Network Working Group R. Fielding Request for Comments: 2616 ...

A client MUST include a Host header field in all HTTP/1.1 requestmessages . If the requested URI does not include an Internet hostname for the service being requested, then the Host header field MUSTbe given with an empty value. An HTTP/1.1 proxy MUST ensure that anyrequest message it forwards does contain an appropriate Host headerfield that identifies the service being requested by the proxy. AllInternet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)status code to any HTTP/1.1 request message which lacks a Host headerfield.See sections 5.2 and 19.6.1.1 for other requirements relating toHost.14.24 If-MatchThe If-Match request-header field is used with a method to make itconditional. A client that has one or more entities previouslyobtained from the resource can verify that one of those entities iscurrent by including a list of their associated entity tags in theIf-Match header field. Entity tags are defined in section 3.11. Thepurpose of this feature is to allow efficient updates of cachedinformation with a minimum amount of transaction overhead. It is alsoused, on updating requests, to prevent inadvertent modification ofthe wrong version of a resource. As a special case, the value "*"matches any current entity of the resource.If-Match = "If-Match" ":" ( "*" | 1#entity-tag )If any of the entity tags match the entity tag of the entity thatwould have been returned in the response to a similar GET request(without the If-Match header) on that resource, or if "*" is givenFielding, et al. Standards Track [Page 129]RFC 2616 HTTP/1.1 June 1999and any current entity exists for that resource, then the server MAYperform the requested method as if the If-Match header field did notexist.A server MUST use the strong comparison function (see section 13.3.3)to compare the entity tags in If-Match.If none of the entity tags match, or if "*" is given and no currententity exists, the server MUST NOT perform the requested method, andMUST return a 412 (Precondition Failed) response. This behavior ismost useful when the client wants to prevent an updating method, such