Network Working Group R. Allbery Internet-Draft Stanford University Intended status: Informational October 3, 2009 Expires: April 6, 2010 DNS SRV Resource Records for AFS draft-allbery-afs-srv-records-00 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and 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 April 6, 2010. Copyright Notice Copyright (c) 2009 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 in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Abstract This document specifies how to use DNS (Domain Name Service) SRV RRs (Resource Records) to locate services for the AFS distributed file system and how the priority and weight values of the SRV RR should be Allbery Expires April 6, 2010 [Page 1] Internet-Draft DNS SRV Resource Records for AFS October 2009 interpreted in the server weighting system used by AFS. It deprecates use of the AFSDB RR to locate AFS cell database servers and provides guidance for backward compatibility. Internet Draft Comments Comments are solicited. Please include the AFS Standardization mailing list at afs3-standardization@openafs.org as a recipient of any comments. Table of Contents 1. Overview and Rationale . . . . . . . . . . . . . . . . . . . . 3 2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Requirements Notation . . . . . . . . . . . . . . . . . . . . . 4 4. DNS SRV RRs for AFS . . . . . . . . . . . . . . . . . . . . . . 4 5. Use of AFSDB RRs . . . . . . . . . . . . . . . . . . . . . . . 6 6. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 Allbery Expires April 6, 2010 [Page 2] Internet-Draft DNS SRV Resource Records for AFS October 2009 1. Overview and Rationale AFS (a registered trademark of IBM Corporation) is a distributed file system organized administratively into cells. Each AFS cell consists of one or more Volume Location Database (VLDB) servers, one or more Protection Service (PTS) servers, and one or more pairs of file servers and volume servers, plus possibly additional services not relevant to this document. Data stored in AFS is divided into collections of files called volumes. An AFS protocol client, when accessing a file within a specific AFS cell, first contacts a VLDB server for that cell to determine the file server for the AFS volume in which that file is located, and then contacts that file server directly to access the file. A client may also need to contact a PTS server for that cell to register before accessing files in that cell. An AFS client therefore needs to determine, for a given AFS cell, the VLDB and possibly the PTS servers for that cell. (Traditionally, the VLDB and PTS servers are provided by the same host.) Once the client is in contact with the VLDB server, it locates file and volume servers through AFS protocol queries to the VLDB server. Originally, VLDB server information was configured separately on each client in a file called the CellServDB file. [RFC1183] specified the DNS RR (Resource Record) AFSDB to locate VLDB servers for AFS. Subsequent to [RFC1183], a general DNS RR was defined by [RFC2782] for service location for any service. This DNS SRV RR has several advantages over the AFSDB RR: o AFSDB RRs do not support priority or ranking, leaving AFS cell administrators without a way to control which VLDB servers clients prefer. o AFSDB RRs do not include protocol or port information, implicitly assuming that all VLDB servers will be contacted over the standard port and the UDP protocol. Future changes to the AFS protocol may require separate VLDB server lists for UDP and TCP traffic, and some uses of AFS, such as providing VLDB service for multiple cells from the same systems, require use of different ports. o Clients using AFSDB RRs must assume that VLDB and PTS services are provided by the same host, but it may be useful to separate VLDB servers from PTS servers. o DNS SRV RRs are in widespread use, whereas AFSDB RRs are a little- known and little-supported corner of the DNS protocol. For those reasons, it is desirable to move AFS service location from the AFSDB RR to DNS SRV RRs. Allbery Expires April 6, 2010 [Page 3] Internet-Draft DNS SRV Resource Records for AFS October 2009 2. Scope This document describes the format and use of DNS SRV RRs for AFS service location and deprecates the AFSDB RR. It also provides guidance for transition from the AFSDB RR to DNS SRV RRs and recommendations for backward compatibility. Documentation of the AFS protocol, the exact purpose and use of the VLDB and PTS services, and other information about AFS is outside the scope of this document. AFSDB RRs may also be used for locating servers for the Open Software Foundation's (OSF) Distributed Computing Environment (DCE) authenticated naming system, as described in [RFC1183]. Service location for DCE servers is outside the scope of this document and not modified by this specification. 3. Requirements Notation 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]. 4. DNS SRV RRs for AFS The label of a DNS SRV RR, as defined in [RFC2782], is: _._. The following values for advertise servers providing AFS services: afs3-vlserver: servers providing AFS VLDB services. afs3-prserver: servers providing AFS PTS services. Other AFS services, such as file and volume management services, are located through the VLDB service and therefore do not use DNS SRV RRs. MUST be "udp" for the current AFS protocol, which uses Rx over UDP. The value "tcp" may be used for future revisions of the AFS protocol using Rx over TCP. MUST be the AFS cell name for which the identified server provides AFS services. Clients MUST query DNS SRV RRs only for a Allbery Expires April 6, 2010 [Page 4] Internet-Draft DNS SRV Resource Records for AFS October 2009 value exactly matching the AFS cell of interest. They MUST not remove leading components to search for more general DNS SRV RRs. The AFS cell "prod.example.com" and the AFS cell "example.com" are entirely different cells in the AFS protocol and VLDB servers for the latter cannot provide information for the former. NOTE: As with AFSDB RRs, this means that DNS SRV RRs can only be used to locate AFS services for cells whose naming matches the structure of DNS. This is not a requirement of the AFS protocol, but sites creating new AFS cells SHOULD use names that follow the structure of DNS and which result in DNS SRV RRs under their administrative control. This both permits use of DNS SRV RRs instead of client configuration and helps avoid naming conflicts between separate AFS cells. DNS SRV RRs include a priority and a weight. As defined in the DNS SRV RR specification [RFC2782], clients MUST attempt to contact the target host with the lowest-numbered priority they can reach. AFS clients which use a weighted algorithm to determine which server to contact MUST therefore assign a sufficiently distinct weight to targets with different priorities such that targets with a higher- numbered priority are only contacted if all targets with a lower- numbered priority are inaccessible. If there are multiple targets with an equal priority, the weight value of the DNS SRV RR SHOULD be used as input to a weighted algorithm for selecting servers. As specified by [RFC2782], larger weights SHOULD be given a proportionately higher probability of being selected. In the presence of records containing weights greater than 0, records with weight 0 should have a very small chance of being selected. A weight of 0 SHOULD be used if all targets with that priority are weighted equally. Several AFS implementations use a weighted algorithm that assigns numbers to each server when the client first contacts that AFS cell and then prefers the server with the lowest weight unless that server goes down. Clients using this algorithm should assign their weights as follows: 1. Sort targets by priority and assign a base weight to each target based on its priority. Each base weight MUST be sufficiently different from the base weight assigned to any higher-numbered priority that higher-numbered targets will only be attempted if lower-numbered targets cannot be reached. 2. For each group of targets with the same priority, follow the algorithm in [RFC2782] to order those targets. Then, assign those targets weights formed by incrementing the base weight for Allbery Expires April 6, 2010 [Page 5] Internet-Draft DNS SRV Resource Records for AFS October 2009 that priority such that the first selected target has the lowest weight, the second selected target has the next lowest weight, and so on. 5. Use of AFSDB RRs Since many AFS client implementations currently support AFSDB RRs but do not support DNS SRV RRs, AFS cells providing DNS SRV RRs SHOULD also provide AFSDB RRs. However, be aware that AFSDB RRs do not provide priority or weighting information; all servers listed in ASFDB RRs are treated as equal. AFSDB RRs also do not provide port information. An AFS cell using DNS SRV RRs SHOULD therefore also provide an AFSDB RR listing all AFS servers for which the following statements are all true: o The server provides both VLDB and PTS service on the standard ports (7003 and 7002) respectively. o The server provides these services over UDP. o The server either has the lowest-numbered priority of those listed in the DNS SRV RRs or the AFS cell administrator believes it reasonable for clients using AFSDB RRs to use this server by preference. The above is a default recommendation. AFS cell administrators MAY use different lists of servers in the AFSDB RRs and DNS SRV RRs if desired for specific effects based on local knowledge of which clients use AFSDB RRs and which clients use DNS SRV RRs. However, AFS servers SHOULD NOT be advertised with AFSDB RRs unless they provide VLDB and PTS services via UDP on the standard ports. An AFS cell SHOULD have at least one VLDB and at least one PTS server providing service on the standard ports of 7003 and 7002, respectively, since clients without DNS SRV RR support cannot locate servers on non-standard ports. Clients SHOULD query DNS SRV RRs by default but SHOULD then fall back on AFSDB RRs if no DNS SRV RRs are found. All servers listed in an AFSDB RR of 1 SHOULD be treated as equivalent to the following pair of DNS SRV RRs: _afs3-vlserver._udp. IN SRV 0 0 7003 _afs3-prserver._udp. IN SRV 0 0 7002 Allbery Expires April 6, 2010 [Page 6] Internet-Draft DNS SRV Resource Records for AFS October 2009 is the label of the AFSDB RR, is its TTL, and is the value, following the specification in [RFC1183]. 6. Example The following example includes TCP AFS services, separation of a PTS server from a VLDB server, and use of non-standard ports, all features that either assume future AFS protocol development or are not widely supported by current clients. This example is intended to show the range of possibilities for AFS DNS SRV RRs, not as a practical example for an existing cell. This is a part of the zone file for a fictional example.com domain with AFS services. $ORIGIN example.com. @ SOA server.example.com. root.example.com. ( 2009100201 3600 3600 604800 86400 ) NS server.example.com. _afs3-vlserver._udp SRV 0 2 7003 afsdb1.example.com. _afs3-vlserver._udp SRV 0 4 7003 afsdb2.example.com. _afs3-vlserver._udp SRV 1 0 7008 afsdb3.example.com. _afs3-vlserver._tcp SRV 0 0 7003 afsdb3.example.com. _afs3-prserver._udp SRV 0 0 7002 afsdb1.example.com. _afs3-prserver._tcp SRV 0 0 7002 afsdb3.example.com. AFSDB 1 afsdb1.example.com. afsdb1 A 172.30.79.10 afsdb2 A 172.30.79.11 afsdb3 A 172.30.79.12 In this example, the AFS cell name is example.com. afsdb1, afsdb2, and afsdb3 all provide VLDB service via UDP. The first two have the same priority but have weights indicating that afsdb1 should get about twice as many clients as afsdb2. afsdb3 should only be used for UDP VLDB service if afsdb1 and afsdb2 are not accessible and provides that service on a non-standard port (7008). Only one host, afsdb1, provides UDP PTS service. afsdb3 provides a theoretical TCP version of AFS VLDB and PTS service on the standard ports and is the only server providing these services over TCP for this cell. Allbery Expires April 6, 2010 [Page 7] Internet-Draft DNS SRV Resource Records for AFS October 2009 An AFSDB RR is provided for backward compatibility with older clients. It lists only afsdb1, since only that host provides both VLDB and PTS service over UDP on the standard ports. 7. Security Considerations Use of DNS SRV RRs for AFS service location pose the same security issues as the existing AFSDB RRs. Specifically, unless the integrity and authenticity of the DNS response is checked, an attacker may forge DNS replies and thereby direct clients at a VLDB or PTS server under the control of the attacker. From there, the attacker may decieve an AFS client about the volumes and file servers in a cell and about the contents of files and directories in that cell. If the client uses cell data in a trusted way, such as by executing programs out of that AFS cell or using data from the cell as input to other programs, the attacker may be able to further compromise the security of the client and trick it into taking actions under the attacker's control. This attack can be ameliorated if the client is authenticated using the Kerberos-based authentication provided by the AFS protocol, since the client can then detect a failure to authenticate to the attacker's servers and thereby detect possible impersonation. However, this applies only to authenticated AFS access, and much AFS access is unauthenticated. Furthermore, clients after failure to authenticate may fall back to unauthenticated access, which the attacker's servers may permit. This vulnerability is inherent in the current AFS protocol and may be exploited in ways other than DNS spoofing, such as by spoofing the results of VLDB queries for an AFS cell. Addressing it properly requires changes to the AFS protocol allowing clients to always authenticate AFS services and discard unauthenticated data. Addition of DNS SRV RRs does not make this vulnerability more severe, only opens another equivalent point of attack. 8. References [RFC1183] Everhart, C., Mamakos, L., Ullmann, R., and P. Mockapetris, "New DNS RR Definitions", RFC 1183, October 1990. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for Allbery Expires April 6, 2010 [Page 8] Internet-Draft DNS SRV Resource Records for AFS October 2009 specifying the location of services (DNS SRV)", RFC 2782, February 2000. Author's Address Russ Allbery Stanford University P.O. Box 20066 Stanford, CA 94309 US Email: rra@stanford.edu URI: http://www.eyrie.org/~eagle/ Allbery Expires April 6, 2010 [Page 9]