Lesson 10 · Case-insensitive header names

Case-insensitive header names

HTTP header names ignore case - "Content-Type" and "content-type" are the same field. Today you make lookups case-insensitive so a handler can ask for a header without guessing how the client capitalized it.

The goal

Make header lookup succeed regardless of the case used when storing or querying the name.

Start here - the target

TO DO

Scenario: Looking up a header by any casing

Givena header stored from the line "Content-Type: text/html"

Whenit is looked up as "content-type"

Thenthe value is "text/html"

Andlooking it up as "CONTENT-TYPE" also gives "text/html"

Background

The HTTP spec says field names are case-insensitive: a client may send Content-Type, content-type, or CONTENT-TYPE and mean the same header. If your map keys on the exact bytes received, a handler asking for "Content-Length" will miss a client that wrote "content-length".

The fix is to pick one canonical form — lowercasing is simplest — and apply it both when storing and when looking up. Then every casing collapses to the same key. Note this keeps your earlier exact-case lookups working too, since Get("Host") still lowercases to the same entry.

Make it work

// canonicalize on the way in AND on the way out
func (h Headers) Get(name string) string {
    return h[strings.ToLower(name)]
}
// when building: headers[strings.ToLower(name)] = value

CheckpointDONE

Header lookup ignores case. Commit.