``` Filename: 117-ipv6-exits.txt Title: IPv6 exits Author: coderman Created: 10-Jul-2007 Status: Closed Target: 0.2.4.x Implemented-In: 0.2.4.7-alpha Overview Extend Tor for TCP exit via IPv6 transport and DNS resolution of IPv6 addresses. This proposal does not imply any IPv6 support for OR traffic, only exit and name resolution. Contents 0. Motivation As the IPv4 address space becomes more scarce there is increasing effort to provide Internet services via the IPv6 protocol. Many hosts are available at IPv6 endpoints which are currently inaccessible for Tor users. Extending Tor to support IPv6 exit streams and IPv6 DNS name resolution will allow users of the Tor network to access these hosts. This capability would be present for those who do not currently have IPv6 access, thus increasing the utility of Tor and furthering adoption of IPv6. 1. Design 1.1. General design overview There are three main components to this proposal. The first is a method for routers to advertise their ability to exit IPv6 traffic. The second is the manner in which routers resolve names to IPv6 addresses. Last but not least is the method in which clients communicate with Tor to resolve and connect to IPv6 endpoints anonymously. 1.2. Router IPv6 exit support In order to specify exit policies and IPv6 capability new directives in the Tor configuration will be needed. If a router advertises IPv6 exit policies in its descriptor this will signal the ability to provide IPv6 exit. There are a number of additional default deny rules associated with this new address space which are detailed in the addendum. When Tor is started on a host it should check for the presence of a global unicast IPv6 address and if present include the default IPv6 exit policies and any user specified IPv6 exit policies. If a user provides IPv6 exit policies but no global unicast IPv6 address is available Tor should generate a warning and not publish the IPv6 policies in the router descriptor. It should be noted that IPv4 mapped IPv6 addresses are not valid exit destinations. This mechanism is mainly used to interoperate with both IPv4 and IPv6 clients on the same socket. Any attempts to use an IPv4 mapped IPv6 address, perhaps to circumvent exit policy for IPv4, must be refused. 1.3. DNS name resolution of IPv6 addresses (AAAA records) In addition to exit support for IPv6 TCP connections, a method to resolve domain names to their respective IPv6 addresses is also needed. This is accomplished in the existing DNS system via AAAA records. Routers will perform both A and AAAA requests when resolving a name so that the client can utilize an IPv6 endpoint when available or preferred. To avoid potential problems with caching DNS servers that behave poorly all NXDOMAIN responses to AAAA requests should be ignored if a successful response is received for an A request. This implies that both AAAA and A requests will always be performed for each name resolution. For reverse lookups on IPv6 addresses, like that used for RESOLVE_PTR, Tor will perform the necessary PTR requests via IP6.ARPA. All routers which perform DNS resolution on behalf of clients (RELAY_RESOLVE) should perform and respond with both A and AAAA resources. [NOTE: In a future version, when we extend the behavior of RESOLVE to encapsulate more of real DNS, it will make sense to allow more flexibility here. -nickm] 1.4. Client interaction with IPv6 exit capability 1.4.1. Usability goals There are a number of behaviors which Tor can provide when interacting with clients that will improve the usability of IPv6 exit capability. These behaviors are designed to make it simple for clients to express a preference for IPv6 transport and utilize IPv6 host services. 1.4.2. SOCKSv5 IPv6 client behavior The SOCKS version 5 protocol supports IPv6 connections. When using SOCKSv5 with hostnames it is difficult to determine if a client wishes to use an IPv4 or IPv6 address to connect to the desired host if it resolves to both address types. In order to make this more intuitive the SOCKSv5 protocol can be supported on a local IPv6 endpoint, [::1] port 9050 for example. When a client requests a connection to the desired host via an IPv6 SOCKS connection Tor will prefer IPv6 addresses when resolving the host name and connecting to the host. Likewise, RESOLVE and RESOLVE_PTR requests from an IPv6 SOCKS connection will return IPv6 addresses when available, and fall back to IPv4 addresses if not. [NOTE: This means that SocksListenAddress and DNSListenAddress should support IPv6 addresses. Perhaps there should also be a general option to have listeners that default to 127.0.0.1 and 0.0.0.0 listen additionally or instead on ::1 and :: -nickm] 1.4.3. MAPADDRESS behavior The MAPADDRESS capability supports clients that may not be able to use the SOCKSv4a or SOCKSv5 hostname support to resolve names via Tor. This ability should be extended to IPv6 addresses in SOCKSv5 as well. When a client requests an address mapping from the wildcard IPv6 address, [::0], the server will respond with a unique local IPv6 address on success. It is important to note that there may be two mappings for the same name if both an IPv4 and IPv6 address are associated with the host. In this case a CONNECT to a mapped IPv6 address should prefer IPv6 for the connection to the host, if available, while CONNECT to a mapped IPv4 address will prefer IPv4. It should be noted that IPv6 does not provide the concept of a host local subnet, like 127.0.0.0/8 in IPv4. For this reason integration of Tor with IPv6 clients should consider a firewall or filter rule to drop unique local addresses to or from the network when possible. These packets should not be routed, however, keeping them off the subnet entirely is worthwhile. 1.4.3.1. Generating unique local IPv6 addresses The usual manner of generating a unique local IPv6 address is to select a Global ID part randomly, along with a Subnet ID, and sharing this prefix among the communicating parties who each have their own distinct Interface ID. In this style a given Tor instance might select a random Global and Subnet ID and provide MAPADDRESS assignments with a random Interface ID as needed. This has the potential to associate unique Global/Subnet identifiers with a given Tor instance and may expose attacks against the anonymity of Tor users. To avoid this potential problem entirely MAPADDRESS must always generate the Global, Subnet, and Interface IDs randomly for each request. It is also highly suggested that explicitly specifying an IPv6 source address instead of the wildcard address not be supported to ensure that a good random address is used. 1.4.4. DNSProxy IPv6 client behavior A new capability in recent Tor versions is the transparent DNS proxy. This feature will need to return both A and AAAA resource records when responding to client name resolution requests. The transparent DNS proxy should also support reverse lookups for IPv6 addresses. It is suggested that any such requests to the deprecated IP6.INT domain should be translated to IP6.ARPA instead. This translation is not likely to be used and is of low priority. It would be nice to support DNS over IPv6 transport as well, however, this is not likely to be used and is of low priority. 1.4.5. TransPort IPv6 client behavior Tor also provides transparent TCP proxy support via the Trans* directives in the configuration. The TransListenAddress directive should accept an IPv6 address in addition to IPv4 so that IPv6 TCP connections can be transparently proxied. 1.5. Additional changes The RedirectExit option should be deprecated rather than extending this feature to IPv6. 2. Spec changes 2.1. Tor specification In '6.2. Opening streams and transferring data' the following should be changed to indicate IPv6 exit capability: "No version of Tor currently generates the IPv6 format." In '6.4. Remote hostname lookup' the following should be updated to reflect use of ip6.arpa in addition to in-addr.arpa. "For a reverse lookup, the OP sends a RELAY_RESOLVE cell containing an in-addr.arpa address." In 'A.1. Differences between spec and implementation' the following should be updated to indicate IPv6 exit capability: "The current codebase has no IPv6 support at all." [NOTE: the EXITPOLICY end-cell reason says that it can hold an ipv4 or an ipv6 address, but doesn't say how. We may want a separate EXITPOLICY2 type that can hold an ipv6 address, since the way we encode ipv6 addresses elsewhere ("0.0.0.0 indicates that the next 16 bytes are ipv6") is a bit dumb. -nickm] [Actually, the length field lets us distinguish EXITPOLICY. -nickm] 2.2. Directory specification In '2.1. Router descriptor format' a new set of directives is needed for IPv6 exit policy. The existing accept/reject directives should be clarified to indicate IPv4 or wildcard address relevance. The new IPv6 directives will be in the form of: "accept6" exitpattern NL "reject6" exitpattern NL The section describing accept6/reject6 should explain that the presence of accept6 or reject6 exit policies in a router descriptor signals the ability of that router to exit IPv6 traffic (according to IPv6 exit policies). The "[::]/0" notation is used to represent "all IPv6 addresses". "[::0]/0" may also be used for this representation. If a user specifies a 'reject6 [::]/0:*' policy in the Tor configuration this will be interpreted as forcing no IPv6 exit support and no accept6/reject6 policies will be included in the published descriptor. This will prevent IPv6 exit if the router host has a global unicast IPv6 address present. It is important to note that a wildcard address in an accept or reject policy applies to both IPv4 and IPv6 addresses. 2.3. Control specification In '3.8. MAPADDRESS' the potential to have to addresses for a given name should be explained. The method for generating unique local addresses for IPv6 mappings needs explanation as described above. When IPv6 addresses are used in this document they should include the brackets for consistency. For example, the null IPv6 address should be written as "[::0]" and not "::0". The control commands will expect the same syntax as well. In '3.9. GETINFO' the "address" command should return both public IPv4 and IPv6 addresses if present. These addresses should be separated via \r\n. 2.4. Tor SOCKS extensions In '2. Name lookup' a description of IPv6 address resolution is needed for SOCKSv5 as described above. IPv6 addresses should be supported in both the RESOLVE and RESOLVE_PTR extensions. A new section describing the ability to accept SOCKSv5 clients on a local IPv6 address to indicate a preference for IPv6 transport as described above is also needed. The behavior of Tor SOCKSv5 proxy with an IPv6 preference should be explained, for example, preferring IPv6 transport to a named host with both IPv4 and IPv6 addresses available (A and AAAA records). 3. Questions and concerns 3.1. DNS A6 records A6 is explicitly avoided in this document. There are potential reasons for implementing this, however, the inherent complexity of the protocol and resolvers make this unappealing. Is there a compelling reason to consider A6 as part of IPv6 exit support? [IMO not till anybody needs it. -nickm] 3.2. IPv4 and IPv6 preference The design above tries to infer a preference for IPv4 or IPv6 transport based on client interactions with Tor. It might be useful to provide more explicit control over this preference. For example, an IPv4 SOCKSv5 client may want to use IPv6 transport to named hosts in CONNECT requests while the current implementation would assume an IPv4 preference. Should more explicit control be available, through either configuration directives or control commands? Many applications support a inet6-only or prefer-family type option that provides the user manual control over address preference. This could be provided as a Tor configuration option. An explicit preference is still possible by resolving names and then CONNECTing to an IPv4 or IPv6 address as desired, however, not all client applications may have this option available. 3.3. Support for IPv6 only transparent proxy clients It may be useful to support IPv6 only transparent proxy clients using IPv4 mapped IPv6 like addresses. This would require transparent DNS proxy using IPv6 transport and the ability to map A record responses into IPv4 mapped IPv6 like addresses in the manner described in the "NAT-PT" RFC for a traditional Basic-NAT-PT with DNS-ALG. The transparent TCP proxy would thus need to detect these mapped addresses and connect to the desired IPv4 host. The IPv6 prefix used for this purpose must not be the actual IPv4 mapped IPv6 address prefix, though the manner in which IPv4 addresses are embedded in IPv6 addresses would be the same. The lack of any IPv6 only hosts which would use this transparent proxy method makes this a lot of work for very little gain. Is there a compelling reason to support this NAT-PT like capability? 3.4. IPv6 DNS and older Tor routers It is expected that many routers will continue to run with older versions of Tor when the IPv6 exit capability is released. Clients who wish to use IPv6 will need to route RELAY_RESOLVE requests to the newer routers which will respond with both A and AAAA resource records when possible. One way to do this is to route RELAY_RESOLVE requests to routers with IPv6 exit policies published, however, this would not utilize current routers that can resolve IPv6 addresses even if they can't exit such traffic. There was also concern expressed about the ability of existing clients to cope with new RELAY_RESOLVE responses that contain IPv6 addresses. If this breaks backward compatibility, a new request type may be necessary, like RELAY_RESOLVE6, or some other mechanism of indicating the ability to parse IPv6 responses when making the request. 3.5. IPv4 and IPv6 bindings in MAPADDRESS It may be troublesome to try and support two distinct address mappings for the same name in the existing MAPADDRESS implementation. If this cannot be accommodated then the behavior should replace existing mappings with the new address regardless of family. A warning when this occurs would be useful to assist clients who encounter problems when both an IPv4 and IPv6 application are using MAPADDRESS for the same names concurrently, causing lost connections for one of them. 4. Addendum 4.1. Sample IPv6 default exit policy reject 0.0.0.0/8 reject 169.254.0.0/16 reject 127.0.0.0/8 reject 192.168.0.0/16 reject 10.0.0.0/8 reject 172.16.0.0/12 reject6 [0000::]/8 reject6 [0100::]/8 reject6 [0200::]/7 reject6 [0400::]/6 reject6 [0800::]/5 reject6 [1000::]/4 reject6 [4000::]/3 reject6 [6000::]/3 reject6 [8000::]/3 reject6 [A000::]/3 reject6 [C000::]/3 reject6 [E000::]/4 reject6 [F000::]/5 reject6 [F800::]/6 reject6 [FC00::]/7 reject6 [FE00::]/9 reject6 [FE80::]/10 reject6 [FEC0::]/10 reject6 [FF00::]/8 reject *:25 reject *:119 reject *:135-139 reject *:445 reject *:1214 reject *:4661-4666 reject *:6346-6429 reject *:6699 reject *:6881-6999 accept *:* # accept6 [2000::]/3:* is implied 4.2. Additional resources 'DNS Extensions to Support IP Version 6' http://www.ietf.org/rfc/rfc3596.txt 'DNS Extensions to Support IPv6 Address Aggregation and Renumbering' http://www.ietf.org/rfc/rfc2874.txt 'SOCKS Protocol Version 5' http://www.ietf.org/rfc/rfc1928.txt 'Unique Local IPv6 Unicast Addresses' http://www.ietf.org/rfc/rfc4193.txt 'INTERNET PROTOCOL VERSION 6 ADDRESS SPACE' http://www.iana.org/assignments/ipv6-address-space 'Network Address Translation - Protocol Translation (NAT-PT)' http://www.ietf.org/rfc/rfc2766.txt ```