DNS SRV Records for HTTP
draft-jennings-http-srv-01
Status of this Memo
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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.
Legal
This documents and the information contained therein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION THEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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 http://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 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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
3. Mechanisms
Applications compliant with this specification MUST perform an SRV lookup as specified in [RFC2782] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) 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] (Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1,” June 1999.). 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] (Rescorla, E., “HTTP Over TLS,” May 2000.).
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
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:
- 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
- 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 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 (TXT, HTML, XML). |
| [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 (TXT, PS, PDF, HTML, XML). |
| [RFC2782] | Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” RFC 2782, February 2000 (TXT). |
| [RFC2818] | Rescorla, E., “HTTP Over TLS,” RFC 2818, May 2000 (TXT). |
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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