syslog Working Group A. Okmianski Internet-Draft Cisco Systems, Inc. Expires: May 5, 2005 November 2004 Transmission of syslog messages over UDP draft-ietf-syslog-transport-udp-04 Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. 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://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 5, 2005. Copyright Notice Copyright (C) The Internet Society (2004). Abstract This document describes the transport for syslog messages over UDP/ IPv4 or UDP/IPv6. The syslog protocol layered architecture provides for support of any number of transport mappings. However, for interoperability purposes, syslog protocol implementors are required to support this transport protocol. Okmianski Expires May 5, 2005 [Page 1] Internet-Draft syslog udp transport November 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. One Message Per Datagram . . . . . . . . . . . . . . . . . . 3 3. Message Size . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Source and Target Ports . . . . . . . . . . . . . . . . . . 4 5. Source IP Address . . . . . . . . . . . . . . . . . . . . . 4 6. UDP/IP Structure . . . . . . . . . . . . . . . . . . . . . . 4 7. UDP Checksums . . . . . . . . . . . . . . . . . . . . . . . 4 8. Reliability Considerations . . . . . . . . . . . . . . . . . 5 8.1 Lost Datagrams . . . . . . . . . . . . . . . . . . . . . . 5 8.2 Message Corruption . . . . . . . . . . . . . . . . . . . . 5 8.3 Congestion Control . . . . . . . . . . . . . . . . . . . . 5 8.4 Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 6 9. Security Considerations . . . . . . . . . . . . . . . . . . 6 9.1 Sender Authentication . . . . . . . . . . . . . . . . . . 6 9.2 Message Forgery . . . . . . . . . . . . . . . . . . . . . 6 9.3 Message Observation . . . . . . . . . . . . . . . . . . . 7 9.4 Replaying . . . . . . . . . . . . . . . . . . . . . . . . 7 9.5 Unreliable Delivery . . . . . . . . . . . . . . . . . . . 7 9.6 Message Prioritization and Differentiation . . . . . . . . 7 9.7 Denial of Service . . . . . . . . . . . . . . . . . . . . 8 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 8 11. Notice to RFC Editor . . . . . . . . . . . . . . . . . . . . 8 12. Working Group . . . . . . . . . . . . . . . . . . . . . . . 8 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8 14. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 14.1 Normative References . . . . . . . . . . . . . . . . . . 9 14.2 Informative References . . . . . . . . . . . . . . . . . 9 Author's Address . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . 10 Okmianski Expires May 5, 2005 [Page 2] Internet-Draft syslog udp transport November 2004 1. Introduction The informational RFC 3164[7] originally described the syslog protocol as it was observed in existing implementations. It described both the format of syslog messages and a UDP[1] transport. Subsequently, the syslog protocol has been formally defined in the standards track RFC-protocol[2]. The RFC-protocol specified a layered architecture that provided for support of any number of transport layer protocols for transmitting syslog messages. This standards track RFC describes the UDP transport for the syslog protocol. This protocol can be used for transmitting syslog messages over both IPv4[3] and IPv6[4]. This transport protocol is REQUIRED for all syslog protocol implementations for interoperability purposes. Network administrators and architects should be aware of the significant reliability and security issues of this protocol, which stem from the use of UDP. They are documented in this specification. However, this protocol is lightweight and is built upon the existing popular use of UDP for syslog. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119[5]. The words 'byte' and 'octet' are used interchangeably in this specification. 2. One Message Per Datagram Each syslog UDP datagram MUST contain one and only one syslog message. The message MUST be formatted according to the RFC- protocol[2]. Additional data MUST NOT be present in the datagram payload. 3. Message Size This protocol supports transmission of syslog messages up to 65536 bytes in size. This limit stems from the maximum supported UDP payload of 65536 kilobytes specified in the RFC 768[1]. IPv4 syslog receivers MUST be able to receive datagrams with message size up to and including 480 bytes. IPv6 syslog receivers MUST be able to receive datagrams with message size up to and including 1180 bytes. All syslog receivers SHOULD be able to receive datagrams with messages size of at least 2048 bytes. Okmianski Expires May 5, 2005 [Page 3] Internet-Draft syslog udp transport November 2004 The above restrictions and recommendations establish a baseline for interoperability. The minimum required message size support was determined based on the minimum MTU size that internet hosts are required to support: 576 bytes for IPv4[3] and 1280 bytes for IPv6[4]. Datagrams that fall within these limits have the greatest chance of being delivered because they do not require fragmentation. It is RECOMMENDED that application developers restrict message sizes such that IP datagrams do not exceed the smallest MTU of the network in use. This avoids datagram fragmentation and possible issues surrounding fragmentation such as incorrect MTU discovery. Fragmentation may be undesirable because it increases the risk of the message being lost due to loss of just one datagram fragment. When network MTU is not known in advance and cannot be reliably determined using path MTU discovery [8], the safest assumption is to restrict messages to 480 bytes for IPv4 and 1180 bytes for IPv6. 4. Source and Target Ports Syslog receivers MUST support accepting syslog datagrams on a well- known UDP port 514, but MAY be configurable to listen on a different port. Syslog senders MUST support sending syslog message datagrams to the UDP port 514, but MAY be configurable to send messages to a different port. Syslog senders MAY use any source UDP port for transmitting messages. 5. Source IP Address The source IP address of the UDP datagrams SHOULD NOT be interpreted as the identifier for the host that originated the syslog message. The entity sending the syslog message may be merely a relay. The syslog message itself contains the identifier of the originator of the message. 6. UDP/IP Structure Each UDP/IP datagram sent by the transport layer MUST completely adhere to the structure specified in the UDP RFC 768[1] and either IPv4 RFC 791[3] or IPv6 RFC 2640[4] depending on which protocol is used. 7. UDP Checksums Use of UDP checksums was defined as optional in RFC 768[1]. IPv6 has subsequently made UDP checksums required in RFC 2460[4]. Syslog senders MUST use valid UDP checksums when sending messages over IPv6. Syslog senders SHOULD use UDP checksums when sending Okmianski Expires May 5, 2005 [Page 4] Internet-Draft syslog udp transport November 2004 messages over IPv4. Syslog receivers MUST check the checksums whenever they are present and discard messages with incorrect checksums. Note that this is typically accomplished by the UDP layer implementation, and some UDP implementations allow for checksum validation to be enabled or disabled. 8. Reliability Considerations The UDP is an unreliable low-overhead protocol. This section discusses reliability issues inherent in UDP that implementers and users MUST be aware of. 8.1 Lost Datagrams This transport protocol does not provide any mechanism to detect and correct loss of datagrams. Datagrams may be lost in transit due to congestion, corruption, or any other intermittent network problem. IP fragmentation exacerbates this problem because loss of a single fragment will result in the entire message being discarded. In some circumstances the sender may receive an ICMP error message or other indication of a transmission problem. If the sender receives a reasonable indication that a datagram may have been lost, it MAY retransmit the datagram. 8.2 Message Corruption The UDP/IP datagrams may get corrupted in transit due to software, hardware, or network errors. This protocol specifies use of UDP checksums to enable corruption detection in addition to checksums used in IP and Layer 2 protocols. However, checksums do not guarantee corruption detection, and this protocol does not provide for message retransmission when a corrupt message is detected. A special case of corruption is corruption introduced by the UDP implementation itself. For example, several earlier UDP implementations defaulted to a buffer size of less than 65536 bytes and truncated larger payloads upon reception [9]. By following the message size recommendations of this protocol, application developers can significantly reduce the risk of this type of error. 8.3 Congestion Control The UDP does not provide for congestion control. Any network host or router can discard UDP packets when it is overloaded, and it may or may not provide an ICMP error to indicate this. One or multiple Okmianski Expires May 5, 2005 [Page 5] Internet-Draft syslog udp transport November 2004 syslog senders can maliciously or inadvertently overload the receiver or the network infrastructure and cause loss of syslog messages. 8.4 Sequenced Delivery The IP transport used by the UDP does not guarantee that the sequence of datagram delivery will match the order in which the datagrams were sent. The time stamp contained within each syslog message may serve as a rough guide in establishing sequence order, but it will not help in cases when multiple messages were generated during the same time slot, the sender cannot generate a time stamp, or messages originated from different hosts whose clocks are not synchronized. The order of syslog message arrival via this transport SHOULD NOT be used as an authoritative guide in establishing an absolute or relative sequence of events on the syslog sender hosts. 9. Security Considerations Several syslog security considerations are discussed in RFC- protocol[2]. This section focuses on security considerations specific to the syslog transport over UDP. 9.1 Sender Authentication This transport protocol does not provide for strong sender authentication. The receiver of the syslog message will not be able to ascertain that the message was indeed sent from the reported sender, or whether the packet was sent from another device. This may also lead to a case of mistaken identity if a misconfigured machine sends syslog messages to a receiver representing itself as another machine. 9.2 Message Forgery This transport protocol does not provide protection against syslog message forgery. An attacker may transmit syslog messages (either from the machine from which the messages are purportedly sent or from any other machine) to a receiver. 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 Okmianski Expires May 5, 2005 [Page 6] Internet-Draft syslog udp transport November 2004 untrue indications of the status of systems. 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 not been restarted. 9.3 Message Observation This transport protocol does not provide confidentiality of the messages in transit. If syslog messages are in clear text, this is how they will be transferred. In most cases passing clear-text human-readable messages is a benefit to the administrators. 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 these messages to compromise a machine. It is RECOMMENDED that no sensitive information be transmitted via this transport protocol or that transmission of such information be restricted to properly secured networks. 9.4 Replaying Message forgery and observation can be combined into a replay attack. An attacker may record a set of messages that indicate normal activity of a machine. At a later time, an attacker may remove that machine from the network and replay the syslog messages with new time stamps. The administrators may find nothing unusual in the received messages, and their receipt would falsely indicate normal activity of the machine. 9.5 Unreliable Delivery As was previously discussed in the Reliability Considerations section, the UDP transport is not reliable, and packets containing syslog message datagrams can be lost in transit without any notice. There can be security consequences to the loss of one or more syslog messages. 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. 9.6 Message Prioritization and Differentiation This transport protocol does not mandate prioritization of syslog messages on the wire or when processed on the receiving host based on their severity. The security implication of such behavior is that the syslog receiver or network devices may get overwhelmed with low- severity messages and be forced to discard potentially high-severity messages. High-severity messages may contain an indication of Okmianski Expires May 5, 2005 [Page 7] Internet-Draft syslog udp transport November 2004 serious security problems, but they will not get a higher priority. 9.7 Denial of Service An attacker may overwhelm a receiver by sending more messages to it than can be handled by the infrastructure or the device itself. Implementers SHOULD attempt to provide features that minimize this threat such as optionally restricting reception of messages to a set of know source IP addresses. 10. IANA Considerations IANA must reserve UDP port 514 for this transport. 11. Notice to RFC Editor This is a notice to the RFC editor. This ID is submitted along with ID draft-ietf-syslog-protocol and they cross-reference each other. When RFC numbers are determined for each of these IDs, please replace all references to "RFC-protocol" with the RFC number of draft-ietf-syslog-protocol ID. Please remove this section after editing. 12. Working Group The working group can be contacted via the mailing list: syslog-sec@employees.org The current Chair of the Working Group may be contacted at: Chris Lonvick Cisco Systems Email: clonvick@cisco.com 13. Acknowledgements The author gratefully acknowledges the contributions of: Chris Lonvick, Rainer Gerhards, David Harrington, Andrew Ross, Albert Mietus, Bernie Volz, Mickael Graham, Greg Morris, Alexandra Fedorova, Devin Kowatch, Richard Graveman, and all others who have commented on the various versions of this proposal. 14. References Okmianski Expires May 5, 2005 [Page 8] Internet-Draft syslog udp transport November 2004 14.1 Normative References [1] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [2] Gerhards, R., "The syslog Protocol", RFC RFC-protocol. [3] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [4] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [5] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [6] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. 14.2 Informative References [7] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001. [8] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [9] Stevens, W., "TCP/IP Illustrated Volume 1. The Protocols.", January 1994. Author's Address Anton Okmianski Cisco Systems, Inc. 1414 Massachusetts Ave Boxborough, MA 01719-2205 USA Phone: +1-978-936-1612 Email: aokmians@cisco.com Okmianski Expires May 5, 2005 [Page 9] Internet-Draft syslog udp transport November 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. 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