syslog Working Group A. Okmianski Internet-Draft Cisco Systems, Inc. Expires: October 30, 2004 May 2004 Transmission of syslog messages over UDP draft-ietf-syslog-transport-udp-02 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://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 October 30, 2004. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Abstract This document describes the transport for syslog messages over UDP/ IPv4 or UDP/IPv6. While several transport mappings are envisioned for the syslog protocol, syslog protocol implementors are required to support the transport mapping described in this document. This transport specification overcomes limitations of UDP/IP datagram size by introducing support for fragmentation of large messages. Okmianski Expires October 30, 2004 [Page 1] Internet-Draft syslog udp transport May 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Transport Protocol Overview . . . . . . . . . . . . . . . . . 4 2.1 Definitions and Architecture . . . . . . . . . . . . . . . 4 2.2 Required Transport Protocol . . . . . . . . . . . . . . . 5 2.3 Encapsulation Layers . . . . . . . . . . . . . . . . . . . 5 3. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1 Basic Header Format . . . . . . . . . . . . . . . . . . . 6 3.2 Extended Header Format . . . . . . . . . . . . . . . . . . 7 3.2.1 Message Identifier . . . . . . . . . . . . . . . . . . 7 3.2.2 Total Length . . . . . . . . . . . . . . . . . . . . . 8 3.2.3 Fragment Offset . . . . . . . . . . . . . . . . . . . 8 3.2.4 Extended Header Example . . . . . . . . . . . . . . . 8 3.3 Payload . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.4 Supported Message Length . . . . . . . . . . . . . . . . . 9 4. UDP/IP Layer Considerations . . . . . . . . . . . . . . . . . 10 4.1 Target Port . . . . . . . . . . . . . . . . . . . . . . . 10 4.2 Source Port . . . . . . . . . . . . . . . . . . . . . . . 10 4.3 Source IP Address . . . . . . . . . . . . . . . . . . . . 10 4.4 UDP/IP Headers . . . . . . . . . . . . . . . . . . . . . . 10 5. Fragmentation and Reassembly . . . . . . . . . . . . . . . . . 11 5.1 Message Fragmentation . . . . . . . . . . . . . . . . . . 11 5.2 Message Reassembly . . . . . . . . . . . . . . . . . . . . 12 5.3 Avoiding Fragmentation . . . . . . . . . . . . . . . . . . 12 6. Reliability Considerations . . . . . . . . . . . . . . . . . . 13 6.1 Lost Datagrams . . . . . . . . . . . . . . . . . . . . . . 13 6.2 Message Corruption and Checksums . . . . . . . . . . . . . 13 6.3 Congestion Control . . . . . . . . . . . . . . . . . . . . 13 6.4 Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 13 6.5 Sender Authentication . . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 7.1 Message Authenticity . . . . . . . . . . . . . . . . . . . 14 7.2 Message Forgery . . . . . . . . . . . . . . . . . . . . . 14 7.3 Message Observation . . . . . . . . . . . . . . . . . . . 15 7.4 Replaying . . . . . . . . . . . . . . . . . . . . . . . . 15 7.5 Unreliable Delivery . . . . . . . . . . . . . . . . . . . 15 7.6 Message Prioritization and Differentiation . . . . . . . . 15 7.7 Denial of Service . . . . . . . . . . . . . . . . . . . . 16 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 9. Notice to RFC Editor . . . . . . . . . . . . . . . . . . . . . 16 10. Working Group . . . . . . . . . . . . . . . . . . . . . . . 16 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 12.1 Normative References . . . . . . . . . . . . . . . . . . . . 17 12.2 Informative References . . . . . . . . . . . . . . . . . . . 17 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 18 A. Rational For Transport Message Size Restrictions . . . . . . . 18 Okmianski Expires October 30, 2004 [Page 2] Internet-Draft syslog udp transport May 2004 Intellectual Property and Copyright Statements . . . . . . . . 20 Okmianski Expires October 30, 2004 [Page 3] Internet-Draft syslog udp transport May 2004 1. Introduction The original syslog protocol has been described in informational RFC 3164[1] as observed in existing implementations. It describes both the semantics of the syslog message format as well as a UDP transport. Subsequently, the syslog protocol has been formally defined in a standards track RFC-protocol[2]. The RFC-protocol[2] has provided for support of any number of transport layer protocols for transmitting syslog messages and left it to subsequent RFCs to specify transport protocols. This standards track RFC describes the UDP transport for the syslog protocol. This transport protocol is REQUIRED for all syslog protocol implementations. This transport protocol was designed to work on top of UDP [3] over both IPv4 [4] and IPv6 [5]. This protocol overcomes the data size restrictions of the UDP protocol by supporting message fragmentation. Support for fragmentation is only REQUIRED for implementations wishing to support messages which exceed certain size limits outline in this specification. This protocol has significant reliability and security issues stemming from the use of UDP. They are documented in this specification. This protocol also does not support acknowledgements of message receipt by the receiver and does not incorporate any reliable retransmission mechanism for lost datagrams. However, this protocol is lightweight and extends on the existing popular use of UDP for syslog. Network administrators and architects should be aware of the shortcomings of this protocol and plan accordingly. 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[6]. The words 'byte' and 'octet' are used interchangeably in this specification. 2. Transport Protocol Overview 2.1 Definitions and Architecture The following definitions will be used in this document: o An application that can generate syslog messages will be referred to as a "sender"; o An application that can receive syslog messages will be referred to as a "receiver". An application can function in dual capacity. For example, a syslog Okmianski Expires October 30, 2004 [Page 4] Internet-Draft syslog udp transport May 2004 relay may receive and forward messages. A single system can host any number of syslog senders. Only one syslog receiver can be hosted on a single system using the standard listening port. 2.2 Required Transport Protocol This document describes the UDP transport layer protocol for the syslog protocol RFC-protocol[2]. Every syslog sender and receiver implementation which adheres to the RFC-protocol[2] MUST fully implement the transport protocol specified in this document. An implementation does not have to support both IPv4 and IPv6 if it is designed to be used over only one of these protocols. 2.3 Encapsulation Layers This syslog transport carries syslog messages as a generic payload encapsulated with a syslog transport header, UDP header and an IP header. Below is a summary of syslog UDP/IP packet structure as used by this transport protocol: +--------------------------------+ | IPv4 or IPv6 Header | | (20 or more bytes) | +--------------------------------+ | UDP Header | | (8 bytes) | +--------------------------------+ | Syslog Transport Header | | (5 or 32 bytes) | +--------------------------------+ | Syslog Message Payload | | (1 to 1191 bytes) | +--------------------------------+ Some syslog messages may be transmitted using one UDP/IP datagram per message. Syslog protocol [2] allows messages as large as 16777216 bytes, while UDP/IP datagram cannot exceed a total size of 65526 bytes [3] and most existing protocols restrict the size of UDP data to much less. In order to support transmitting large messages over UDP/IP, this transport protocol supports fragmentation of large syslog messages into multiple UDP/IP datagrams for transmission and reassembly on the receiving end. Each syslog UDP/IP datagram MUST contain one and only one complete syslog message or one fragment of a message. Transmitting multiple messages or multiple fragments of different messages in a single UDP datagram is not supported by this protocol. Okmianski Expires October 30, 2004 [Page 5] Internet-Draft syslog udp transport May 2004 3. Message Format The syslog transport message consists of a transport header and a syslog message payload. The format of the transport header is different for fragmented and non-fragmented messages. The basic transport header format is used for non-fragmented messages and the extended transport header format is used for fragmented messages. The receiver MUST discard messages with incorrectly formatted headers. An ASCII-based encoding was chosen for the syslog transport for consistency with the RFC-protocol[2]. Syslog transport messages have the following format in ABNF[7] notation: SyslogTransportMessage = ( BasicHeader / ExtendedHeader ) SP Payload BasicHeader = Version SP "0" Version = %d118 1*3DIGIT ; "v1" in this version ExtendedHeader = Version SP "1" SP MessageId SP TotalLength SP FragmentOffset MessageId = 1*8DIGIT ; 0 to 16777215 TotalLength = 1*8DIGIT ; 1 to 16777216 FragmentOffset = 1*8DIGIT ; 0 to 16777215 Payload = 1*1191OCTET OCTET = %d00-255 DIGIT = %d48-57 SP = %d32 3.1 Basic Header Format When no fragmentation is used and the entire syslog message is transferred as a single UDP/IP datagram, a basic syslog transport header MUST be used. The version for this protocol is "1". It must be followed by one space, a "0" to indicate that this is a basic header and a trailing space. Therefore, the only possible value for the basic header in this protocol is as follows: "v1 0 " Example of a syslog message without the transport header (message is Okmianski Expires October 30, 2004 [Page 6] Internet-Draft syslog udp transport May 2004 wrapped for display): "v1 888 3 2003-10-11T22:14:15.003Z host.domain.com dns: configuration error" Example of the same message with the transport header (message is wrapped for display): "v1 0 v1 888 4 2003-10-11T22:14:15.003Z host.domain.com dns: configuration error" 3.2 Extended Header Format When a syslog message is fragmented by the sender, multiple UDP datagrams MUST be used and each datagram MUST contain an extended syslog transport header. The version for this protocol is "1". The version field MUST be followed by a single space and a "1" to indicate that this is an extended header. Thus, an extended header MUST always begin with "v1 1 ", but MUST also have additional fields which aid in reassembly. The MessageId, TotalLength and FragmentOffset fields are used solely for fragmentation of long messages and reassembly. They MUST NOT be used for other purposes. 3.2.1 Message Identifier The MessageId field (along with the source UDP port and the IP address) is used to identify the message such that fragments of a single syslog message can be reassembled by the receiver into a complete message. The MessageId field MUST be a numeric value in the range of 0 to 16777215. Leading zeros MUST NOT be present in the MessageId field. Each syslog sender process MUST choose a random MessageId value within the supported range for its first message. The sender SHOULD increment the MessageId by 1 up to 16777215 for each subsequent message and then continue at 0. Incrementing the value each time ensures that MessageId is unique and does not repeat over a long range of values. Using random value for the first MessageId helps reduce the possibility of potential errors in message reassembly. Refer to discussion about message reassembly (Section 5.2) for more details. All datagrams which represent parts of a given fragmented syslog Okmianski Expires October 30, 2004 [Page 7] Internet-Draft syslog udp transport May 2004 message MUST have the same MessageId value. 3.2.2 Total Length The TotalLength field MUST be a numeric value in the range of 1 to 16777216. It MUST indicate the length of a complete syslog message in bytes before it was fragmented and before it was encapsulated with transport headers. The same TotalLength field value MUST be present in all UDP datagrams which represent fragments of the same syslog message. Leading zeros MUST not be present in the TotalLength field. Note that the TotalLength field is used to identify the total length of a complete syslog message, which is transmitted using multiple fragments and multiple datagram packets. The fragment length is not specified in the transport header because it can be inferred from the size of the IP packet containing the UDP datagram. 3.2.3 Fragment Offset The FragmentOffset field MUST be a numeric value in the range of 0 to 16777215. It MUST indicate the byte offset of the fragment data in the complete syslog message. The offset index starts at 0 for the first fragment. For example, suppose we want to fragment a 700 byte syslog message into 480 and 220 byte parts. Then, the FragmentOffset in the first message will be 0 and in the second - 480. Note that fragments don't have to be the same size. Leading zeros MUST not be present in the FragmentOffset field. 3.2.4 Extended Header Example The following is an example of a syslog message without the transport header (message is wrapped for display): "v1 888 4 2003-10-11T22:14:15.003Z host.domain.com dns: configuration error" Suppose this message had to be fragmented by transport layer into two parts at an arbitrary point. This would result in two separate UDP datagrams being sent - one for each fragment. Below is the content of each of the syslog transport UDP messages with syslog transport headers but without UDP/IP headers: "v1 1 45612221 74 0 v1 888 4 2003-10-11T22:14:15.003Z host.dom" "v1 1 45612221 74 42 ain.com dns: configuration error" Okmianski Expires October 30, 2004 [Page 8] Internet-Draft syslog udp transport May 2004 In the above example, the leading "v1" is the version of the transport protocol, "1" indicates that this is an extended header (fragmentation in use), "45612221" is the MessageId, "74" is the TotalLength of the message, while "0" and "42" are FragmentOffset fields. Everything following the FragmentOffset and a space is the Payload of each respective message. 3.3 Payload The Payload field of the syslog transport message is an entire syslog message or one fragment. The maximum Payload size depends on the IP protocol used and the type header that is used. Maximum Payload size: With IPv4 and basic header: 507 bytes With IPv4 and extended header: 480 bytes With IPv6 and basic header: 1191 bytes With IPv6 and extended header: 1164 bytes The receiver MUST discard messages with Payload sizes exceeding the above restrictions. The Payload size restrictions above effectively mean that the largest syslog message that can be sent non-fragmented is 507 bytes for transport via IPv4 and 1191 bytes for transport via IPv6. For a discussion of the rational behind the above size restrictions please refer to Appendix A. 3.4 Supported Message Length The maximum syslog message length supported by this protocol is the maximum value of the TotalLength field, which is 16777216 bytes. However, not all deployment scenarios for syslog will be on hosts with hardware capable of supporting this maximum length of messages. Additionally, extremely large messages may not be needed in many environments. Therefore, implementations are NOT REQUIRED to support the maximum message length allowed by this protocol. All implementations MUST support sending and receiving syslog messages up to and including the size which does not require fragmentation (507 bytes for IPv4 and 1191 bytes for IPv6). This size excludes the overhead of the syslog transport and UDP/IP headers. Support for larger messages is encouraged. Implementors SHOULD clearly state the maximum supported message size in documentation. Okmianski Expires October 30, 2004 [Page 9] Internet-Draft syslog udp transport May 2004 The receiver which receives a message greater than it can handle SHOULD discard the message. A diagnostic message MAY be logged by the receiver, but care SHOULD be taken not to expose this behavior as an additional vulnerability for denial of service attack. 4. UDP/IP Layer Considerations 4.1 Target Port Syslog receivers MUST support accepting syslog message datagrams on the well-known UDP port 514. Syslog senders MUST support sending syslog message datagrams to the UDP port 514. 4.2 Source Port Syslog senders can use any source UDP port for transmitting messages. Senders MAY randomly select a source port, but MUST use the port in an exclusive fashion. No concurrent port reuse on the same host is allowed. Since source port is used to identify parts of a fragmented message, the sender MUST use the same port to send all fragments of a given message. If due to an error or other condition, the sender is unable to do that, the sender MAY resend all message fragments and if it does so, it MUST use the new port and a new MessageId field value. 4.3 Source IP Address The source IP address of the UDP datagrams is one of the data elements used to identify parts of a fragmented message. Therefore, a syslog sender MUST attempt to use the same source IP address to send all fragments of a given syslog message. If due to an error, reconfiguration or other condition it is unable to do so, the sender MAY resend all fragments of the syslog message and, if it does so, it MUST use the new source IP address and a new MessageId value. 4.4 UDP/IP Headers Each UDP/IP datagram sent by the transport layer MUST completely adhere to the structure specified in the UDP RFC 768[3] and either IPv4 RFC 791[4] or IPv6 RFC 2640[5] depending on which protocol is used. Use of UDP checksums was defined as optional in RFC 768[3]. IPv6 has subsequently made UDP checksums required [5]. Syslog senders MUST utilize valid UDP checksums when sending messages over IPv6 and SHOULD do it when sending over IPv4. Syslog receivers MUST check the checksums whenever they are present and discard messages with Okmianski Expires October 30, 2004 [Page 10] Internet-Draft syslog udp transport May 2004 incorrect checksums. Note that this is typically accomplished by the UDP layer implementation, and some implementations allow for checksum checks to be enabled or disabled. Enabling use of checksums serves as an extra measure of corruption detection in addition to checksums performed by IP and Layer 2 protocols such as Ethernet. None of the above checksums provide a complete guarantee of corruption detection. Utilizing checksums on multiple layers reduces the chance of a corruption error not being detected. 5. Fragmentation and Reassembly 5.1 Message Fragmentation The syslog transport layer MUST perform fragmentation if the size of a given syslog message exceeds the maximum allowed Payload size. Fragmentation SHOULD NOT be used if message can fit into the maximum allowed Payload size. Syslog messages SHOULD be fragmented such that all but last message utilize the Payload to its maximum capacity. For example, when using IPv4, a 700 byte syslog message SHOULD be fragmented into 480 and 220 byte parts because the maximum Payload size with IPv4 and extended header is 480 bytes. Each message fragment MUST be sent as a separate UDP/IP datagram with an extended syslog transport header. The sender MUST use the same MessageId value, TotalLength value, source port and source IP address for all fragments of a given message. These three field together uniquely identify fragments belonging to a given message. On a system with short-lived sender processes, it may be possible that fragments with the same MessageId value, TotalLength value, source port and source IP address will get generated in short time proximity. This can be possible because a new process may re-use the source port that was freed up by another process that just dies. Such behavior could confuse the receiver if the datagrams were received out of order or some datagrams got lost. In order to reduce the risk of such mistaken identity errors, section 3.2.1 specified that each process must start with a random value for MessageId field. Given a relatively large range of MessageId values and the unlikely event of a coincidence of having the same MessageId and TotalLength values combined with re-used source port and UDP errors, the window for potential mistaken identity errors during message reassembly is very small and tolerable. The users take a greater risk by using this protocol due to general UDP reliability Okmianski Expires October 30, 2004 [Page 11] Internet-Draft syslog udp transport May 2004 issues discussed later in this specification. 5.2 Message Reassembly The reassembly process uses the source IP address from the IP header, the source port from the UDP header, the MessageId and TotalLength field values to identify fragments of a given message. It then uses data from the TotalLength and FragmentOffset fields to re-assemble fragments into a complete message. If one of the fragments of the message is not received, all other fragments of the message SHOULD be discarded. Typically, an implementation of fragmentation reassembly involves allocating a buffer for the message when any fragment with a new combination of source IP address, source port, MessageId and TotalLength values is received. A timer is used to expire the message reassembly and clean the buffer if all fragments are not received within a certain time period. As each fragment is received, it is placed into the buffer at the appropriate offset and a check is performed to determine if all fragments have been received using additional data structures. The receiver SHOULD make the timeout interval used for message reassembly configurable for the administrator. The receiver SHOULD also be able to limit the total amount of memory used for buffers such that it does not run out of resources under a simple denial of service attack involving just one message fragment with a large TotalLength field value. Degrading the service under heavy load or attack is better than crashing and potentially making the service completely unavailable. The receiver MUST validate the FragmentOffset and fragment length against the TotalLength of the message to ensure that the fragment fits into the buffer. This would prevent a typical buffer overflow exploit by attackers. 5.3 Avoiding Fragmentation Fragmentation and reassembly of messages incurs substantial processing overhead on both the sender and the receiver hosts. It also increases the risk of lost messages due to loss of just one fragment. It is RECOMMENDED that syslog senders which anticipate sending messages over this transport protocol attempt to reduce the number of messages which require fragmentation by only sending messages which are small enough not to require fragmentation. Okmianski Expires October 30, 2004 [Page 12] Internet-Draft syslog udp transport May 2004 6. Reliability Considerations The UDP is an unreliable low-overhead protocol. This section discusses reliability issues inherent to UDP that implementers and users MUST be aware of. 6.1 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. The transport protocol fragmentation and IP fragmentation exacerbate the 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 some datagram may have been lost, it MAY retransmit previously sent messages by either retransmitting the datagram(s) or by transmitting the message with a new MessageId value. 6.2 Message Corruption and Checksums 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 utilized in IP and Layer 2 layers. However, checksums do not guarantee corruption detection and this protocol does not provide for message retransmission when a corrupt message is detected. 6.3 Congestion Control The UDP does not provide for congestion control. Some systems (hosts or routers) may generate ICMP source quench error, but they are not required to do so [9]. Any network host can discard UDP packets when it is overloaded. Due to lack of congestion control one or multiple syslog senders can maliciously or inadvertently overload the receiver or the network infrastructure and cause loss of syslog messages. 6.4 Sequenced Delivery The IP transport utilized 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 some guide in establishing sequence order, but it will not help in cases when multiple messages were generated during the same time slot or when messages originated from different Okmianski Expires October 30, 2004 [Page 13] Internet-Draft syslog udp transport May 2004 hosts whose clocks are not synchronized. The order of syslog message arrival via the this syslog transport SHOULD NOT be used as an authoritative guide in establishing the sequence of events on the syslog sender hosts. 6.5 Sender Authentication The UDP syslog transport does not strongly associate the message with the message sender. The receiver of the syslog message 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. One possible consequence of this behavior is that a misconfigured machine may send syslog messages to a receiver representing itself as another machine. The administrators may not be able to readily discern that there are two or more machines representing themselves as the same machine. 7. Security Considerations Several syslog security considerations have been discussed in RFC-protocol[2] and in the original RFC 3164[1]. This section focuses on security considerations specific to the syslog transport over UDP. 7.1 Message Authenticity This transport protocol does not strongly authenticate the identity of the message sender and does not provide any assurance that the message was not modified in transit. The receiver of the syslog message 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. 7.2 Message Forgery Syslog messages can be easily forged. 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. Okmianski Expires October 30, 2004 [Page 14] Internet-Draft syslog udp transport May 2004 Additionally, an attacker may generate false syslog messages to give untrue indications of 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 been restarted. 7.3 Message Observation The 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 those messages to compromise a machine or do other damage. 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. 7.4 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, that attacker may remove that machine from the network and replay the syslog messages to the collector 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. 7.5 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. 7.6 Message Prioritization and Differentiation The transport protocol described in this document does not require 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 Okmianski Expires October 30, 2004 [Page 15] Internet-Draft syslog udp transport May 2004 potentially high severity messages. High severity messages may contain an indication of serious security problems, but they will not get a higher priority. It is difficult to make sure that high severity messages get higher end-to-end delivery priority, especially over an unreliable UDP transport which provides no congestion control. 7.7 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 only receiving syslog messages from known IP addresses. 8. IANA Considerations IANA must reserve UDP port 514 for this transport. 9. 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. 10. 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 11. 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 and all others who have commented on the various versions of this proposal. Okmianski Expires October 30, 2004 [Page 16] Internet-Draft syslog udp transport May 2004 12. References 12.1 Normative References [1] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001. [2] Gerhards, R., "The syslog Protocol", RFC RFC-protocol. [3] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August 1980. [4] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [5] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [7] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [8] Braden, R., "Requirements for Internet Hosts - Communication Layers", STD 3, RFC 1122, October 1989. 12.2 Informative References [9] Stevens, W., "TCP/IP Illustrated Volume 1. The Protocols.", January 1994. [10] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [11] Hedrick, C., "Routing Information Protocol", RFC 1058, June 1988. [12] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [13] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350, July 1992. [14] Kent, C. and J. Mogul, ""Fragmentation Considered Harmful," Computer Communications Review, vol.17, no.5, pp.390-401", August 1987. [15] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and Okmianski Expires October 30, 2004 [Page 17] Internet-Draft syslog udp transport May 2004 IPv6 Headers", RFC 2474, December 1998. 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 Appendix A. Rational For Transport Message Size Restrictions This appendix provides the rational behind the Payload size restrictions for this protocol. The Payload restrictions outlined in the specification, ensure that the transport message size does not exceed 512 bytes (without UDP/IP headers) for transport via IPv4 and does not exceed 1196 bytes for transport via IPv6. These restrictions put an upper boundary on the UDP/IP datagram size for this protocol, which accomplishes two goals. First, they insure interoperability between various UDP/IP implementations. Even though the maximum IP datagram size is specified as 65536 bytes, many UDP/IP implementations have been shown not to work with large datagram sizes [9]. Many established UDP-based protocols restrict UDP datagram data size to 512 bytes. For example, DNS [10] and RIP [11] do that. The DHCPv4 [12] restricts the size to 512 bytes, but allows sides to agree on a larger value through the protocol. The TFTP [13] restricts the UDP data size to 518 bytes, which is slightly larger. The second reason for datagram size restrictions is that it reduces the likelihood of the IP-layer datagram fragmentation. Syslog message can be fragmented on two levels: syslog transport protocol and IP layer. Since fragmentation has significant overhead for message reassembly, it is best to avoid double fragmentation. The likelihood of IP fragmentation can be significantly reduced by sending IP datagrams in sizes which all hosts must be able to process. The minimum MTU of a transport protocol determines the minimum size of packets that hosts must be able to accept. For IPv4, the minimum MTU is 576 bytes [4] and for IPv6 - 1280 bytes [5]. In both cases, the maximum message size fits within the MTU of the transport in all cases except for when extremely large IP headers are used. IPv4 Okmianski Expires October 30, 2004 [Page 18] Internet-Draft syslog udp transport May 2004 header can range from 20 to 60 bytes in length and UDP header is fixed at 8 bytes. Thus, the message size restrictions ensure that in all cases except for when the IP header is 56 bytes or greater, the size of the packet will be within the size of the transport MTU. For IPv6, the specification provides for the same amount of padding for UDP/IP headers as was conventionally done for IPv4 in DNS, RIP and DHCPv4 with an additional padding of extra 20 bytes to accommodate a larger IPv6 header. This follows the methodology suggested in the IPv6 specification for calculating upper-layer payload limits [5]. Path MTU discovery can generally be used to discover the MTU of the link. Unfortunately, using path MTU discovery with UDP is not a reliable option because it depends on routers providing ICMP errors and hosts doing retransmission, which are not done consistently. Implementors MUST follow the size restrictions outlined above and not rely on path MTU discovery. 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