Apache Module mod_http2

H2CopyFiles Directive

This directive influences how file content is handled in responses. When off, which is the default, file handles are passed from the requestion processing down to the main connection, using the usual Apache setaside handling for managing the lifetime of the file.

When set to on, file content is copied while the request is still being processed and the buffered data is passed on to the main connection. This is better if a third party module is injecting files with different lifetimes into the response.

An example for such a module is mod_wsgi that may place Python file handles into the response. Those files get close down when Python thinks processing has finished. That may be well before mod_http2 is done with them.

H2Direct Directive

This directive toggles the usage of the HTTP/2 Direct Mode. This should be used inside a <VirtualHost> section to enable direct HTTP/2 communication for that virtual host.

Direct communication means that if the first bytes received by the server on a connection match the HTTP/2 preamble, the HTTP/2 protocol is switched to immediately without further negotiation. This mode is defined in RFC 7540 for the cleartext (h2c) case. Its use on TLS connections not mandated by the standard.

When a server/vhost does not have h2 or h2c enabled via Protocols, the connection is never inspected for a HTTP/2 preamble. H2Direct does not matter then. This is important for connections that use protocols where an initial read might hang indefinitely, such as NNTP.

For clients that have out-of-band knowledge about a server supporting h2c, direct HTTP/2 saves the client from having to perform an HTTP/1.1 upgrade, resulting in better performance and avoiding the Upgrade restrictions on request bodies.

This makes direct h2c attractive for server to server communication as well, when the connection can be trusted or is secured by other means.

H2Direct on

H2EarlyHint Directive

H2EarlyHint allows adding a response header before the real request processing is started. Such headers are picked up for "103 Early Hints" intermediate responses. The main purpose is to send "preload" information to client browsers.

name and value must be valid HTTP header fields or will lead to failed responses. H2EarlyHints must still be enabled to allow 103 intermediate responses to be sent. This directive can be repeated several times and header fields of the same names add.

H2EarlyHint Link "</my.css>;rel=preload;as=style"

H2EarlyHints Directive

This setting controls if HTTP status 103 interim responses are forwarded to the client or not. By default, this is currently not the case since a range of clients still have trouble with unexpected interim responses.

When set to on, PUSH resources announced with H2PushResource will trigger an interim 103 response before the final response. The 103 response will carry Link headers that advise the preload of such resources.

H2MaxDataFrameLen Directive

H2MaxDataFrameLen limits the maximum amount of response body bytes placed into a single HTTP/2 DATA frame. Setting this to 0 places no limit (but the max size allowed by the protocol is observed).

The module, by default, tries to use the maximum size possible, which is somewhat around 16KB. This sets the maximum. When less response data is availble, smaller frames will be sent.

H2MaxSessionStreams Directive