What HTTP response headers are required

It depends on what you define as being required: there are no header fields that must be sent with every response no matter what the circumstances are, but there are header fields that you really should send. The only header field that comes close is Date, but even it has circumstances under which it is not required.

In the parlance of RFC 2119, the term MUST means that something is a requirement of the specification and not meeting the requirement would be invalid. There are no header fields defined by RFCs 7230, 7231, 7232, 7233, 7234, or 7235 that MUST be sent by an origin server in all cases.

The following headers, for example, can be omitted (though you probably should send them):

7.1.1.2. Date

An origin server MUST NOT send a Date header field if it does not have a clock capable of providing a reasonable approximation of the current instance in Coordinated Universal Time. An origin server MAY send a Date header field if the response is in the 1xx (Informational) or 5xx (Server Error) class of status codes. An origin server MUST send a Date header field in all other cases.

Note the last sentence of the quote. The Date header field MUST be sent if the origin server is capable of providing a "reasonable approximation" of the date in UTC, but there is nothing stopping a server from misrepresenting itself.

7.4.2. Server

An origin server MAY generate a Server field in its responses.

3.3.2. Content-Length

Aside from [a finite number of predefined cases], in the absence of Transfer-Encoding, an origin server SHOULD send a Content-Length header field when the payload body size is known prior to sending the complete header section.

On the subject of Content-Length and Transfer-Encoding, note that neither can be sent, in which case the length of the response is "determined by the number of octets received prior to the server closing the connection."

3.1.1.5. Content-Type

If a Content-Type header field is not present, the recipient MAY either assume a media type of application/octet-stream (RFC2046, Section 4.5.1) or examine the data to determine its type.

There are circumstances under which particular headers can be required, for example:

• An origin server that does not support persistent connections MUST send the Connection: close in every response that does not have a 1xx status code.
• An origin server MUST generate an Allow header in a 405 (Method Not Allowed) response.
• An origin server generating a 401 (Unauthorized) response MUST send a WWW-Authenticate header field containing at least one challenge.

It depends on the specifics of the response, but generally, a response from an origin server should have:

• Date
• Content-Type
• Server

and either Content-Length, Transfer-Encoding or Connection: close.

If you want to do caching, add Cache-Control (e.g., with max-age); Expires isn't generally necessary any more. If you want clients to be able to validate, add Last-Modified or ETag.