capitalize_hook(Status, Headers, Body, Req) -> Headers2 = [{cowboy_bstr:capitalize_token(N), V} || {N, V} <- Headers], cowboy_req:reply(Status, Headers2, Body, Req).
There exists a very large number of implementations for the HTTP protocol. Most widely used clients, like browsers, follow the standard quite well, but others may not. In particular custom enterprise clients tend to be very badly written.
Cowboy tries to follow the standard as much as possible, but is not trying to handle every possible special cases. Instead Cowboy focuses on the cases reported in the wild, on the public Web.
That means clients that ignore the HTTP standard completely may fail to understand Cowboy’s responses. There are of course workarounds. This chapter aims to cover them.
Cowboy converts all headers it receives to lowercase, and similarly sends back headers all in lowercase. Some broken HTTP clients have issues with that.
A simple way to solve this is to create an onresponse hook
that will format the header names with the expected case.
capitalize_hook(Status, Headers, Body, Req) -> Headers2 = [{cowboy_bstr:capitalize_token(N), V} || {N, V} <- Headers], cowboy_req:reply(Status, Headers2, Body, Req).
Note that HTTP/2 clients do not have that particular issue because the specification explicitly says all headers are lowercase, unlike HTTP which allows any case but treats them as case insensitive.
Sometimes it is desirable to keep the actual case used by
clients, for example when acting as a proxy between two broken
implementations. There is no easy solution for this other than
forking the project and editing the cowboy_protocol file
directly.
Sometimes an HTTP client advertises itself as HTTP/1.1 but does not support chunked transfer-encoding. This is invalid behavior, as HTTP/1.1 clients are required to support it.
A simple workaround exists in these cases. By changing the
Req object response state to waiting_stream, Cowboy will
understand that it must use the identity transfer-encoding
when replying, just like if it was an HTTP/1.0 client.
Req2 = cowboy_req:set(resp_state, waiting_stream).