Apache HTTP Server Version 2.5
Description: | Support for the HTTP/2 transport layer |
---|---|
Status: | Extension |
Module Identifier: | http2_module |
Source File: | mod_http2.c |
Compatibility: | Available in version 2.4.17 and later |
This module provides HTTP/2 (RFC 7540) support for the Apache HTTP Server.
This module relies on libnghttp2 to provide the core http/2 engine.
You must enable HTTP/2 via Protocols
in order to use the functionality described in this document. The
HTTP/2 protocol does not require the use of encryption so two schemes are available:
h2
(HTTP/2 over TLS) and h2c
(HTTP/2 over TCP).
Two useful configuration schemes are:
Protocols h2 http/1.1
Allows HTTP/2 negotiation (h2) via TLS ALPN in a secure
<VirtualHost>
.
HTTP/2 preamble checking (Direct mode, see H2Direct
) is disabled by default for h2
.
Protocols h2 h2c http/1.1
Allows HTTP/2 negotiation (h2) via TLS ALPN for secure
<VirtualHost>
. Allows
HTTP/2 cleartext negotiation (h2c) upgrading from an initial HTTP/1.1
connection or via HTTP/2 preamble checking (Direct mode, see
H2Direct
).
Refer to the official HTTP/2 FAQ for any doubt about the protocol.
Enabling HTTP/2 on your Apache Server has impact on the resource consumption and if you have a busy site, you may need to consider carefully the implications.
The first noticeable thing after enabling HTTP/2 is that your server processes will start additional threads. The reason for this is that HTTP/2 gives all requests that it receives to its own Worker threads for processing, collects the results and streams them out to the client.
In the current implementation, these workers use a separate thread
pool from the MPM workers that you might be familiar with. This is
just how things are right now and not intended to be like this forever.
(It might be forever for the 2.4.x release line, though.) So, HTTP/2
workers, or shorter H2Workers, will not show up in mod_status
. They
are also not counted against directives such as ThreadsPerChild
. However
they take ThreadsPerChild
as default if you have not configured something
else via H2MinWorkers
and
H2MaxWorkers
.
Another thing to watch out for is is memory consumption. Since HTTP/2
keeps more state on the server to manage all the open request, priorities
for and dependencies between them, it will always need more memory
than HTTP/1.1 processing. There are three directives which steer the
memory footprint of a HTTP/2 connection:
H2MaxSessionStreams
,
H2WindowSize
and
H2StreamMaxMemSize
.
H2MaxSessionStreams
limits the
number of parallel requests that a client can make on a HTTP/2 connection.
It depends on your site how many you should allow. The default is 100 which
is plenty and unless you run into memory problems, I would keep it this
way. Most requests that browsers send are GETs without a body, so they
use up only a little bit of memory until the actual processing starts.
H2WindowSize
controls how much
the client is allowed to send as body of a request, before it waits
for the server to encourage more. Or, the other way around, it is the
amount of request body data the server needs to be able to buffer. This
is per request.
And last, but not least, H2StreamMaxMemSize
controls how much response data shall be buffered. The request sits in
a H2Worker thread and is producing data, the HTTP/2 connection tries
to send this to the client. If the client does not read fast enough,
the connection will buffer this amount of data and then suspend the
H2Worker.
Many sites use the same TLS certificate for multiple virtual hosts. The certificate either has a wildcard name, such as '*.example.org' or carries several alternate names. Browsers using HTTP/2 will recognize that and reuse an already opened connection for such hosts.
While this is great for performance, it comes at a price: such vhosts need more care in their configuration. The problem is that you will have multiple requests for multiple hosts on the same TLS connection. And that makes renegotiation impossible, in face the HTTP/2 standard forbids it.
So, if you have several virtual hosts using the same certificate and want to use HTTP/2 for them, you need to make sure that all vhosts have exactly the same SSL configuration. You need the same protocol, ciphers and settings for client verification.
If you mix things, Apache httpd will detect it and return a special response code, 421 Misdirected Request, to the client.
This module can be configured to provide HTTP/2 related information
as additional environment variables to the SSI and CGI namespace, as well
as in custom log configurations (see %{VAR_NAME}e
).
Variable Name: | Value Type: | Description: |
---|---|---|
HTTP2 | flag | HTTP/2 is being used. |
H2PUSH | flag | HTTP/2 Server Push is enabled for this connection and also supported by the client. |
H2_PUSH | flag | alternate name for H2PUSH |
H2_PUSHED | string | empty or PUSHED for a request being pushed by the server. |
H2_PUSHED_ON | number | HTTP/2 stream number that triggered the push of this request. |
H2_STREAM_ID | number | HTTP/2 stream number of this request. |
H2_STREAM_TAG | string | HTTP/2 process unique stream identifier, consisting of connection id and stream id separated by - . |
Description: | Determine file handling in responses |
---|---|
Syntax: | H2CopyFiles on|off |
Default: | H2CopyFiles off |
Context: | server config, virtual host, directory, .htaccess |
Override: | FileInfo |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.24 and later. |
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.
Description: | H2 Direct Protocol Switch |
---|---|
Syntax: | H2Direct on|off |
Default: | H2Direct on for h2c, off for h2 protocol |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
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
Description: | Determine sending of 103 status codes |
---|---|
Syntax: | H2EarlyHints on|off |
Default: | H2EarlyHints off |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.24 and later. |
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.
Description: | Maximum number of active streams per HTTP/2 session. |
---|---|
Syntax: | H2MaxSessionStreams n |
Default: | H2MaxSessionStreams 100 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
This directive sets the maximum number of active streams per HTTP/2 session (e.g. connection)
that the server allows. A stream is active if it is not idle
or
closed
according to RFC 7540.
H2MaxSessionStreams 20
Description: | Maximum number of seconds h2 workers remain idle until shut down. |
---|---|
Syntax: | H2MaxWorkerIdleSeconds n |
Default: | H2MaxWorkerIdleSeconds 600 |
Context: | server config |
Status: | Extension |
Module: | mod_http2 |
This directive sets the maximum number of seconds a h2 worker may
idle until it shuts itself down. This only happens while the number of
h2 workers exceeds H2MinWorkers
.
H2MaxWorkerIdleSeconds 20
Description: | Maximum number of worker threads to use per child process. |
---|---|
Syntax: | H2MaxWorkers n |
Context: | server config |
Status: | Extension |
Module: | mod_http2 |
This directive sets the maximum number of worker threads to spawn
per child process for HTTP/2 processing. If this directive is not used,
mod_http2
will chose a value suitable for the mpm
module loaded.
H2MaxWorkers 20
Description: | Minimal number of worker threads to use per child process. |
---|---|
Syntax: | H2MinWorkers n |
Context: | server config |
Status: | Extension |
Module: | mod_http2 |
This directive sets the minimum number of worker threads to spawn
per child process for HTTP/2 processing. If this directive is not used,
mod_http2
will chose a value suitable for the mpm
module loaded.
H2MinWorkers 10
Description: | Require HTTP/2 connections to be "modern TLS" only |
---|---|
Syntax: | H2ModernTLSOnly on|off |
Default: | H2ModernTLSOnly on |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.18 and later. |
This directive toggles the security checks on HTTP/2 connections
in TLS mode (https:). This can be used server wide or for specific
<VirtualHost>
s.
The security checks require that the TSL protocol is at least TLSv1.2 and that none of the ciphers listed in RFC 7540, Appendix A is used. These checks will be extended once new security requirements come into place.
The name stems from the Security/Server Side TLS definitions at mozilla where "modern compatibility" is defined. Mozilla Firefox and other browsers require modern compatibility for HTTP/2 connections. As everything in OpSec, this is a moving target and can be expected to evolve in the future.
One purpose of having these checks in mod_http2
is to enforce this
security level for all connections, not only those from browsers. The other
purpose is to prevent the negotiation of HTTP/2 as a protocol should
the requirements not be met.
Ultimately, the security of the TLS connection is determined by the
server configuration directives for mod_ssl
.
H2ModernTLSOnly off
Description: | Determine the range of padding bytes added to payload frames |
---|---|
Syntax: | H2Padding numbits |
Default: | H2Padding 0 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.39 and later. |
With the default 0, no padding bytes are added to any payload frames, e.g. HEADERS, DATA and PUSH_PROMISE. This is the behaviour of previous versions. It means that under certain conditions, an observer of network traffic can see the length of those frames in the TLS stream.
When configuring numbits of 1-8, a random number in range [0, 2^numbits[ are added to each frame. The random value is chosen independantly for each frame that the module sends back to the client.
While more padding bytes give better message length obfuscation, they are also additional traffic. The optimal number therefore depends on the kind of web traffic the server carries.
The default of 0, e.g. no padding, was chosen for maximum backward compatibility. There might be deployments where padding bytes are unwanted or do harm. The most likely cause would be a client that has a faults implementation.
Description: | H2 Server Push Switch |
---|---|
Syntax: | H2Push on|off |
Default: | H2Push on |
Context: | server config, virtual host, directory, .htaccess |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.18 and later. |
This directive toggles the usage of the HTTP/2 server push protocol feature.
The HTTP/2 protocol allows the server to push other resources to a client when it asked for a particular one. This is helpful if those resources are connected in some way and the client can be expected to ask for it anyway. The pushing then saves the time it takes the client to ask for the resources itself. On the other hand, pushing resources the client never needs or already has is a waste of bandwidth.
Server pushes are detected by inspecting the Link
headers of
responses (see https://tools.ietf.org/html/rfc5988 for the
specification). When a link thus specified has the rel=preload
attribute, it is treated as a resource to be pushed.
Link headers in responses are either set by the application or
can be configured via H2PushResource
or
using mod_headers
as:
<Location /index.html> Header add Link "</css/site.css>;rel=preload" Header add Link "</images/logo.jpg>;rel=preload" </Location>
As the example shows, there can be several link headers added to a response, resulting in several pushes being triggered. There are no checks in the module to avoid pushing the same resource twice or more to one client. Use with care.
HTTP/2 server pushes are enabled by default. On a server or virtual host, you may enable/disable this feature for any connection to the host. In addition, you may disable PUSH for a set of resources in a Directory/Location. This controls which resources may cause a PUSH, not which resources may be sent via PUSH.
H2Push off
Last but not least, pushes happen only when the client signals its willingness to accept those. Most browsers do, some, like Safari 9, do not. Also, pushes also only happen for resources from the same authority as the original response is for.
Description: | H2 Server Push Diary Size |
---|---|
Syntax: | H2PushDiarySize n |
Default: | H2PushDiarySize 256 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.19 and later. |
This directive toggles the maximum number of HTTP/2 server pushes
that are remembered per HTTP/2 connection. This can be used inside the
<VirtualHost>
section to influence the number for all connections to that virtual host.
The push diary records a digest (currently using a 64 bit number) of pushed resources (their URL) to avoid duplicate pushes on the same connection. These value are not persisted, so clients opening a new connection will experience known pushes again. There is ongoing work to enable a client to disclose a digest of the resources it already has, so the diary maybe initialized by the client on each connection setup.
If the maximum size is reached, newer entries replace the oldest ones. A diary entry uses 8 bytes, letting a default diary with 256 entries consume around 2 KB of memory.
A size of 0 will effectively disable the push diary.
Description: | H2 Server Push Priority |
---|---|
Syntax: | H2PushPriority mime-type [after|before|interleaved] [weight] |
Default: | H2PushPriority * After 16 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.18 and later. For having an effect, a nghttp2 library version 1.5.0 or newer is necessary. |
This directive defines the priority handling of pushed responses based on the content-type of the response. This is usually defined per server config, but may also appear in a virtual host.
HTTP/2 server pushes are always related to a client request. Each such request/response pairs, or streams have a dependency and a weight, together defining the priority of a stream.
When a stream depends on another, say X depends on Y, then Y gets all bandwidth before X gets any. Note that this does not mean that Y will block X. If Y has no data to send, all bandwidth allocated to Y can be used by X.
When a stream has more than one dependant, say X1 and X2 both depend on Y, the weight determines the bandwidth allocation. If X1 and X2 have the same weight, they both get half of the available bandwidth. If the weight of X1 is twice as large as that for X2, X1 gets twice the bandwidth of X2.
Ultimately, every stream depends on the root stream which gets all the bandwidth available, but never sends anything. So all its bandwidth is distributed by weight among its children. Which either have data to send or distribute the bandwidth to their own children. And so on. If none of the children have data to send, that bandwidth get distributed somewhere else according to the same rules.
The purpose of this priority system is to always make use of available bandwidth while allowing precedence and weight to be given to specific streams. Since, normally, all streams are initiated by the client, it is also the one that sets these priorities.
Only when such a stream results in a PUSH, gets the server to decide what the initial priority of such a pushed stream is. In the examples below, X is the client stream. It depends on Y and the server decides to PUSH streams P1 and P2 onto X.
The default priority rule is:
H2PushPriority * After 16
which reads as 'Send a pushed stream of any content-type depending on the client stream with weight 16'. And so P1 and P2 will be send after X and, as they have equal weight, share bandwidth equally among themselves.
H2PushPriority text/css Interleaved 256
which reads as 'Send any CSS resource on the same dependency and
weight as the client stream'. If P1 has content-type 'text/css',
it will depend on Y (as does X) and its effective weight will be
calculated as P1ew = Xw * (P1w / 256)
. With P1w being
256, this will make the effective weight the same as the weight
of X. If both X and P1 have data to send, bandwidth will be allocated
to both equally.
With Pw specified as 512, a pushed, interleaved stream would get double the weight of X. With 128 only half as much. Note that effective weights are always capped at 256.
H2PushPriority application/json Before
This says that any pushed stream of content type 'application/json' should be send out before X. This makes P1 dependent on Y and X dependent on P1. So, X will be stalled as long as P1 has data to send. The effective weight is inherited from the client stream. Specifying a weight is not allowed.
Be aware that the effect of priority specifications is limited by the available server resources. If a server does not have workers available for pushed streams, the data for the stream may only ever arrive when other streams have been finished.
Last, but not least, there are some specifics of the syntax to be used in this directive:
H2PushPriority application/json 32 # an After rule H2PushPriority image/jpeg before # weight inherited H2PushPriority text/css interleaved # weight 256 default
Description: | Declares resources for early pushing to the client |
---|---|
Syntax: | H2PushResource [add] path [critical] |
Context: | server config, virtual host, directory, .htaccess |
Override: | FileInfo |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.24 and later. |
When added to a directory/location HTTP/2 PUSHes will be attempted for all paths added via this directive. This directive can be used several times for the same location.
This directive pushes resources much earlier than adding
Link
headers via mod_headers
.
mod_http2
announces these resources in a
103 Early Hints
interim response to the client.
That means that clients not supporting PUSH will still get
early preload hints.
In contrast to setting Link
response headers
via mod_headers
, this directive will only
take effect on HTTP/2 connections.
By adding critical
to such a resource, the server
will give processing it more preference and send its data, once
available, before the data from the main request.
Description: | Serialize Request/Response Processing Switch |
---|---|
Syntax: | H2SerializeHeaders on|off |
Default: | H2SerializeHeaders off |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
This directive toggles if HTTP/2 requests shall be serialized in
HTTP/1.1 format for processing by httpd
core or if
received binary data shall be passed into the request_rec
s
directly.
Serialization will lower performance, but gives more backward compatibility in case custom filters/hooks need it.
H2SerializeHeaders on
Description: | Maximum amount of output data buffered per stream. |
---|---|
Syntax: | H2StreamMaxMemSize bytes |
Default: | H2StreamMaxMemSize 65536 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
This directive sets the maximum number of outgoing data bytes buffered in memory for an active streams. This memory is not allocated per stream as such. Allocations are counted against this limit when they are about to be done. Stream processing freezes when the limit has been reached and will only continue when buffered data has been sent out to the client.
H2StreamMaxMemSize 128000
Description: | Configure the number of seconds of idle time on TLS before shrinking writes |
---|---|
Syntax: | H2TLSCoolDownSecs seconds |
Default: | H2TLSCoolDownSecs 1 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.18 and later. |
This directive sets the number of seconds of idle time on a TLS
connection before the TLS write size falls back to small (~1300 bytes)
length.
This can be used server wide or for specific
<VirtualHost>
s.
See H2TLSWarmUpSize
for a
description of TLS warmup. H2TLSCoolDownSecs
reflects the fact
that connections may deteriorate over time (and TCP flow adjusts)
for idle connections as well. It is beneficial to overall performance
to fall back to the pre-warmup phase after a number of seconds that
no data has been sent.
In deployments where connections can be considered reliable, this timer can be disabled by setting it to 0.
The following example sets the seconds to zero, effectively disabling any cool down. Warmed up TLS connections stay on maximum record size.
H2TLSCoolDownSecs 0
Description: | Configure the number of bytes on TLS connection before doing max writes |
---|---|
Syntax: | H2TLSWarmUpSize amount |
Default: | H2TLSWarmUpSize 1048576 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
Compatibility: | Available in version 2.4.18 and later. |
This directive sets the number of bytes to be sent in small
TLS records (~1300 bytes) until doing maximum sized writes (16k)
on https: HTTP/2 connections.
This can be used server wide or for specific
<VirtualHost>
s.
Measurements by google performance labs show that best performance on TLS connections is reached, if initial record sizes stay below the MTU level, to allow a complete record to fit into an IP packet.
While TCP adjust its flow-control and window sizes, longer TLS records can get stuck in queues or get lost and need retransmission. This is of course true for all packets. TLS however needs the whole record in order to decrypt it. Any missing bytes at the end will stall usage of the received ones.
After a sufficient number of bytes have been send successfully, the TCP state of the connection is stable and maximum TLS record sizes (16 KB) can be used for optimal performance.
In deployments where servers are reached locally or over reliable connections only, the value might be decreased with 0 disabling any warmup phase altogether.
The following example sets the size to zero, effectively disabling any warmup phase.
H2TLSWarmUpSize 0
Description: | H2 Upgrade Protocol Switch |
---|---|
Syntax: | H2Upgrade on|off |
Default: | H2Upgrade on for h2c, off for h2 protocol |
Context: | server config, virtual host, directory, .htaccess |
Status: | Extension |
Module: | mod_http2 |
This directive toggles the usage of the HTTP/1.1 Upgrade method
for switching to HTTP/2. This
should be used inside a
<VirtualHost>
section to enable Upgrades to HTTP/2 for that virtual host.
This method of switching protocols is defined in HTTP/1.1 and uses the "Upgrade" header (thus the name) to announce willingness to use another protocol. This may happen on any request of a HTTP/1.1 connection.
This method of protocol switching is enabled by default on cleartext (potential h2c) connections and disabled on TLS (potential h2), as mandated by RFC 7540.
Please be aware that Upgrades are only accepted for requests
that carry no body. POSTs and PUTs with content will never
trigger an upgrade to HTTP/2.
See H2Direct
for an
alternative to Upgrade.
This mode only has an effect when h2 or h2c is enabled via
the Protocols
.
H2Upgrade on
Description: | Size of Stream Window for upstream data. |
---|---|
Syntax: | H2WindowSize bytes |
Default: | H2WindowSize 65535 |
Context: | server config, virtual host |
Status: | Extension |
Module: | mod_http2 |
This directive sets the size of the window that is used for flow control from client to server and limits the amount of data the server has to buffer. The client will stop sending on a stream once the limit has been reached until the server announces more available space (as it has processed some of the data).
This limit affects only request bodies, not its meta data such as headers. Also, it has no effect on response bodies as the window size for those are managed by the clients.
H2WindowSize 128000