Internet DRAFT - draft-andrews-http-srv
draft-andrews-http-srv
Network Working Group M. Andrews
Internet-Draft ISC
Expires: November 21, 2014 T. Kottelin
May 20, 2014
Use of SRV records in conjuction with HTTP and URIs"
draft-andrews-http-srv-02
Abstract
The combined use of SRV records for HTTP along with URIs is not as
straight forward as it would appear at first glance. This document
looks at the issues involved and recommends solutions.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. URIs without an explicit port specification . . . . . . . . . . 3
3. URIs with a explicit port specification . . . . . . . . . . . . 4
4. Transitioning Considerations . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Normative References . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Many of todays HTTP sites are virtual, that is they are hosted on a
machine that is not known by the name the HTTP site is known by.
This leads to the problem of how to rationally give these HTTP sites
IP addresses. This has traditionally been done by using CNAMES
[RFC1034] [RFC1035] or by using explicit IP address records where
CNAMES are illegal due to restrictions in the DNS.
Both of these solutions have undesired side effects. CNAMES are not
protocol specific. Using IP address records is a logistic nightmare
for large servers with many virtual sites. This is becoming a bigger
problem as companies move away from identifying their HTTP site with
a "www" prefix and just use their delegated domain name, e.g.
"http://example.com/".
Using SRV [RFC2782] records would seem to be a natural solution to
this problem in that they are protocol specific and will work where
CNAMES are illegal in the DNS.
There are problems with doing this without thought however in that
URIs [RFC3986] can specify a port and SRV records do specify a port.
When this occurs which one do you honour?
In addition to this SRV records provide for load balancing. For most
protocols this is straight forward as there will only be a single
connection made. For HTTP however there are often many connections
made in a session. Should each of these individual connections be
load balanced or should the load balancing be on a per session basis?
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", "NOT
RECOMMENDED", and "MAY" in this document are to be interpreted as
described in [RFC2119].
2. URIs without an explicit port specification
If the URI does not explicitly specify a port to connect to, i.e. the
URI does not contain a ":<port>" part, there is no port conflict. In
this case a client MUST follow the logic specified in [RFC2782],
including the server selection mechanism provided by the priority and
weight fields. If SRV records do not exist then the client MUST fall
back to looking for IP address records.
Once a server is selected it SHOULD be continued to be used for the
rest of the session if possible after an initial connection is made.
If a server has multiple addresses the client SHOULD continue to use
the same address while possible taking into consideration ttl values
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on address records. If connections to this address fail it SHOULD
try the other addresses for the server first before attempting other
servers.
The use of a SRV record does not affect the contents of the "Host:"
field of the HTTP transaction. Its only effect is to potentially
change the address and port the client connects to. All other parts
of the HTTP transaction are not affected by the presence of a SRV
record.
Examples:
Single SRV record:
URI: http://example.com/
SRV RR: _http._tcp.example.com. SRV 10 0 8080 host1.example.com.
A RRs: example.com. A 10.0.0.2
host1.example.com. A 10.0.1.1
Connect to: 10.0.1.1 port 8080
Multiple SRV records:
URI: http://example.com/
SRV RRs: _http._tcp.example.com. SRV 10 1 8080 host1.example.com.
_http._tcp.example.com. SRV 10 3 8080 host2.example.com.
_http._tcp.example.com. SRV 20 0 8080 host3.example.com.
A RRs: example.com. A 10.0.0.4
host1.example.com. A 10.0.1.2
host2.example.com. A 10.0.2.2
host3.example.com. AAAA 1080::8:800:200C:417A
Connect to: 10.0.1.2 port 8080 or 10.0.2.2 port 8080 if either is
available (the probability of being selected should be 25% for
10.0.1.2 port 8080, and 75% for 10.0.2.2 port 8080); otherwise, try
1080::8:800:200C:417A port 8080.
3. URIs with a explicit port specification
If the URI does explicitly specify a port, other than the default
port, to connect to then there is a potential conflict in the port
specification between the URI and the SRV records, and the SRV record
is ignored. In this case the user agent MUST query for address
records for the host name in the URI (instead of SRV records).
If the server has multiple addresses the client SHOULD continue to
use the same address while possible taking into consideration ttl
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values on address records.
Note [RFC3986], Section 6.2.3. Scheme-Based Normalization states
that URIs with a port value equal to the default port (80) are
identical to those with no port or a empty port.
Examples:
Default port specified:
URI: http://example.com:80/
SRV RR: _http._tcp.example.com. SRV 10 1 8080 host2.example.com.
A RRs: example.com. A 10.0.0.1
host2.example.com. A 10.0.2.2
Connect to: 10.0.0.2 port 8080
Non-default port specified:
URI: http://example.com:8080/
SRV RR: _http._tcp.example.com. SRV 10 1 80 host2.example.com.
CNAME RR: example.com. CNAME host1.example.com.
A RRs: host1.example.com. A 10.0.0.1
host2.example.com. AAAA 1080::8:800:200C:417A
Connect to: 10.0.0.1 port 8080
4. Transitioning Considerations
When transitioning from using a non-SRV solution to using a SRV based
solution old, non-SRV aware, clients will continue to look for
address records. It may be necessary to use redirection at the HTTP
layer to direct these clients to the new servers if the SRV records
point to a different <address, port> tuple.
It will also be necessary to continue to provide the existing address
/ CNAME records until there is a significant percentage of SRV aware
clients. Experience has shown that this should be within one to two
years of the introduction of the first SRV aware client.
In cases where you are just trying to replace the A or CNAME record
referring to a service providers machine with a SRV record the
following should suffice.
The service provider is hosting the service on machine.example.net
and you are example.com.
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example.com. A <IP address of machine.example.net>
_http._tcp.example.com. SRV 0 0 80 machine.example.net.
5. Security Considerations
The authors believe the algorithm described in this document to not
cause any new security problems. However care should be taken as SRV
and non-SRV aware clients may be directed to different locations.
6. IANA Considerations
A well known label has to be allocated for the first label of the
http SRV record. This document has used "_http".
7. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[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
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
Authors' Addresses
M. Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
US
Email: marka@isc.org
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T. Kottelin
Laivalahden puistotie 10 C 37
Helsinki FIN-00810
Finland
Email: thor@anta.net
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