Standard for adding multiple values of a single HTTP Header to a request or response

Question

If I want to add a list of values as an HTTP Header, is there a standard way to do this? I couldn't find anything (that I could easily understand) in RFC 822. For example, is comma separated values standard or semi-colon separated values. Is there a standard at all?

Example:

Key: value1;value2;value3 

Answer 1

You'll want to take a look at the HTTP spec RFC 2616 where it says:

Multiple message-header fields with the same field-name MAY be present in a message if and only if the entire field-value for that header field is defined as a comma-separated list [i.e., #(values)]. It MUST be possible to combine the multiple header fields into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field-value to the first, each separated by a comma. The order in which header fields with the same field-name are received is therefore significant to the interpretation of the combined field value, and thus a proxy MUST NOT change the order of these field values when a message is forwarded.

What this means is that you can send the same header multiple times in a response with different values, as long as those values can be appended to each other using a comma. This also means that you can send multiple values in a single header by concatenating them with commas.

So in your case it will be:

Key: value1,value2,value3