syslog Working Group                                         R. Gerhards
Internet-Draft                                          December 2, 2003
Expires: June 1, 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 June 1, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). 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 a backwards-compatible
   and easily extensible syslog protocol.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.    Definitions and Architecture . . . . . . . . . . . . . . . .  4
   3.    Transport Layer Protocol . . . . . . . . . . . . . . . . . .  7
   4.    Required syslog Format . . . . . . . . . . . . . . . . . . .  8
   4.1   PRI Part . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.2   HEADER Part  . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.2.1 TIMESTAMP  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.2.2 HOSTNAME . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.2.3 TAG  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.3   MSG  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.3.1 COOKIE . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   4.3.2 PAYLOAD  . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   4.4   TRAILER  . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   4.5   Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   5.    Security Considerations  . . . . . . . . . . . . . . . . . . 19
   5.1   Packet Parameters  . . . . . . . . . . . . . . . . . . . . . 19
   5.2   Message Authenticity . . . . . . . . . . . . . . . . . . . . 19
   5.3   Authentication Problems  . . . . . . . . . . . . . . . . . . 19
   5.4   Message Forgery  . . . . . . . . . . . . . . . . . . . . . . 20
   5.5   Sequenced Delivery . . . . . . . . . . . . . . . . . . . . . 20
   5.5.1 Single Source to a Destination . . . . . . . . . . . . . . . 21
   5.5.2 Multiple Sources to a Destination  . . . . . . . . . . . . . 21
   5.5.3 Multiple Sources to Multiple Destinations  . . . . . . . . . 21
   5.6   Replaying  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   5.7   Reliable Delivery  . . . . . . . . . . . . . . . . . . . . . 22
   5.8   Message Integrity  . . . . . . . . . . . . . . . . . . . . . 22
   5.9   Message Observation  . . . . . . . . . . . . . . . . . . . . 23
   5.10  Message Prioritization and Differentiation . . . . . . . . . 23
   5.11  Misconfiguration . . . . . . . . . . . . . . . . . . . . . . 24
   5.12  Forwarding Loop  . . . . . . . . . . . . . . . . . . . . . . 24
   5.13  Load Considerations  . . . . . . . . . . . . . . . . . . . . 25
   5.14  Denial of Service  . . . . . . . . . . . . . . . . . . . . . 25
   5.15  Covert Channels  . . . . . . . . . . . . . . . . . . . . . . 25
   6.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 26
   7.    Authors and Working Group Chair  . . . . . . . . . . . . . . 27
   8.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
         References . . . . . . . . . . . . . . . . . . . . . . . . . 29
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 30
         Intellectual Property and Copyright Statements . . . . . . . 31

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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


      TRAILER         = [CR] LF
      PRI             = "<" PRIVALUE ">"
      PRIVALUE        = (0..191) / (1*3DIGIT "," 0..7)
                        ; the alternate form is based on Albert Mietus comments...
      HOSTNAME        = 1*64PRINTUSASCII  ; a FQDN,
                        ;adopt international domain names later (too political issue,
                        ; takes too long)?
      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       = TIMESTAMP-3164 / TIMESTAMP-3339
      TIMESTAMP-3164  = MON-3164 SP DAY-3164 SP TIME-3164
      MON-3164        = %d74.97.110  / ; "Jan"

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                        %d70.101.97  / ; "Feb"
                        %d77.97.114  / ; "Mar"
                        %d65.112.114 / ; "Apr"
                        %d77.97.121  / ; "May"
                        %d74.117.110 / ; "Jun"
                        %d74.117.108 / ; "Jul"
                        %d65.117.103 / ; "Aug"
                        %d83.101.112 / ; "Sep"
                        %d79.99.116  / ; "Oct"
                        %d78.111.118 / ; "Nov"
                        %d68.101.99    ; "Dec"
      DAY-3164        = (SP 1..9) / (10..31)
      TIME-3164       = time-hour ":" time-minute ":" time-second-nl

      TIMESTAMP-3339  = full-date "T" full-time
      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-second-nl  = 2DIGIT  ; 00-59 no leap seconds!
      time-secfrac    = "." 1*DIGIT
      time-numoffset  = ("+" / "-") time-hour ":" time-minute
      time-offset     = "Z" / time-numoffset

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

      COOKIE-IANA     = COOKIE-ID ; IANA-Assigned
      COOKIE-EXPER    = "X-" COOKIE-ID ; experimental
                        ; a valid domain name owned by the vendor
      COOKIE-ID       = 4*6PRINTUSASCII ; MUST NOT begin with "V-" or "X-"
      PAYLOAD         = *((%d32-126) / (%d128-254))
                        ; VALID UTF-8 String of PRINTABLE characters

                        ; parameters defined by the extension using the cookie

      LF              = %d10

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

4.1 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 [14]. These are the ASCII codes as
   defined in "USA Standard Code for Information Interchange"
   ANSI.X3-4.1968 [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 defined in RFC
   3164. and are repeated here.

          Numerical             Facility

              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)

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             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

           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

              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" follows the "<" is
   for the Priority value of "0". Otherwise, leading "0"s MUST NOT be

   An alternate form for the PRI part has been recommended by Albert
   Mietus. It is described in the ABNF above as a basis for discussion.
   In this form, the facility and severity is split across two fields,
   with the facility defined to have up to a thousand values (0..999).

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   This would enable newer emitors to provide more detailed information
   on which subsystem caused the syslog message. In this form, the
   facility is followed by a comma and then the severity as a single
   digit. An example of a message from the (traditional) mail subsystem
   with error severity would be "<2,3>".

4.2 HEADER Part

   The HEADER part contains a time stamp, an indication of the hostname
   or IP address of the device, and a string indicating the source of
   the message. 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 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 TIMESTAMP field is either a timestamp as defined in RFC 3164
   denoted as TIMESTAMP-3164, or as a formalized timestamp as taken from
   RFC 3339 [21].  A sender SHOULD format the timestamp as a
   TIMESTAMP-3339.  A receiver MUST accept both formats. The formal
   definition for both timestamp formats can be found in the ABNF above.

   Note well: RFC 3339 makes allowances for multiple syntaxes for a
   timestamp to be used in various cases.  This document mandates a
   single syntax.  The primary characteristics of TIMESTAMP-3339 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

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      clock accuracy permits.

   o  the entire length of the TIMESTAMP-3339 field MUST NOT exceed 32

   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.

   Receivers parsing the date format SHOULD check if the TIMESTAMP is a
   TIMESTAMP-3339. The "T" character at position 11 of the string can be
   used as a rough indication for this. However, the receiver MUST NOT
   rely solely on the "T" character but also parse the other data for
   validity. A receiver SHOULD check for TIMESTAMP-3339 format first
   and, if unsuccessful, assume a TIMESTAMP-3164. If it is also not a
   TIMESTAMP-3164 format, the receiver MUST NOT try any other timestamp
   format but consider the TIMESTAMP to be invalid or missing from the
   received syslog message.

   If a relay receives a TIMESTAMP-3164, it SHOULD forward the message
   with a TIMESTAMP-3164 but MAY reformat it to a TIMESTAMP-3339 if
   configured to do so. Relays should be aware that the TIMESTAMP-3339
   may be longer than the TIMESTAMP-3164 and a replacement of the
   TIMESTAMP-3164 with a TIMESTAMP-3339 may increase the length of the
   entire packet beyond 1024 bytes.  If a relay receives a
   TIMESTAMP-3339 it MUST forward the message with a TIMESTAMP-3339. It
   MUST NOT reformat it to a TIMESTAMP-3164.

   There is one minor - but eventually important - difference in regard
   to the second representation between a TIMESTAMP-3164 and a
   TIMESTAMP-3339. In a TIMESTAMP-3339, the second part may have the
   value "60" to indicate a leap second. No such value is permitted in a
   TIMESTAMP-3164 second part. If a relay receives a value of "60" in a
   TIMESTAMP-3339 AND is configured to rewrite this to a TIMESTAMP-3164
   (for whatever reasons), it MUST represent the second part with the
   value "59" and otherwise leave the TIMESTAMP time unmodified. The
   author beliefs this handling causes the least confusion and potential
   code errors. It should occur seldom enough to not cause any issue at

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   The HOSTNAME field contains an indication of the originator of the
   message in one of four formats:  only the hostname, the hostname and
   domainname, the IPv4 address, or the IPv6 address.  The preferred
   value is the hostname and domainname in the format specified in STD
   13 [5].  This format will be referred to in this document as
   HOSTNAME-STD13.  If only the hostname is used, the HOSTNAME field
   MUST contain the hostname only of the device as specified in STD 13.
   This format is discouraged but provides for legacy compatibility with
   the format described in RFC 3164.  This format will be referred to in
   this document as HOSTNAME-3164.  In this format, 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
   [6], and will be referred to as HOSTNAME-IPV4.  If an IPv6 address is
   used, any valid representation used in RFC 2373 [15] MAY be used and
   will be referred to as HOSTNAME-IPV6. A single space character MUST
   also follow the HOSTNAME field.

4.2.3 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. It
   is RECOMMENDED to terminate the TAG with a colon (':'), which if
   used, is part of the TAG.

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

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   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.

   Receivers SHOULD, to be consistent with the format described in
   RFC3164, accept TAGs that terminate with a single colon, without a
   space following it. Then the colon is both the last character of that
   TAG, and the field separator with the next field (MSG).

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

4.3 MSG

   The MSG part contains an optional COOKIE and the actual PAYLOAD.

   If the MSG part contains a COOKIE, optional cookie parameters follow
   after the cookie and after that the original message.

   NOTE WELL: MSG is a recursive structure. As such, a MSG may contain a
   COOKIE and another MSG which in turn also contains a COOKIE and yet
   another MSG. To clarify things, we call a MSG that does not contain
   any COOKIE the actual PAYLOAD (see below).

   There is no hard limit of how many levels of COOKIE/MSG constructs
   are used inside a single message. The only limit is that the whole
   construct must fit within the syslog size limitation. Practically,
   however, it is recommended to limit nesting to those cases where it
   is absolutely necessary and there is good reasoning for it.

   NOTE WELL: there is an inherent risk with the nesting of COOKIES: As
   specified, a receiver must assume a valid cookie only if he knows the
   full COOKIE, including COOKIE-ID. If he does not know that specific
   cookie, it MUST be treated as ordinary data, thus turning the message
   from MSG to PAYLOAD. As such, no parsing for further COOKIES in
   PAYLOAD is allowed nor desired. In consequence, COOKIES nested in
   deeper layers will not be seen and processed.

   The author beliefs this potential shortcoming is acceptable. If
   inner-layer cookies would be tried to parse, this would potentially
   conflict with existing syslog data as well as introduce a number of
   potential bugs, as the format and thus validity of the outer level
   cookie is not know. It is assumed that if the outer layer cookie is
   not know, the receiver will most probably not understand the
   inner-layer cookie.

   To minimize this risk, more generic cookies should be at the outer
   layers and less specific cookies on the inner layers.

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4.3.1 COOKIE

   The COOKIE is an optional part of the message. It is used to identify
   optional features inside a syslog message. A cookie can either be
   assigned via IANA (COOKIE-IANA), be experimental but intended to be
   vendor-neutral (COOKIE-EXPER) or be vendor-specific (COOKIE-VENDOR).

   If there is a cookie present, it MUST start with the sequence "@#" at
   the first character of the payload block. The COOKIE-ID must be at
   least 4 characters, so that the overall minimum COOKIE size is 6
   characters. These requirements makes it highly unlikely that a string
   sequence in an "old-style" syslog message will be misinterpreted as a
   cookie. However, there is a slight chance that this may happen. It
   may also be deliberately done as part of a malicious message. As
   such, an implementation MUST NOT rely solely on the "@#" sequence to
   judge whether it is a valid cookie or not. It MUST parse the whole
   cookie to see if it is known or not and then act accordingly. Unknown
   cookies should be treated as ordinary data and not be acted upon.
   This implies that an implementation MUST not attempt to find further
   cookies inside the MSG. COOKIE-ID

   The COOKIE-ID uniquely identifies the cookie. It is a 4 to 6
   character wide string of printable characters. It is case-sensitive.
   The 4 character minimum size requirement is introduced to reduce the
   likelihood that a cookie is mistakenly being recognized. The
   COOKIE-ID alone MUST NOT be used to detect a cookie. It can, however,
   be handy for human discussion.

   The COOKIE-ID MUST NOT begin with "V-" or "X-". IANA and Experimental Cookies

   These are vendor-neutral cookies. IANA-assigned cookie values have
   undergone the consensus process and are well-defined. Experimental
   cookies are for vendor-neutral functionality that is currently in
   development. A syslog extension that is expected to be
   vendor-specific SHOULD NOT use experimental cookies, it SHOULD use
   vendor-specific cookies instead. As a rule of thumb, only cookies
   used for functionalities discussed on IETF mailing lists should be
   treated as vendor-neutral.

   When new experimental cookies are designed, they SHOULD use a
   COOKIE-ID not yet assigned by IANA. This will facilitate the later
   transition as the experimental COOKIED-ID could eventually be used as
   an IANA COOKIE-ID once consensus has been reached and the discussed
   functionality is mature enough.

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   These cookie values are reserved for vendor extensions. A general
   issue with namespaces and vendor extensions is that multiple vendors
   may (accidentally) decide to use the same value as their extension
   ID. To avoid this, we prefix each vendor-specific COOKIE-ID with a
   VENDORURI. This should be a long-lasting Internet domain name that
   the vendor owns.

   An example: "Example Inc" has two software products called
   "GreatestSyslog" and "EvenGreaterSyslog". It owns the domains
   "", "GreatestSyslog.example" and
   "EvenGreaterSyslog.example". Now, "Example Inc" decides to introduce
   a new cookie for exclusive use by "EvenGreaterSyslog". It is
   recommended that the company's main domain is used for building the
   vendor cookie. If they used the COOKIE-ID "MyTag", the complete
   vendor cookie would look like this: "".

   The VENDOR-URI is case-insensitive. However, it is good practice to
   send it consistently in the same case. It SHOULD be sent in lower

   If cookies are nested, vendor cookies MUST be used on the innermost
   layer, only.


   The PAYLOAD part contains the details of the message. This has
   traditionally been a freeform message that gives some detailed
   information of the event.

   The PAYLOAD part of the syslog packet MUST contain visible (printing)
   characters. The code set traditionally and most often used has been
   seven-bit ASCII in an eight-bit field.  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 PAYLOAD is required, nor is it expected. Other code sets MAY be
   used as long as the characters used in the MSG part are exclusively
   visible characters and spaces similar to those described above.  For
   example, the UTF-8 RFC 2279 [13] character set may be used.

   The selection of a code set used in the PAYLOAD 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. As such, it is strongly RECOMMENDED to
   use a standard mechanism to indicate the code set used to the

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   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).

4.5 Examples

   The following examples are given.

   Example 1

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

   In this example, as it was originally described in RFC 3164, the PRI
   part is "<34>". In this work, however, the HEADER part consists of
   the TIMESTAMP, the HOSTNAME, and the TAG fields. The TIMESTAMP is
   "Oct 11 22:14:15 ", the HOSTNAME is "mymachine ", and the TAG value
   is "su:". The CONTENT field is " 'su root' failed for lonvick...".
   The CONTENT field starts with a leading space character in this case.

   Example 2

         <165>Aug 24 05:34:00 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 PRI part is <165> denoting 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. The HOSTNAME is an IPv4
   address and the TAG field is "myproc[10]:". The MSG part starts with
   "%% It's time to make the do-nuts. %%  Ingredients: Mix=OK, ..." this
   time without a leading space character.

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5. Security Considerations

   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.

5.1 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-3339 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

5.2 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.

5.3 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

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   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.

5.4 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

5.5 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.

   Strict adherence to the use of TIMESTAMP-3339 will help
   administrators to place received messages in their proper order.

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5.5.1 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.

5.5.2 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.

5.5.3 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
   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

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   these have coordinated timestamps, then there will be some indication
   of the time of receipt of the individual messages.

5.6 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.

5.7 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

5.8 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
   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.

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5.9 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.

5.10 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],
   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,

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   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.

5.11 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.

5.12 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
   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

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   must take care to not cause such a death spiral.

5.13 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

5.14 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.

5.15 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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6. 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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7. 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

   The author of this draft is:

         Rainer Gerhards

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

         Adiscon GmbH
         Mozartstrasse 21
         97950 Grossrinderfeld

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8. Acknowledgements

   The authors wish to thank Chris Lonvick, Jon Callas, Andrew Ross,
   Albert Mietus, Anton Okmianski 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


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Internet-Draft            The syslog Protocol              December 2003



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