IPv6 Operations E. Davies Internet-Draft Consultant Expires: January 12, 2006 J. Mohacsi NIIF/HUNGARNET July 11, 2005 Best Current Practice for Filtering ICMPv6 Messages in Firewalls draft-davies-v6ops-icmpv6-filtering-bcp-00.txt Status of this Memo 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 becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 January 12, 2006. Copyright Notice Copyright (C) The Internet Society (2005). Abstract In networks supporting IPv6 the Internet Control Message Protocol version 6 (ICMPv6) plays a fundamental role with a large number of functions, and a correspondingly large number of message types and options. A number of security risks are associated with uncontrolled forwarding of ICMPv6 messages, and it is desirable to configure site firewalls to intercept inappropriate usages of ICMPv6 which might allow an attacker outside a site to probe or compromise the site Davies & Mohacsi Expires January 12, 2006 [Page 1] Internet-Draft ICMPv6 Filtering BCP July 2005 network. On the other hand, compared with IPv4 and the corresponding protocol ICMP, ICMPv6 is essential to the functioning of IPv6 rather than a useful auxiliary. Hence too aggressive filtering of ICMPv6 messages can be detrimental to the establishment of IPv6 communications. This means that effective filtering of ICMPv6 requires a more complex configuration than was needed for ICMP. This document provides some recommendations for ICMPv6 firewall filter configuration that will allow propagation of ICMPv6 messages that are needed to maintain the functioning of the network but drop messages which are potential security risks. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Classifying ICMPv6 Messages . . . . . . . . . . . . . . . . 5 2.1 Error and Informational ICMPv6 Messages . . . . . . . . . 5 2.2 Addressing of ICMPv6 . . . . . . . . . . . . . . . . . . . 6 2.3 Network Topology and Address Scopes . . . . . . . . . . . 6 2.4 Role in Establishing Communication . . . . . . . . . . . . 6 3. Security Concerns for ICMPv6 Controllable by Firewall Configuration . . . . . . . . . . . . . . . . . . . . . . . 7 3.1 Denial of Service Attacks . . . . . . . . . . . . . . . . 7 3.2 Probing . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.3 Redirection Attacks . . . . . . . . . . . . . . . . . . . 8 3.4 Renumbering Attacks . . . . . . . . . . . . . . . . . . . 8 4. Filtering Recommendations . . . . . . . . . . . . . . . . . 8 4.1 Common Considerations . . . . . . . . . . . . . . . . . . 8 4.2 ICMPv6 Echo Request and Echo Response . . . . . . . . . . 9 4.3 Destination Unreachable Error Message . . . . . . . . . . 10 4.4 Packet Too Big Error Message . . . . . . . . . . . . . . . 10 4.5 Time Exceeded Error Message . . . . . . . . . . . . . . . 11 4.6 Parameter Problem Error Message . . . . . . . . . . . . . 12 4.7 Neighbor Solicitation and Neighbor Advertisement Messages . . . . . . . . . . . . . . . . . . . . . . . . . 13 4.8 Router Solicitation and Router Advertisement Messages . . 14 4.9 Redirect Messages . . . . . . . . . . . . . . . . . . . . 14 4.10 Multicast Listener Discovery Messages . . . . . . . . . 14 4.11 Router Renumbering Messages . . . . . . . . . . . . . . 15 4.12 Node Information Query and Reply . . . . . . . . . . . . 15 4.13 Mobile IPv6 Messages . . . . . . . . . . . . . . . . . . 16 4.14 Unused and Experimental Messages . . . . . . . . . . . . 16 4.15 Problems Resulting from ICMPv6 Transparency . . . . . . 17 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 17 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17 7. Security Considerations . . . . . . . . . . . . . . . . . . 17 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17 8.1 Normative References . . . . . . . . . . . . . . . . . . . 17 8.2 Informative References . . . . . . . . . . . . . . . . . . 18 Davies & Mohacsi Expires January 12, 2006 [Page 2] Internet-Draft ICMPv6 Filtering BCP July 2005 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 19 Intellectual Property and Copyright Statements . . . . . . . 20 Davies & Mohacsi Expires January 12, 2006 [Page 3] Internet-Draft ICMPv6 Filtering BCP July 2005 1. Introduction When a network supports IPv6 [RFC2460], the Internet Control Message Protocol version 6 (ICMPv6) [RFC2463], [I-D.ietf-ipngwg-icmp-v3] plays a fundamental role including being an essential component in establishing communications both at the interface level and for sessions to remote nodes. This means that overly aggressive filtering of ICMPv6 may have a detrimental effect on the establishment of IPv6 communications. On the other hand, allowing indiscriminate passage of all ICMPv6 messages can be a major security risk. This document recommends a set of rules which seek to balance effective IPv6 communication against the needs of site security. [Author's note: The new versions of RFC2461, RFC2462 and RFC2463 have been taken into account in this draft, but not necessarily referenced as yet.] ICMPv6 has a large number of functions defined in a number of sub- protocols, and there are a correspondingly large number of messages and options within these messages. The functions currently defined are: o Returning error messages to the source if a packet could not be delivered. Four different error messages are specified in [RFC2463]. o Simple monitoring of connectivity through echo requests and responses used by the ping and traceroute utilities. The Echo Request and Echo Response messages are specified in [RFC2463]. o Finding neighbors (both routers and hosts) connected to the same link and determining their IP and link layer addresses. These messages are also used to check the uniqueness of any addresses that an interface proposes to use (Duplicate Address Detection - DAD)) - DAD can be turned off if the network administrator believes that the configuration method used is bound to generate unique addresses. Four messages - Neighbor Solicitation (NS), Neighbor Advertisement (NA), Router Solicitation (RS) and Router Advertisement (RA) - are specified in [RFC2461]. o Ensuring that neighbors remain reachable using the same IP and link layer addresses after initial discovery (Neighbor Unreachability Discovery - NUD) and notifying neighbors of changes to link layer addresses. Uses NS and NA [RFC2461]. o Finding routers and determining how to obtain IP addresses to join the subnets supported by the routers. Uses RS and RA [RFC2461]. o If stateless auto-configuration of hosts is enabled, communicating prefixes and other configuration information (including the link MTU and suggested hop count default) from routers to hosts. Uses RS and RA [RFC2462]. o Redirecting packets to a more appropriate router on the local link for the destination address or pointing out that a destination is actually on the local link even if it is not obvious from the IP Davies & Mohacsi Expires January 12, 2006 [Page 4] Internet-Draft ICMPv6 Filtering BCP July 2005 address (where a link supports multiple subnets). This facility could be used by a malicious sender to divert packets and nodes should provide configuration options to prevent the messages being sent by routers and acted on by hosts. The redirect message is specified in [RFC2461]. o Supporting renumbering of networks by allowing the prefixes advertised by routers to be altered. Uses NS, NA, RS and RA together with the Router Renumbering message specified in [RFC2894]. o Determining the Maximum Transmission Unit (MTU) along a path. The Packet Too Big error message is essential to this function [RFC1981]. o Communicating which multicast groups have listeners on a link to the multicast capable routers connected to the link. Uses messages Multicast Listener Query, Multicast Listener Report (two versions) and Multicast Listener Done (version 1 only) as specified in Multicast Listener Discovery MLDv1 [RFC2710] and MLDv2[RFC3810]. o Providing support for some aspects of Mobile IPv6 especially dealing with the IPv6 Mobile Home Agent functionality provided in routers and needed to support a Mobile node homed on the link. The Home Agent Address Discovery Request and reply; and Mobile Prefix Solicitation and Advertisement messages are specified in [RFC3775] o ICMPv6 can provide some basic information about nodes to interested parties [I-D.ietf-ipngwg-icmp-name-lookups]. Many of these messages should only be used in a link-local context rather than end-to-end, and filters need to be concerned with the type of addresses in ICMPv6 packets as well as the specific source and destination addresses. Compared with the corresponding IPv4 protocol, ICMP, ICMPv6 cannot be treated as an auxiliary function with packets that can be dropped in most cases without damaging the functionality of the network. This means that firewall filters for ICMPv6 have to be more carefully configured than was the case for ICMP, where typically a small set of blanket rules could be applied. 2. Classifying ICMPv6 Messages 2.1 Error and Informational ICMPv6 Messages ICMPv6 messages contain an eight bit Type field interpreted as an integer between 0 and 255. Messages with Type values less than or equal to 127 are Error Messages. The remainder are Informational Messages. In general terms, Error Messages with well-known (standardized) Type values would normally be expected to be allowed Davies & Mohacsi Expires January 12, 2006 [Page 5] Internet-Draft ICMPv6 Filtering BCP July 2005 to be sent to or pass through firewalls, and may be essential to the establishment of communications (see Section 2.4 whereas Informational Messages will generally be the subject of policy rules, and those passing through firewalls can, in many but by no means all cases, be dropped without damaging IPv6 communications. 2.2 Addressing of ICMPv6 ICMPv6 messages are sent using various kinds of source and destination address types. The source address is usually a unicast address, but during address autoconfiguration message exchanges, the unspecified address :: is also used as a source address [RFC2462]. The destination address can be either a well-known multicast address, a generated multicast address such as the solicited-node multicast address, an anycast address or a unicast address. While many ICMPv6 messages use multicast addresses most of the time, some also use unicast addresses sometimes. For instance, the Router Advertisement messages are sent to the all-nodes multicast address when unsolicited, but can also be sent to a unicast address in response to a specific Router Solicitation. 2.3 Network Topology and Address Scopes ICMPv6 messages can be classified according to whether they are meant for end-to-end communications or communications within a link. There are also messages that we can classify as 'any-to-end', which can be sent from any point within a path back to the source; typically these are used to announce an error in processing the original packet. For instance, the address resolution messages are solely for local communications [RFC2461], whereas the Destination Unreachable messages are any-to-end in nature. Generally end-to-end and any-to- end messages might be expected to pass through firewalls depending on policies but local communications must not Local communications will use link-local addresses in many cases but may also use global unicast addresses for example when configuring global addresses. Also some ICMPv6 messages in local communications may contravene the usual rules requiring compatible scopes for source and destination addresses. 2.4 Role in Establishing Communication Many ICMPv6 messages have a role in establishing communications to and from the firewall and such messages have to be accepted by firewalls for local delivery. Which messages need to be accepted depend on whether the firewall is also acting as a router. This type of communication establishment messages should not be passed through a firewall as they are normally intended for use within a link. Davies & Mohacsi Expires January 12, 2006 [Page 6] Internet-Draft ICMPv6 Filtering BCP July 2005 On the other hand, some ICMPv6 error messages, which are sent either end-to-end or any-to-end. are essential to the establishment of communications. For example the Packet Too Big error message is needed to establish the MTU along a path. These messages must be passed through firewalls and might also be sent to and from firewalls to assist with establishment of communications. The remaining ICMPv6 messages which are not associated with communication establishment will normally be legitimately attempting to pass through a firewall from inside to out or vice versa, but decisions as to whether to allow them to pass or not can be made on the basis of local policy without interfering with the establishment of IPv6 communications. The filtering rules for the various message roles will generally be different. 3. Security Concerns for ICMPv6 Controllable by Firewall Configuration A major concern with most ICMPv6 messages is that it is generally not possible to use IPsec or other means to authenticate the sender and validate the contents of some ICMPv6 messages. To a large extent this is because a site can legitimately expect to receive certain error and other messages from almost any location in the wider Internet, and these messages may occur as a result of the first message sent to a destination. Establishing security associations with all possible sources of ICMPv6 messages is therefore impossible. The inability to establish security associations to protect some messages that are needed to establish communications means that alternative means have to used to reduce the vulnerability of sites to ICMPv6 based attacks. The most common way of doing this is to establish strict filtering policies in site firewalls to limit the unauthenticated ICMPv6 messages that can pass between the site and the wider Internet. This makes control of ICMPv6 filtering a delicate balance between protecting the site by dropping most of the ICMPv6 traffic passing through the firewall and allowing enough of the traffic through to make sure that efficient communication can be established. Firewalls will normally be concerned to monitor ICMPv6 to control the following security concerns: 3.1 Denial of Service Attacks ICMPv6 can be used to cause a Denial of Service(DoS) in a number of ways, including simply sending excessive number sof ICMPv6 packets to destinations in the site and sending error messages which disrupt Davies & Mohacsi Expires January 12, 2006 [Page 7] Internet-Draft ICMPv6 Filtering BCP July 2005 established commnuications by causing sessions to be dropped. Also if spurious communication establishment messages can be passed on to link it might be possible to disrupt established communications. 3.2 Probing A major security consideration is preventing attackers probing the site to determine the topology and identify hosts that might be vulnerable to attack. Carefully crafted but, often, malformed messages can be used to provoke ICMPv6 responses from hosts thereby informing attackers of potential targets for future attacks. 3.3 Redirection Attacks These attacks would normally have to be carried out locally on a link, but it is important to ensure that Redirect messages are not allowed through the firewall. Redirection could be used simply for DoS as well as endeavouring to redirect traffic to a compromised node. 3.4 Renumbering Attacks Spurious Renumbering messages could lead to the disruption of a site and should not be allowed through a firewall in general. 4. Filtering Recommendations 4.1 Common Considerations Depending on the classification of the message to be fitered (see Section 2), ICMPv6 messages should be filtered based on the ICMPv6 type of the message and the type (unicast, multicast, etc.) and scope (link-local, global unicast, etc) of source and destination addresses. In some cases, where deeper packet inspection is possible, it may be desirable to filter on, for example, the Code field of ICMPv6 error messages. Messages that are authenticated by means of an IPsec AH or ESP header may be subject to less strict policies than unauthenticated messages. In the remainder of this section, we are generally considering what should be configured for unauthenticated messages. In many cases it is not realistic to expect more than a tiny fraction of the messages to be authenticated. Where messages are not essential to the establishment of communications, local policy can be used to determine whether a message should be allowed or dropped. Davies & Mohacsi Expires January 12, 2006 [Page 8] Internet-Draft ICMPv6 Filtering BCP July 2005 Depending on the capabilities of the firewall being configured, it may be possible for the firewall to maintain state about packets that may result in error messages being returned or about ICMPv6 packets (e.g., Echo Requests) that are expected to receive a specific response. This state may allow the firewall to perform more precise checks based on this state, and to apply limits on the number of ICMPv6 packets accepted incoming or outgoing as a result of a packet travelling in the opposite direction. The capabilities of firewalls to perform such stateful packet inspection vary from model to model, and it is not assumed that firewalls are uniformly capable in this respect. Unless otherwise specified, the scopes of source and destination addresses of ICMPv6 messages should be matched, and packets with mismatched addresses should be dropped. 4.2 ICMPv6 Echo Request and Echo Response Echo Request (Type 128) uses unicast addresses as source addresses, but may be sent to any legal IPv6 address, even multicast and anycast addresses [RFC2463]. Echo Requests travel end-to-end but never have a role in establishing communications. Similarly Echo Responses (Type 129) travel end-to-end and would have a unicast address as destination and either a unicast or anycast address as source. They are used in combination for monitoring and debugging connectivity. o It is desirable that Echo Requests are allowed to pass outwards through firewalls. o Echo Requests may be allowed to pass inwards only towards selected hosts which are providing well-known services to the rest of the Internet. o If possible, it is desirable to maintain state about Echo Requests passing through the firewall which can be used to limit the possible corresponding Echo Responses that may be returned. o If policy requires, the possible source addresses for Echo Requests can be limited to certain machines, but in any case the source address should be an address owned by the site. o If state is maintained, only one response should be allowed corresponding to each request. Echo Responses should only be passed outwards from machines to which Echo Requests can be sent, and should only be passed inwards towards hosts which are allowed to originate Echo Requests. o Both Echo Requets and Echo Responses should be rate limited in line with [RFC2463] and, if possible, the overall incoming rate from all sources destined for any given host should be limited to a value that the host is known to be able to handle. o Messages with link local addresses in either source or destination should be dropped, except for messages coming from the inside directed to the firewall itself. Davies & Mohacsi Expires January 12, 2006 [Page 9] Internet-Draft ICMPv6 Filtering BCP July 2005 o Echo Response messages must have a unicast address as destination and may have a unicast, or anycast address as source. 4.3 Destination Unreachable Error Message Destination Unreachable (Type 1) error messages [RFC2463] are sent any-to-end between unicast addresses. The message can be generated from any node which a packet traverses on the path when the node is unable to forward the packet for any reason except congestion. o Incoming ICMPv6 Destination Unreachable messages may be passed through the firewall for debugging purposes where they relate to outgoing IPv6 packets that have previously been sent out through the firewall. For preference, this should be implemented by means of a stateful packet inspection mechanism. These messages should not be sent if the outgoing message was sent to a multicast address: if possible any error message that is returned from a multicast address should be discarded. o Outgoing ICMPv6 Destination Unreachable messages may be generated and passed out for all packets that have been allowed through the firewall. o At most one Destination Unreachable message should be passed through the firewall in response to each packet that might result in an error if the firewall is able to manage this. Destination Unreachable messages are useful for debugging but are also important to speed up cycling through possible addresses, as they can avoid the need to wait through timeouts and hence can be part of the process of establishing communications. It is a common practice in IPv4, to refrain from generating ICMP Destination Unreachable messages to attempt to hide the networking topology and/or service structure. The same rule can be applied to IPv6 but this can slow down connection if a host has multiple addresses some of which are deprecated as when using privacy addresses [RFC3041]. If policy allows the generation of ICMPv6 Destination Unreachable messages, it is important to provide the correct reason code, one of: no route to destination, administratively prohibited, beyond scope of source address, address unreachable, port unreachable, source address failed ingress/egress policy, reject route to destination. 4.4 Packet Too Big Error Message Packet Too Big (Type 2) error messages [RFC2463] are sent any-to-end between unicast addresses. The message can be generated from any node which a packet traverses on the path when the node is unable to forward the packet because the packet is too large for the MTU of the next link. This message is vital to the correct functioning of Path MTU Discovery and hence is part of the establishment of communications. Since routers are not allowed to fragment packets, Davies & Mohacsi Expires January 12, 2006 [Page 10] Internet-Draft ICMPv6 Filtering BCP July 2005 informing sources of the need to fragment large packets is more important than for IPv4. If these messages are not generated when appropriate, hosts will continue to send packets which are too large or may assume that the route is congested. Effectively parts of the Internet will become inaccessible. o It is essential to allow incoming ICMPv6 Packet Too Big messages as responses to outgoing IPv6 packets so that Path MTU Discovery will operate properly. o If possible, stateful packet inspection should be used to limit incoming Packet Too Big messages to legitimate responses to outgoing packets and to limit error messages to at most one per outgoing packet. Note that this message can be geenrated as a result of messages sent to multicast addresses. o It is essential to allow outgoing ICMPv6 Packet Too Big messages to be generated and passed through the firewall if the MTU is smaller on any link anywhere within the network protected by the firewall than the MTU on the link between the firewall and the ISP which connects the site to the rest of the Internet. In transit networks, it will be necessary to pass Packet Too Big messages through the network. If a network chooses to generate packets that are no larger than the Guaranteed Minimum MTU (1280 octets) and the site's links to the wider internet have corresponding MTUs, Packet Too Big messages should not be expected at the firewall and can be dropped if they arrive. 4.5 Time Exceeded Error Message Time Exceeded (Type 3) error messages [RFC2463] can occur in two contexts: o Code 0 are generated at any node on the path being taken by the packet and sent any-to-end between unicast addresses if the Hop Limit value is decremented to zero at any point on the path. o Code 1 messages are generated at the destination node and sent end-to-end between unicast addresses if all the segments of a fragmented message are not received within the reassembly time limit Code 0 messages can be needed as part of the establishment of communications if the path to a particular destination requires an unusually large number of hops. It is therefore essential to generate and forward Code 0 Time Exceeded messages for effective operation of IPv6 networks. Code 1 messages will generally only result from congestion in the network and it is less essential to propagate these messages. Davies & Mohacsi Expires January 12, 2006 [Page 11] Internet-Draft ICMPv6 Filtering BCP July 2005 o It is essential to allow incoming ICMPv6 Time Exceeded Code 0 messages to be able discover destination systems not reachable due to a low Hop Limit value in the outgoing packets, so that the Hop Limit can be selectively increased for these packets. o Incoming ICMPv6 Time Exceeded Code 1 messages may be enabled by policy if desired. o If possible, stateful packet inspection should be used to limit incoming Time Exceeded messages to legitimate responses to outgoing packets and to limit error messages to at most one per outgoing packet. o It is essential to generate and allow outgoing ICMPv6 Time Exceeded Code 0 messages to ensure that communications can be established to sites which generate packets with small hop limits or are traversing very long paths. o Generation of outgoing ICMPv6 Time Exceeded Code 1 messages and allowing them through the firewall may be enabled by policy if desired. 4.6 Parameter Problem Error Message The great majority of Parameter Problem (Type 4) error messages will be generated by the destination node when processing destination options and other extension headers, and hence are sent end-to-end between unicast addresses. Exceptionally, these messages might be generated by any node on the path if a faulty or unrecognized hop-by- hop option is included or from any routing waypoint if there are faulty or unrecognized destination options associated with a Type 0 routing header. In these cases the message will be sent any-to-end using unicast source and destination addresses. Parameter Problem Code 1 (Unrecognized Next Header) and Code 2 (Unrecognized IPv6 Option) messages may result if a node on the path (usually the destination) is unable to process a correctly formed extension header or option. If these messages are not returned to the source communication cannot be established, as the source would need to adapt its choice of options probably because the destination does not implement these capabilities. Hence these messages need to be generated and allowed for effective IPv6 communications. Code 0 (Erroneous Header) messages indicate a malformed extension header generally as a result of incorrectly generated packets. Hence these messages are useful for debugging purposes but it is unlikely that a node generating such packets could establish communications without human intervention to correct the problem. Code 2 messages, only, can be generated for packets with multicast destinatio addresses. Davies & Mohacsi Expires January 12, 2006 [Page 12] Internet-Draft ICMPv6 Filtering BCP July 2005 o It is essential that incoming ICMPv6 Parameter Problem messages with Code 1 or Code 2 are allowed as responses to outgoing IPv6 packets to allow establishment of communications to sites which do not support unusual options. o Incoming ICMPv6 Parameter Problem messages with Code 1 may be allowed by policy if desired. o If possible, stateful packet inspection should be used to limit incoming Parameter Problem messages to legitimate responses to outgoing packets and to limit error messages to at most one per outgoing packet. o It is essential to generate and allow outgoing ICMPv6 Parameter Problem Code 1 and Code 2 messages to ensure that communications can be established if it is known that nodes are not able to support certain options or extension headers. Some risks associated with blanket generation and forwarding of responses of this type are discussed at the end of this section. o Generation of outgoing ICMPv6 Parameter Problem Code 0 messages and allowing them through the firewall may be enabled by policy if desired. It is possible that attackers may seek to probe or scan a network by deliberately generating packets with unknown extension headers or options, or faulty headers. If nodes generate Parameter Problem error messages in all cases and these outgoing messages are allowed through firewalls, the attacker may be able to identify active addresses that can be probed further or learn about the network topology. The vulnerability could be mitigated whilst helping to establish communications if the firewall was able to examine such error messages in depth and was configured to only allow Parameter Problem messages for headers which had been standardized but were not supported in the protected network. If the network administrator believes that all nodes in the network support all legitimate extension headers then it would be reasonable to drop all outgoing Parameter Problem messages. 4.7 Neighbor Solicitation and Neighbor Advertisement Messages ICMPv6 Neighbor Solicitation and Neighbor Advertisement (Type 135 and 136) messages are essential to establishment of communications on the local link. Firewalls need to generate and accept these messages to allow them to establish interfaces onto their connected links. o Firewalls must accept and generate ICMPv6 neighbor Solicitation and Neighbor Advertisement messages for local delivery on all interfaces as described in [RFC2461] and [RFC2462]. o Firewalls must not allow any Neighbor Solicitation or Neighbor Advertisement messages to pass through the firewall, either incoming or outgoing. Davies & Mohacsi Expires January 12, 2006 [Page 13] Internet-Draft ICMPv6 Filtering BCP July 2005 Note that the address scopes of the source and destination addresses on Neighbor Solicitations and Neighbor Advertisements may not match. The exact functions which these messages will be carrying out depends on the mechanism being used to configure IPv6 addresses on the link (Stateless, Stateful or Static configuration). 4.8 Router Solicitation and Router Advertisement Messages ICMPv6 Router Solicitation and Router Advertisement(Type 133 and 134) messages are essential to establishment of communications on the local link. Firewalls need to generate (since the firewall will generally be behaving as a router) and accept these messages to allow them to establish interfaces onto their connected links. o Firewalls must accept and generate ICMPv6 Router Solicitation and Router Advertisement messages for local delivery on all interfaces as described in [RFC2461] and [RFC2462]. o Firewalls must not allow any Router Solicitation or Router Advertisement messages to pass through the firewall, either incoming or outgoing. 4.9 Redirect Messages ICMPv6 Redirect Messages(Type 137) are used on the local link to indicate that nodes are actually link-local and communications need not go via a router, or to indicate a more appropriate first hop router. Although they can be used to make communications more efficient, they are not essential to the establishment of communications and may be a security vulnerability. o Firewalls should generally implement a policy of not generating or accepting ICMPv6 Redirect messages for local delivery on any interface. o Firewalls must not allow ICMPv6 Redirect messages to pass through the firewall, either incoming or outgoing. 4.10 Multicast Listener Discovery Messages Multicast Listener Discovery (MLD) version 1 [RFC2710] (Listener Query, Listener Report and Listener Done - Types 130, 131 and 132) and version 2 [RFC3810] (Listener Query and Listener Report Version 2 - Types 130 and 143) messages are sent on the local link to communicate between multicast capable routers and nodes which wish to join or leave specific multicast groups. o Firewalls must be able to generate ICMPv6 Listener Report messages on their interfaces for Solicited Node Multicast Groups as part of the address configuration process. They may also have to generate ICMPv6 Listener Report version 2 messages and Listener Done messages for the same reason [Author's Note: The current versions of the standards are not specific about this.]. Davies & Mohacsi Expires January 12, 2006 [Page 14] Internet-Draft ICMPv6 Filtering BCP July 2005 o Firewalls which are also acting as multicast routers need to be able to receive both types of ICMPv6 Listener Report and Listener Done messages, and generate ICMPv6 Listener Query messages on relevant interfaces. o Firewalls must not allow any of the MLD messages to pass throigh the firewall, wither incoming or outgoing. 4.11 Router Renumbering Messages ICMPv6 Router Renumbering (Type 138) command messages can be received and results messages sent by routers to change the prefixes which they advertise as part of Stateless Address Configuration [RFC2461], [RFC2462]. These messages are sent end-to-end to either the all- routers multicast address (site or local scope) or specific unicast addresses from a unicast address. Router Renumbering messages are required to be protected by IPsec authentication since they could be readily misused by attackers to disrupt or divert site communications. Renumbering messages should be confined to sites for this reason. o Firewalls which are acting as routers on sites which implement router renumbering functionality must accept for local delivery and generate appropriate ICMPv6 Router Renumbering messages on inward facing interfaces. Such messages must be protected by IPsec authentication as described in [RFC2894]. o Firewalls must not accept ICMPv6 Router Renumbering messages on outward facing interfaces. o Firewalls must not allow any Router Renumbering messages to pass through the firewall, either incoming or outgoing. 4.12 Node Information Query and Reply ICMPv6 Node Information Query and Reply (Type 139 and 140) messages are sent end-to-end between unicast addresses. They can, in theory, be sent from any node to any other but it would generally not be desirable for nodes outside the local site to be able to send queries to nodes within the site. Also these messages are not required to be authenticated. o Firewall policy may allow ICMPv6 Node Information Query messages to be accepted for local delivery on inward facing interfaces and allow generation of corresponding Node Information reply messages on these interfaces. o Firewalls must not accept or generate ICMPv6 Node Information messages on outward facing interfaces. o Firewalls must not allow any ICMPv6 Node Information messages to pass through the firewall, either incoming or outgoing. Davies & Mohacsi Expires January 12, 2006 [Page 15] Internet-Draft ICMPv6 Filtering BCP July 2005 4.13 Mobile IPv6 Messages Mobile IPv6 [RFC3775] defines four ICMPv6 messages which are used to support mobile operations: Home Agent Address Discovery Request, Home Agent Address Discovery Reply, Mobile Prefix Solicitation and ICMP Mobile Prefix Advertisement(Type 144, 145, 146 and 147) messages. These messages are sent end-to-end between unicast addresses of a mobile node and its home agent. They must be expected to be sent from outside a site. The two Mobile prefix messages should be protected by the use of IPsec authentication. o If the site provides home agents for mobile nodes, the firewall must allow incoming Home Agent Address Discovery Request and Mobile Prefix Solicitation messages., and outgoing Home Agent Address Discovery Reply and ICMP Mobile Prefix Advertisement messages. It may be desirable to limit the destination addresses for the incoming messages to links that are known to support home agents. o If the site is prepared to host roaming mobile nodes, the firewall must allow outgoing Home Agent Address Discovery Request and Mobile Prefix Solicitation messages., and incoming Home Agent Address Discovery Reply and ICMP Mobile Prefix Advertisement messages. Administrators may find it desirable to prevent fixed nodes which are normally resident on the site from behaving as mobile nodes by dropping outgoing messages from these nodes. o If possible, stateful packet inspection should be used to match incoming and outgoing messages. 4.14 Unused and Experimental Messages A large number of ICMPv6 Type values are currently unused. These values have not had a specific function registered with IANA. This section describes how to treat messages which attempt to use these Type values in a way of which the network administrator (and hence the firewall) is not aware. [I-D.ietf-ipngwg-icmp-v3] defines a number of experimental Type values for ICMPv6 Error and Informational messages, which could be used in site specific ways. These values should be treated in the same way as values which are not registered by IANA unless the network administrator is explicitly made aware of usage. Any ICMPv6 Informational messages of which the firewall is not aware should not be allowed to pass through the firewall or be accepted for local delivery on any of its interfaces. Any incoming ICMPv6 Error messages of which the firewall is not aware may be allowed through the firewall in line with the specification in [RFC2463], which requests delivery of unknown error messages to Davies & Mohacsi Expires January 12, 2006 [Page 16] Internet-Draft ICMPv6 Filtering BCP July 2005 higher layer protocol processes. However, administrators may wish to disallow forwarding of these incoming messages as a potential security risk. Unknown outgoing Error messages must be dropped as the administrator should be aware of all messages that could be generated on the site. 4.15 Problems Resulting from ICMPv6 Transparency Because some ICMPv6 error packets need to be passed through a firewall in both directions. This means that the ICMPv6 error packets can be exchanged between inside and outside without any filtering. Using this feature, malicious users can communicate between the inside and outside of a firewall bypassing the administrator's inspection (proxy, firewall etc.). For example in might be possible to carry out a covert conversation through the payload of ICMPv6 error messages or tunnel inappropriate encapsulated IP packets in ICMPv6 error messages. This problem can be alleviated by filtering ICMPv6 errors using a stateful packet inspection mechanism to ensure that the packet carried as a payload is associated with legitimate traffic to or from the protected network. 5. IANA Considerations There are no IANA considerations defined in this document. 6. Acknowledgements Pekka Savola created the original IPv6 Security Overview document which contained suggestions for ICMPv6 filter setups. This information has been incorporated into this document. Some analysis of the classification of ICMPv6 messages and the term 'any-to-end' were used by Jari Arkko in a draft relating to ICMPv6 and IKE. 7. Security Considerations This memo recommends filtering configurations for firewalls designed to minimize the security vulnerabilities that can arise in using the many different sub-protocols of ICMPv6 in support of IPv6 communication. 8. References 8.1 Normative References [I-D.ietf-ipngwg-icmp-name-lookups] Crawford, M. and B. Haberman, "IPv6 Node Information Davies & Mohacsi Expires January 12, 2006 [Page 17] Internet-Draft ICMPv6 Filtering BCP July 2005 Queries", draft-ietf-ipngwg-icmp-name-lookups-11 (work in progress), May 2005. [I-D.ietf-ipngwg-icmp-v3] Conta, A., "Internet Control Message Protocol (ICMPv6)for the Internet Protocol Version 6 (IPv6) Specification", draft-ietf-ipngwg-icmp-v3-06 (work in progress), November 2004. [RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery for IP version 6", RFC 1981, August 1996. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998. [RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [RFC2463] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998. [RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast Listener Discovery (MLD) for IPv6", RFC 2710, October 1999. [RFC2894] Crawford, M., "Router Renumbering for IPv6", RFC 2894, August 2000. [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004. [RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. 8.2 Informative References [RFC3041] Narten, T. and R. Draves, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6", RFC 3041, January 2001. Davies & Mohacsi Expires January 12, 2006 [Page 18] Internet-Draft ICMPv6 Filtering BCP July 2005 Authors' Addresses Elwyn B. Davies Consultant Soham, Cambs UK Phone: +44 7889 488 335 Email: elwynd@dial.pipex.com Janos Mohacsi NIIF/HUNGARNET Victor Hugo u. 18-22 Budapest, H-1132 Hungary Phone: +36 1 4503070 Email: mohacsi@niif.hu Davies & Mohacsi Expires January 12, 2006 [Page 19] Internet-Draft ICMPv6 Filtering BCP July 2005 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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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Davies & Mohacsi Expires January 12, 2006 [Page 20]