Applications Area Arnt Gulbrandsen
INTERNET-DRAFT Troll Technologies
<draft-ietf-dnsind-rfc2052bis-02.txt> Paul Vixie
Obsoletes: RFC 2052 Internet Software Consortium
January 1999
A DNS RR for specifying the location of services (DNS SRV)
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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Abstract
This document describes a DNS RR which specifies the location of the
server(s) for a specific protocol and domain (like a more general
form of MX).
Overview and rationale
Currently, one must either know the exact address of a server to
contact it, or broadcast a question. This has led to, for example,
ftp.whatever.com aliases [RFC 2219], the SMTP-specific MX RR, and
using MAC-level broadcasts to locate servers.
The SRV RR allows administrators to use several servers for a single
domain, to move services from host to host with little fuss, and to
designate some hosts as primary servers for a service and others as
backups.
Clients ask for a specific service/protocol for a specific domain
(the word domain is used here in the strict RFC 1034 sense), and get
back the names of any available servers.
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Note that where this document refers to "address records", it means A
RR's, AAAA RR's, or their most modern equivalent.
Introductory example
If a SRV-cognizant web-browser wants to retrieve
it does a lookup of
and retrieves the document from one of the servers in the reply. The
example zone file near the end of this memo contains answering RRs
for this query.
Definitions
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT" and "MAY"
used in this document are to be interpreted as specified in BCP 14.
Other terms used in this document are defined in the DNS
specification, RFC 1034.
The format of the SRV RR
Here is the format of the SRV RR, whose DNS type code is 33:
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
(There is an example near the end of this document.)
Service
The symbolic name of the desired service, as defined in Assigned
Numbers [STD 2] or locally. An underscore (_) is prepended to
the service identifier to avoid collisions with DNS labels that
occur in nature.
Some widely used services, notably POP, don't have a single
universal name. If Assigned Numbers names the service
indicated, that name is the only name which is legal for SRV
lookups. Only locally defined services may be named locally.
The Service is case insensitive.
Proto
The symbolic name of the desired protocol, with an underscore
(_) 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
for this field, though any name defined by Assigned Numbers or
locally may be used (as for Service). The Proto is case
insensitive.
Name
The domain this RR refers to. The SRV RR is unique in that the
name one searches for is not this name; the example near the end
shows this clearly.
TTL
Standard DNS meaning [RFC 1035].
Class
Standard DNS meaning [RFC 1035]. SRV records occur in the IN
Class.
Priority
As for MX, the priority of this target host. A client MUST
attempt to contact the target host with the lowest-numbered
priority it can reach; target hosts with the same priority
SHOULD be tried in an order defined by the weight field. The
range is 0-65535. This is a 16 bit binary integer in network
byte order.
Weight
A load balancing mechanism. When selecting a target host among
the those that have the same priority, the chance of trying this
one first SHOULD be proportional to its weight, as specified
below. Larger weights lead to a higher probability of being
selected. The range of this number is 0-65535. This is a 16
bit binary integer in network byte order. Domain administrators
are urged to use Weight 0 when there isn't any load balancing to
do, to make the RR easier to read for humans (less noisy). In
the presence records containing weights greater than 0, records
with weight 0 have a very small chance of being selected.
To choose the target, the client SHOULD implement the effect of
this algorithm. This permits administrators to plan weights to
achieve the load distribution desired. Each time a target is
needed, the client should order the remaining (not previously
used) SRV RRs at the current priority in any random fashion,
except placing all those with weight 0 at the beginning of the
list. Compute the sum of the weights of those RRs, and with
each RR associate the running sum in the selected order. Then
choose a random number (not necessarily of integral value)
between 0 and the sum computed (inclusive), and select the RR
whose running sum value is the first in the selected order which
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is greater than or equal to the random number selected.
Port
The port on this target host of this service. The range is
0-65535. This is a 16 bit binary integer in network byte order.
This is often as specified in Assigned Numbers but need not be.
Target
As for MX, the domain name of the target host. There MUST be
one or more address records for this name, the name MUST NOT be
an alias (in the sense of RFC 1034 or RFC 2181). Implementors
are urged, but not required, to return the address record(s) in
the Additional Data section. Unless and until permitted by
future standards action, name compression is not to be used for
this field.
A Target of "." means that the service is decidedly not
available at this domain.
Applicability Statement
In general, it is expected that SRV records will be used by clients
for applications where the relevant protocol specification indicates
that clients should use the SRV record. The examples in this
document use familiar protocols as an aid in understanding. It is
not intended that those protocols will necessarily use SRV records.
Domain administrator advice
Expecting everyone to update their client applications when the first
internet site adds a SRV RR for some server is futile (even if
desirable). Therefore SRV would have to coexist with address record
lookups for existing protocols, and DNS administrators should try to
provide address records to support old clients:
- Where the services for a single domain are spread over several
hosts, it seems advisable to have a list of address records at
the same DNS node as the SRV RR, listing reasonable (if perhaps
suboptimal) fallback hosts for Telnet, NNTP and other protocols
likely to be used with this name. Note that some programs only
try the first address they get back from e.g. gethostbyname(),
and we don't know how widespread this behavior is.
- Where one service is provided by several hosts, one can either
provide address records for all the hosts (in which case the
round-robin mechanism, where available, will share the load
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
server(s) is/are down, it probably shouldn't be listed in
address records.
- Hosts that are referenced by backup address records must use the
port number specified in Assigned Numbers for the service.
- Designers of future protocols for which "secondary servers" is
not useful (or meaningful) may choose to not use SRV's support
for secondary servers. Clients for such protocols may use or
ignore SRV RRs with Priority higher than the RR with the lowest
Priority for a domain.
Currently there's a practical limit of 512 bytes for DNS replies.
Until all resolvers can handle larger responses, domain
administrators are strongly advised to keep their SRV replies below
512 bytes.
All round numbers, wrote Dr. Johnson, are false, and these numbers
are very round: A reply packet has a 30-byte overhead plus the name
of the service ("_telnet._tcp.example.com" for instance); each SRV RR
adds 20 bytes plus the name of the target host; each NS RR in the NS
section is 15 bytes plus the name of the name server host; and
finally each A RR in the additional data section is 20 bytes or so,
and there are A's for each SRV and NS RR mentioned in the answer.
This size estimate is extremely crude, but shouldn't underestimate
the actual answer size by much. If an answer may be close to the
limit, using a DNS query tool (e.g. "dig") to look at the actual
answer is a good idea.
The "Weight" field
Weight, the load balancing field, is not quite satisfactory, but the
actual load on typical servers changes much too quickly to be kept
around in DNS caches. It seems to the authors that offering
administrators a way to say "this machine is three times as fast as
that one" is the best that can practically be done.
The only way the authors can see of getting a "better" load figure is
asking a separate server when the client selects a server and
contacts it. For short-lived services like SMTP an extra step in the
connection establishment seems too expensive, and for long-lived
services like telnet, the load figure may well be thrown off a minute
after the connection is established when someone else starts or
finishes a heavy job.
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The Port number
Currently, the translation from service name to port number happens
at the client, often using a file such as /etc/services.
Moving this information to the DNS makes it less necessary to update
these files on every single computer of the net every time a new
service is added, and makes it possible to move standard services out
of the "root-only" port range on unix.
Usage rules
A SRV-cognizant client SHOULD use this procedure to locate a list of
servers and connect to the preferred one:
Do a lookup for QNAME=_service._protocol.target, QCLASS=IN,
QTYPE=SRV.
If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV
RR which specifies the requested Service and Protocol in the
reply:
If there is precisely one SRV RR, and its Target is "."
(the root domain), abort.
Else, for all such RR's, build a list of (Priority, Weight,
Target) tuples
Sort the list by priority (lowest number first)
Create a new empty list
For each distinct priority level
While there are still elements left at this priority
level
Select an element randomly, with probability
Weight, as specified above, and move it to the
tail of the new list
For each element in the new list
query the DNS for address records for the Target or
use any such records found in the Additional Data
section of the earlier SRV response.
for each address record found, try to connect to the
(protocol, address, service).
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else if the service desired is SMTP (and SMTP has been defined
elsewhere to expect SRV lookups)
skip to RFC 974 (MX).
else
Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A
for each address record found, try to connect to the
(protocol, address, service)
Notes:
- Port numbers SHOULD NOT be used in place of the symbolic service
or protocol names (for the same reason why variant names cannot
be allowed: Applications would have to do two or more lookups).
- If a truncated response comes back from an SRV query, the rules
described in [RFC2181] shall apply.
- A client MAY use means other than Weight to choose among target
hosts with equal Priority.
- A client MUST parse all of the RR's in the reply.
- If the Additional Data section doesn't contain address records
for all the SRV RR's and the client may want to connect to the
target host(s) involved, the client MUST look up the address
record(s). (This happens quite often when the address record
has shorter TTL than the SRV or NS RR's.)
- Future protocols could be designed to use SRV RR lookups as the
means by which clients locate their servers.
Fictional example
This is (part of) the zone file for example.com, a still-unused
domain:
$ORIGIN example.com.
@ SOA server.example.com. root.example.com. (
1995032001 3600 3600 604800 86400 )
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_ftp._tcp SRV 0 0 21 server.example.com.
_finger._tcp SRV 0 0 79 server.example.com.
; telnet - use old-slow-box or new-fast-box if either is
; available, make three quarters of the logins go to
; new-fast-box.
_telnet._tcp SRV 0 1 23 old-slow-box.example.com.
SRV 0 3 23 new-fast-box.example.com.
; if neither old-slow-box or new-fast-box is up, switch to
; using the sysdmin's box and the server
SRV 1 0 23 sysadmins-box.example.com.
SRV 1 0 23 server.example.com.
; HTTP - server is the main server, new-fast-box is the backup
; (On new-fast-box, the HTTP daemon runs on port 8000)
_http._tcp SRV 0 0 80 server.example.com.
SRV 10 0 8000 new-fast-box.example.com.
; since we want to support both http://example.com/ and
; http://www.example.com/ we need the next two RRs as well
_http._tcp.www SRV 0 0 80 server.example.com.
SRV 10 0 8000 new-fast-box.example.com.
; SMTP - mail goes to the server, and to the IP provider if
; the net is down
_smtp._tcp SRV 0 0 25 server.example.com.
SRV 1 0 25 mailhost.ip-provider.net.
@ MX 0 server.example.com.
; NNTP - use the IP provider's NNTP server
_nntp._tcp SRV 0 0 119 nntphost.ip-provider.net.
; IDB is an locally defined protocol
_idb._tcp SRV 0 0 2025 new-fast-box.example.com.
; addresses
server A 172.30.79.10
old-slow-box A 172.30.79.11
sysadmins-box A 172.30.79.12
new-fast-box A 172.30.79.13
; backup address records - new-fast-box and old-slow-box are
; included, naturally, and server is too, but might go
; if the load got too bad
@ A 172.30.79.10
A 172.30.79.11
A 172.30.79.13
; backup address record for www.example.com
www A 172.30.79.10
; NO other services are supported
*._tcp SRV 0 0 0 .
*._udp SRV 0 0 0 .
In this example, a telnet connection to "example.com." needs an SRV
lookup of "_telnet._tcp.example.com." and possibly A lookups of "new-
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fast-box.example.com." and/or the other hosts named. The size of the
SRV reply is approximately 365 bytes:
30 bytes general overhead
20 bytes for the query string, "_telnet._tcp.example.com."
130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new-
fast-box", "old-slow-box", "server" and "sysadmins-box" -
"example.com" in the query section is quoted here and doesn't
need to be counted again.
75 bytes for 3 NS RRs, 15 bytes each plus the lengths of "server",
"ns1.ip-provider.net." and "ns2" - again, "ip-provider.net." is
quoted and only needs to be counted once.
120 bytes for the 6 address records (assuming IPv4 only) mentioned
by the SRV and NS RR's.
IANA Considerations
The IANA has assigned RR type value 33 to the SRV RR. No other IANA
services are required by this document.
Changes from RFC 2052
This document obsoletes RFC 2052. The major change from that
previous, experimental, version of this specification is that now the
protocol and service labels are prepended with an underscore, to
lower the probability of an accidental clash with a similar name used
for unrelated purposes. Aside from that, changes are only intended
to increase the clarity and completeness of the document.
Security Considerations
The authors believes this RR to not cause any new security problems.
Some problems become more visible, though.
- The ability to specify ports on a fine-grained basis obviously
changes how a router can filter packets. It becomes impossible
to block internal clients from accessing specific external
services, slightly harder to block internal users from running
unauthorized services, and more important for the router
operations and DNS operations personnel to cooperate.
- There is no way a site can keep its hosts from being referenced
as servers (as, indeed, some sites become unwilling secondary
MXes today). This could lead to denial of service.
- With SRV, DNS spoofers can supply false port numbers, as well as
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host names and addresses. Because this vunerability exists
already, with names and addresses, this is not a new
vunerability, merely a slightly extended one, with little
practical effect.
References
STD 2: Reynolds, J., Postel, J., "Assigned Numbers", STD 2, RFC 1700,
October 1994 (as currently updated by the IANA).
RFC 1034: Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
RFC 1035: Mockapetris, P., "Domain names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
RFC 974: Partridge, C., "Mail routing and the domain system", RFC
974, January 1986.
BCP 14: Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
RFC 2181: Elz, R., Bush, R., "Clarifications to the DNS
Specification", RFC 2181, July 1997
RFC 2219: Hamilton, M., Wright, R., "Use of DNS Aliases for Network
Services", BCP 17, RFC 2219, October 1997
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 Paul Vixie
Troll Tech Internet Software Consortium
Postboks 6133 Etterstad 950 Charter Street
N-0602 Oslo, Norway Redwood City, CA 94063
+47 22646966 +1 650 779 7001
<agulbra@troll.no> <paul@vix.com>
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