Network Working Group C. Jennings
Internet-Draft Cisco Systems
Intended status: Informational February 22, 2009
Expires: August 26, 2009
DNS SRV Records for HTTP
draft-jennings-http-srv-01
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Abstract
This document specifies a new URI scheme called http+srv which uses a
DNS SRV lookup to locate a HTTP server. The http+srv scheme operates
in the same way as an http scheme but instead of the normal DNS A
record lookup that a http scheme would use, it uses an DNS SRV
lookup. This memo also defines a https+srv scheme that operates in
the same was a an https URI but uses DNS SRV lookups.
The draft is being discussed on the apps-discuss@ietf.org list.
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1. Introduction
Web services often define APIs where software running on machine in a
data center acts as an HTTP client and performs some http
transactions to another HTTP server. For example, a portal such as
Facebook can act as a http client and call specific HTTP-based APIs
on other http servers. The reality of current networks is a large
portion of the hosts have NATed addresses and often can not run on
port 80. This is likely to become more common with the deployment of
Carrier Grade NAT. DNS SRV records allow a DNS lookup of a name like
www.example.com to provide both a port and the IP addresses of the
HTTP server.
This specification defines two new URI schemes, http+srv, and https+
srv which are like http and https respectively. When a http client
uses one of theses schemes to locate a web server, it starts by doing
a DNS SRV record lookup and if one is found, uses that result. If no
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SRV record is found, it falls back to a DNS A record. The
specification does not update or modify HTTP in any way.
It is not expected that most web browsers would support these schemes
for generic web use. It would instead be used for particular
applications using HTTP such as specific web APIs. These APIs would
be defined to require the use of this specification. In this
situation, the end user's web browser might not do the SRV lookup
when it browsed to the portal web pages, but the HTTP calls that the
portal made out to other sites to generate the content would use this
mechanism. As such architectures become more common, DNS SRV would
allow many servers that are just providing an API to run on ports
other than 80 even though main portal sites may still be running on
the well known ports. Eventually, web browsers may end up supporting
these SRV lookups, as the implementation is trivial and has very
little downside.
This technique is useful where users wish to run a web server behind
a NAT but cannot control which port the NAT will allocate for the
service. It is also useful where several users want to run different
web servers on the same machine. A third use case for HTTP SRV is a
situation in which all requests should be sent to a primary server,
but if that server is down, then requests should fall back to an
alternative server.
2. Terminology
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 RFC 2119 [RFC2119].
3. Mechanisms
Applications compliant with this specification MUST perform an SRV
lookup as specified in [RFC2782] when resolving the host portion of a
http+srv or https+srv URI. As defined in the IANA port numbers
registry, the service names used are _http and _https respectively.
As described in RFC 2782, if no SRV record is present, the resolution
will fall back on using other DNS records that would be used by a
http scheme as defined in HTTP[RFC2616]. The rest of the http+srv
URI is processed in the same way as an http URI in RFC 2616 while the
rest of a https+srv scheme URI is processed the same way as a https
URI as defined in [RFC2818].
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4. Example
In the following example, the client will do a lookup on the URI,
which finds the SRV record that then points at the A record that
points at the IP address.
URI: https+srv://example.com
DNS SRV RR: _http._tcp.example.com. SRV 1 0 8080 host1.example.com.
DNS A RR: host1.example.com. A 192.0.2.88
Figure 1
In this case the client would form a TCP connection to 192.0.2.88:
8080 then start TLS over that connection. Note that the certificate
in the TLS handshake would be matched to example.com as that was the
names used in the URI and it would not be matched to
host1.example.com.
5. IANA Considerations
This specification registers two provisional URI schemes.
5.1. http+srv URI scheme
URI scheme name:
http+srv
Status:
provisional
URI scheme syntax:
Identical to http URI as defined in RFC 2616 but using the 'http+
srv' protocol identifier in place of the 'http' protocol
identifier
URI scheme semantics:
See draft-jennings-http-uri
Encoding considerations:
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No special considerations
Applications/protocols that use this URI scheme name:
Applications which need to lookup http servers using DNS SRV
Interoperability considerations.:
None known
Security considerations.:
Same as http URI. See RFC 2616
Contact.:
Cullen Jennings <fluffy@cisco.com>
Author/Change controller.:
Cullen Jennings <fluffy@cisco.com>
References.:
draft-jennings-http-srv
RFC 3986
RFC 2616
5.2. https+srv URI scheme
URI scheme name:
https+srv
Status:
provisional
URI scheme syntax:
Identical to http URI as defined in RFC 2818 but using the 'https+
srv' protocol identifier in place of the 'https' protocol
identifier
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URI scheme semantics:
See draft-jennings-http-uri
Encoding considerations:
No special considerations
Applications/protocols that use this URI scheme name:
Applications which need to lookup http servers using DNS SRV
Interoperability considerations.:
None known
Security considerations.:
Same as https URI. See RFC 2818
Contact.:
Cullen Jennings <fluffy@cisco.com>
Author/Change controller.:
Cullen Jennings <fluffy@cisco.com>
References.:
draft-jennings-http-srv
RFC 3986
RFC 2818
6. Security Considerations
This introduces no new security considerations beyond the common
usage of HTTP. It is analogous to DNS CNAME records that redirect to
other A records.
7. Acknowledgements
Variants of this idea has been proposed by many people, including
Mark Andrews and Thor Kottelin in an internet draft in 2000. Some
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text came from various documents by Ted Hardie.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
Author's Address
Cullen Jennings
Cisco Systems
170 West Tasman Drive
Mailstop SJC-30
San Jose, CA 95134
USA
Phone: +1 408 902-3341
Email: fluffy@cisco.com
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