RFC 7126
Recommendations on Filtering of IPv4 Packets Containing IPv4 Options
Internet Engineering Task Force (IETF) F. Gont Request for Comments: 7126 UTN-FRH / SI6 Networks BCP: 186 R. Atkinson Category: Best Current Practice Consultant ISSN: 2070-1721 C. Pignataro Cisco February 2014 Recommendations on Filtering of IPv4 Packets Containing IPv4 OptionsAbstract
This document provides advice on the filtering of IPv4 packets based on the IPv4 options they contain. Additionally, it discusses the operational and interoperability implications of dropping packets based on the IP options they contain. Status of This Memo This memo documents an Internet Best Current Practice. This document is a product of the Internet Engineering Task Force (IETF). It has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7126. Copyright Notice Copyright (c) 2014 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology and Conventions Used in This Document . . . . 3 1.2. Operational Focus . . . . . . . . . . . . . . . . . . . . 4 2. IP Options . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. General Security Implications of IP Options . . . . . . . . . 5 3.1. Processing Requirements . . . . . . . . . . . . . . . . . 5 4. Advice on the Handling of Packets with Specific IP Options . 7 4.1. End of Option List (Type = 0) . . . . . . . . . . . . . . 7 4.2. No Operation (Type = 1) . . . . . . . . . . . . . . . . . 7 4.3. Loose Source and Record Route (LSRR) (Type = 131) . . . . 8 4.4. Strict Source and Record Route (SSRR) (Type = 137) . . . 10 4.5. Record Route (Type = 7) . . . . . . . . . . . . . . . . . 11 4.6. Stream Identifier (Type = 136) (obsolete) . . . . . . . . 12 4.7. Internet Timestamp (Type = 68) . . . . . . . . . . . . . 13 4.8. Router Alert (Type = 148) . . . . . . . . . . . . . . . . 14 4.9. Probe MTU (Type = 11) (obsolete) . . . . . . . . . . . . 15 4.10. Reply MTU (Type = 12) (obsolete) . . . . . . . . . . . . 16 4.11. Traceroute (Type = 82) . . . . . . . . . . . . . . . . . 16 4.12. DoD Basic Security Option (Type = 130) . . . . . . . . . 17 4.13. DoD Extended Security Option (Type = 133) . . . . . . . . 20 4.14. Commercial IP Security Option (CIPSO) (Type = 134) . . . 22 4.15. VISA (Type = 142) . . . . . . . . . . . . . . . . . . . . 23 4.16. Extended Internet Protocol (Type = 145) . . . . . . . . . 24 4.17. Address Extension (Type = 147) . . . . . . . . . . . . . 25 4.18. Sender Directed Multi-Destination Delivery (Type = 149) . 25 4.19. Dynamic Packet State (Type = 151) . . . . . . . . . . . . 26 4.20. Upstream Multicast Pkt. (Type = 152) . . . . . . . . . . 26 4.21. Quick-Start (Type = 25) . . . . . . . . . . . . . . . . . 27 4.22. RFC3692-Style Experiment (Types = 30, 94, 158, and 222) . 28 4.23. Other IP Options . . . . . . . . . . . . . . . . . . . . 29 5. Security Considerations . . . . . . . . . . . . . . . . . . . 31 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 31 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 31 7.1. Normative References . . . . . . . . . . . . . . . . . . 31 7.2. Informative References . . . . . . . . . . . . . . . . . 32
1. Introduction
This document discusses the filtering of IPv4 packets based on the IPv4 options they contain. Since various protocols may use IPv4 options to some extent, dropping packets based on the options they contain may have implications on the proper functioning of such protocols. Therefore, this document attempts to discuss the operational and interoperability implications of such dropping. Additionally, it outlines what a network operator might do in typical enterprise or Service Provider environments. This document also draws and is partly derived from [RFC6274], which also received review from the operational community. We note that data seems to indicate that there is a current widespread practice of blocking IPv4 optioned packets. There are various plausible approaches to minimize the potential negative effects of IPv4 optioned packets while allowing some option semantics. One approach is to allow for specific options that are expected or needed, and have a default deny. A different approach is to deny unneeded options and have a default allow. Yet a third possible approach is to allow for end-to-end semantics by ignoring options and treating packets as un-optioned while in transit. Experiments and currently available data tend to support the first or third approaches as more realistic. Some results regarding the current state of affairs with respect to dropping packets containing IP options can be found in [MEDINA] and [FONSECA]. Additionally, [BREMIER-BARR] points out that the deployed Internet already has many routers that do not process IP options. We also note that while this document provides advice on dropping packets on a "per IP option type", not all devices (routers, security gateways, and firewalls) may provide this capability with such granularity. Additionally, even in cases in which such functionality is provided, an operator might want to specify a dropping policy with a coarser granularity (rather than on a "per IP option type" granularity), as indicated above. Finally, in scenarios in which processing of IP options by intermediate systems is not required, a widespread approach is to simply ignore IP options and process the corresponding packets as if they do not contain any IP options.1.1. Terminology and Conventions Used in This Document
The terms "fast path", "slow path", and associated relative terms ("faster path" and "slower path") are loosely defined as in Section 2 of [RFC6398].
Because of the security-oriented nature of this document, we are deliberately including some historical citations. The goal is to explicitly retain and show history, as well as remove ambiguity and confusion. 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 [RFC2119].1.2. Operational Focus
All of the recommendations in this document have been made in an effort to optimize for operational community consensus, as best the authors have been able to determine that. This has included not only accepting feedback from public lists, but also accepting off-list feedback from people at various network operators (e.g. Internet Service Providers, content providers, educational institutions, commercial firms).2. IP Options
IP options allow for the extension of the Internet Protocol. As specified in [RFC0791], there are two cases for the format of an option: o Case 1: A single byte of option-type. o Case 2: An option-type byte, an option-length byte, and the actual option-data bytes. IP options of Case 1 have the following syntax: +-+-+-+-+-+-+-+-+- - - - - - - - - | option-type | option-data +-+-+-+-+-+-+-+-+- - - - - - - - - The length of IP options of Case 1 is implicitly specified by the option-type byte. IP options of Case 2 have the following syntax: +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - | option-type | option-length | option-data +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - In this case, the option-length byte counts the option-type byte and the option-length byte, as well as the actual option-data bytes.
All current and future options, except "End of Option List" (Type = 0) and "No Operation" (Type = 1), are of Class 2. The option-type has three fields: o 1 bit: copied flag. o 2 bits: option class. o 5 bits: option number. The copied flag indicates whether this option should be copied to all fragments in the event the packet carrying it needs to be fragmented: o 0 = not copied. o 1 = copied. The values for the option class are: o 0 = control. o 1 = reserved for future use. o 2 = debugging and measurement. o 3 = reserved for future use. This format allows for the creation of new options for the extension of the Internet Protocol (IP). Finally, the option number identifies the syntax of the rest of the option. The "IP OPTION NUMBERS" registry [IANA-IP] contains the list of the currently assigned IP option numbers.3. General Security Implications of IP Options
3.1. Processing Requirements
Historically, most IP routers used a general-purpose CPU to process IP packets and forward them towards their destinations. This same CPU usually also processed network management traffic (e.g., SNMP), configuration commands (e.g., command line interface), and various routing protocols (e.g., RIP, OSPF, BGP, IS-IS) or other control protocols (e.g., RSVP, ICMP). In such architectures, it has been common for the general-purpose CPU also to perform any packet
filtering that has been enabled on the router (or router interface).
An IP router built using this architecture often has a significant
Distributed Denial-of-Service (DDoS) attack risk if the router
control plane (e.g., CPU) is overwhelmed by a large number of IPv4
packets that contain IPv4 options.
From about 1995 onwards, a growing number of IP routers have
incorporated silicon specialized for IP packet processing (i.e.,
Field-Programmable Gate Array (FPGA), Application-Specific Integrated
Circuit (ASIC)), thereby separating the function of IP packet
forwarding from the other functions of the router. Such router
architectures tend to be more resilient to DDoS attacks that might be
seen in the global public Internet. Depending upon various
implementation and configuration details, routers with a silicon
packet-forwarding engine can handle high volumes of IP packets
containing IP options without any adverse impact on packet-forwarding
rates or on the router's control plane (e.g., general-purpose CPU).
Some implementations have a configuration knob simply to forward all
IP packets containing IP options at wire-speed in silicon, as if the
IP packet did not contain any IP options ("ignore options &
forward"). Other implementations support wire-speed silicon-based
packet filtering, thereby enabling packets containing certain IP
options to be selectively dropped ("drop"), packets containing
certain other IP options to have those IP options ignored ("ignore
options & forward"), and other packets containing different IP
options to have those options processed, either on a general-purpose
CPU or using custom logic (e.g., FPGA, ASIC), while the packet is
being forwarded ("process option & forward").
Broadly speaking, any IP packet that requires processing by an IP
router's general-purpose CPU can be a DDoS risk to that router's
general-purpose CPU (and thus to the router itself). However, at
present, the particular architectural and engineering details of the
specific IP router being considered are important to understand when
evaluating the operational security risks associated with a
particular IP packet type or IP option type.
Operators are urged to consider the capabilities of potential IP
routers for IP option filtering and handling as they make deployment
decisions in the future.
Additional considerations for protecting the control plane from
packets containing IP options can be found in [RFC6192].
Finally, in addition to advice to operators, this document also
provides advice to router, security gateway, and firewall
implementers in terms of providing the capability to filter packets
with different granularities: both on a "per IP option type" granularity (to maximize flexibility) as well as more coarse filters (to minimize configuration complexity).4. Advice on the Handling of Packets with Specific IP Options
The following subsections contain a description of each of the IP options that have so far been specified, a discussion of possible interoperability implications if packets containing such options are dropped, and specific advice on whether to drop packets containing these options in a typical enterprise or Service Provider environment.4.1. End of Option List (Type = 0)
4.1.1. Uses
This option is used to indicate the "end of options" in those cases in which the end of options would not coincide with the end of the Internet Protocol header.4.1.2. Option Specification
Specified in RFC 791 [RFC0791].4.1.3. Threats
No specific security issues are known for this IPv4 option.4.1.4. Operational and Interoperability Impact if Blocked
Packets containing any IP options are likely to include an End of Option List. Therefore, if packets containing this option are dropped, it is very likely that legitimate traffic is blocked.4.1.5. Advice
Routers, security gateways, and firewalls SHOULD NOT drop packets because they contain this option.4.2. No Operation (Type = 1)
4.2.1. Uses
The no-operation option is basically meant to allow the sending system to align subsequent options in, for example, 32-bit boundaries.
4.2.2. Option Specification
Specified in RFC 791 [RFC0791].4.2.3. Threats
No specific security issues are known for this IPv4 option.4.2.4. Operational and Interoperability Impact if Blocked
Packets containing any IP options are likely to include a No Operation option. Therefore, if packets containing this option are dropped, it is very likely that legitimate traffic is blocked.4.2.5. Advice
Routers, security gateways, and firewalls SHOULD NOT drop packets because they contain this option.4.3. Loose Source and Record Route (LSRR) (Type = 131)
RFC 791 states that this option should appear at most once in a given packet. Thus, if a packet contains more than one LSRR option, it should be dropped, and this event should be logged (e.g., a counter could be incremented to reflect the packet drop). Additionally, packets containing a combination of LSRR and SSRR options should be dropped, and this event should be logged (e.g., a counter could be incremented to reflect the packet drop).4.3.1. Uses
This option lets the originating system specify a number of intermediate systems a packet must pass through to get to the destination host. Additionally, the route followed by the packet is recorded in the option. The receiving host (end-system) must use the reverse of the path contained in the received LSRR option. The LSSR option can be of help in debugging some network problems. Some Internet Service Provider (ISP) peering agreements require support for this option in the routers within the peer of the ISP.4.3.2. Option Specification
Specified in RFC 791 [RFC0791].
4.3.3. Threats
The LSRR option has well-known security implications [RFC6274]. Among other things, the option can be used to: o Bypass firewall rules. o Reach otherwise unreachable internet systems. o Establish TCP connections in a stealthy way. o Learn about the topology of a network. o Perform bandwidth-exhaustion attacks. Of these attack vectors, the one that has probably received least attention is the use of the LSRR option to perform bandwidth exhaustion attacks. The LSRR option can be used as an amplification method for performing bandwidth-exhaustion attacks, as an attacker could make a packet bounce multiple times between a number of systems by carefully crafting an LSRR option. This is the IPv4 version of the IPv6 amplification attack that was widely publicized in 2007 [Biondi2007]. The only difference is that the maximum length of the IPv4 header (and hence the LSRR option) limits the amplification factor when compared to the IPv6 counterpart. Additionally, some implementations have been found to fail to include proper sanity checks on the LSRR option, thus leading to security issues. These specific issues are believed to be solved in all modern implementations. [Microsoft1999] is a security advisory about a vulnerability arising from improper validation of the Pointer field of the LSRR option. Finally, we note that some systems were known for providing a system- wide toggle to enable support for this option for those scenarios in which this option is required. However, improper implementation of such a system-wide toggle caused those systems to support the LSRR option even when explicitly configured not to do so. [OpenBSD1998] is a security advisory about an improper implementation of such a system-wide toggle in 4.4BSD kernels. This issue was resolved in later versions of the corresponding operating system.
4.3.4. Operational and Interoperability Impact if Blocked
Network troubleshooting techniques that may employ the LSRR option (such as ping or traceroute with the appropriate arguments) would break when using the LSRR option. (Ping and traceroute without IPv4 options are not impacted.) Nevertheless, it should be noted that it is virtually impossible to use the LSRR option for troubleshooting, due to widespread dropping of packets that contain the option.4.3.5. Advice
Routers, security gateways, and firewalls SHOULD implement an option- specific configuration knob to select whether packets with this option are dropped, packets with this IP option are forwarded as if they did not contain this IP option, or packets with this option are processed and forwarded as per [RFC0791]. The default setting for this knob SHOULD be "drop", and the default setting MUST be documented. Please note that treating packets with LSRR as if they did not contain this option can result in such packets being sent to a different device than the initially intended destination. With appropriate ingress filtering, this should not open an attack vector into the infrastructure. Nonetheless, it could result in traffic that would never reach the initially intended destination. Dropping these packets prevents unnecessary network traffic and does not make end-to-end communication any worse.4.4. Strict Source and Record Route (SSRR) (Type = 137)
4.4.1. Uses
This option allows the originating system to specify a number of intermediate systems a packet must pass through to get to the destination host. Additionally, the route followed by the packet is recorded in the option, and the destination host (end-system) must use the reverse of the path contained in the received SSRR option. This option is similar to the Loose Source and Record Route (LSRR) option, with the only difference that in the case of SSRR, the route specified in the option is the exact route the packet must take (i.e., no other intervening routers are allowed to be in the route). The SSRR option can be of help in debugging some network problems. Some ISP peering agreements require support for this option in the routers within the peer of the ISP.
4.4.2. Option Specification
Specified in RFC 791 [RFC0791].4.4.3. Threats
The SSRR option has the same security implications as the LSRR option. Please refer to Section 4.3 for a discussion of such security implications.4.4.4. Operational and Interoperability Impact if Blocked
Network troubleshooting techniques that may employ the SSRR option (such as ping or traceroute with the appropriate arguments) would break when using the SSRR option. (Ping and traceroute without IPv4 options are not impacted.) Nevertheless, it should be noted that it is virtually impossible to use the SSRR option for trouble-shooting, due to widespread dropping of packets that contain such option.4.4.5. Advice
Routers, security gateways, and firewalls SHOULD implement an option- specific configuration knob to select whether packets with this option are dropped, packets with this IP option are forwarded as if they did not contain this IP option, or packets with this option are processed and forwarded as per [RFC0791]. The default setting for this knob SHOULD be "drop", and the default setting MUST be documented. Please note that treating packets with SSRR as if they did not contain this option can result in such packets being sent to a different device that the initially intended destination. With appropriate ingress filtering this should not open an attack vector into the infrastructure. Nonetheless, it could result in traffic that would never reach the initially intended destination. Dropping these packets prevents unnecessary network traffic, and does not make end-to-end communication any worse.4.5. Record Route (Type = 7)
4.5.1. Uses
This option provides a means to record the route that a given packet follows.4.5.2. Option Specification
Specified in RFC 791 [RFC0791].
4.5.3. Threats
This option can be exploited to map the topology of a network. However, the limited space in the IP header limits the usefulness of this option for that purpose.4.5.4. Operational and Interoperability Impact if Blocked
Network troubleshooting techniques that may employ the RR option (such as ping with the RR option) would break when using the RR option. (Ping without IPv4 options is not impacted.) Nevertheless, it should be noted that it is virtually impossible to use such techniques due to widespread dropping of packets that contain RR options.4.5.5. Advice
Routers, security gateways, and firewalls SHOULD implement an option- specific configuration knob to select whether packets with this option are dropped, packets with this IP option are forwarded as if they did not contain this IP option, or packets with this option are processed and forwarded as per [RFC0791]. The default setting for this knob SHOULD be "drop", and the default setting MUST be documented.4.6. Stream Identifier (Type = 136) (obsolete)
The Stream Identifier option originally provided a means for the 16-bit SATNET stream Identifier to be carried through networks that did not support the stream concept. However, as stated by Section 3.2.1.8 of RFC 1122 [RFC1122] and Section 4.2.2.1 of RFC 1812 [RFC1812], this option is obsolete. Therefore, it must be ignored by the processing systems. See also [IANA-IP] and [RFC6814]. RFC 791 states that this option appears at most once in a given datagram. Therefore, if a packet contains more than one instance of this option, it should be dropped, and this event should be logged (e.g., a counter could be incremented to reflect the packet drop).4.6.1. Uses
This option is obsolete. There is no current use for this option.
4.6.2. Option Specification
Specified in RFC 791 [RFC0791], and deprecated in RFC 1122 [RFC1122] and RFC 1812 [RFC1812]. This option has been formally obsoleted by [RFC6814].4.6.3. Threats
No specific security issues are known for this IPv4 option.4.6.4. Operational and Interoperability Impact if Blocked
None.4.6.5. Advice
Routers, security gateways, and firewalls SHOULD drop IP packets containing a Stream Identifier option.4.7. Internet Timestamp (Type = 68)
4.7.1. Uses
This option provides a means for recording the time at which each system (or a specified set of systems) processed this datagram, and it may optionally record the addresses of the systems providing the timestamps.4.7.2. Option Specification
Specified by RFC 791 [RFC0791].4.7.3. Threats
The timestamp option has a number of security implications [RFC6274]. Among them are: o It allows an attacker to obtain the current time of the systems that process the packet, which the attacker may find useful in a number of scenarios. o It may be used to map the network topology in a similar way to the IP Record Route option. o It may be used to fingerprint the operating system in use by a system processing the datagram.
o It may be used to fingerprint physical devices by analyzing the
clock skew.
[Kohno2005] describes a technique for fingerprinting devices by
measuring the clock skew. It exploits, among other things, the
timestamps that can be obtained by means of the ICMP timestamp
request messages [RFC0791]. However, the same fingerprinting method
could be implemented with the aid of the Internet Timestamp option.
4.7.4. Operational and Interoperability Impact if Blocked
Network troubleshooting techniques that may employ the Internet
Timestamp option (such as ping with the Timestamp option) would break
when using the Timestamp option. (Ping without IPv4 options is not
impacted.) Nevertheless, it should be noted that it is virtually
impossible to use such techniques due to widespread dropping of
packets that contain Internet Timestamp options.
4.7.5. Advice
Routers, security gateways, and firewalls SHOULD drop IP packets
containing an Internet Timestamp option.
4.8. Router Alert (Type = 148)
4.8.1. Uses
The Router Alert option has the semantic "routers should examine this
packet more closely, if they participate in the functionality denoted
by the Value of the option".
4.8.2. Option Specification
The Router Alert option is defined in RFC 2113 [RFC2113] and later
updates to it have been clarified by RFC 5350 [RFC5350]. It contains
a 16-bit Value governed by an IANA registry (see [RFC5350]).
4.8.3. Threats
The security implications of the Router Alert option have been
discussed in detail in [RFC6398]. Basically, the Router Alert option
might be exploited to perform a DoS attack by exhausting CPU
resources at the processing routers.
4.8.4. Operational and Interoperability Impact if Blocked
Applications that employ the Router Alert option (such as RSVP
[RFC2205]) would break.
4.8.5. Advice
This option SHOULD be allowed only in controlled environments, where the option can be used safely. [RFC6398] identifies some such environments. In unsafe environments, packets containing this option SHOULD be dropped. A given router, security gateway, or firewall system has no way of knowing a priori whether this option is valid in its operational environment. Therefore, routers, security gateways, and firewalls SHOULD, by default, ignore the Router Alert option. Additionally, routers, security gateways, and firewalls SHOULD have a configuration setting that governs their reaction in the presence of packets containing the Router Alert option. This configuration setting SHOULD allow to honor and process the option, ignore the option, or drop packets containing this option.4.9. Probe MTU (Type = 11) (obsolete)
4.9.1. Uses
This option originally provided a mechanism to discover the Path-MTU. It has been declared obsolete.4.9.2. Option Specification
This option was originally defined in RFC 1063 [RFC1063] and was obsoleted with RFC 1191 [RFC1191]. This option is now obsolete, as RFC 1191 obsoletes RFC 1063 without using IP options.4.9.3. Threats
This option is obsolete. This option could have been exploited to cause a host to set its Path MTU (PMTU) estimate to an inordinately low or an inordinately high value, thereby causing performance problems.4.9.4. Operational and Interoperability Impact if Blocked
None This option is NOT employed with the modern "Path MTU Discovery" (PMTUD) mechanism [RFC1191], which employs special ICMP messages (Type 3, Code 4) in combination with the IP DF bit. Packetization Layer PMTUD (PLPMTUD) [RFC4821] can perform PMTUD without the need for any special packets.
4.9.5. Advice
Routers, security gateways, and firewalls SHOULD drop IP packets that contain a Probe MTU option.4.10. Reply MTU (Type = 12) (obsolete)
4.10.1. Uses
This option originally provided a mechanism to discover the Path-MTU. It is now obsolete.4.10.2. Option Specification
This option was originally defined in RFC 1063 [RFC1063] and was obsoleted with RFC 1191 [RFC1191]. This option is now obsolete, as RFC 1191 obsoletes RFC 1063 without using IP options.4.10.3. Threats
This option is obsolete. This option could have been exploited to cause a host to set its PMTU estimate to an inordinately low or an inordinately high value, thereby causing performance problems.4.10.4. Operational and Interoperability Impact if Blocked
None This option is NOT employed with the modern "Path MTU Discovery" (PMTUD) mechanism [RFC1191], which employs special ICMP messages (Type 3, Code 4) in combination with the IP DF bit. PLPMTUD [RFC4821] can perform PMTUD without the need of any special packets.4.10.5. Advice
Routers, security gateways, and firewalls SHOULD drop IP packets that contain a Reply MTU option.4.11. Traceroute (Type = 82)
4.11.1. Uses
This option originally provided a mechanism to trace the path to a host.
4.11.2. Option Specification
This option was originally specified by RFC 1393 [RFC1393] as "experimental", and it was never widely deployed on the public Internet. This option has been formally obsoleted by [RFC6814].4.11.3. Threats
This option is obsolete. Because this option required each router in the path both to provide special processing and to send an ICMP message, it could have been exploited to perform a DoS attack by exhausting CPU resources at the processing routers.4.11.4. Operational and Interoperability Impact if Blocked
None4.11.5. Advice
Routers, security gateways, and firewalls SHOULD drop IP packets that contain a Traceroute option.4.12. DoD Basic Security Option (Type = 130)
4.12.1. Uses
This option [RFC1108] is used by Multi-Level Secure (MLS) end-systems and intermediate systems in specific environments to: o transmit from source to destination in a network standard representation the common security labels required by computer security models [Landwehr81], o validate the datagram as appropriate for transmission from the source and delivery to the destination, and, o ensure that the route taken by the datagram is protected to the level required by all protection authorities indicated on the datagram. The DoD Basic Security Option (BSO) was implemented in IRIX [IRIX2008] and is currently implemented in a number of operating systems (e.g., Security-Enhanced Linux [SELinux2008], Solaris [Solaris2008], and Cisco IOS [Cisco-IPSO]). It is also currently deployed in a number of high-security networks. These networks are typically either in physically secure locations, protected by military/governmental communications security equipment, or both.
Such networks are typically built using commercial off-the-shelf (COTS) IP routers and Ethernet switches, but they are not normally interconnected with the global public Internet. MLS systems are much more widely deployed now than they were at the time the then-IESG decided to remove IPSO (IP Security Options) from the IETF Standards Track. Since nearly all MLS systems also support IPSO BSO and IPSO ESO, this option is believed to have more deployment now than when the IESG removed this option from the IETF Standards Track. [RFC5570] describes a similar option recently defined for IPv6 and has much more detailed explanations of how sensitivity label options are used in real-world deployments.4.12.2. Option Specification
It is specified by RFC 1108 [RFC1108], which obsoleted RFC 1038 [RFC1038] (which in turn obsoleted the Security Option defined in RFC 791 [RFC0791]). RFC 791 [RFC0791] defined the "Security Option" (Type = 130), which used the same option type as the DoD Basic Security option discussed in this section. Later, RFC 1038 [RFC1038] revised the IP security options, and in turn was obsoleted by RFC 1108 [RFC1108]. The "Security Option" specified in RFC 791 is considered obsolete by Section 3.2.1.8 of RFC 1122 [RFC1122] and Section 4.2.2.1 of RFC 1812 [RFC1812], and therefore the discussion in this section is focused on the DoD Basic Security option specified by RFC 1108 [RFC1108]. Section 4.2.2.1 of RFC 1812 states that routers "SHOULD implement [this option]". Some private IP networks consider IP router-based per-interface selective filtering of packets based on (a) the presence of an IPSO option (including BSO and ESO) and (b) the contents of that IPSO option to be important for operational security reasons. The recent IPv6 Common Architecture Label IPv6 Security Option (CALIPSO) specification discusses this in additional detail, albeit in an IPv6 context [RFC5570]. Such private IP networks commonly are built using both commercial and open-source products -- for hosts, guards, firewalls, switches, routers, etc. Some commercial IP routers support this option, as do some IP routers that are built on top of MLS operating systems (e.g., on top of Trusted Solaris [Solaris2008] or Security-Enhanced Linux [SELinux2008]).
For example, many Cisco routers that run Cisco IOS include support
for selectively filtering packets that contain the IP Security
Options (IPSO) with per-interface granularity. This capability
has been present in many Cisco routers since the early 1990s
[Cisco-IPSO-Cmds]. Some government-sector products reportedly
also support the IP Security Options (IPSO), for example, CANEWARE
[RFC4949].
Support for the IPSO Basic Security Option also is included in the
"IPsec Configuration Policy Information Model" [RFC3585] and in
the "IPsec Security Policy Database Configuration MIB" [RFC4807].
Section 4.6.1 of the IP Security Domain of Interpretation
[RFC2407] includes support for labeled IPsec security associations
compatible with the IP Security Options. (Note: RFC 2407 was
obsoleted by [RFC4306], which was obsoleted by [RFC5996].)
4.12.3. Threats
Presence of this option in a packet does not by itself create any
specific new threat. Packets with this option ought not normally be
seen on the global public Internet.
4.12.4. Operational and Interoperability Impact if Blocked
If packets with this option are blocked or if the option is stripped
from the packet during transmission from source to destination, then
the packet itself is likely to be dropped by the receiver because it
is not properly labeled. In some cases, the receiver might receive
the packet but associate an incorrect sensitivity label with the
received data from the packet whose BSO was stripped by an
intermediate router or firewall. Associating an incorrect
sensitivity label can cause the received information either to be
handled as more sensitive than it really is ("upgrading") or as less
sensitive than it really is ("downgrading"), either of which is
problematic.
4.12.5. Advice
A given IP router, security gateway, or firewall has no way to know a
priori what environment it has been deployed into. Even closed IP
deployments generally use exactly the same commercial routers,
security gateways, and firewalls that are used in the public
Internet.
Since operational problems result in environments where this option is needed if either the option is dropped or IP packets containing this option are dropped, but no harm results if the option is carried in environments where it is not needed, the default configuration SHOULD NOT (a) modify or remove this IP option or (b) drop an IP packet because the IP packet contains this option. A given IP router, security gateway, or firewall MAY be configured to drop this option or to drop IP packets containing this option in an environment known to not use this option. For auditing reasons, routers, security gateways, and firewalls SHOULD be capable of logging the numbers of packets containing the BSO on a per-interface basis. Also, routers, security gateways, and firewalls SHOULD be capable of dropping packets based on the BSO presence as well as the BSO values.
(page 20 continued on part 2)
