306 Switch Proxy
The HTTP specifications, starting from RFC2616 have a gap between
the 305 and 307 status codes.
The specification states:
The 306 status code was defined in a previous version of this specification, is no longer used, and the code is reserved.
Naturally I was curious what it used to be, so I went through all the drafts of RFC2616 and RFC2068, but couldn’t find a mention of it, except that it was reserved.
After some googling and digging into old mailing list archives, I found a
separate draft unrelated to the core specification. This document from
1996 wants to define 306 as 306 Switch Proxy.
The purpose of the response is a bit different from 305. The idea is that if
a HTTP client already used a proxy, this response can be used to tell a client
that the client should start using a new proxy.
This was deemed as a security risk, and ultimately dropped from the
specifications. 306 should not be used by anyone. It’s still an interesting
bit of HTTP history though!
Example
HTTP/1.1 306 Switch Proxy
Set-Proxy: SET; proxyURI="https://proxy.example.org:8080/" scope="http://", seconds=100
The preceding response would tell a client to use proxy.example.org for all
http:// requests for the next 100 seconds.
The scope could also be limited to specific domains, specific paths or
everything using *.
References
- RFC2616, Section-10.3.7 -
306 (Unused). - draft-cohen-http-305-306-responses.