Network Working Group Bill Manning draft-manning-opcode-discover-02.txt ep.net Expires: May 2006 Paul Vixie ISC 16 November 2005 DISCOVER: Supporting Multicast DNS Queries 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 XXXXXX. Distribution of this memo is unlimited. Abstract This document describes the DISCOVER opcode, an experimental extension to the Domain Name System (DNS) to use multicast queries for resource discovery. A client multicasts a DNS query using the DISCOVER opcode and processes the multiple responses that may result. 1. Introduction In the standard Domain Name System (DNS) [3] [4], queries are always unicast using the QUERY opcode. The TBDS research project, funded under DARPA grant F30602-99-1-0523, explored the use of multicast DNS [3][4] queries for resource discovery. Multicast queries may return multiple replies, while the standard DNS QUERY operation [5] expects a single reply. Instead of extending the QUERY opcode, the project developed and tested a new query operation, DISCOVER, that is designed to accommodate multiple responses from a multicast query. This memo documents the processing rules for DISCOVER, for possible incorporation in a future revision of the DNS specification. 2. DISCOVER Processing Rules A requester will send a DISCOVER query message to a multicast destination address, with some particular multicast scope. The requester must be prepared to receive multiple replies from multiple responders, although we expect that there will be a single reply per responder. DISCOVER responses (i.e., response messages from DISCOVER queries) have standard Answer, Authority, and Additional sections. For example, the DISCOVER response is the same as the response to a QUERY operation. Zero-content answers should not be sent, to avoid badly formed or unfulfilled requests. Responses should be sent to th unicast address of the requester, and the source address should reflect the unicast address of the responder. DISCOVER responses may echo the request's Question section or leave it blank, just as for QUERY. DISCOVER works like QUERY, except: 1. The Question section of a DISCOVER operation contains <QNAME=zonename,QTYPE=SOA> tuples, if the section is present. Within TBDS, this structure was augmented with: <QNAME=service,QTYPE=SRV>. While this worked, it would be cleaner to ask the SRV question in a separate pass, and any future work should take this into consideration. 2. If QDCOUNT equals 0, then only servers willing to do recursion should answer; other servers must silently discard a DISCOVER request with QDCOUNT equals 0. 3. if QDCOUNT is not equal to 0, then only servers that are authoritative for the zones named by some QNAME should answer. Hence, replies to DISCOVER queries will always be authoritative or else have RA (Recursion Available) set. 3. Using DISCOVER Queries 3.1 Performing Host Lookups To perform a hostname lookup using DISCOVER, a node could: o Compute the zone name of the enclosing in-addr.arpa, ip6.int, or ip6.arpa domain. o DISCOVER whether any in-scope server(s) are authoritative for this zone. If so, query these authoritative servers for local in-addr/ip6 names. o If not, DISCOVER whether there are recursive servers available. If so, query these recursive servers for local in-addr/ip6 names. The requester can determine from the replies whether there are any DNS servers that are authoritative (or support recursion) for the zone. o Once the host's FQDN is known, repeat the process to discover the closest enclosing authoritative server for this local name. o Cache all NS and A data learned in this process, respecting TTL's. 3.2 Performing Service Lookups To lookup a service name using DISCOVER, the following steps may be used: o Use DISCOVER as outlines in Section 3.1 to perform gethostbyaddr() and then gethostbyname() on one's own link-local address. This gives a list of local authoritative servers. o Assume that the closest enclosing zone for which an authoritative server responds to an in-scope DISCOVER message is this host's "parent domain", and compute the SRV name as _service._transport.*.parentdomain. This is a change to the definition as defined in RFC 1034 [3]. A wildcard label ("*") in the QNAME used in a DNS message with op-code DISCOVER should be evaluated with special rules: the wildcardshould match any label for which the DNS server data is authoritative. For example 'x.*.example.com.' would match 'x.y.example.com.' and 'x.yy.example.com.', provided that the server was authoritative for 'example.com.' o Finally, send a SRV query for this SRV name to the discovered local authoritative servers, to complete the getservbyname() call. This call returns a structure that can be populated by response values, as follows: s_name The name of the service, "_service" without the preceding underscore. s_aliases The names returned in the SRV RRs in replies to the query. s_port The port number in the SRV RRs replies to the query. If these port numbers disagree - one of the port numbers is chosen, and only those names which correspond are returned. s_proto The transport protocol from named by the "_transport" label, without the preceding underscore. 3.3 Using DISCOVER for Disconnected Names DISCOVER allows discovery of a host (for example, a printer offering LPD services) whose DNS server answers authoritatively for a domain name that hasn't been delegated to it, but is defined within some local scope. Since DISCOVER is explicitly defined to discover undelegated zones for tightly-scoped queries, this behavior isn't a violation of DNS's coherency principles. Note that a responder to DISCOVER might not be traditional DNS software, it could be special-purpose software. DISCOVER usage for disconnected networks with no authoritative servers can be achieved using the following conditions. o Hosts run a "stub server" that acts as though its FQDN were a zone name. o The computed SOA gives the host's FQDN as the MNAME, "." as the ANAME, seconds-since-1Jan2000 as the SERIAL, and low constants for EXPIRE and the other SOA timers. o NS is used as the host's FQDN. o The glue is computed as the host's link-local address, or hosts may run a "DNS stub server" that acts as though its FQDN were a zone name. The rules governing the behavior of this stub server are given elsewhere [1] [2]. Such stub servers should answer DISCOVER packets for its zone, and will be found by the iterative "discover closest enclosing authority server" by DISCOVER clients, in either the gethostbyname() or SRV cases described above. Note that stub servers answer only with zone names which exactly match QNAME's, not with zone names which are owned by QNAME's. 4. IANA Considerations The IANA will need to assign a numeric value for the DISCOVER opcode. 5. Security Considerations No new security considerations are known to be introduced with a new DNS query operation. However, using multicast for service discovery has the potential for denial of service from flooding attacks. It may also be possible to enable deliberate misconfiguration of clients simply by running a malicious DNS server that falsely claims to be authoritative for delegations. One possible way to mitigate this threat is to use credentials, such as CERT resource records within an RR set. The TBDS project took this approach. TBDS did not directly utilize DNSSEC and so possible interactions with DNSSEC aware/capable servers are unknown. 6. Acknowledgments This material was generated in discussions on the mdns maili list hosted by Zocalo in March 2000 and updated by discussions in September/October 2003 on a closed mailing list. David Lawrence, Scott Rose, Stuart Cheshire, Bill Woodcock, Erik Guttman were active contributors. Suzanne Woolf was part of the original implementation team and an invaluable sanity checker. 7. References [1] Esibov, L., Aboba, B., Thaler, D., "Multicast DNS", Work in Progress, November 2000. [2] Woodcock, B., Manning, B., "Multicast Domain Name Service", Work in Progress, August 2000. [3] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES", RFC 1034, November 1987. [4] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND SPECIFICATION", RFC 1035, November 1987. [5] QUERY opcode -- defined in section 3.7, 4.3, and section 5 of RFC 1034 and in section 4.1.1 of RFC 1035. 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