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649 lines
23 KiB
Plaintext
Network Working Group A. Gulbrandsen
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Request for Comments: 2782 Troll Technologies
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Obsoletes: 2052 P. Vixie
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Category: Standards Track Internet Software Consortium
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L. Esibov
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Microsoft Corp.
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February 2000
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A DNS RR for specifying the location of services (DNS SRV)
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Status of this Memo
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Copyright Notice
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Copyright (C) The Internet Society (2000). All Rights Reserved.
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Abstract
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This document describes a DNS RR which specifies the location of the
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server(s) for a specific protocol and domain.
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Overview and rationale
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Currently, one must either know the exact address of a server to
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contact it, or broadcast a question.
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The SRV RR allows administrators to use several servers for a single
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domain, to move services from host to host with little fuss, and to
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designate some hosts as primary servers for a service and others as
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backups.
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Clients ask for a specific service/protocol for a specific domain
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(the word domain is used here in the strict RFC 1034 sense), and get
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back the names of any available servers.
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Note that where this document refers to "address records", it means A
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RR's, AAAA RR's, or their most modern equivalent.
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Gulbrandsen, et al. Standards Track [Page 1]
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RFC 2782 DNS SRV RR February 2000
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Definitions
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The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY"
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used in this document are to be interpreted as specified in [BCP 14].
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Other terms used in this document are defined in the DNS
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specification, RFC 1034.
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Applicability Statement
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In general, it is expected that SRV records will be used by clients
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for applications where the relevant protocol specification indicates
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that clients should use the SRV record. Such specification MUST
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define the symbolic name to be used in the Service field of the SRV
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record as described below. It also MUST include security
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considerations. Service SRV records SHOULD NOT be used in the absence
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of such specification.
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Introductory example
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If a SRV-cognizant LDAP client wants to discover a LDAP server that
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supports TCP protocol and provides LDAP service for the domain
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example.com., it does a lookup of
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_ldap._tcp.example.com
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as described in [ARM]. The example zone file near the end of this
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memo contains answering RRs for an SRV query.
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Note: LDAP is chosen as an example for illustrative purposes only,
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and the LDAP examples used in this document should not be considered
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a definitive statement on the recommended way for LDAP to use SRV
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records. As described in the earlier applicability section, consult
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the appropriate LDAP documents for the recommended procedures.
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The format of the SRV RR
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Here is the format of the SRV RR, whose DNS type code is 33:
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_Service._Proto.Name TTL Class SRV Priority Weight Port Target
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(There is an example near the end of this document.)
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Service
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The symbolic name of the desired service, as defined in Assigned
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Numbers [STD 2] or locally. An underscore (_) is prepended to
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the service identifier to avoid collisions with DNS labels that
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occur in nature.
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Gulbrandsen, et al. Standards Track [Page 2]
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RFC 2782 DNS SRV RR February 2000
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Some widely used services, notably POP, don't have a single
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universal name. If Assigned Numbers names the service
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indicated, that name is the only name which is legal for SRV
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lookups. The Service is case insensitive.
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Proto
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The symbolic name of the desired protocol, with an underscore
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(_) prepended to prevent collisions with DNS labels that occur
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in nature. _TCP and _UDP are at present the most useful values
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for this field, though any name defined by Assigned Numbers or
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locally may be used (as for Service). The Proto is case
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insensitive.
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Name
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The domain this RR refers to. The SRV RR is unique in that the
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name one searches for is not this name; the example near the end
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shows this clearly.
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TTL
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Standard DNS meaning [RFC 1035].
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Class
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Standard DNS meaning [RFC 1035]. SRV records occur in the IN
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Class.
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Priority
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The priority of this target host. A client MUST attempt to
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contact the target host with the lowest-numbered priority it can
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reach; target hosts with the same priority SHOULD be tried in an
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order defined by the weight field. The range is 0-65535. This
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is a 16 bit unsigned integer in network byte order.
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Weight
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A server selection mechanism. The weight field specifies a
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relative weight for entries with the same priority. Larger
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weights SHOULD be given a proportionately higher probability of
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being selected. The range of this number is 0-65535. This is a
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16 bit unsigned integer in network byte order. Domain
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administrators SHOULD use Weight 0 when there isn't any server
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selection to do, to make the RR easier to read for humans (less
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noisy). In the presence of records containing weights greater
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than 0, records with weight 0 should have a very small chance of
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being selected.
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In the absence of a protocol whose specification calls for the
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use of other weighting information, a client arranges the SRV
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RRs of the same Priority in the order in which target hosts,
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Gulbrandsen, et al. Standards Track [Page 3]
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RFC 2782 DNS SRV RR February 2000
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specified by the SRV RRs, will be contacted. The following
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algorithm SHOULD be used to order the SRV RRs of the same
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priority:
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To select a target to be contacted next, arrange all SRV RRs
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(that have not been ordered yet) in any order, except that all
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those with weight 0 are placed at the beginning of the list.
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Compute the sum of the weights of those RRs, and with each RR
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associate the running sum in the selected order. Then choose a
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uniform random number between 0 and the sum computed
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(inclusive), and select the RR whose running sum value is the
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first in the selected order which is greater than or equal to
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the random number selected. The target host specified in the
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selected SRV RR is the next one to be contacted by the client.
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Remove this SRV RR from the set of the unordered SRV RRs and
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apply the described algorithm to the unordered SRV RRs to select
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the next target host. Continue the ordering process until there
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are no unordered SRV RRs. This process is repeated for each
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Priority.
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Port
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The port on this target host of this service. The range is 0-
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65535. This is a 16 bit unsigned integer in network byte order.
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This is often as specified in Assigned Numbers but need not be.
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Target
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The domain name of the target host. There MUST be one or more
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address records for this name, the name MUST NOT be an alias (in
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the sense of RFC 1034 or RFC 2181). Implementors are urged, but
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not required, to return the address record(s) in the Additional
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Data section. Unless and until permitted by future standards
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action, name compression is not to be used for this field.
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A Target of "." means that the service is decidedly not
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available at this domain.
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Domain administrator advice
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Expecting everyone to update their client applications when the first
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server publishes a SRV RR is futile (even if desirable). Therefore
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SRV would have to coexist with address record lookups for existing
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protocols, and DNS administrators should try to provide address
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records to support old clients:
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- Where the services for a single domain are spread over several
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hosts, it seems advisable to have a list of address records at
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the same DNS node as the SRV RR, listing reasonable (if perhaps
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Gulbrandsen, et al. Standards Track [Page 4]
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RFC 2782 DNS SRV RR February 2000
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suboptimal) fallback hosts for Telnet, NNTP and other protocols
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likely to be used with this name. Note that some programs only
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try the first address they get back from e.g. gethostbyname(),
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and we don't know how widespread this behavior is.
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- Where one service is provided by several hosts, one can either
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provide address records for all the hosts (in which case the
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round-robin mechanism, where available, will share the load
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equally) or just for one (presumably the fastest).
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- If a host is intended to provide a service only when the main
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server(s) is/are down, it probably shouldn't be listed in
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address records.
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- Hosts that are referenced by backup address records must use the
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port number specified in Assigned Numbers for the service.
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- Designers of future protocols for which "secondary servers" is
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not useful (or meaningful) may choose to not use SRV's support
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for secondary servers. Clients for such protocols may use or
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ignore SRV RRs with Priority higher than the RR with the lowest
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Priority for a domain.
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Currently there's a practical limit of 512 bytes for DNS replies.
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Until all resolvers can handle larger responses, domain
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administrators are strongly advised to keep their SRV replies below
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512 bytes.
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All round numbers, wrote Dr. Johnson, are false, and these numbers
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are very round: A reply packet has a 30-byte overhead plus the name
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of the service ("_ldap._tcp.example.com" for instance); each SRV RR
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adds 20 bytes plus the name of the target host; each NS RR in the NS
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section is 15 bytes plus the name of the name server host; and
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finally each A RR in the additional data section is 20 bytes or so,
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and there are A's for each SRV and NS RR mentioned in the answer.
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This size estimate is extremely crude, but shouldn't underestimate
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the actual answer size by much. If an answer may be close to the
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limit, using a DNS query tool (e.g. "dig") to look at the actual
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answer is a good idea.
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The "Weight" field
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Weight, the server selection field, is not quite satisfactory, but
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the actual load on typical servers changes much too quickly to be
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kept around in DNS caches. It seems to the authors that offering
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administrators a way to say "this machine is three times as fast as
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that one" is the best that can practically be done.
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Gulbrandsen, et al. Standards Track [Page 5]
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RFC 2782 DNS SRV RR February 2000
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The only way the authors can see of getting a "better" load figure is
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asking a separate server when the client selects a server and
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contacts it. For short-lived services an extra step in the
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connection establishment seems too expensive, and for long-lived
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services, the load figure may well be thrown off a minute after the
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connection is established when someone else starts or finishes a
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heavy job.
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Note: There are currently various experiments at providing relative
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network proximity estimation, available bandwidth estimation, and
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similar services. Use of the SRV record with such facilities, and in
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particular the interpretation of the Weight field when these
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facilities are used, is for further study. Weight is only intended
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for static, not dynamic, server selection. Using SRV weight for
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dynamic server selection would require assigning unreasonably short
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TTLs to the SRV RRs, which would limit the usefulness of the DNS
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caching mechanism, thus increasing overall network load and
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decreasing overall reliability. Server selection via SRV is only
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intended to express static information such as "this server has a
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faster CPU than that one" or "this server has a much better network
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connection than that one".
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The Port number
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Currently, the translation from service name to port number happens
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at the client, often using a file such as /etc/services.
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Moving this information to the DNS makes it less necessary to update
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these files on every single computer of the net every time a new
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service is added, and makes it possible to move standard services out
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of the "root-only" port range on unix.
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Usage rules
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A SRV-cognizant client SHOULD use this procedure to locate a list of
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servers and connect to the preferred one:
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Do a lookup for QNAME=_service._protocol.target, QCLASS=IN,
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QTYPE=SRV.
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If the reply is NOERROR, ANCOUNT>0 and there is at least one
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SRV RR which specifies the requested Service and Protocol in
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the reply:
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If there is precisely one SRV RR, and its Target is "."
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(the root domain), abort.
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Gulbrandsen, et al. Standards Track [Page 6]
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RFC 2782 DNS SRV RR February 2000
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Else, for all such RR's, build a list of (Priority, Weight,
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Target) tuples
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Sort the list by priority (lowest number first)
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Create a new empty list
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For each distinct priority level
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While there are still elements left at this priority
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level
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Select an element as specified above, in the
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description of Weight in "The format of the SRV
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RR" Section, and move it to the tail of the new
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list
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For each element in the new list
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query the DNS for address records for the Target or
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use any such records found in the Additional Data
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section of the earlier SRV response.
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for each address record found, try to connect to the
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(protocol, address, service).
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else
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Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A
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for each address record found, try to connect to the
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(protocol, address, service)
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Notes:
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- Port numbers SHOULD NOT be used in place of the symbolic service
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or protocol names (for the same reason why variant names cannot
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be allowed: Applications would have to do two or more lookups).
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- If a truncated response comes back from an SRV query, the rules
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described in [RFC 2181] shall apply.
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- A client MUST parse all of the RR's in the reply.
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- If the Additional Data section doesn't contain address records
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for all the SRV RR's and the client may want to connect to the
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target host(s) involved, the client MUST look up the address
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record(s). (This happens quite often when the address record
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has shorter TTL than the SRV or NS RR's.)
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Gulbrandsen, et al. Standards Track [Page 7]
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RFC 2782 DNS SRV RR February 2000
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- Future protocols could be designed to use SRV RR lookups as the
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means by which clients locate their servers.
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Fictional example
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This example uses fictional service "foobar" as an aid in
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understanding SRV records. If ever service "foobar" is implemented,
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it is not intended that it will necessarily use SRV records. This is
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(part of) the zone file for example.com, a still-unused domain:
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$ORIGIN example.com.
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@ SOA server.example.com. root.example.com. (
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1995032001 3600 3600 604800 86400 )
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NS server.example.com.
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NS ns1.ip-provider.net.
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NS ns2.ip-provider.net.
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; foobar - use old-slow-box or new-fast-box if either is
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; available, make three quarters of the logins go to
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; new-fast-box.
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_foobar._tcp SRV 0 1 9 old-slow-box.example.com.
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SRV 0 3 9 new-fast-box.example.com.
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; if neither old-slow-box or new-fast-box is up, switch to
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; using the sysdmin's box and the server
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SRV 1 0 9 sysadmins-box.example.com.
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SRV 1 0 9 server.example.com.
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server A 172.30.79.10
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old-slow-box A 172.30.79.11
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sysadmins-box A 172.30.79.12
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new-fast-box A 172.30.79.13
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; NO other services are supported
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*._tcp SRV 0 0 0 .
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*._udp SRV 0 0 0 .
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Gulbrandsen, et al. Standards Track [Page 8]
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RFC 2782 DNS SRV RR February 2000
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In this example, a client of the "foobar" service in the
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"example.com." domain needs an SRV lookup of
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"_foobar._tcp.example.com." and possibly A lookups of "new-fast-
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box.example.com." and/or the other hosts named. The size of the SRV
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reply is approximately 365 bytes:
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30 bytes general overhead
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20 bytes for the query string, "_foobar._tcp.example.com."
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130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new-
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fast-box", "old-slow-box", "server" and "sysadmins-box" -
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"example.com" in the query section is quoted here and doesn't
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need to be counted again.
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75 bytes for 3 NS RRs, 15 bytes each plus the lengths of "server",
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"ns1.ip-provider.net." and "ns2" - again, "ip-provider.net." is
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quoted and only needs to be counted once.
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120 bytes for the 6 address records (assuming IPv4 only) mentioned
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by the SRV and NS RR's.
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IANA Considerations
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The IANA has assigned RR type value 33 to the SRV RR. No other IANA
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services are required by this document.
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Changes from RFC 2052
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This document obsoletes RFC 2052. The major change from that
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previous, experimental, version of this specification is that now the
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protocol and service labels are prepended with an underscore, to
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lower the probability of an accidental clash with a similar name used
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for unrelated purposes. Aside from that, changes are only intended
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to increase the clarity and completeness of the document. This
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document especially clarifies the use of the Weight field of the SRV
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records.
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Security Considerations
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The authors believe this RR to not cause any new security problems.
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Some problems become more visible, though.
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- The ability to specify ports on a fine-grained basis obviously
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changes how a router can filter packets. It becomes impossible
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to block internal clients from accessing specific external
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services, slightly harder to block internal users from running
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unauthorized services, and more important for the router
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operations and DNS operations personnel to cooperate.
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- There is no way a site can keep its hosts from being referenced
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as servers. This could lead to denial of service.
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Gulbrandsen, et al. Standards Track [Page 9]
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RFC 2782 DNS SRV RR February 2000
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- With SRV, DNS spoofers can supply false port numbers, as well as
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host names and addresses. Because this vulnerability exists
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already, with names and addresses, this is not a new
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vulnerability, merely a slightly extended one, with little
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practical effect.
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References
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STD 2: Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
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1700, October 1994.
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RFC 1034: Mockapetris, P., "Domain names - concepts and facilities",
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STD 13, RFC 1034, November 1987.
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RFC 1035: Mockapetris, P., "Domain names - Implementation and
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Specification", STD 13, RFC 1035, November 1987.
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RFC 974: Partridge, C., "Mail routing and the domain system", STD
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14, RFC 974, January 1986.
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BCP 14: Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
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RFC 2181: Elz, R. and R. Bush, "Clarifications to the DNS
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Specification", RFC 2181, July 1997.
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RFC 2219: Hamilton, M. and R. Wright, "Use of DNS Aliases for Network
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Services", BCP 17, RFC 2219, October 1997.
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BCP 14: Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", BCP 14, RFC 2119, March 1997.
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ARM: Armijo, M., Esibov, L. and P. Leach, "Discovering LDAP
|
||
Services with DNS", Work in Progress.
|
||
|
||
KDC-DNS: Hornstein, K. and J. Altman, "Distributing Kerberos KDC and
|
||
Realm Information with DNS", Work in Progress.
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
Gulbrandsen, et al. Standards Track [Page 10]
|
||
|
||
RFC 2782 DNS SRV RR February 2000
|
||
|
||
|
||
Acknowledgements
|
||
|
||
The algorithm used to select from the weighted SRV RRs of equal
|
||
priority is adapted from one supplied by Dan Bernstein.
|
||
|
||
Authors' Addresses
|
||
|
||
Arnt Gulbrandsen
|
||
Troll Tech
|
||
Waldemar Thranes gate 98B
|
||
N-0175 Oslo, Norway
|
||
|
||
Fax: +47 22806380
|
||
Phone: +47 22806390
|
||
EMail: arnt@troll.no
|
||
|
||
|
||
Paul Vixie
|
||
Internet Software Consortium
|
||
950 Charter Street
|
||
Redwood City, CA 94063
|
||
|
||
Phone: +1 650 779 7001
|
||
|
||
|
||
Levon Esibov
|
||
Microsoft Corporation
|
||
One Microsoft Way
|
||
Redmond, WA 98052
|
||
|
||
EMail: levone@microsoft.com
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
Gulbrandsen, et al. Standards Track [Page 11]
|
||
|
||
RFC 2782 DNS SRV RR February 2000
|
||
|
||
|
||
Full Copyright Statement
|
||
|
||
Copyright (C) The Internet Society (2000). All Rights Reserved.
|
||
|
||
This document and translations of it may be copied and furnished to
|
||
others, and derivative works that comment on or otherwise explain it
|
||
or assist in its implementation may be prepared, copied, published
|
||
and distributed, in whole or in part, without restriction of any
|
||
kind, provided that the above copyright notice and this paragraph are
|
||
included on all such copies and derivative works. However, this
|
||
document itself may not be modified in any way, such as by removing
|
||
the copyright notice or references to the Internet Society or other
|
||
Internet organizations, except as needed for the purpose of
|
||
developing Internet standards in which case the procedures for
|
||
copyrights defined in the Internet Standards process must be
|
||
followed, or as required to translate it into languages other than
|
||
English.
|
||
|
||
The limited permissions granted above are perpetual and will not be
|
||
revoked by the Internet Society or its successors or assigns.
|
||
|
||
This document and the information contained herein is provided on an
|
||
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
|
||
TASK FORCE DISCLAIMS 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.
|
||
|
||
Acknowledgement
|
||
|
||
Funding for the RFC Editor function is currently provided by the
|
||
Internet Society.
|