Can Apache rewrite a request's Host header based on the URL?

Question

Apache is a reverse-proxy to my app. A client requests http://cdn.example.com/foo/images/logo.png:

GET /foo/images/logo.png HTTP/1.1
Host: cdn.example.com

I want Apache to modify the request so that the app on the other side of the reverse proxy receives it in the format http://foo.example.com/images/logo.png:

GET /images/logo.png HTTP/1.1
Host: foo.example.com

Rewriting the URL is easy, but I haven't found a way to modify the Host header with a value extracted from the URL. Is this possible?

Answer 1

Yes, you can change headers based on URIs. It's ugly though -- This is the only way I know of to do it:

1. Use SetEnvIF to set an environment variable if the URI matches what you want to rewrite.

2. Use mod_headers' RequestHeader directive to reset the appropriate Request Header (Host:) if that environment variable is set.

Answer 2

Using the SetEnvIf and Header mentioned above, here is what I did to rewrite the "Accept-Encoding" header to reduce the caching copies created by mod_cache, tested working.

# rewrite variation of the Accept-Encoding header to the same one
# to reduce the caching copies
UnsetEnv compression_ok
SetEnvIfNoCase Accept-Encoding ".*gzip.*deflate.*" compression_ok=1
RequestHeader set Accept-Encoding "gzip,deflate" env=compression_ok

What this does: Different browser sets the Accept-Encoding slightly differently, like "gzip,deflate" vs "gzip, deflate" (with extra space), and these causes the mod_cache to create different copies of the content. By rewrite it to the same value, the mod_cache only generates one copy. (note: my server only care about "gzip,deflate", yours might be different).