of the sending device and the process that originated it in the messages. However, none of those are stringently required.

It should be assumed that any process on any device might generate an event message. This may include processes on machines that do not have any local storage - e.g., printers, routers, hubs, switches, and diskless workstations. In that case, it may be imperative that event messages are transported to a collector so that they may be recorded and hopefully viewed by an operator.

1.2 Operations of the Message Receivers

It is beyond the scope of this document to specify how event messages should be processed when they are received. Like the operations described in Section 1.1, they generally may be displayed to the appropriate people, saved onto disk, further forwarded, or any combination of these. The rules for determining the disposition of received messages have been seen to be identical to the rules for determining the disposition of locally generated messages.

As a very general rule, there are usually many devices sending messages to relatively fewer collectors. This fan-in process allows an administrator to aggregate messages into relatively few repositories.

2. Transport Layer Protocol

syslog uses the user datagram protocol (UDP) [1] as its underlying transport layer mechanism. The UDP port that has been assigned to syslog is 514. It is RECOMMENDED that the source port also be 514 to indicate that the message is from the syslog process of the sender, but there have been cases seen where valid syslog messages have come from a sender with a source port other than 514. If the sender uses a source port other than 514 then it is RECOMMENDED and has been considered to be good form that subsequent messages are from a single consistent port.

3. Definitions and Architecture

The following definitions will be used in this document.

A machine that can generate a message will be called a "device".

A machine that can receive the message and forward it to another machine will be called a "relay".

A machine that receives the message and does not relay it to any other machines will be called a "collector". This has been commonly known as a "syslog server".

Any device or relay will be known as the "sender" when it sends a message.

Any relay or collector will be known as the "receiver" when it receives the message.

The architecture of the devices may be summarized as follows:

Senders send messages to relays or collectors with no knowledge of whether it is a collector or relay.

Senders may be configured to send the same message to multiple receivers.

Relays may send all or some of the messages that they receive to a subsequent relay or collector. In the case where they do not forward all of their messages, they are acting as both a collector and a relay. In the following diagram, these devices will be designated as relays.

Relays may also generate their own messages and send them on to subsequent relays or collectors. In that case it is acting as a device. These devices will also be designated as a relay in the following diagram.

The following architectures shown in Diagram 1 are valid while the first one has been known to be the most prevalent. Other arrangements of these examples are also acceptable. As noted above, in the following diagram relays may pass along all or some of the messages that they receive along with passing along messages that they internally generate. 

         +------+         +---------+
         |Device|---->----|Collector|
         +------+         +---------+
         
         +------+         +-----+         +---------+
         |Device|---->----|Relay|---->----|Collector|
         +------+         +-----+         +---------+

+------+ +-----+ +-----+ +---------+

|Device|-->--|Relay|-->--..-->--|Relay|-->--|Collector|

+------+ +-----+ +-----+ +---------+

         +------+         +-----+         +---------+
         |Device|---->----|Relay|---->----|Collector|
         |      |-\       +-----+         +---------+
         +------+  \
                    \      +-----+         +---------+
                     \-->--|Relay|---->----|Collector|
                           +-----+         +---------+
         
         +------+         +---------+
         |Device|---->----|Collector|
         |      |-\       +---------+
         +------+  \
                    \      +-----+         +---------+
                     \-->--|Relay|---->----|Collector|
                           +-----+         +---------+
         
         +------+         +-----+            +---------+
         |Device|---->----|Relay|---->-------|Collector|
         |      |-\       +-----+         /--|         |
         +------+  \                     /   +---------+
                    \      +-----+      /
                     \-->--|Relay|-->--/
                           +-----+

Diagram 1. Some Possible syslog Architectures

4. Packet Format and Contents

The payload of any IP packet that has a UDP destination port of 514 MUST be treated as a syslog message. There MAY be differences between the format of an originally transmitted syslog message and the format of a relayed message. In essence, it is RECOMMENDED to transmit a syslog message in the format specified in this document, but it is not required. If a relay is able to recognize the message as adhering to that format then it MUST retransmit the message without making any changes to it. However, if a relay receives a message but cannot discern the proper implementation of the format, it is REQUIRED to modify the message so that it conforms to that format before it retransmits it. Section 4.1 will describe the RECOMMENDED format for syslog messages. Section 4.2 will describe the requirements for originally transmitted messages and Section 4.3 will describe the requirements for relayed messages.

4.1 syslog Message Parts

The full format of a syslog message seen on the wire has three discernable parts. The first part is called the PRI, the second part is the HEADER, and the third part is the MSG. The total length of the packet MUST be 1024 bytes or less. There is no minimum length of the syslog message although sending a syslog packet with no contents is worthless and SHOULD NOT be transmitted.

4.1.1 PRI Part

The PRI part MUST have three, four, or five characters and will be bound with angle brackets as the first and last characters. The PRI part starts with a leading "<" ('less-than' character), followed by a number, which is followed by a ">" ('greater-than' character). The code set used in this part MUST be seven-bit ASCII in an eight-bit field as described in RFC 2234 [2]. These are the ASCII codes as defined in "USA Standard Code for Information Interchange" [3]. In this, the "<" character is defined as the Augmented Backus-Naur Form (ABNF) %d60, and the ">" character has ABNF value %d62. The number contained within these angle brackets is known as the Priority value and represents both the Facility and Severity as described below. The Priority value consists of one, two, or three decimal integers (ABNF DIGITS) using values of %d48 (for "0") through %d57 (for "9").

The Facilities and Severities of the messages are numerically coded with decimal values. Some of the operating system daemons and processes have been assigned Facility values. Processes and daemons that have not been explicitly assigned a Facility may use any of the "local use" facilities or they may use the "user-level" Facility. Those Facilities that have been designated are shown in the following table along with their numerical code values. 

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

Table 1. syslog Message Facilities 

        Note 1 - Various operating systems have been found to utilize
           Facilities 4, 10, 13 and 14 for security/authorization,
           audit, and alert messages which seem to be similar.
        Note 2 - Various operating systems have been found to utilize
           both Facilities 9 and 15 for clock (cron/at) messages.

Each message Priority also has a decimal Severity level indicator. These are described in the following table along with their numerical values. 

        Numerical         Severity
          Code
        
           0       Emergency: system is unusable
           1       Alert: action must be taken immediately
           2       Critical: critical conditions
           3       Error: error conditions
           4       Warning: warning conditions
           5       Notice: normal but significant condition
           6       Informational: informational messages
           7       Debug: debug-level messages

Table 2. syslog Message Severities

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. The only time a value of "0" will follow the "<" is for the Priority value of "0". Otherwise, leading "0"s MUST NOT be used.

4.1.2 HEADER Part of a syslog Packet

The HEADER part contains a timestamp and an indication of the hostname or IP address of the device. The HEADER part of the syslog packet MUST contain visible (printing) characters. The code set used MUST also be seven-bit ASCII in an eight-bit field like that used in the PRI part. In this code set, the only allowable characters are the ABNF VCHAR values (%d33-126) and spaces (SP value %d32).

The HEADER contains two fields called the TIMESTAMP and the HOSTNAME. The TIMESTAMP will immediately follow the trailing ">" from the PRI part and single space characters MUST follow each of the TIMESTAMP and HOSTNAME fields. HOSTNAME will contain the hostname, as it knows itself. If it does not have a hostname, then it will contain its own IP address. If a device has multiple IP addresses, it has usually been seen to use the IP address from which the message is transmitted. An alternative to this behavior has also been seen. In that case, a device may be configured to send all messages using a single source IP address regardless of the interface from which the message is sent. This will provide a single consistent HOSTNAME for all messages sent from a device.

The TIMESTAMP field is the local time and is in the format of "Mmm dd hh:mm:ss" (without the quote marks) where:

Mmm is the English language abbreviation for the month of the year with the first character in uppercase and the other two characters in lowercase. The following are the only acceptable values:

Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec

dd is the day of the month. If the day of the month is less than 10, then it MUST be represented as a space and then the number. For example, the 7th day of August would be represented as "Aug 7", with two spaces between the "g" and the "7".

hh:mm:ss is the local time. The hour (hh) is represented in a 24-hour format. Valid entries are between 00 and 23, inclusive. The minute (mm) and second (ss) entries are between 00 and 59 inclusive.

A single space character MUST follow the TIMESTAMP field.

The HOSTNAME field will contain only the hostname, the IPv4 address, or the IPv6 address of the originator of the message. The preferred value is the hostname. If the hostname is used, the HOSTNAME field MUST contain the hostname of the device as specified in STD 13 [4]. It should be noted that this MUST NOT contain any embedded spaces. The Domain Name MUST NOT be included in the HOSTNAME field. If the IPv4 address is used, it MUST be shown as the dotted decimal notation as used in STD 13 [5]. If an IPv6 address is used, any valid representation used in RFC 2373 [6] MAY be used. A single space character MUST also follow the HOSTNAME field.

4.1.3 MSG Part of a syslog Packet

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. There is no ending delimiter to this part. The MSG part of the syslog packet MUST contain visible (printing) characters. The code set traditionally and most often used has also been seven-bit ASCII in an eight-bit field like that used in the PRI and HEADER parts. In this code set, the only allowable characters are the ABNF VCHAR values (%d33-126) and spaces (SP value %d32). However, no indication of the code set used within the MSG is required, nor is it expected. Other code sets MAY be used as long as the characters used in the MSG are exclusively visible characters and spaces similar to those described above. The selection of a code set used in the MSG part SHOULD be made with thoughts of the intended receiver. A message containing characters in a code set that cannot be viewed or understood by a recipient will yield no information of value to an operator or administrator looking at it.

The MSG part has two fields known as the TAG field and the 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. This has traditionally been a freeform message that gives some detailed information of the event. The TAG is a string of ABNF alphanumeric characters that MUST NOT exceed 32 characters. Any non-alphanumeric character will terminate the TAG field and will be assumed to be the starting character of the CONTENT field. Most commonly, the first character of the CONTENT field that signifies the conclusion of the TAG field has been seen to be the left square bracket character ("["), a colon character (":"), or a space character. This is explained in more detail in Section 5.3.

4.2 Original syslog Packets Generated by a Device

There are no set requirements on the contents of the syslog packet as it is originally sent from a device. It should be reiterated here that the payload of any IP packet destined to UDP port 514 MUST be considered to be a valid syslog message. It is, however, RECOMMENDED that the syslog packet have all of the parts described in Section 4.1 - PRI, HEADER and MSG - as this enhances readability by the recipient and eliminates the need for a relay to modify the message.

For implementers that do choose to construct syslog messages with the RECOMMENDED format, the following guidance is offered.

If the originally formed message has a TIMESTAMP in the HEADER part, then it SHOULD be the local time of the device within its timezone.

If the originally formed message has a HOSTNAME field, then it will contain the hostname as it knows itself. If it does not have a hostname, then it will contain its own IP address.

If the originally formed message has a TAG value, then that will be the name of the program or process that generated the message.

4.3 Relayed syslog Packets

When a relay receives a packet, it will check for a valid PRI. If the first character is not a less-than sign, the relay MUST assume that the packet does not contain a valid PRI. If the 3rd, 4th, or 5th character is not a right angle bracket character, the relay again MUST assume that the PRI was not included in the original message. If the relay does find a valid PRI part then it must check for a valid TIMESTAMP in the HEADER part. From these rules, there will be three general cases of received messages. Table 3 gives the general characteristics of these cases and lists the subsequent section of this document that describes the handling of that case. 

              Case                                         Section
         Valid PRI and TIMESTAMP                            4.3.1
         Valid PRI but no TIMESTAMP or invalid TIMESTAMP    4.3.2
         No PRI or unidentifiable PRI                       4.3.3

Table 3. Cases of Received syslog Messages

4.3.1 Valid PRI and TIMESTAMP

If the relay does find a valid PRI and a valid TIMESTAMP, then it will check its internal configuration. Relays MUST be configured to forward syslog packets on the basis of their Priority value. If the relay finds that it is configured to forward the received packet, then it MUST do so without making any changes to the packet. To emphasize the point one more time, it is for this reason that it is RECOMMENDED that the syslog message originally transmitted adhere to the format described in Section 4.1.

It should be noted here that the message receiver does not need to validate the time in the TIMESTAMP field. The assumption may be made that a device whose date has not been correctly set will still have the ability to send valid syslog messages. Additionally, the relay does not need to validate that the value in the HOSTNAME field matches the hostname or IP address of the device sending the message. A reason for this behavior may be found in Section 4.1.2.

4.3.2 Valid PRI but no TIMESTAMP or invalid TIMESTAMP

If a relay does not find a valid TIMESTAMP in a received syslog packet, then it MUST add a TIMESTAMP and a space character immediately after the closing angle bracket of the PRI part. It SHOULD additionally add a HOSTNAME and a space character after the TIMESTAMP. These fields are described here and detailed in Section 4.1.2. The remainder of the received packet MUST be treated as the CONTENT field of the MSG and appended. Since the relay would have no way to determine the originating process from the device that originated the message, the TAG value cannot be determined and will not be included.

The TIMESTAMP will be the current local time of the relay.

The HOSTNAME will be the name of the device, as it is known by the relay. If the name cannot be determined, the IP address of the device will be used.

If the relay adds a TIMESTAMP, or a TIMESTAMP and HOSTNAME, after the PRI part, then it MUST check that the total length of the packet is still 1024 bytes or less. If the packet has been expanded beyond 1024 bytes, then the relay MUST truncate the packet to be 1024 bytes. This may cause the loss of vital information from the end of the original packet. It is for this reason that it is RECOMMENDED that the PRI and HEADER parts of originally generated syslog packets contain the values and fields documented in Section 4.1.

4.3.3 No PRI or Unidentifiable PRI

If the relay receives a syslog message without a PRI, or with an unidentifiable PRI, then it MUST insert a PRI with a Priority value of 13 as well as a TIMESTAMP as described in Section 4.3.2. The relay SHOULD also insert a HOSTNAME as described in Section 4.3.2. The entire contents of the received packet will be treated as the CONTENT of the relayed MSG and appended.

An example of an unidentifiable PRI would be "<00>", without the double quotes. It may be that these are the first 4 characters of the message. To continue this example, if a relay does receive a syslog message with the first four characters of "<00>", then it will consult its configuration. If it is configured to forward syslog messages with a Priority value of 13 to another relay or collector, then it MUST modify the packet as described above. The specifics of doing this, including the RECOMMENDED insertion of the HOSTNAME, are given below. 

   Originally received message
     <00>...
   Relayed message
     <13>TIMESTAMP HOSTNAME <00>...

If the relay adds a TIMESTAMP, or a TIMESTAMP and HOSTNAME, after the PRI part, then it MUST check that the total length of the packet is still 1024 bytes or less. If the packet has been expanded beyond 1024 bytes, then the relay MUST truncate the packet to be 1024 bytes. This may cause the loss of vital information from the end of the original packet. It is for this reason that it is RECOMMENDED that the PRI and HEADER parts of originally generated syslog packets contain the values and fields documented in Section 4.1.

5. Conventions

Although Section 4 of this document specifies all requirements for the syslog protocol format and contents, certain conventions have come about over time for the inclusion of additional information within the syslog message. It must be plainly stated that these items are not mandated but may be considered by implementers for completeness and to give the recipient some additional clues of their origin and nature.

5.1 Dates and Times

It has been found that some network administrators like to archive their syslog messages over long periods of time. It has been seen that some original syslog messages contain a more explicit time stamp in which a 2 character or 4 character year field immediately follows the space terminating the TIMESTAMP. This is not consistent with the original intent of the order and format of the fields. If implementers wish to contain a more specific date and time stamp within the transmitted message, it should be within the CONTENT field. Implementers may wish to utilize the ISO 8601 [7] date and time formats if they want to include more explicit date and time information.

Additional methods to address this desire for long-term archiving have been proposed and some have been successfully implemented. One such method is that the network administrators may choose to modify the messages stored on their collectors. They may run a simple script to add the year, and any other information, to each stored record. Alternatively, the script may replace the stored time with a format more appropriate for the needs of the network administrators. Another alternative has been to insert a record into the file that contains the current year. By association then, all other records near that informative record should have been received in that same year. Neither of these however, addresses the issue of associating a correct timezone with each record.

5.2 Domain Name and Address

To readily identify the device that originated the message, it may be a good practice to include its fully qualified domain name (FQDN) and its IP address within the CONTENT field. Traditionally, however, only the hostname has been included in the HOSTNAME field.

5.3 Originating Process Information

It has also been considered to be a good practice to include some information about the process on the device that generated the message - if that concept exists. This is usually the process name and process id (often known as the "pid") for robust operating systems. The process name is commonly displayed in the TAG field. Quite often, additional information is included at the beginning of the CONTENT field. The format of "TAG[pid]:" - without the quote marks - is common. The left square bracket is used to terminate the TAG field in this case and is then the first character in the CONTENT field. If the process id is immaterial, it may be left off.

In that case, a colon and a space character usually follow the TAG. This would be displayed as "TAG: " without the quotes. In that case, the colon is the first character in the CONTENT field.

5.4 Examples

As examples, these are valid messages as they may be observed on the wire between two devices. In the following examples, each message has been indented, with line breaks inserted in this document for readability. 

        Example 1
        
        <34>Oct 11 22:14:15 mymachine su: 'su root' failed for lonvick
        on /dev/pts/8

This example shows an authentication error in an attempt to acquire additional privileges. It also shows the command attempted and the user attempting it. This was recorded as an original message from the device called mymachine. A relay receiving this would not make any changes before sending it along as it contains a properly formatted PRI part and TIMESTAMP field in the HEADER part. The TAG value in this example is the process "su". The colon has terminated the TAG field and is the first character of the CONTENT field. In this case, the process id (pid) would be considered transient and anyone looking at this syslog message would gain no useful information from knowing the pid. It has not been included so the first two characters of the CONTENT field are the colon and a space character. 

        Example 2
        
        Use the BFG!

While this is a valid message, it has extraordinarily little useful information. This message does not have any discernable PRI part. It does not contain a timestamp or any indication of the source of the message. If this message is stored on paper or disk, subsequent review of the message will not yield anything of value.

This example is obviously an original message from a device. A relay MUST make changes to the message as described in Section 4.3 before forwarding it. The resulting relayed message is shown below. 

        <13>Feb  5 17:32:18 10.0.0.99 Use the BFG!

In this relayed message, the entire message has been treated as the CONTENT portion of the MSG part. First, a valid PRI part has been added using the default priority value of 13. Next, a TIMESTAMP has been added along with a HOSTNAME in the HEADER part. Subsequent relays will not make any further changes to this message. It should be noted in this example that the day of the month is less than 10. Since single digits in the date (5 in this case) are preceded by a space in the TIMESTAMP format, there are two spaces following the month in the TIMESTAMP before the day of the month. Also, the relay appears to have no knowledge of the host name of the device sending the message so it has inserted the IPv4 address of the device into the HOSTNAME field. 

        Example 3
        
         <165>Aug 24 05:34:00 CST 1987 mymachine myproc[10]: %% It's
         time to make the do-nuts.  %%  Ingredients: Mix=OK, Jelly=OK #
         Devices: Mixer=OK, Jelly_Injector=OK, Frier=OK # Transport:
         Conveyer1=OK, Conveyer2=OK # %%

This message does have a valid PRI part with a Priority value indicating that it came from a locally defined facility (local4) with a severity of Notice. The HEADER part has a proper TIMESTAMP field in the message. A relay will not modify this message before sending it. However, the HOSTNAME and TAG fields are not consistent with the definitions in Section 4. The HOSTNAME field would be construed to be "CST" and the beginning of the MSG part would be "1987".

It should be noted that the information contained in the CONTENT of this example is not telemetry data, nor is it supervisory control or data acquisition information. Due to the security concerns listed in Section 6 of this document, information of that nature should probably not be conveyed across this protocol. 

        Example 4
        
         <0>1990 Oct 22 10:52:01 TZ-6 scapegoat.dmz.example.org 10.1.2.3
         sched[0]: That's All Folks!

This example has a lot of extraneous information throughout. A human or sufficiently adaptable automated parser would be able to determine the date and time information as well as a fully qualified domain name (FQDN) [4] and IP address. The information about the nature of the event is, however, limited. Due to the indicated severity of the event, the process may not have been able to gather or send anything more informative. It may have been fortunate to have generated and sent this message at all.

This example is obviously an original message from a device. Since the first field in the HEADER part is not a TIMESTAMP in the format defined in Section 4.1.2, it MUST be modified by a relay. A relay will add a TIMESTAMP and SHOULD add a HOSTNAME as follows and will treat the entire received packet after the PRI part from the original packet as the CONTENT field of the new packet. The value used in the HOSTNAME field is only the hostname without the domain name as it is known by the relay. A TAG value will not be added to the relayed packet. While the inclusion of the domain name and IPv4 address in the original message is a noble endeavor, it is not consistent with the use of the field as described in Section 4.1.2. 

         <0>Oct 22 10:52:12 scapegoat 1990 Oct 22 10:52:01 TZ-6
         scapegoat.dmz.example.org 10.1.2.3 sched[0]: That's All Folks!

6. Security Considerations

An odor may be considered to be a message that does not require any acknowledgement. People tend to avoid bad odors but are drawn to odors that they associate with good food. The acknowledgement of the receipt of the odor or scent is not required and indeed it may be the height of discretion to totally ignore some odors. On the other hand, it is usually considered good civility to acknowledge the prowess of the cook merely from the ambiance wafting from the kitchen. Similarly, various species have been found to utilize odors to attract mates. One species of moth uses this scent to find each other. However, it has been found that bolas spiders can mimic the odor of the female moths of this species. This scent will then attract male moths, which will follow it with the expectation of finding a mate. Instead, when they arrive at the source of the scent, they will be eaten [8]. This is a case of a false message being sent out with inimical intent.

In its local use, the syslog process places event notification messages into files on that system. This relies upon the integrity of the system for the protection of the messages. The subsequent configuration of the syslog process to use the syslog protocol to transport the messages to a remote collector was an extension of the delivery of event notification messages and it exhibits the same trust of the network. There are several security consequences of the fundamental simplicity of syslog and there are some concerns about the applicability of this protocol in situations that require robust delivery. Along the lines of the analogy, computer event messages may be sent accidentally, erroneously and even maliciously. At the time of this writing, however, there have not been any reports of any networked device consuming any other device.

6.1 Packet Parameters

As was described above, the message length MUST NOT exceed 1024 bytes. Attacks have seen where syslog messages are sent to a receiver that have message lengths greater than 1024 bytes. In some older versions of syslog, the receipt of syslog packets that had a message greater than 1024 bytes caused problems. syslog message receivers must not malfunction upon the receipt of packets where the message length is greater than 1024 bytes. Various behaviors have been seen on receivers that do receive messages greater than 1024 bytes. Some have been seen to log the entire contents of the message, while others have been seen to log only portions of the message. Still others have been known to discard the message altogether. Devices MUST NOT retransmit messages whose received length exceeds 1024 bytes.

Similarly, the receiver must rigidly enforce the correctness of the message body. syslog collectors must not malfunction if received messages do not have the less-than and greater-than characters around a valid Priority value. They MUST treat these messages as the unformatted CONTENT as was described in Section 4.3.3 if they relay it.

Also, received messages must contain printable text in the message as was described throughout Section 4. Devices must not malfunction if they receive a message containing characters other than the characters described above.

6.2 Message Authenticity

The syslog delivery mechanism does not strongly associate the message with the message sender. The receiver of that packet will not be able to ascertain that the message was indeed sent from the reported sender, or if the packet was sent from another device. It should be noted here that the message receiver does not need to verify that the HOSTNAME in the HEADER part match the name of the IP address contained in the Source Address field of the IP packet.

6.2.1 Authentication Problems

One possible consequence of this behavior is that a misconfigured machine may send syslog messages to a collector representing itself as another machine. The administrative staff may become confused that the status of the supposed sender of the messages may not be accurately reflected in the received messages. The administrators may not be able to readily discern that there are two or more machines representing themselves as the same machine.

It should also be noted that some cases of filling the HOSTNAME field in the HEADER part might only have local significance and that may only be ephemeral. If the device had obtained an IP address from a DHCP pool, then any association between an identifier and an actual source would not always hold true. The inclusion of a fully qualified domain name in the CONTENT may give the administrators the best chance of identifying the source of each message if it can always be associated with an IP address or if it can always be associated with a unique machine.

6.2.2 Message Forgery

Malicious exploits of this behavior have also been noted. An attacker may transmit syslog messages (either from the machine from which the messages are purportedly sent or from any other machine) to a collector. In one case, an attacker may hide the true nature of an attack amidst many other messages. As an example, an attacker may start generating forged 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. Additionally, an attacker may generate false syslog messages to give untrue indications of status or of events. As an example, an attacker may stop a critical process on a machine, which may generate a notification of exit. The attacker may subsequently generate a forged notification that the process had been restarted. The system administrators may accept that misinformation and not verify that the process had indeed been restarted.

6.3 Sequenced Delivery

As a general rule, the forensics of a network anomaly rely upon reconstructing the sequence of events. In a perfect world, the messages would be received on the syslog collector in the order of their generation from the other devices and anyone looking at these records would have an accurate picture of the sequence of events. Unfortunately, the syslog process and protocol do not ensure ordered delivery. This section details some of the problems that may be encountered from this.

6.3.1 Single Source to a Destination

The syslog records are usually presented (placed in a file, displayed on the console, etc.) in the order in which they are received. This is not always in accordance with the sequence in which they were generated. As they are transported across an IP network, some out of order receipt should be expected. This may lead to some confusion as messages may be received that would indicate that a process has stopped before it was started. This may be somewhat rectified if the originating process had timestamped or numbered each of the messages before transmission. In this, the sending device should utilize an authoritative time source. It should be remembered, however, that not all devices are capable of receiving time updates, and not all devices can timestamp their messages.

6.3.2 Multiple Sources to a Destination

In syslog, there is no concept of unified event numbering. Single devices are free to include a sequence number within the CONTENT but that can hardly be coordinated between multiple devices. In such cases, multiple devices may report that each one is sending message number one. Again, this may be rectified somewhat if the sending devices utilize a timestamp from an authoritative source in their messages. As has been noted, however, even messages from a single device to a single collector may be received out of order. This situation is compounded when there are several devices configured to send their syslog messages to a single collector. Messages from one device may be delayed so the collector receives messages from another device first even though the messages from the first device were generated before the messages from the second. If there is no timestamp or coordinated sequence number, then the messages may be presented in the order in which they were received which may give an inaccurate view of the sequence of actual events.

6.3.3 Multiple Sources to Multiple Destinations

The plethora of configuration options available to the network administrators may further skew the perception of the order of events. It is possible to configure a group of devices to send the status messages -or other informative messages- to one collector, while sending messages of relatively higher importance to another collector. Additionally, the messages may be sent to different files on the same collector. If the messages do not contain timestamps from the source, it may be difficult to order the messages if they are kept in different places. An administrator may not be able to determine if a record in one file occurred before or after a record in a different file. This may be somewhat alleviated by placing marking messages with a timestamp into all destination files. If these have coordinated timestamps, then there will be some indication of the time of receipt of the individual messages.

6.3.4 Replaying

Without any sequence indication or timestamp, messages may be recorded and replayed at a later time. 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 collector. Even with a TIMESTAMP field in the HEADER part, an attacker may record the packets and could simply modify them to reflect the current time before retransmitting them. The administrators may find nothing unusual in the received messages and their receipt would falsely indicate normal activity of the machine.

6.4 Reliable Delivery

As there is no mechanism within either the syslog process or the protocol to ensure delivery, and since the underlying transport is UDP, some messages may be lost. They may either be dropped through network congestion, or they may be maliciously intercepted and discarded. The consequences of the drop of one or more syslog messages cannot be determined. If the messages are simple status updates, then their non-receipt may either not be noticed, or it may cause an annoyance for the system operators. On the other hand, if the messages are more critical, then the administrators may not become aware of a developing and potentially serious problem. Messages may also be intercepted and discarded by an attacker as a way to hide unauthorized activities.

6.5 Message Integrity

Besides being discarded, syslog messages may be damaged in transit, or an attacker may maliciously modify them. In the case of a packet containing a syslog message being damaged, there are various mechanisms built into the link layer as well as into the IP [9] and UDP protocols which may detect the damage. An intermediary router may discard a damaged IP packet [10]. Damage to a UDP packet may be detected by the receiving UDP module, which may silently discard it. In any case, the original contents of the message will not be delivered to the collector. Additionally, if an attacker is positioned between the sender and collector of syslog messages, they may be able to intercept and modify those messages while in-transit to hide unauthorized activities.

6.6 Message Observation

While there are no strict guidelines pertaining to the event message format, most syslog messages are generated in human readable form with the assumption that capable administrators should be able to read them and understand their meaning. Neither the syslog protocol nor the syslog application have mechanisms to provide confidentiality of the messages in transit. In most cases passing clear-text messages is a benefit to the operations staff if they are sniffing the packets off of the wire. The operations staff may be able to read the messages and associate them with other events seen from other packets crossing the wire to track down and correct problems. Unfortunately, an attacker may also be able to observe the human- readable contents of syslog messages. The attacker may then use the knowledge gained from those messages to compromise a machine or do other damage.

6.7 Message Prioritization and Differentiation

While the processes that create the messages may signify the importance of the events through the use of the message Priority value, there is no distinct association between this value and the importance of delivery of the packet. As an example of this, consider an application that generates two event messages. The first is a normal status message but the second could be an important message denoting a problem with the process. This second message would have an appropriately higher Severity value associated with the importance of that event. If the operators had configured that both of these messages be transported to a syslog collector then they would, in turn, be given to UDP for transmission. Under normal conditions, no distinction would be made between them and they would be transmitted in their order.

Again, under normal circumstances, the receiver would accept syslog messages as they are received. If many devices are transmitting normal status messages, but one is transmitting an important event message, there is no inherent mechanism within the syslog protocol to prioritize the important message over the other messages.

On a case-by-case basis, device operators may find some way to associate the different levels with the quality of service identifiers. As an example, the operators may elect to define some linkage between syslog messages that have a specific Priority value with a specific value to be used in the IPv4 Precedence field [9], the IPv6 Traffic Class octet [11], or the Differentiated Services field [12]. In the above example, the operators may have the ability to associate the status message with normal delivery while associating the message indicating a problem with a high reliability, low latency queue as it goes through the network. This would have the affect of prioritizing the essential messages before the normal status messages. Even with this hop-by-hop prioritization, this queuing mechanism could still lead to head of line blocking on the transmitting device as well as buffer starvation on the receiving device if there are many near-simultaneous messages being sent or received. This behavior is not unique to syslog but is endemic to all operations that transmit messages serially.

There are security concerns for this behavior. Head of line blocking of the transmission of important event messages may relegate the conveyance of important messages behind less important messages. If the queue is cleared appropriately, this may only add seconds to the transmission of the important message. On the other hand, if the queue is not cleared, then important messages may not be transmitted. Also at the receiving side, if the syslog receiver is suffering from buffer starvation due to large numbers of messages being received near-simultaneously, important messages may be dropped indiscriminately along with other messages. While these are problems with the devices and their capacities, the protocol security concern is that there is no prioritization of the relatively more important messages over the less important messages.

6.8 Misconfiguration

Since there is no control information distributed about any messages or configurations, it is wholly the responsibility of the network administrator to ensure that the messages are actually going to the intended recipient. Cases have been noted where devices were inadvertently configured to send syslog messages to the wrong receiver. In many cases, the inadvertent receiver may not be configured to receive syslog messages and it will probably discard them. In certain other cases, the receipt of syslog messages has been known to cause problems for the unintended recipient [13]. If messages are not going to the intended recipient, then they cannot be reviewed or processed.

6.9 Forwarding Loop

As it is shown in Figure 1, machines may be configured to relay syslog messages to subsequent relays before reaching a collector. In one particular case, an administrator found that he had mistakenly configured two relays to forward messages with certain Priority values to each other. When either of these machines either received or generated that type of message, it would forward it to the other relay. That relay would, in turn, forward it back. This cycle did cause degradation to the intervening network as well as to the processing availability on the two devices. Network administrators must take care to not cause such a death spiral.

6.10 Load Considerations

Network administrators must take the time to estimate the appropriate size of the syslog receivers. An attacker may perform a Denial of Service attack by filling the disk of the collector with false messages. Placing the records in a circular file may alleviate this but that has the consequence of not ensuring that an administrator will be able to review the records in the future. Along this line, a receiver or collector must have a network interface capable of receiving all messages sent to it.

Administrators and network planners must also critically review the network paths between the devices, the relays, and the collectors. Generated syslog messages should not overwhelm any of the network links.

7. IANA Considerations

The syslog protocol has been assigned UDP port 514. This port assignment will be maintained by IANA exclusively for this protocol.

The syslog protocol provides for the definition of named attributes to indicate the Severity of each message and the Facility that generated the message as described in Section 4. The name space identifiers for these attributes are defined as numbers. The protocol does not define the specific assignment of the name space for these numbers; the application developer or system vendor is allowed to define the attribute, its semantics, and the associated numbers. This name space will not be controlled to prevent collisions as systems are expected to use the same attributes, semantics and associated numbers to describe events that are deemed similar even between heterogeneous devices.

8. Conclusion and Other Efforts

The syslog protocol may be effectively used to transport event notification messages across a network. In all cases, it is important that the syslog message receiver embody the principle of "be liberal in what you accept". It is highly recommended that the network operators who choose to use this understand the characteristics of the protocol and its security implications.

There have been attempts in the past to standardize the format of the syslog message. The most notable attempt culminated in a BOF at the Fortieth Internet Engineering Task Force meeting in 1997. This was the Universal Logging Protocol (ulp) BOF and the minutes of their meeting are on-line at the IETF Proceedings web site [14].

Many good thoughts came from that effort and interested implementers may want to find some of the notes or papers produced from that effort.

At the time of this writing, efforts are underway to allow the usage of international character sets in applications that have been traditionally thought of as being text-only. The HOSTNAME and TIMESTAMP fields described above are representative of this. Also, the entire CONTENT field has traditionally been printing characters and spaces in the code set known as US-ASCII. It is hoped that the proponents of these internationalization efforts will find a suitable way to allow the use of international character sets within syslog messages without being disruptive. It should also be hoped that implementers will allow for the future acceptance of additional code sets and that they may make appropriate plans. Again, it must be cautioned that the simplicity of the existing system has been a tremendous value to its acceptance. Anything that lessens that simplicity may diminish that value.

Acknowledgements

The following people provided content feedback during the writing of this document: 

         Jon Knight <J.P.Knight@lboro.ac.uk>
         Magosanyi Arpad <mag@bunuel.tii.matav.hu>
         Balazs Scheidler <bazsi@balabit.hu>
         Jon Callas <jon@counterpane.com>
         Eliot Lear <lear@cisco.com>
         Petter Reinholdtsen <pere@hungry.com>
         Darren Reed <darrenr@reed.wattle.id.au>
         Alfonso De Gregorio <dira@speedcom.it>
         Eric Allman <eric@sendmail.com>
         Andrew Ross <andrew@kiwi-enterprises.com>
         George Maslyar <george.maslyar@primark.com>
         Albert Mietus <albert@ons-huis.net>
         Russ Allbery <rra@stanford.edu>
         Titus D. Winters <titus@cs.hmc.edu>
         Edwin P. Boon <Edwin.Boon@consul.com>
         Jeroen M. Mostert <Jeroen.Mostert@consul.com>

Eric Allman is the original inventor and author of the syslog daemon and protocol. The author of this memo and the community at large would like to express their appreciation for this work and for the usefulness that it has provided over the years.

A large amount of additional information about this de-facto standard operating system feature may usually be found in the syslog.conf file as well as in the man pages for syslog.conf, syslog, syslogd, and logger, of many Unix and Unix-like devices.

References

   1  Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980.
   
   2  Crocker, D. and P. Overell, "Augmented BNF for Syntax
      Specifications: ABNF", RFC 2234, November 1997.
   
   3  USA Standard Code for Information Interchange, USASI X3.4-1968
   
   4  Mockapetris, P., "Domain Names - Concepts and Facilities", STD 13,
      RFC 1034, November 1987.
   
   5  Mockapetris, P., "Domain names - Implementation and
      Specification", STD 13, RFC 1035, November 1987.
   
   6  Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture",
      RFC 2373, July 1998.
   
   7  Data elements and interchange formats - Information exchange -
      Representation of dates and times, International Organization for
      Standardization, Reference number ISO 8601 : 1988 (E), 1988
   
   8  Stowe, M., et al, "Chemical Mimicry: Bolas Spiders Emit Components
      of Moth Prey Species Sex Pheromones", Science, 1987
   
   9  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
   
   10 Baker, F., "Requirements for IP Version 4 Routers", RFC 1812, June
      1995.
   
   11 Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 2460, December 1998.
   
   12 Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the
      Differentiated Services Field (DS Field) in the IPv4 and IPv6
      Headers", RFC 2474, December 1998.
  1. Cisco Systems Product Security Incident Response Team (PSIRT), "Field Notice: Cisco IOS(r) Syslog Crash", January 11, 1999 http://www.cisco.com/warp/public/707/advisory.html
   14 Walker, D., IETF Secretariat, "Proceedings of the Fortieth
      Internet Engineering Task Force, Washington, DC, USA, December 8-
      12, 1997
      http://www.ietf.org/proceedings/97dec/index.html

Author's Address

Chris Lonvick
Cisco Systems
12515 Research Blvd.
Austin, TX, USA 

   Phone:  +1.512.378.1182
   EMail:  clonvick@cisco.com

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