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syslog Working Group                                         R. Gerhards
Internet-Draft                                          February 2, 2004
Expires: August 2, 2004

                          The syslog Protocol

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on August 2, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   This document describes the syslog protocol. The syslog protocol has
   been used throughout the years to convey event notifications. This
   documents describes a layered architecture for an easily extensible
   syslog protocol.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    Definitions and Architecture . . . . . . . . . . . . . . . .  5
   3.    Transport Layer Protocol . . . . . . . . . . . . . . . . . .  8
   4.    Required syslog Format . . . . . . . . . . . . . . . . . . .  9
   4.1   HEADER Part  . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.1.1 VERSION  . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   4.1.2 enterpriseID . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.1.3 FACILITY . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.1.4 SEVERITY . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.1.5 TIMESTAMP  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4.1.6 HOSTNAME . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.1.7 TAG  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.2   MSG  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   4.3   TRAILER  . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   4.4   Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   5.    Structured Data  . . . . . . . . . . . . . . . . . . . . . . 19
   5.1   Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
   5.2   MSG with just Structured Data  . . . . . . . . . . . . . . . 21
   5.3   Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 21
   6.    Fragmentation  . . . . . . . . . . . . . . . . . . . . . . . 23
   6.1   SD-ID fragment . . . . . . . . . . . . . . . . . . . . . . . 23
   6.1.1 msgid  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   6.1.2 fragnum  . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   6.1.3 fragcount  . . . . . . . . . . . . . . . . . . . . . . . . . 24
   6.2   Cryptographically signing fragmented Messages  . . . . . . . 25
   6.3   Fragmentation Examples . . . . . . . . . . . . . . . . . . . 25
   7.    Security Considerations  . . . . . . . . . . . . . . . . . . 28
   7.1   Packet Parameters  . . . . . . . . . . . . . . . . . . . . . 28
   7.2   Message Authenticity . . . . . . . . . . . . . . . . . . . . 28
   7.3   Authentication Problems  . . . . . . . . . . . . . . . . . . 28
   7.4   Message Forgery  . . . . . . . . . . . . . . . . . . . . . . 29
   7.5   Sequenced Delivery . . . . . . . . . . . . . . . . . . . . . 29
   7.5.1 Single Source to a Destination . . . . . . . . . . . . . . . 30
   7.5.2 Multiple Sources to a Destination  . . . . . . . . . . . . . 30
   7.5.3 Multiple Sources to Multiple Destinations  . . . . . . . . . 30
   7.6   Replaying  . . . . . . . . . . . . . . . . . . . . . . . . . 31
   7.7   Reliable Delivery  . . . . . . . . . . . . . . . . . . . . . 31
   7.8   Message Integrity  . . . . . . . . . . . . . . . . . . . . . 31
   7.9   Message Observation  . . . . . . . . . . . . . . . . . . . . 32
   7.10  Message Prioritization and Differentiation . . . . . . . . . 32
   7.11  Misconfiguration . . . . . . . . . . . . . . . . . . . . . . 33
   7.12  Forwarding Loop  . . . . . . . . . . . . . . . . . . . . . . 33
   7.13  Load Considerations  . . . . . . . . . . . . . . . . . . . . 34
   7.14  Denial of Service  . . . . . . . . . . . . . . . . . . . . . 34
   7.15  Covert Channels  . . . . . . . . . . . . . . . . . . . . . . 34
   8.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 35

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   9.    Authors and Working Group Chair  . . . . . . . . . . . . . . 36
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
         References . . . . . . . . . . . . . . . . . . . . . . . . . 38
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 39
         Intellectual Property and Copyright Statements . . . . . . . 40

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1. Introduction

   The informational document RFC 3164 [19] describes a general format
   of syslog messages as they have been seen on the wire, and as the
   original author intended.  Over time that format has been modified
   and extended in several ways, usually to meet new requirements.  This
   document describes the semantics of the syslog protocol and provides
   a standard format for all syslog messages, that adheres to the
   original intent of the message format but also contains enhancements
   that are consistent with many of the innovations put forth through
   the years. Some components have been adjusted in this document to
   allow for backwards compatibility.  However, the greatest benefit to
   automated log message parsers and people reading the log messages
   will come from adherence to the newly defined fields laid out in this
   document. The adherence of syslog messages to the format defined in
   this document may present problems to older syslog message receivers
   even though efforts were made to keep the message format similar to
   the format described in RFC 3164 [19].  People deploying devices that
   generate messages following the protocol described here should verify
   that they don't present problems to their existing syslog receivers.

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2. 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

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

      There are machines that both receive messages and forward them to
      another machine AND generate syslog messages themselfs. An example
      for this may be an application that operates as a syslog relay as
      one service while at the same time running other services. These
      services may be monitored by the same application, generating new
      syslog messages. Such a machine acts both as a relay AND a device.
      This case is specifically mentioned as the role a machine plays
      has special significance, for example on formatting. A machine as
      described here may thus have two separate configurations for each
      of the machine's operations modes.

   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

      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.

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   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.

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

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

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

            +------+         +-----+         +---------+
            |      |-\       +-----+         +---------+
            +------+  \
                       \      +-----+         +---------+
                              +-----+         +---------+

            +------+         +---------+
            |      |-\       +---------+
            +------+  \
                       \      +-----+         +---------+
                              +-----+         +---------+

            +------+         +-----+            +---------+
            |      |-\       +-----+         /--|         |
            +------+  \                     /   +---------+
                       \      +-----+      /

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            +------+          +-----+               +---------+
            |      |-\        +-----+            /--|         |
            +------+  \                         /   +---------+
                       \      +--------+       /
                        \     |+------+|      /
                         \-->-||Relay ||->---/
                              |+------||    /

   Diagram 1.  Some Possible syslog Architectures

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3. Transport Layer Protocol

   This document DOES NOT specify or enforce a specific transport layer
   protocol. Instead, it describes the format of a syslog message in a
   transport layer independent way.

   As long as there are no transport mappings defined, the relevant
   parts of RFC 3164 should be used for UDP-based transport and the
   relevant parts of RFC 3195 should be used for TCP-based transport.

   Transport mappings being defined MUST ensure that a message formatted
   according to this document can be transported unaltered over the
   mapping. If the mapping needs to perform temporary transformations,
   it must be guaranteed that the message received at the final
   destination is an exact copy of the message sent from the initial
   originator. This is vital because otherwise cryptographic verifiers
   (like signatures) would be broken.

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4. Required syslog Format

   The traditional format of a syslog message is defined in RFC 3164.
   There is a concept in that document that anything delivered to UDP
   port 514 will be accepted as a valid syslog message.  However, this
   document REQUIRES a defined format for syslog messages.

   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.

   The definitions of the fields are slightly changed in this document
   from RFC 3164. While the format described in RFC 3164 is correct for
   packet formation, the Working Group evaluating this work determined
   that it would be better if the TAG field were to become a part of the
   HEADER part rather than the CONTENT part. While IETF documentation
   does not allow the specification of an API, people developing code to
   adhere to this specification have found it helpful to think about the
   parts in this format.

   The syslog message has the following ABNF [14] definition:

      ; The general syslog message format


      HEADER          = "V" VERSION SP enterpriseID SP FACILITY SP
      TRAILER         = LF
      VERSION         = 1*3DIGIT
      enterpriseID    = 1*10DIGIT  ; range 0..2147483648
      FACILITY        = 1*10DIGIT  ; range 0..2147483648
      SEVERITY        = "0" / "1" / "2" / "3" / "4" / "5" /
                        "6" / "7"
      HOSTNAME        = 1*255PRINTUSASCII  ; a FQDN

      TAG             = static-id  [full-dyn-id] [":"] ; 64 chars max
      static-id       = 1*VISUAL
      full-dyn-id     = "[" proc-id [thread-sep thread-id] "]"
      proc-id         = 1*ALFANUM  ; recommended: number
      thread-sep      = VISUAL / %d58     ; recommended: ",", or ':', or '.'
      thread-id       = 1*ALFANUM  ; recommended: number
      VISUAL          = (%d33-57/%d59-126) ; all but SP and ":"

      TIMESTAMP       = full-date "T" full-time

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      date-fullyear   = 4DIGIT
      date-month      = 2DIGIT  ; 01-12
      date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on
                                ; month/year
      time-hour       = 2DIGIT  ; 00-23
      time-minute     = 2DIGIT  ; 00-59
      time-second     = 2DIGIT  ; 00-58, 00-59, 00-60 based on leap
                                ; second rules
      time-secfrac    = "." 1*6DIGIT
      time-offset     = "Z" / time-numoffset
      time-numoffset  = ("+" / "-") time-hour ":" time-minute

      partial-time    = time-hour ":" time-minute ":" time-second
      full-date       = date-fullyear "-" date-month "-" date-mday
      full-time       = partial-time time-offset

      MSG             = *((%d32-126) / (%d128-254))
                        ; VALID UTF-8 String of PRINTABLE characters

      LF              = %d10
      CR              = %d13
      SP              = %d32
      PRINTUSASCII    = %d33-126
      ALFANUM         = %d48..57 / %d65..90 / %d97..122

4.1 HEADER Part

   The header MUST contain the token identifying the message as a syslog
   message complying with this specification, the version of the
   specification it complies to, the enterpriseID of the original
   sender, the facility, the severity, the timestamp, the hostname and
   the tag. Each of this fields MUST be present and MUST be of correct
   syntax. The code set used in the HEADER MUST be seven-bit ASCII in an
   eight- bit field as described in RFC 2234 [14]. These are the ASCII
   codes as defined in "USA Standard Code for Information Interchange"
   ANSI.X3-4.1968 [3].

   If the header is not syntactically correct, the receiver MUST NOT try
   to sub-parse some of the header fields in order to find a "good"
   interpretation. However, the receiver MAY assume it is a RFC3164
   compliant message and MAY decide to process it as such. In this case,
   RFC3164 semantics MUST be used.

   As a note to implementors, the "V" token at the very beginning of the
   message MAY be used as a rough indication whether or not the message
   complies to this document. However, it is not sufficient to assume it

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   complies to this document just because the first character is a "V".
   As written above, the full header MUST be validated to assume this.

   The HEADER contains three fields called the TIMESTAMP, the HOSTNAME,
   and the TAG fields. The TIMESTAMP immediately follows the trailing
   ">" from the PRI part and single space characters MUST follow each of
   the TIMESTAMP and HOSTNAME fields. HOSTNAME contains the hostname, as
   it knows itself. If it does not have a hostname, then it contains 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 provides a single consistent HOSTNAME for all
   messages sent from a device.


   The Version field denotes the version of the syslog protcol
   specification the message is formatted to. It is used to uniquely
   identify the message format should later versions of the syslog
   protocol specification change it.

   Note well: this document is the first to specify this format,
   including the VERSION in the header. Any previous syslog
   specification had a different header. As outlined under HEADER above,
   an invalid HEADER will automatically tell the receiver that the
   message is NOT compliant to this specification. As such, all version
   information is well defined (absence of version information means
   legacy syslog by the fact that the header is invalid).

   The VERSION MUST be a numerical value. It MUST be one of the IANA
   assigned valid VERSION numbers. It starts at 1, which means the
   format specified in this document. The VERSION number MUST be
   incremented for each new syslog protocol specification that changes
   the format. If MUST NOT be incremented if a new syslog protocol
   specification does not change the syntax and semantics of the message

   The sender of the syslog message MUST specify the VERSION of the
   format that the message was formatted to.

   The receiver MUST check the VERSION. If the VERSION is within the set
   of format versions supported by the receiver, the receiver MUST parse
   the message according to the correct syslog protcol specification. A
   receiver MUST NOT parse a previous version with some parsing rules
   from a later specification.

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   If the receiver does not support the specified VERSION, it SHOULD log
   a diagnostic message. It SHOULD NOT parse beyond the VERSION field.
   This is because the header format may have changed in a newer
   version. It SHOULD NOT try to process the message, but I MAY try this
   if the administrator has configured the receiver to do so. In the
   later case, the results may be undefined. If the administrator has
   instructed the receiver to parse non-supported version, it SHOULD
   assume that these messages are legacy syslog messages and parse and
   process them in respect to RFC 3164. Again, the administrator MAY
   configure the receiver to use a different algorithm.

   To be precise, a receiver receiving an unknown VERSION number, MUST,
   by default, ignore it. The administrator may configure it to not
   ignore it. Then, the receiver MUST, by default, parse it according to
   RFC 3164. The administrator may again override this setting. In this
   case, the receiver MAY use whatever method the administor has
   choosen. In this case, the receiver MUST ensure that no application
   reliability issues occur. If there is a chance for this, it MUST NOT
   allow the administrator to select an insecure mode.

   The spirit of this behaviour is that the administrator may sometime
   need the power to allow overriding of version-specific parsing, but
   this should be done in the most secure and reliable way. Therefore,
   the receiver MUST use the appropriate defaults specified above. This
   document is so specific on the defaults and modes because it is
   common experience that parsing unknown formats often leads to
   security issues.

4.1.2 enterpriseID

   The enterprise ID unquily identifies the vender whom's software or
   device created the message. This is to support log-parsers
   sub-parsing vendor-specifc information from the message part.

   The enterprise ID is an integer. It MUST be the enterprise ID
   assigned by IANA to the vendor whoms software or device created the
   syslog message.


   The facility is primarily a way to filter messages at the receiver.
   It is a numerical value. There exist some traditional facility code
   semantics for the codes in the range from 0 to 23. These semantics
   are not closely followed by all vendors, softwares and devices.
   Therefore, no specifc semantics for facility codes are implied in
   this document.

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   FACILITY is just a sender-supplied numerical identifier that can be
   used for filtering by the receiver. The facility in itself does not
   have any semantics. Semantics MUST be applied by site configuration
   (through the site's administrator).

   Any implementation of this document MUST support free configuration
   of the FACILITY on the sender.


   The SEVERITY field is used to indicate the severity that the sender
   of the message assgined to it. It is a numerical value with just few
   values. The traditional syslog severity values are reused, because
   they have prooven to be useful and sufficient in reality.

   SEVERITY is a numerical field, which MUST contain one of the digits
   from 0 to 7. Any other value is invalid and MUST NOT be used.

   Each of the numerical codes has been assigned the follwing semantics:

           Numerical         Severity

              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

   All implementations SHOULD try to assign the most appropriate
   severity to their message. Most importantly, test aid like messages
   or programm debugging information SHOULD be assiged severity 7.
   Severity 0 SHOULD be reserved for high-priviledge core processes of
   very high importance (like serious hardware failures or a very soon
   power failure). An implementation MAY use severities 0 and 7 for
   other purposes if this is configured by the administrator.

   In general, a receiver should abide to the fact that severities are
   often very subjective. As such, a receiver MUST not assume that all
   senders have the same sense of severities.


   The TIMESTAMP field is a formalized timestamp as taken from RFC 3339

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   Note well: RFC 3339 makes allowances for multiple syntaxes for a
   timestamp to be used in various cases.  This document mandates a
   restricted set of syntaxes.  The primary characteristics of TIMESTAMP
   used in this document are as follows.

   o  the "T" and "Z" characters in this syntax MUST be upper case.

   o  usage of the "T" character is mandatory. It MUST NOT be replaced
      by any other character (like a SP character).

   o  the sender SHOULD include time-secfrac (fractional seconds) if its
      clock accuracy and performance permits.

   o  the entire length of the TIMESTAMP field MUST NOT exceed 32

   Please also note that RFC 3339 permits the value "60" in the second
   part to indicate a leap second. This must not be misinterpreted. As a
   suggestion for application developer, it is advised to replace the
   value "60" if seen in the header, with the value "59" if it otherwise
   can not be processed, e.g. stored to a database. It SHOULD NOT be
   converted to the first second of the next minute.

   Two samples of this format are:



   The first represents 20 minutes and 50.52 seconds after the 23rd hour
   of April 12th, 1985 in UTC.  The second represents the same time but
   expressed in the Eastern US timezone (daylight savings time being

   A single space character MUST follow the TIMESTAMP field.


   The HOSTNAME field contains the original creator of the syslog

   The HOSTNAME field SHOULD contain the host name and the domain name
   of the originator in the format specified in STD 13 [5]. This format
   will be referred to in this document as HOSTNAME-STD13.

   If the HOSTNAME-STD13 is not known to the orginator, it MUST use

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   either its IPv4 address or its IPv6 address.

   If the IPv4 address is used, it MUST be shown as the dotted decimal
   notation as used in STD 13 [6], and will be referred to as
   HOSTNAME-IPV4.  If an IPv6 address is used, any valid textual
   representation used in RFC 2373 [15], section 2 MAY be used and will
   be referred to as HOSTNAME-IPV6.

   If a device has multiple IP addresses, it SHOULD use a consistent
   value in the HOSTNAME field. This consistent value MUST be one of its
   actual IP addresses. As an alternative, it MAY use the IP address of
   the interface that is used to send the message.

   A single space character MUST follow the HOSTNAME field.

4.1.7 TAG

   The TAG is a string of visible (printing) characters excluding SP,
   that MUST NOT exceed 64 characters in length. The first occurrence of
   a SP (space) will terminate the TAG field, but is not part of it.

   Note well: the colon (":") is no special character inside the TAG. It
   may occur anywhere within it and may occur muliple times. The TAG is
   terminated by the first SP, NOT the colon character.

   The TAG is used to denote the sender of the message. It MUST be in
   the syntax shown in the ABNF above.

   A typical example of a TAG is: (without the quotes)


   Another example with a dynamic id may be:


   Another example (from VMS) is: (without the quotes)


   Please note that in this example,
   "DKA0:[MYDIR.SUBDIR1.SUBDIR2]MYFILE.TXT;1" is the static-id while
   "[123,456]" is still the full-dyn-id. This shows that a receiver must
   be prepared for special characters like '[' and ':' to be present
   inside the static part.

   As a note to implementors: the beginning of the full-dyn-id is not
   the first but the LAST occurrence of '[' inside the tag and this ONLY

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   if the tag ends in either "]" or "]". If these conditions are not
   met, the '[' is part of the static-id.

   Systems that use both process-ID's and thead-IDs, SHOULD fill both
   the proc-id and the thread-part. For other systems it is RECOMMENDED
   to use the proc-id only.

   No specific format inside the tag is required. However, a sender
   SHOULD use a consistent tag value.

4.2 MSG

   The MSG part contains the details of the message. This has
   traditionally been a freeform message that gives some detailed
   information of the event. It MAY also contain structured data as
   described in Structured Data (Section 5) below.

   The MSG part of the syslog packet MUST contain visible (printing)
   characters. The code set used must be UNICODE. It MUST be encoded in
   UTF-8 as specified in RFC 2279 [13]. Only non-control characters and
   spaces MUST be used inside the MSG part. Specifically, ABNF values
   %d00..31 are NOT permitted inside the MSG part.


   The trailer is an optional part that is being introduced to preserve
   compatibility to legacy syslog implementations. It is observed
   behavior that some emitors send a trailer after the MSG part. Their
   syslog-message is otherwise well-formed. In order to provide
   backwards compatibility, receivers MUST accept messages with trailers
   as valid syslog messages. A relay receiving a trailer MUST NOT
   reformat the message to remove the trailer. An emitor SHOULD NOT
   include the trailer inside the syslog message. It MAY be configured
   to include it, if the receiver it is sending to requires a trailer
   (which is unlikely).

4.4 Examples

   The following examples are given.

   Example 1

         V1 0 888 4 2003-10-11T22:14:15.003Z mymachine.example.com su: 'su
         root' failed for lonvick on /dev/pts/8

   In this example, the VERSION is 1 (formatted according this
   document), the enterprise ID is 0 (IETF), the FACILITY has the value
   of 888 (whatever this means is up to the sender and recipient). The

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   message was created The timestamp is in UTC. on October, 11th 2003 at
   10:14:15pm, 3 milliseconds into the next second. Please note that the
   sender had millisecond resolution. The sender may have actually had a
   better resolution, but by providing just three digits for the
   fractional settings, he does not tell us this. The message orignated
   from a host that calls itself "mymachine.example.com". The TAG is
   "su:". Note that the colon is part of the tag. The MSG is "'su root
   failed for lonvick...". Please note that the SP after the TAG is NOT
   part of the MSG - it is the seperator between TAG and MSG.

   As a note to implementors: please note that the sender had
   millisecond time resolution in this example. A common coding bug is
   that leading zeros are not written for fractional seconds. Very
   often, the above timestamp is errornously being written as:
   "2003-10-11T22:13:14.3". This would indicate 300 milliseconds instead
   of the 3 milliseconds that are actually meant. Please make sure that
   an implementation handles this correctly.

   Example 2

         V1 0 20 6 2003-08-24T05:14:15.000003-09:00
         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 # %%

   In this example, the VERSION is again 1 and the enterprise ID 0. The
   FACILITY is within the legacy syslog range (20), as such we assume
   the user has specifically configure the sender to use this FACILITY.
   The severity is 6 ("Notice" semantics). The timestamp now has
   microsecond resolution, indicated by the additional digits in the
   fractional seconds. The sender indicates that its local time is -9
   hours from UTC. Given the date stamp, we can assume the sender is in
   the US Pacific time zone during daylight savings time. The HOSTNAME
   is "", so the sender did not know its host- and domainname
   and used the V4 IP address instead. The TAG is "myproc[10]:%%" - we
   can speculate that the sender actually wanted the tag to be
   "myproc[10]:", but because there was no SP following it, the TAG
   continues until the first SP. The message is "It's time to make the

   Example 3 - An Invalid Message

         V1 0 20 6 2003-08-24T05:14:15.000000003-09:00
         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 # %%

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   This example just just like Example 2, but this time the sender is
   overdoing with the clock resolution. It is supplying nanosecond
   resolution. This will result in the TIME-SECFRAC part to be longer
   than the allowed 6 digits, which invalidates the header and thus the
   message. A receiver MUST NOT try to "fix" this error. It MUST detect
   this as an invalid message and SHOULD log a diagnostic entry. If the
   receiver is capable of processing legacy syslog messages, it MUST
   assume that this message is legacy syslog and act accordingly.

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5. Structured Data

   While syslog traditionally contains freeform data, there may be
   structured data present in the MSG part of a syslog message.
   Structured data are special, well defined data elements designed to
   be easily computer-parsable. They may transport meta data for the
   syslog protocol as well as application-defined information (like
   traffic counters, IP addresses and other well-defined elements).

   There is a certain set of structured data that is under IANA control.
   These structured data elements are described in this and other RFCs.
   A second set of structured data elements is not under IANA-control.
   This set MUST be used for experimental or vendor-specific elements.

   A syslog message may contain none, one or multiple structured data

5.1 Format

   Structured data can be present anywhere within the MSG part and
   follows this ABNF:

      ; Format of structured data element
      STRUCTURED-DATA = "[@#" SD-ID 0*(1*SP SD-PARAM) *SP "]"
      SD-ID           = SD-ID-IANA / SD-ID-EXPERI
      SD-ID-IANA      = 1*1ID-CHAR [1*1ID-CHARNOSLASH [1*62ID-CHAR]]
      SD-ID-EXPERI    = %d120 "-" 1*62ID-CHAR ; "x-" (lower case 'x'!)
      ID-CHAR         = %d32-33 / %d35-60 / %d62-92 / %d94-126 /
                        ; all US-ASCII but '"' (%d34), '=' (%d61), ']'
                        ; (%d93)
      ID-CHARNOSLASH  = %d32-33 / %d35-44 / %d46-60 / %d62-92 /
                        %d94-126 / %d128-254
                        ; same as ID-CHAR but without '-' (%d45)
      SD-PARAM        = PARAM-ID "=%d34" PARAM-VALUE "%d34"
      PARAM-ID        = 1*64ID-CHAR
      SAFE-CHAR       = *((%d32-33) / (%d35-46) / (%d48-92) /
                        (%d94-126) / (%d128-254))
      ESCAPED-CHAR    = ("\\" / %d47.34 / "]")  ; 47.34 is \"

   Each structured data element MUST begin with the token "[@#". This
   designates it as a special entity. This three-character sequence is
   highly likely not to be confused with traditional syslog message

   The beginning token MUST immediatly be followed by the ID of the
   structured data element. No space is allowed between the beginning

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   token and the SD-ID. The SD-ID uniquely identifies the type and
   purpose of the element.

   IANA controls ALL SD-IDs without a hyphen '-' in the second character
   position. Experimental or vendor-specific SD-IDs MUST start with
   "x-". Values with a hypen on the second character position and the
   first character position not being a lower case "x" are undfined and
   SHOULD NOT be used. Receivers MAY accept them.

   If a receiver receives a well-formed but unknown SD-ID, the receiver
   SHOULD ignore this element. It MUST NOT malfunction because of this
   unknown SD-ID.

   The SD-ID is followed by none, one or many optional parameter/value
   pairs. Each of them MUST start with the parameter name, MUST be
   followed by an equal sign and quote sign. There MUST NOT be any space
   between the SD-ID, the equal and the quote sign. This is followed by
   the parameter value and then another quote sign.

   The parameter value may contain any character, but the three special
   characters '"', '\' and ']' MUST be escaped. This is neccessary to
   avoid parsing errors. Please note that escaping ']' would actually
   not be necessary but is required in order to avoid parser
   implementation errors. Each of these three characters MUST be escaped
   as '\"', '\\' and '\]' respectively. If a receiver receives an

   A backslash ('\') followed by none of the three described characters
   is considered an invalid escape sequence. Upon reception of such an
   invalid message, the receiver MUST replace the two-character sequence
   with just the second character received. It is recommended that the
   receivers logs a diagnostic message in this case. The receiver MUST
   otherwise ignore the invalid escape sequence.

   Parameter/Value pairs MUST be separated by at least one SP character.

   The structured data element MUST be terminated by the character ']',
   the ending token. This MUST follow the last parameter/value pair.
   There SHOULD be no SP in front of the ending token, but there MAY be
   one or multiple SP in front of it.

   If multiple structured data elements are written, it is RECOMMENDED
   that they are all sequentially written and no SP be written between
   those elements. However, they MAY occur at any position inside MSG.

   The order of structured data elements inside the MSG is irrelevant,
   except for IANA-assigned SD-IDs which specifically require a certain
   order. The same SD-ID MAY exist more than once inside a MSG if this

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   is permitted by the SD-ID type.

5.2 MSG with just Structured Data

   Any syslog MSG may contain structured as well as traditional free
   form data. The free form (or unstructured) part of the syslog MSG is
   obtained by omiting all the structured data elements from the MSG.

   The resulting free from part of the MSG may consist purely of one or
   more SPs. This is considered as a MSG with just structured data

   As far as this specification goes, there is no implied special action
   to be taken on messages without a free form content in the MSG field.
   This case is just defined so that it may be used for
   implementation-specifc (and probably user-configurable) actions.

5.3 Examples

   All examples show the MSG part of the syslog message only. All
   examples should be considered to be on one line. They are wrapped on
   multiple lines for readabily purposes, only.

   Example 1

        [@#x-adiscon-iut iut="3" EventSource="Application"
        EventID="1011"]This is event 1011

   This example is a MSG with an experimental SD-ID of type
   "x-adiscon-iut" which has two parameters. This is followed by the
   free form text "This is event 1011".

   Example 2

        [@#x-adiscon-iut iut="3" EventSource="Application"
        EventID="1011"]This is event 1011 [@#x-adiscon-priority

   This is the same example, but with a second structured data element.
   Please note that the structured data element does not immediately
   follow the first one. Also note that the free form message is
   different from the example 1. It now is "This is event 1011>SP<" -
   notice the extra space character at the end.

   Example 3

        This is event[@#x-adiscon-priority class="high"] 1
        [@#x-adiscon-iut iut="3" EventSource="Application

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   In this example, <SP> is actually a single space character. Although
   all elements are re-odered and the free form message is intermixed
   with structured data, it is still exactly the same message as in
   example 2. The message formatting shown in example three SHOULD be
   avoided by syslog senders. However, receivers MUST accept messages
   formatted in that way.

   Example 4

        [ @#x-adiscon-iut iut="3" EventSource="Application"
        EventID="1011"]This is event 1011

   Example four looks very much like example one. However, it is totally
   different because example four does NOT contain any structured data
   element at all. This is because there is a SP between the bracket and
   the rest of the beginning token "@#". As such, the three-character
   beginning token is not identifiable and not parsed as such. Receivers
   receiving this format MUST NOT assume structured data. This is
   especially important as legacy syslog data may very well contain a
   sequence as shown above which actually is no structured data.

   Example 5

        [@#sigSig Ver="1" RSID="1234" ... Signature="......"]

   Example 5 is not a full example. It shows how a hypothetical IANA
   assigned SD-ID may be used inside an otherwise empty message. Please
   note that the dots denote missing fields, which have been left out
   for brevity.

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6. Fragmentation

   Fragmentation is an optional feature. It may be used if the amount of
   syslog data to be transported is larger than the maximum of 1024
   characters allowed. Fragmentation is implemented using
   STRUCTURED-DATA elements.

   A syslog message that is fragemented is split into a number of
   fragments that will be transmitted as separate messages via syslog.
   If fragmentation is used, special processing needs to take place. In
   order to avoid complexity, the receiver MUST reassemble the orginal
   message before parsing the message content. This original message
   MUST NOT contain the fragmentation structured data elements.

   It is RECOMMENDED that fragmentation is only be used if the full
   message does not fit into a single syslog message. If it does,
   fragmentatin SHOULD NOT be used. However, a sender may still choose
   to use it in this case. Thus, a receiver MUST accept a fragmented
   message with just a single fragment.

6.1 SD-ID fragment

   Fragmentation is done via the IANA-reserved "fragment" SD-ID.

   The "fragment" SD-ID is a structured data element with 3 parameters.
   It describes a single fragment of a fragmented syslog message. It
   MUST begin immediately after the HEADER of the syslog message. The
   fragment of the original message immediatly begins AFTER the closing
   token (']') of the fragment SD-ID. The MUST NOT be any SP or other
   character between the closing token and the begin of the fragment.

   The receiver of a fragment MUST NOT try to parse structured data
   elements inside a single fragment. This MUST only be done on the
   fully re-assembled message. The reason for this is that a single
   fragment may be missing important tokens that will lead to
   misdetection of structured data elements.

   The "fragement" SD-ID has three parameters: msgid, fragnum,
   fragcount. Each of them is described in detail in the following

   All fragments of a single syslog message MUST have the exact same
   syslog message header, most importantly the exact same timestamp. It
   is RECOMMENDED that a sender implementing fragementation uses
   TIMESTAMP and provides better-than-second time-resolution inside it.

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6.1.1 msgid

   The parameter "msgid" is an integer value in the range 0..2147483648.
   It MUST uniquely identify a message for a given TIMESTAMP. It SHOULD
   at least uniquely identify a message between two reboots of the
   syslog sender.

   It is RECOMMENDED that an incrementing value is used, which MAY be
   reset to 0 at the time of the syslog sender's startup. If the value
   is incremented and the maximum value is reached, than it is
   RECOMMENDED to reset the msgid to 0.

   Two otherwise identical msgid received in different fragments with
   different TIMESTAMP in the header MUST be considered to be two
   different msgid.

6.1.2 fragnum

   The parameter "fragnum" is an integer value in the range
   1..2147483648. This value MUST start at one for the first message
   fragment and MUST be incremented by one for each subsequent

   The "fragnum" counter MUST be processed on a per-message basis. That
   is, when the next full syslog message is to be fragemented, its first
   fragment again starts with "fragnum" set to 1.

   The "fragnum" counter MUST never be greater than "fragcount". If it
   ever is greater, all message fragments MUST be considered invalid. It
   is RECOMMENDED that a diagnostic message is logged in that case.

   If the "fragnum" is outside the defined range, all message fragments
   MUST be considered invalid. It is RECOMMENDED that a diagnostic
   message is logged in that case.

6.1.3 fragcount

   The parameter "fragcount" is an integer value in the range
   1..2147483648. It specifies into how many fragments the message has
   been split into.

   The "fragcount" parameter must remain the same for all fragments of a
   fragmented message. If "fragcount" is not the same in all message
   fragments, all fragments MUST be considered invalid. It is
   RECOMMENDED that a diagnostic message is logged in that case.

   If the "fragcount" is outside the defined range, all message
   fragments MUST be considered invalid. It is RECOMMENDED that a

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   diagnostic message is logged in that case.

6.2 Cryptographically signing fragmented Messages

   While the author of this draft does not intend to specify how
   messages can be signed, he would like to offer a suggestion on the
   implications of fragmented messages.

   Fragmented messages need to be transmitted in fragmented form and
   need to be reassmebled to be processed. During reassembly, parts of
   the fragment's message text are stripped from the message. This poses
   a problem to cryptographically signing the messages.

   An obvious solution to keeping the message signature intact is that
   only the original, full-size message is signed. Then, the individual
   fragments are transmitted without a specifc signature attached to
   them. Only the re-assembled message will then be used for verifying
   the signature.

   This mode will probably be very efficient, as the ultimate goal is to
   guarantee the integrity of the original message. Any modification to
   the fragments will either result in a protocol error or a
   modification of the signed original message. Both of this will be
   detected by verifying the signature in the original, reassembled,

   One problem, however, may be caused to signature verifiers who work
   on raw logs. Raw logs will most probably include only the individual
   fragments. If this is an issue, it may be worth thinking about a
   signature protocol which both signs the original message as well as
   each individual fragments. So the message would effectively be signed

   The author of this draft thinks it is NOT advisable to only sign the
   individual message fragments. While this would guarantee the
   authenticy of the individual fragments, no authentic signature could
   be provided for the reassembled message. This may cause serious
   issues with higher-level signature verifiers.

   Again, these are just thoughts about implementing signatures.
   Depending on the signature specification used, there may be different
   solutions. It is RECOMMENDED that authors of signature specifications
   specifically describe how their specification deals with fragmented

6.3 Fragmentation Examples

   To conserve some space, we use an abbreviated sample, where not all

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   data is shown:

   Base Example

        <34>2004-01-19T22:14:15.002 mymachine mwagent:
        [@#x-adiscon-iut iut="3" EventSource="Application"
           (some lenghty params) EventID="1011"][@#x-adiscon-priority
        class="high"]This is event 1011.
        (lengthy data) This is the end.

   We assume that the lengthy data is longer than does fit into a single
   syslog message. As such, it needs to be fragmented. To keep it
   simple, we assume that "(some lengthy paramters)" and "(lenghty
   data)" are the lengthy parts and that their length forces us to
   create three fragments.

   The initial fragment will just contain structured data, the second
   fragment some structured data and some free form data and the last
   part only free form data. This is how they look:

   Example Fragment One

        <34>2004-01-19T22:14:15.002 mymachine mwagent:
        [@#fragment msgid="12" fragcount="3" fragnum="1"]
        [@#x-adiscon-iut iut="3" EventSource="Application"
           (some lenghty params) EventID="1011"][@#x-adiscon-prior

   Example Fragment Two

        <34>2004-01-19T22:14:15.002 mymachine mwagent:
        [@#fragment msgid="12" fragnum="2" fragcount="3"]
        ity class="high"]This is event 1011.  (lengthy
        data) This i

   Example Fragment Three

        <34>2004-01-19T22:14:15.002 mymachine mwagent:
        [@#fragment msgid="12" fragnum="3"
        fragcount="3"]s the end.

   There are some things worth noting when looking at the examples:

   o  The header, and most importantly the TIMESTAMP is the same for all
      three messages, even though fragments two and three are most
      probably send at a slightly later time. Please also note that a
      TIMESTAMP is used to facilitate msgid uniquenes.

   o  The value for "msgid", 12, is just taken randomly for this

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   o  The sequence of "fragnum" and "fragcount" is not fixed - their
      order is different in message fragment one than in fragments two
      and three. This is irrelevant.

   o  The "x-adiscon-priority" SD-ID is split between fragment one and
      two. This is the reason why parsing structured data should only be
      done on the re-assmbled message. Parsing the fragements themselfs
      may seriously confuse the parser.

   o  Note how the freeform message part is split between fragment two
      and three. In fragment three, it starts with "]s" to complete the
      "its" from the nonfragmented message. Please note that if in
      fragment three it had been "] s", this would have been reassembled
      to be "it s" (with a SP in between).

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7. Security Considerations

   This section is to be updated once the rest of the document has been
   confirmed. The current content is incomplete and potentially not in
   sync with the rest of the draft.

   Many security considerations were described in the informational RFC
   3164 [19] and are repeated here for completeness. Additional
   considerations are also included in this section.

7.1 Packet Parameters

   The message length must not exceed 1024 bytes.  Various problems may
   result if a device sends out messages with a length greater than 1024
   bytes.  In this case, as with all others, it is best to be
   conservative with what you send but liberal in what you receive, and
   accept more than 1024 bytes.

   Similarly, the fragmentation features introduced in this document may
   be misused to overrun a receiver or a log analyzer with a gigantic
   message. Any process reassembling fragmented messages MUST properly
   check the maximum re-assembled message size it supports. Oversize
   data SHOULD be dropped.

   Similarly, senders must rigidly enforce the correctness of the
   message body. It is hoped that all devices adopt the newly defined
   HOSTNAME-STD13 and TIMESTAMP formats.  However, until that happens,
   receivers may become upset at the receipt of messages with these
   fields.  Knowledgeable humans should review the senders and receivers
   to ensure that no problems arise from this.

   Finally, receivers must not malfunction if they receive syslog
   messages containing characters other than those specified in this

7.2 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.

7.3 Authentication Problems

   One possible consequence of this behavior is that a misconfigured

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   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.

7.4 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

7.5 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.

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   Strict adherence to the use of TIMESTAMP will help administrators to
   place received messages in their proper order.

7.5.1 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 a
   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.

7.5.2 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.

7.5.3 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

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   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.

7.6 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.

7.7 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.

   RFC 3195 may be used for the reliable delivery of all syslog

7.8 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

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   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.

7.9 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.

7.10 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],

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   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.

7.11 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.

7.12 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

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   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.

7.13 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

7.14 Denial of Service

   As with any system, an attacker may just overwhelm a receiver by
   sending more messages to it than can be handled by the infrastructure
   or the device itself. Implementors should attempt to provide features
   that minimize this threat. Such as only receiving syslog messages
   from known IP addresses.

7.15 Covert Channels

   Nothing in this protocol attempts to eliminate covert channels.
   Indeed, the unformatted message syntax in the packets could be very
   amenable to sending embedded secret messages.  In fact, just about
   every aspect of syslog messages lends itself to the conveyance of
   covert signals.  For example, a collusionist could send odd and even
   PRI values to indicate Morse Code dashes and dots.

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8. IANA Considerations

   This document also upholds the Facilities and Severities listed in
   RFC 3164 [19].  Those values range from 0 to 191.  This document also
   instructs the IANA to reserve all other possible values of the
   Severities and Facilities above the value of 191 and to distribute
   them via the consensus process as defined in RFC 2434 [16].

   IANA must also maintain a registry of cookie values.

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9. Authors and Working Group Chair

   The working group can be contacted via the mailing list:


   The current Chair of the Working Group may be contacted at:

         Chris Lonvick
         Cisco Systems
         Email: clonvick@cisco.com

   The author of this draft is:

         Rainer Gerhards
         Email: rgerhards@hq.adiscon.com

         Phone: +49-9349-92880
         Fax: +49-9349-928820

         Adiscon GmbH
         Mozartstrasse 21
         97950 Grossrinderfeld

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10. Acknowledgements

   The authors wish to thank Chris Lonvick, Jon Callas, Andrew Ross,
   Albert Mietus, Anton Okmianski, Tina Bird, David Harrington and all
   other people who commented on various versions of this proposal.

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   [1]   National Institute of Standards and Technology, "Digital
         Signature Standard", FIPS PUB 186-1, December 1998, <http://

   [2]   National Institute of Standards and Technology, "Secure Hash
         Standard", FIPS PUB 180-1, April 1995, <http://

   [3]   American National Standards Institute, "USA Code for
         Information Interchange", ANSI X3.4, 1968.

   [4]   Menezes, A., van Oorschot, P. and S. Vanstone, ""Handbook of
         Applied Cryptography", CRC Press", 1996.

   [5]   Mockapetris, P., "Domain names - concepts and facilities", STD
         13, RFC 1034, November 1987.

   [6]   Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, November 1987.

   [7]   Eastlake, D., Crocker, S. and J. Schiller, "Randomness
         Recommendations for Security", RFC 1750, December 1994.

   [8]   Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.

   [9]   Freed, N. and N. Borenstein, "Multipurpose Internet Mail
         Extensions (MIME) Part One: Format of Internet Message Bodies",
         RFC 2045, November 1996.

   [10]  Oehler, M. and R. Glenn, "HMAC-MD5 IP Authentication with
         Replay Prevention", RFC 2085, February 1997.

   [11]  Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing
         for Message Authentication", RFC 2104, February 1997.

   [12]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
         Levels", BCP 14, RFC 2119, March 1997.

   [13]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         2279, January 1998.

   [14]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", RFC 2234, November 1997.

   [15]  Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.

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   [16]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434, October

   [17]  Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
         Message Format", RFC 2440, November 1998.

   [18]  Blumenthal, U. and B. Wijnen, "User-based Security Model (USM)
         for version 3 of the Simple Network Management Protocol
         (SNMPv3)", RFC 2574, April 1999.

   [19]  Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.

   [20]  New, D. and M. Rose, "Reliable Delivery for syslog", RFC 3195,
         November 2001.

   [21]  Klyne, G. and C. Newman, "Date and Time on the Internet:
         Timestamps", RFC 3339, July 2002.

   [22]  Schneier, B., "Applied Cryptography Second Edition: protocols,
         algorithms, and source code in C", 1996.

Author's Address

   Rainer Gerhards

   EMail: rgerhards@hq.adiscon.com

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