Q:

Correct behavior with If-Match the header?

According to RFC 2616, generation of entity tags by HTTP servers is optional. However, I couldn't find what a conditionally compliant HTTP server should do if it receives an If-Match (or If-None-Match) header. Should it just ignore those headers or should it respond with 412 Precondition Failed?  
UPD: Just to clarify, I'm assuming that the server in question does not support entity tags.

A:

There is a handy HTTP response status codes activity diagram that you can use to answer this question.
If you don’t support ETag and the request contains a If-Match value other than *, you would respond with a 412. And If-None-Match with values other than * can be completely ignored.

A:

In contrast to an If-None-Match header (ignorance of which only harms performance), an IF-MATCH request should almost certainly fail and return a HTTP/412 if the server cannot match the requested entity. Probably the most common use of the IF-MATCH header is when the client is doing a Range request, and unless the server can confirm that the resource was not modified, it should not return the requested range because the result could be data corruption on the client.
Now, if the server knows it's not a Range request or knows that the client entity must, in fact, match (e.g. because the server never allows updates to its entities) then acting as if the header wasn't present may make sense in that limited circumstance.

A:

While the RFC2616 is implicit on the matter, you can deduce from, say 14.26 (If-None-Match), that if the server cannot match the resource with the tag, then it should go ahead with request). The 412 code, based upon my understanding of RFC2616 is intended for requests which modify state (e.g. PUT, POST, DELETE).
So, in essence, if the tag doesn't match (and when it's absent on server side is only one of many possible scenarios), then the server should go ahead with the request.

