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H2O.CONF(5) File Formats Manual H2O.CONF(5)

NAME
h2o.conf - The configuration file for H2O, the optimized HTTP/1.x,
HTTP/2 server

SYNOPSIS
/etc/h2o/h2o.conf

DESCRIPTION
h2o.conf h2o.conf is a YAML configuration file.

QUICK START
In order to run the H2O standalone HTTP server, you need to write a
configuration file. The minimal configuration file looks like as fol-
lows.

hosts:
"myhost.example.com":
listen:
port: 80
listen:
port: 443
ssl: {}
listen:
port: 443
ssl: {}
type: quic
paths:
/:
file.dir: /path/to/the/public-files

user: nobody
access-log: /path/to/the/access-log
error-log: /path/to/the/error-log
pid-file: /path/to/the/pid-file

acme:
email: admin@myhost.example.com
accept-tos: YES

The configuration instructs the server to:

serve contents under the hostname: myhost.example.com for HTTP, listen
on TCP port 80 for HTTPS, listen on TCP port 443 and UDP port 443 serve
files under /path/to/the/public-files run under the privileges of no-
body emit access logs to file: /path/to/the/access-log emit error logs
to /path/to/the/error-log store the process id of the server in
/path/to/the/pid-file use ACME to fetch the TLS certificates automati-
cally (note: lego must be installed for this to work).

Enter the command below to start the server.

% sudo h2o -m daemon -c /path/to/the/configuration-file

The command instructs the server to read the configuration file, and
start in daemon mode, which dispatches a pair of master and worker
processes that serves the HTTP requests.

To stop the server, send SIGTERM to the server.

% sudo kill -TERM `cat /path/to/the/pid-file`

Next Step

Now that you know how to start and stop the server, the next step is to
learn the configuration directives and their structure, or see the con-
figuration examples.

SYNTAX AND STRUCTURE
Syntax

H2O uses YAML 1.1 as the syntax of its configuration file.

Levels of Configuration

When using the configuration directives of H2O, it is important to un-
derstand that there are four configuration levels: global, host, path,
extension.

Global-level configurations affect the entire server. Host-level con-
figurations affect the configuration for the specific hostname (i.e.
corresponds to the <VirtualHost> directive of the Apache HTTP Server).
Path-level configurations only affect the behavior of resources spe-
cific to the path.

Extension-level configuration affect how files with certain extensions
are being served. For example, it is possible to map files with .php
extension to the FastCGI handler running the php-cgi command.

Consider the following example.

hosts:
"example.com":
listen:
port: 443
ssl:
certificate-file: etc/site1.crt
key-file: etc/site1.key
paths:
"/":
file.dir: htdocs/site1
"/icons":
file.dir: icons
expires: 1 day
"example.com:80":
listen:
port: 80
paths:
"/":
redirect: "https://example.com/"

In the example, two host-level configurations exist (under the hosts
mapping), each of them listening to different ports. The first host
listens to port 443 using TLS (i.e. HTTPS) using the specified server
certificate and key. It has two path-level configurations, one for /
and the other for /icons, each of them pointing to different local di-
rectories containing the files to be served. The latter also has the
expires directive set, so that Cache-Control: max-age=86400 [1] header
would be sent. The second host accepts connections on port 80 (via the
plain-text HTTP protocol), and redirects all the requests to the first
host using HTTPS.

Certain configuration directives can be used in more than one levels.
For example, the listen can be used either at the global level or at
the host level. Expires can be used at all levels. On the other hand
file.dir can only be used at the path level.

Path-level configuration

Values of the path-level configuration define the action(s) to be taken
when the server processes a request that prefix-matches to the config-
ured paths. Each entry of the mapping associated to the paths is eval-
uated in the order they appear.

Consider the following example. When receiving a request for
https://example.com/foo, the file handler is first executed trying to
serve a file named /path/to/doc-root/foo as the response. In case the
file does not exist, then the FastCGI handler is invoked.

hosts:
"example.com":
listen:
port: 443
ssl:
certificate-file: etc/site1.crt
key-file: etc/site1.key
paths:
"/":
file.dir: /path/to/doc-root
fastcgi.connect:
port: /path/to/fcgi.sock
type: unix

Starting from version 2.1, it is also possible to define the path-level
configuration as a sequence of mappings instead of a single mapping.
The following example is identical to the previous one. Notice the
dashes placed before the handler directives.

hosts:
"example.com":
listen:
port: 443
ssl:
certificate-file: etc/site1.crt
key-file: etc/site1.key
paths:
"/":
- file.dir: /path/to/doc-root
- fastcgi.connect:
port: /path/to/fcgi.sock
type: unix

Using YAML Alias

H2O resolves YAML aliases before processing the configuration file.
Therefore, it is possible to use an alias to reduce the redundancy of
the configuration file. For example, the following configuration
reuses the first paths element (that is given an anchor named de-
fault_paths) in the following definitions.

hosts:
"example.com":
listen:
port: 443
ssl:
certificate-file: /path/to/example.com.crt
key-file: /path/to/example.com.crt
paths: &default_paths
"/":
file.dir: /path/to/doc-root
"example.org":
listen:
port: 443
ssl:
certificate-file: /path/to/example.org.crt
key-file: /path/to/example.org.crt
paths: *default_paths

Using YAML Merge

Since version 2.0, H2O recognizes Merge Key Language-Independent Type
for YAML Version 1.1. Users can use the feature to merge an existing
mapping against another. The following example reuses the TLS configu-
ration of example.com in example.org.

hosts:
"example.com":
listen:
port: 443
ssl: &default_ssl
minimum-version: TLSv1.2
cipher-suite: ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256
certificate-file: /path/to/example.com.crt
key-file: /path/to/example.com.crt
paths:
...
"example.org":
listen:
port: 443
ssl:
<<: *default_ssl
certificate-file: /path/to/example.org.crt
key-file: /path/to/example.org.crt
paths:
...

Including Files

Starting from version 2.1, it is possible to include a YAML file from
the configuration file using !file custom YAML tag. The following ex-
ample extracts the TLS configuration into default_ssl.conf and include
it multiple times in h2o.conf.

Example: default_ssl.conf

minimum-version: TLSv1.2
cipher-suite: ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256
certificate-file: /path/to/example.com.crt
key-file: /path/to/example.com.crt

Example: h2o.conf

hosts:
"example.com":
listen:
port: 443
ssl: !file default_ssl.conf
paths:
...
"example.org":
listen:
port: 443
ssl:
<
Starting from version 2.3, it is possible to refer to an environment variable (intepreted as a scalar) from the configuration file by using !env custom YAML tag.

Example:
h2o.conf

hosts:
"example.com":
listen:
port: !env H2O_PORT
paths:
...

Notes:
[1]1 day is equivalent to 86400 seconds

BASE DIRECTIVES
This document describes the configuration directives common to all the
protocols and handlers.

hosts
Maps host:port to the mappings of per-host configs.

The directive specifies the mapping between the authorities (the host
or host:port section of an URL) and their configurations. The direc-
tive is mandatory, and must at least contain one entry.

When port is omitted, the entry will match the requests targetting the
default ports (i.e. port 80 for HTTP, port 443 for HTTPS) with given
hostname. Otherwise, the entry will match the requests targetting the
specified port.

Since version 1.7, a wildcard character * can be used as the first com-
ponent of the hostname. If used, they are matched using the rule de-
fined in RFC 2818 Section 3.1. For example, *.example.com will match
HTTP requests for both foo.example.com and bar.example.com.

For each HTTP request to be processed, the matching host entry is de-
termined by the steps below:

Among the host elements that do not use wildcards, find the first ele-
ment that matches the host and port being specified by the URI. If
none is found in the previous step, find a matching element among the
entries that use wildcards. If none is found in the previous steps,
use the first host element without a strict-match flag.

When the hostname of the HTTP request is unknown (i.e., processing an
HTTP/1.0 request without a host header field), only the last step is
being used.

Example: A host redirecting all HTTP requests to HTTPS

hosts:
"www.example.com:80":
listen:
port: 80
paths:
"/":
redirect: https://www.example.com/
"www.example.com:443":
listen:
port: 443
ssl:
key-file: /path/to/ssl-key-file
certificate-file: /path/to/ssl-certificate-file
paths:
"/":
file.dir: /path/to/doc-root

paths
Mapping of paths and their configurations.

The mapping is searched using prefix-match. The entry with the longest
path is chosen when more than one matching paths were found. An 404
Not Found error is returned if no matching paths were found.

Example: Configuration with two paths

hosts:
"www.example.com":
listen:
port: 80
paths:
"/":
file.dir: /path/to/doc-root
"/assets":
file.dir: /path/to/assets

In releases prior to version 2.0, all the path entries are considered
as directories. When H2O receives a request that exactly matches to an
entry in paths that does not end with a slash, the server always re-
turns a 301 redirect that appends a slash.

Since 2.0, it depends on the handler of the path whether if a 301 redi-
rect that appends a slash is returned. Server administrators can take
advantage of this change to define per-path configurations (see the ex-
amples in file.file and the FastCGI handler). file.dir is an exception
that continues to perform the redirection; in case of the example
above, access to /assets is redirected to /assets/.

listen
Specifies the port at which the server should listen to.

In addition to specifying the port number, it is also possible to des-
ignate the bind address or the SSL and HTTP/3 (QUIC) configuration.

Example: Various ways of using the Listen Directive

# accept HTTP on port 80 on default address (both IPv4 and IPv6)
listen: 80

# accept HTTP on 127.0.0.1:8080
listen:
host: 127.0.0.1
port: 8080

# accept HTTPS on port 443
listen:
port: 443
ssl:
key-file: /path/to/key-file
certificate-file: /path/to/certificate-file

# accept HTTPS on port 443 (using PROXY protocol)
listen:
port: 443
ssl:
key-file: /path/to/key-file
certificate-file: /path/to/certificate-file
proxy-protocol: ON

To configure HTTP/3 (QUIC), see HTTP/3.

Configuration Levels

The directive can be used either at global-level or at host-level. At
least one listen directive must exist at the global level, or every
host-level configuration must have at least one listen directive.

Incoming connections accepted by global-level listeners will be dis-
patched to one of the host-level contexts with the corresponding
host:port, or to the first host-level context if none of the contexts
were given host:port corresponding to the request.

Host-level listeners specify bind addresses specific to the host-level
context. However it is permitted to specify the same bind address for
more than one host-level contexts, in which case hostname-based lookup
will be performed between the host contexts that share the address.
The feature is useful for setting up a HTTPS virtual host using Server-
Name Indication (RFC 6066).

Example: Using host-level listeners for HTTPS virtual-hosting

hosts:
"www.example.com:443":
listen:
port: 443
ssl:
key-file: /path/to/www_example_com.key
certificate-file: /path/to/www_example_com.crt
paths:
"/":
file.dir: /path/to/doc-root_of_www_example_com
"www.example.jp:443":
listen:
port: 443
ssl:
key-file: /path/to/www_example_jp.key
certificate-file: /path/to/www_example_jp.crt
paths:
"/":
file.dir: /path/to/doc-root_of_www_example_jp

SSL Attribute

The ssl attribute must be defined as a mapping.

The identity of the TLS listener is specified in one of the following
ways:

by specifying the certificate-file and key-file attributes directly un-
der ssl, by specifying a sequence of certificate- and key-file pairs
inside identity, or if neither of the above is used, the identity is
obtained using ACME; see acme.

The certificate-file attribute specifies the path of a PEM file con-
taining either an X.509 certificate chain or a raw public key. When
the latter form is being used, RFC 7250 handshake will be used.

The key-file attribute specifies the path of a PEM file that contains
the corresponding private key.

The identity attribute can be used in place of certificate-file and
key-file to specify more than one pair of certificates and keys. When
a TLS handshake is performed, h2o uses the first pair that contains a
compatible certificate / key. The last pair acts as the fallback.

Example: Using both RSA and ECDSA certificates

ssl:
identity:
- certificate-file: /path/to/ecdsa.crt # prefer ECDSA if client supports both
key-file: /path/to/ecdsa.key
- certificate-file: /path/to/rsa.crt. # use RSA if ECDSA is not supported
key-file: /path/to/rsa.key

In addition, the ssl mapping recognizes the following attributes.

client-ca-file:

path to a file of Certificate Authority (CA) certificates used to ver-
ify client certificates. If set, H2O will request a client certificate
from connecting clients and validate it against these CAs. Connections
lacking a valid client certificate (either not provided or not signed
by one of the given CAs) will be rejected. This enables mutual TLS
(mTLS) authentication.

cipher-suite: list of cipher suites to be passed to OpenSSL via
SSL_CTX_set_cipher_list cipher-suite-tls1.3: list of TLS 1.3 cipher
suites to use; the list must be a YAML sequence where each element
specifies the cipher suite using the name as registered to the IANA TLS
Cipher Suite Registry; e.g., TLS_AES_128_GCM_SHA256,
TLS_CHACHA20_POLY1305_SHA256. cipher-preference:

side of the list that should be used for selecting the cipher-suite;
should be either of: client, server. Default is client.

dh-file:

path of a PEM file containing the Diffie-Hellman parameters to be used.
Use of the file is recommended for servers using Diffie-Hellman key
agreement. (optional)

key-exchange-tls1.3: list of TLS 1.3 key exchange algorithms to use;
the list must be a YAML sequence of algorithms. x25519 and secp256r1
are supported and enabled by default. In addition, secp384r1,
secp521r1, and x25519mlkem768 might be available if they are supported
by libcrypto. max-tickets:

maximum number of TLS/1.3 session tickets to send, when the client re-
quests for them using the ticket_request extension. Default is 4.

minimum-version:

minimum protocol version, should be one of: SSLv2, SSLv3, TLSv1,
TLSv1.1, TLSv1.2, TLSv1.3. Default is TLSv1.

min-version:

synonym of minimum-version (introduced in version 2.2)

maximum-version:

maximum protocol version. Introduced in version 2.2. Default is the
maximum protocol version supported by the server.

max-version:

synonym of maximum-version.

ocsp-max-failures:

number of consecutive OCSP query failures before stopping to send OCSP
stapling data to the client. Default is 3.

ocsp-update-interval:

interval for updating the OCSP stapling data (in seconds), or set to
zero to disable OCSP stapling. Default is 14400 (4 hours).

neverbleed:

unless set to OFF, H2O isolates SSL private key operations to a differ-
ent process by using Neverbleed. Default is ON.

ssl-session-resumption directive is provided for tuning parameters re-
lated to session resumption and session tickets.

ech:

This experimental attribute controls the use of TLS Encrypted Client
Hello extension (draft-15). The attribute takes a sequence of map-
pings, each of them defining one ECH configuration.

Example: Encrypted Clint hello

ssl:
key-file: /path/to/rsa.key
certificate-file: /path/to/rsa.crt
ech:
- key-file: /path/to/ech.key
config-id: 11
public-name: public-name.example.net
cipher-suite: [ HKDF-SHA256/AES-128-GCM ]

The example above defines one ECH configuration that uses
/path/to/ech.key as the semi-static ECDH key with a config-id of 11,
with the public-name being public-name.example.net, and the HPKE Sym-
metricCipherSuite being HKDF-SHA256/AES-128-GCM.

In addition to these four attributes, following attributes may be spec-
ified.

max-name-length specifies the maximum-name-length field of an ECH con-
fguration (default: 64).

advertise takes either YES (default) or NO as the argument. This argu-
ment indicates if given ECH configuration should be advertised as part
of retry_configs (draft-ietf-tls-esni-15; section 5).

When removing a stale ECH configuration, its advertise attribute should
be set at first to NO so that the stale configuration would not be ad-
vertised. Then, after waiting for the expiry of caches containing the
stale configuration, the stale ECH configuration can be removed. This
may take long depending on the TTL of the HTTPS / SVC DNS resource
record advertising the configuration.

The ech attribute must be set only in the first ssl attribute that
binds to a particular address.

The CC Attribute

The CC attribute specifies the congestion controller to be used for in-
coming HTTP connections.

For TCP connections, the congestion controller is set using the
TCP_CONGESTION socket option on platforms that have support for that
socket option. To find out the default and the list of supported con-
gestion controllers, please refer to man 7 tcp. If the platform does
not have support for that socket option, the attribute has no effect.

For QUIC connections, the congestion controller is one of Reno, Cubic,
Pico. The default is Reno.

The Initcwnd Attribute

The initcwnd attribute specifies the initial congestion window of each
incoming HTTP connection in the unit of packets. At the moment, this
option only applies to QUIC. It has no effect for TCP connections.

The Proxy-Protocol Attribute

The proxy-protocol attribute (i.e. the value of the attribute must be
either ON or OFF) specifies if the server should recognize the informa-
tion passed via "the PROXY protocol in the incoming connections. The
protocol is used by L4 gateways such as AWS Elastic Load Balancing to
send peer address to the servers behind the gateways.

When set to ON, H2O standalone server tries to parse the first octets
of the incoming connections as defined in version 1 of the specifica-
tion, and if successful, passes the addresses obtained from the proto-
col to the web applications and the logging handlers. If the first
octets do not accord with the specification, it is considered as the
start of the SSL handshake or as the beginning of an HTTP request de-
pending on whether if the ssl attribute has been used.

Default is OFF.

The Sndbuf and Rcvbuf Attributes

The sndbuf and rcvbuf attributes specify the send and receive buffer
size for each TCP or UNIX socket used for accepting incoming HTTP con-
nections. If set, the values of these attributes are applied to the
sockets using SO_SNDBUF and SO_RCVBUF socket options.

These attributes have no effect for QUIC connections.

Listening to a Unix Socket

If the type attribute is set to unix, then the port attribute is as-
sumed to specify the path of the unix socket to which the standalone
server should bound. Also following attributes are recognized.

owner

username of the owner of the socket file. If omitted, the socket file
will be owned by the launching user.

group

name of the group of the socket file. If omitted, group ID associated
to the socket file will be the group ID of the owner.

permission

an octal number specifying the permission of the socket file. Many op-
erating systems require write permission for connecting to the socket
file. If omitted, the permission of the socket file will reflect the
umask of the calling process.

Example: Listening to a Unix Socket accessible only by www-data

listen:
type: unix
port: /tmp/h2o.sock
owner: www-data
permission: 600

acme
Set the parameters for automatic certificate management using ACME
(e.g., Letsencrypt).

Unless a custom loader program is specified, h2o obtains certificates
from Let's Encrypt using the popular ACME client: Lego. On most dis-
tributions, Lego should be available as pre-build packages.

This directive takes two mandatory and one optional parameters.

email (mandatory) The email address to be registered to the ACME ser-
vice provider. accept-tos (mandatory) Must be set to YES to accept the
terms of service of the ACME service provider. loader (optional) The
program to be launched for issuing and renewing the certificates. If
omitted, H2O_ROOT/share/h2o/acme/lego-loader is used.

The below example shows a configuration that relies on ACME for serving
https://example.org.

Example: Simple h2o.conf using ACME

acme:
email: admin@example.org # email address to be used when contacting Let's Encrypt
accept-tos: YES # consent to the Terms of Service of Let's Encrypt
hosts:
example.org:
listen:
port: 80
listen:
port: 443
ssl: {} # omission on an identity indicates the use of ACME
paths:
...

The certificates and keys being issued or generated are stored under
the $prefix/var/h2o/acme directory. Output of the lego command (in-
cluding errors) are emitted to the error log of h2o.

capabilities
Set capabilities to be added to the process before dropping root privi-
leges.

This directive can be used only on Linux. The argument is a YAML se-
quence of capabilites, where each capability is a name that is accepted
by cap_from_name. See man 7 capabilities for details.

error-log
Path of the file to which error logs should be appended.

Default is stderr.

If the path starts with |, the rest of the path is considered as a com-
mand to which the logs should be piped.

Example: Log errors to file

error-log: /path/to/error-log-file

Example: Log errors through pipe

error-log: "| rotatelogs /path/to/error-log-file.%Y%m%d 86400"

See also:

Reorganizing Website Architecture for HTTP/2 and Beyond pp.14-21

The following describes the configuration directives for controlling
the HTTP/2 protocol handler.

http2-casper
Configures CASPer (cache-aware server-push).

When enabled, H2O maintains a fingerprint of the web browser cache, and
cancels server-push suggested by the handlers if the client is known to
be in possession of the content. The fingerprint is stored in a cookie
named h2o_casper using Golomb-compressed sets (a compressed encoding of
Bloom filter).

If the value is OFF, the feature is disabled. Push requests (made by
the handlers through the use of Link: rel=preload header) are processed
regardless of whether if client already has the responses in its cache.
If the value is ON, the feature is enabled with the defaults value
specified below. If the value is mapping, the feature is enabled, rec-
ognizing the following attributes.

capacity-bits: number of bits used for the fingerprinting. Roughly
speaking, the number of bits should be log2(1/P * number-of-assets-to-
track) where P being the probability of false positives. Default is
13, enough for tracking about 100 asset files with 1/100 chance of
false positives (i.e. log2(100 * 100) =~ 13). tracking-types: speci-
fies the types of the content tracked by casper. If omitted or set to
blocking-assets, maintains fingerprint (and cancels server push) for
resources with mime-type of highest priority. If set to all, tracks
all responses.

It should be noted that the size of the cookie will be log2(P) * num-
ber-of-assets-being-tracked bits multiplied by the overhead of Base 64
encoding (4/3). Therefore with current cookie-based implementation, it
is necessary in many cases to restrict the resources being tracked to
those have significant effect to user-perceived response time.

Example: Enabling CASPer

http2-casper: ON

# `ON` is equivalent to:
# http2-casper:
# capacity-bits: 13
# tracking-types: blocking-assets

See also: file.mime.addtypes, issue #421

http2-debug-state
A directive to turn on the HTTP/2 Implementation Debug State.

This experimental feature serves a JSON document at the fixed path
/.well-known/h2/state, which describes an internal HTTP/2 state of the
H2O server. To know the details about the response fields, please see
the spec. This feature is only for developing and debugging use, so
it's highly recommended that you disable this setting in the production
environment.

The value of this directive specifies the property set contained in the
response. Available values are minimum or hpack. If hpack is speci-
fied, the response will contain the internal hpack state of the same
connection. If minimum is specified, the response doesn't contain the
internal hpack state.

In some circumstances, there may be a risk of information leakage on
providing an internal hpack state. For example, the case that some
proxies exist between the client and the server, and they share the
connections among the clients. Therefore, you should specify hpack
only when the server runs in the environments you can completely con-
trol.

This feature is considered experimental yet. For now, the implementa-
tion conforms to the version draft-01 of the specification.

See also: HTTP/2 Implementation Debug State (draft-01)

http2-idle-timeout
Timeout for idle connections in seconds.

http2-input-window-size
(since v2.3) Default window size for HTTP request body.

The value is the maximum amount of request body (in bytes) that can be
sent by the client in 1 RTT (round-trip time).

http2-max-streams
Maximum number of streams to advertise via HTTP2 SETTINGS_MAX_CONCUR-
RENT_STREAMS.

Limits the number of active requests that the proxy will accept on a
client connection. Also see "http2-max-concurrent-requests-per-connec-
tion" which controls the number of requests that the proxy will process
concurrently.

http2-max-concurrent-requests-per-connection
Maximum number of requests to be handled concurrently within a single
HTTP/2 connection.

The value cannot exceed 256.

http2-max-concurrent-streaming-requests-per-connection
Maximum number of streaming requests to be handled concurrently within
a single HTTP/2 connection.

The value cannot exceed 256.

http2-latency-optimization-min-rtt
(since v2.1) Minimum RTT (in milliseconds) to enable latency optimiza-
tion.

Latency optimization is disabled for TCP connections with smaller RTT
(round-trip time) than the specified value. Otherwise, whether if the
optimization is used depends on other parameters.

Setting this value to 4294967295 (i.e. UINT_MAX) effectively disables
the optimization.

http2-latency-optimization-max-additional-delay
(since v2.1) Maximum additional delay (as the ratio to RTT) permitted
to get latency optimization activated.

Latency optimization is disabled if the additional delay imposed by the
interaction between the OS and the TCP/IP stack is estimated to be
greater than the given threshold. Otherwise, whether if the optimiza-
tion is used depends on other parameters.

http2-latency-optimization-max-cwnd
(since v2.1) Maximum size (in octets) of CWND to get latency optimiza-
tion activated.

CWND is a per-TCP-connection variable that represents the number of
bytes that can be sent within 1 RTT.

The server will not use or stop using latency optimization mode if CWND
becomes greater than the configured value. In such case, average size
of HTTP/2 frames buffered unsent will be slightly above the tcp_not-
sent_lowat sysctl value.

http2-push-preload
(since v2.1) A boolean flag (ON or OFF) indicating whether if the
server should push resources when observing a link: rel=preload header.

http2-reprioritize-blocking-assets
A boolean flag (ON or OFF) indicating if the server should send con-
tents with highest priority before anything else.

To maximize the user-perceived responsiveness of a web page, it is es-
sential for the web server to send blocking assets (i.e. CSS and
JavaScript files in ) before any other files such as images. HTTP/2
provides a way for web browsers to specify such priorities to the web
server. However, as of Sep. 2015, no major web browsers except Mozilla
Firefox take advantage of the feature.

This option, when enabled, works as a workaround for such web browsers,
thereby improving experience of users using the web browsers.

Technically speaking, it does the following:

if the client uses dependency-based prioritization, do not reprioritize
if the client does not use dependency-based prioritization, send the
contents of which their types are given highest priority before any
other responses

See also: file.mime.addtypes, HTTP/2 (and H2O) improves user experience
over HTTP/1.1 or SPDY

http2-graceful-shutdown-timeout
A timeout in seconds. How long to wait before closing the connection on
graceful shutdown. Setting the timeout to 0 deactivates the feature:
H2O will wait for the peer to close the connections.

http2-allow-cross-origin-push
(since v2.3) A boolean flag (ON or OFF) indicating whether if the
server should push resources belonging to a different authority.

http2-dos-delay
HTTP request processing delay to be applied when the client behavior is
suspicious, in the unit of milliseconds.

When the client behavior is suspicious and there is a concern of De-
nial-of-Service attack, h2o stops processing requests arriving on the
suspicious connection for specified amount of time.

HTTP/3
HTTP/3 uses QUIC as the transport protocol. A listen directive with a
type attribute set to quic instructs the standalone server to bind to a
UDP port on which QUIC packets will be sent and received. The binding
must have an ssl attribute, as QUIC uses TLS/1.3 as the handshake pro-
tocol.

The example below setups a server that listens to both TCP port 443 and
UDP port 443 using the same certificate and private key.

First listen directive binds the server to TCP port 443 with specified
credentials, marking that directive as an YAML alias called &lis-
ten_ssl. Then, it reuses (YAML merge) the first listen directive,
adding type: quic to create a UDP port 443 binding for accepting QUIC
connections.

Example: Serving HTTP/1,2 and 3 on port 443

listen: &listen_ssl
port: 443
ssl:
certificate-file: /path/to-ssl-certificate-file
key-file: /path/to/ssl-key-file
listen:
<Fine-tuning QUIC Behavior

To fine tune the behavior of QUIC, the quic attribute should be used in place of the type attribute specifying quic.
The quic attribute accepts following parameters.

amp-limit
Amount of data that can be sent to the client before the client address is validated; see section 8.1 of RFC 9000. Default is 3.
ecn
A boolean flag (either ON or OFF) indicating whether the server should use ECN signals to detect congestion. The default setting is ON. This flag affects the server's sending behavior. Regardless of this configuration, the server sends back ECN signals it receives using ACK_ECN frames.
handshake-timeout-rtt-multiplier
Handshake timeout in the unit of round-trip time. Default is 400.
jumpstart-default
Jumpstart enhances the slow start phase of congestion control by pacing a large number of packets for an entire round-trip time (RTT), allowing the server to assess the network's capacity sooner. This parameter specifies the number of packets sent during the jumpstart phase. The default value is zero, indicating that jumpstart is disabled for new connections.
jumpstart-max
Upon resuming QUIC connections, clients present tokens issued by the server that contain the bandwidth of the previous connection. This information enables the server to instantly match the previous send rate by adjusting the jumpstart window accordingly. This parameter sets a cap on the maximum size of the jumpstart window. If set to zero (the default), the server disregards the bandwidth values in the tokens. Nonetheless, the server may still initiate a jumpstart without using previous information, based on the jumpstart-default parameter.
max-initial-handshake-packets
Maximum number of Initial packets to be sent before the handshake is deemed to have failed. Default is 1,000.
max-streams-bidi
Maximum number of client-initated bi-directional streams. This parameter controls the HTTP request concurrency of a HTTP/3 connection. Default is 100.
max-udp-payload-size
See Section 18.2 of RFC 9000. Default is 1,472.
pacing
A boolean flag (either OFF or ON) indicating whether sent packets should be paced. The default setting is OFF.
qpack-encoder-table-capacity
Size of the QPACK encoder table. Default is 4,096.
respect-app-limited
This boolean flag (either OFF or ON) indicates whether the server should respect the notion of rate limited traffic when adjusting the size of the congestion window, as detailed in RFC 7661. The default setting is ON.
retry
A boolean flag (OFF or ON) indicating if a Retry packet should be used for validating the client address. Use of Retry packets mitigate denial-of-service attacks at the cost of incurring one additional round-trip for processing the handshake.
sndbuf, rcvbuf
Size of send and receive buffers, in the unit of bytes. Unlike the TCP counterparts that are per-connection, these buffers are associated to the listening port and applies to all the connections bound to that port.

The example below reuses a previous binding but sets the retry parameter to ON.

Example:
HTTP/3 endpoint using Retry packets

listen:
<
Also, properties such as congestion controller and initial congestion window can be tuned using the top-level attribute of listen.

HTTP/3 Directives

Aside from QUIC-level properties, configuration directives listed below are provided for tuning HTTP/3 behavior.

http3-graceful-shutdown-timeout
Maximum duration to retain HTTP/3 connections in half-closed state, in
seconds.

When a graceful shutdown of h2o is initiated, h2o at first sends a GO-
AWAY frame indicating the clients that it is initiating shutdown, then
after one second, starts rejecting new HTTP requests (a.k.a. half-
closed state).

This directive controls how long h2o should wait for the peer to close
the QUIC connection in this half-closed state, before exitting.

If set to zero, this timeout is disabled. h2o will not shut down until
all QUIC connections are closed by the clients or times out.

http3-gso
If Generic Segmentation Offload should be used when sending QUIC pack-
ets.

http3-idle-timeout
Idle timeout in the unit of seconds.

Unlike idle timeout of HTTP/1 and HTTP/2, this value should be small
because it is faster to re-establish a new connection using 0-RTT than
migrating to a different port due to NAT rebinding.

http3-input-window-size
Default window size for HTTP request body.

See http2-input-window-size.

http3-max-concurrent-streaming-requests-per-connection
Maximum number of streaming requests to be handled concurrently within
a single HTTP/3 connection.

ACCESS LOG DIRECTIVES
This document describes the configuration directives of the access_log
handler.

access-log
The directive sets the path and optionally the format of the access
log.

If the supplied argument is a scalar, it is treated as the path of the
log file, or if the value starts with a |, it is treated as a command
to which the log should be emitted.

The latter approach (i.e. |) needs to be used for rotating the logs.
This is because the log file is opened (or the command that emits the
log is spawned) before dropping privileges so that it can be owned by
root or any other user; therefore it cannot be reopened by the server
process itself once it starts running.

Example: Emit access log to file

access-log: /path/to/access-log-file

Example: Emit access log through pipe

access-log: "| rotatelogs /path/to/access-log-file.%Y%m%d 86400"

If the supplied argument is a mapping, its path property is considered
as the path of the log file or the pipe command, and the format prop-
erty is treated as the format of the log file. Starting from version
2.2, escape property can be used to specify the escape sequence that
should be used to emit unsafe octets.

Two forms of escape sequences are supported. If apache is specified as
the value of the escape property, unsafe octets are emitted in the form
of , where N is a hexadecimal number in lower case. If json is speci-
fied, unsafe octets are emitted in the form of 00NN. apache is the de-
fault escape method.

Example: Emit access log to file using Common Log Format

access-log:
path: /path/to/access-log-file
format: "%h %l %u %t
escape: apache

The list of format strings recognized by H2O is as follows.

Format StringDescription %%the percent sign %Alocal address (e.g.
4.5.6.7) %bsize of the response body in bytes %Hrequest protocol as
sent by the client (e.g. HTTP/1.1) %hremote address (e.g. 1.2.3.4)
%lremote logname (always -) %mrequest method (e.g. GET, POST) %plocal
port (%{local}p is a synonym that is supported since version 2.2) %{re-
mote}premote port (since version 2.2) %qquery string (? is prepended if
exists, otherwise an empty string) %rrequest line (e.g. GET / HTTP/1.1)
%sstatus code sent to client (e.g. 200) %status code received from up-
stream (or initially generated) %ttime when the request was received in
format: [02/Jan/2006:15:04:05 -0700] %{FORMAT}ttime when the request
was received using the specified format. FORMAT should be an argument
to strftime, or one of:

secnumber of seconds since Epoch msecnumber of milliseconds since Epoch
usecnumber of microseconds since Epoch msec_fracmillisecond fraction
usec_fracmicrosecond fraction

As an example, it is possible to log timestamps in millisecond resolu-
tion using %{%Y/%m/%d:%H:%M:%S}t.%{msec_frac}t, which results in a
timestamp like 2006-01-02:15:04:05.000. %Urequested URL path, not in-
cluding the query string %uremote user if the request was authenticated
(always -) %Vrequested server name (or the default server name if not
specified by the client) %vcanonical server name %{VARNAME}erequest en-
vironment variable (since version 2.3; see Logging Arbitrary Variable)
%{HEADERNAME}ivalue of the given request header (e.g. %{user-agent}i)
%{HEADERNAME}ovalue of the given response header sent to client (e.g.
%{set-cookie}o) %<{HEADERNAME}ovalue of the response header received
from upstream (or initially generated) %{NAME}xvarious extensions.
NAME must be one listed in the following tables. A dash (-) is emitted
if the directive is not applicable to the request being logged.

Access Timings NameDescription connect-timetime spent to establish the
connection (i.e. since connection gets accept(2)-ed until first octet
of the request is received) request-header-timetime spent receiving re-
quest headers request-body-timetime spent receiving request body re-
quest-total-timesum of request-header-time and request-body-time
process-timetime spent after receiving request, before starting to send
response response-timetime spent sending response durationsum of re-
quest-total-time, process-time, response-time total-timesame as dura-
tion (since v2.3)

Proxy Timings (since v2.3) NameDescription proxy.idle-timetime spent
after receiving request, before starting to connect to the upstream
proxy.connect-timetime spent to establish the connection (including SSL
handshake) proxy.request-timetime spent sending request (header and
body) proxy.process-timetime spent after sending request, before start-
ing to receive response proxy.response-timetime spent receiving re-
sponse proxy.total-timesum of proxy-request-time, proxy-process-time,
proxy-response-time

Proxy (since v2.3) NameDescription proxy.request-bytesnumber of bytes
used by the proxy handler for sending the request (above TLS layer)
proxy.request-bytes-headernumber of bytes used by the proxy handler for
sending the request header (above TLS layer) proxy.request-bytes-bo-
dynumber of bytes used by the proxy handler for sending the request
body (above TLS layer) proxy.response-bytesnumber of bytes used by the
proxy handler for receiving the response (above TLS layer) proxy.re-
sponse-bytes-headernumber of bytes used by the proxy handler for re-
ceiving the response header (above TLS layer) proxy.response-bytes-bo-
dynumber of bytes used by the proxy handler for receiving the response
body (above TLS layer)

Connection (since v2.0) NameDescription connection-id64-bit internal ID
assigned to every client connection ssl.protocol-versionSSL protocol
version obtained from SSL_get_version ssl.session-reused1 if the SSL
session was reused, or 0 if not [1] ssl.session-idbase64-encoded value
of the session id used for resuming the session (since v2.2) ssl.ci-
phername of the cipher suite being used, obtained from SSL_CI-
PHER_get_name ssl.cipher-bitsstrength of the cipher suite in bits
ssl.server-namehostname provided in Server Name Indication (SNI) exten-
sion, if any

Upstream Proxy Connection (since v2.3) NameDescription proxy.ssl.proto-
col-versionSSL protocol version obtained from SSL_get_version
proxy.ssl.session-reused1 if the SSL session was reused, or 0 if not
proxy.ssl.ciphername of the cipher suite being used, obtained from
SSL_CIPHER_get_name proxy.ssl.cipher-bitsstrength of the cipher suite
in bits

HTTP/2 (since v2.0) NameDescription http2.stream-idstream ID http2.pri-
ority.receivedcolon-concatenated values of exclusive, parent, weight
http2.priority.received.exclusiveexclusive bit of the most recent pri-
ority specified by the client http2.priority.received.parentparent
stream ID of the most recent priority specified by the client
http2.priority.received.weightweight of the most recent priority speci-
fied by the client

Miscellaneous NameDescription errorrequest-level errors. Unless speci-
fied otherwise by using the error-log.emit-request-errors directive,
the same messages are emitted to the error-log. (since v2.1)

The default format is %h %l %u %t "%r" %s %b "%{Referer}i" "%{User-
agent}i", a.k.a. the NCSA extended/combined log format.

Note that you may need to quote (and escape) the format string as re-
quired by YAML (see Yaml Cookbook).

See also: error-log error-log.emit-request-errors

Notes:
[1]A single SSL connection may transfer more than one HTTP request.

ERRORDOC DIRECTIVES
This document describes the configuration directives of the errordoc
handler.

error-doc
Specifies the content to be sent when returning an error response (i.e.
a response with 4xx or 5xx status code).

The argument must be a mapping containing following attributes, or if
it is a sequence, every element must be a mapping with the following
attributes.

status - three-digit number indicating the status code (or sequence of
that from version 2.3) url - URL of the document to be served

URL can either be absolute or relative. Only content-type, content-
language, set-cookie headers obtained from the specified URL are served
to the client.

Example: Set error document for 404 status

error-doc:
status: 404
url: /404.html

Example: Set error document for 500 and 503 status

error-doc:
- status: 500
url: /internal-error.html
- status: 503
url: /service-unavailable.html

Example: Set error document for 50x statuses (From version 2.3)

error-doc:
status: [500, 502, 503, 504]
url: /50x.html

EXPIRES DIRECTIVES
This document describes the configuration directives of the expires
handler.

expires
An optional directive for setting the Cache-Control: max-age= header.

if the argument is OFF the feature is not used if the value is NUMBER
UNIT then the header is set the units recognized are: second, minute,
hour, day, month, year
the units can also be in plural forms

Example: Set Cache-Control: max-age=86400

expires: 1 day

You can also find an example that conditionally sets the header depend-
ing on the aspects of a request in Modifying the Response section of
the Mruby directives documentation.

FASTCGI DIRECTIVES
This document describes the configuration directives of the FastCGI
handler.

The configuration directives of the FastCGI handler can be categorized
into two groups. Fastcgi.connect and fastcgi.spawn define the address
(or the process) to which the requests should be sent. Other direc-
tives customize how the connections to the FastCGI processes should be
maintained.

fastcgi.connect
The directive specifies the address at where the FastCGI daemon is run-
ning.

If the argument is a mapping, following properties are recognized.

host name (or IP address) of the server running the FastCGI daemon (ig-
nored if type is unix) port TCP port number or path to the unix socket
type either tcp (default) or unix

If the argument is a scalar, the value is considered as a TCP port num-
ber and the host is assumed to be 127.0.0.1.

Example: Map /app to FastCGI daemon listening to /tmp/fcgi.sock

hosts:
"example.com:80":
paths:
"/app":
fastcgi.connect:
port: /tmp/fcgi.sock
type: unix

fastcgi.document_root
Sets the DOCUMENT_ROOT variable to be passed to the FastCGI applica-
tion.

fastcgi.spawn
The directive specifies the command to start the FastCGI process man-
ager.

In contrast to fastcgi.connect that connects to a FastCGI server run-
ning externally, this directive launches a FastCGI process manager un-
der the control of H2O, and terminates it when H2O quits. The argument
is a /bin/sh -c expression to be executed when H2O boots up. The HTTP
server records the process id of the expression, and sends SIGTERM to
the id when it exits.

Example: Map .php files to 10 worker processes of /usr/local/bin/php-
cgi

file.custom-handler:
extension: .php
fastcgi.spawn: "PHP_FCGI_CHILDREN=10 exec /usr/local/bin/php-cgi"

Example: Map any executable file in path /var/www/data/cgi-bin to
fastcgi-cgi wrapper

"/cgi-bin":
file.dir: /var/www/data/cgi-bin
file.custom-handler:
extension: default # means "no extension" in this case
fastcgi.spawn:
command: "exec /usr/local/share/h2o/fastcgi-cgi"

As of version 1.4.0, the spawned process is run under the privileges of
user specified by the user directive (in version 1.3.x, the FastCGI
process was spawned under the privileges that spawned the H2O stand-
alone server). It is possible to specify a different user for running
the FastCGI process, by providing a mapping that contains an attribute
named user together with an attribute named command.

Example: Running FastCGI processes under user fastcgi

file.custom-handler:
extension: .php
fastcgi.spawn:
command: "PHP_FCGI_CHILDREN=10 exec /usr/local/bin/php-cgi"
user: fastcgi

fastcgi.timeout.io
Sets the I/O timeout of connections to the FastCGI process in millisec-
onds.

fastcgi.timeout.keepalive
Sets the keepl-alive timeout for idle connections in milliseconds.

FastCGI connections will not be persistent if the value is set to zero
(default).

fastcgi.send-delegated-uri
Send the modified HTTP_HOST and REQUEST_URI being rewritten in case of
internal redirect.

In H2O, it is possible to perform internal redirects (a.k.a. delega-
tions or URL rewrites) using the redirect directive or by returning X-
Reproxy-URL headers from web applications. The directive specifies
whether to send the original values to the FastCGI process (default),
or if the rewritten values should be sent.

FILE DIRECTIVES
This document describes the configuration directives of the file han-
dler - a handler that for serving static files.

Two directives: file.dir and file.file are used to define the mapping.
Other directives modify the behavior of the mappings defined by the
two.

file.custom-handler
The directive maps extensions to a custom handler (e.g. FastCGI).

The directive accepts a mapping containing configuration directives
that can be used at the extension level, together with a property named
extension specifying a extension (starting with .) or a sequence of ex-
tensions to which the directives should be applied. If all the files
(including those without extensions) shall be mapped, this property
must be set to default. Only one handler must exist within the direc-
tives.

Example: Mapping PHP files to FastCGI

file.custom-handler:
extension: .php
fastcgi.connect:
port: /tmp/fcgi.sock
type: unix

file.dir
The directive specifies the directory under which should be served for
the corresponding path.

Example: Serving files under different paths

paths:
"/":
file.dir: /path/to/doc-root
"/icons":
file.dir: /path/to/icons-dir

See also: file.dirlisting, file.file, file.index

file.dirlisting
A boolean flag (OFF, or ON) specifying whether or not to send the di-
rectory listing in case none of the index files exist.

See also: proxy.ssl.cafile

proxy.timeout.connect
(since v2.3) Sets the timeout before establishing the upstream in mil-
liseconds.

When connecting to a TLS upstream, this timeout will run until the end
of the SSL handshake.

proxy.timeout.first_byte
(since v2.3) Sets the timeout before receiving the first byte from up-
stream.

This sets the maxium time we will wait for the first byte from up-
stream, after the establishment of the connection.

proxy.timeout.io
Sets the upstream I/O timeout in milliseconds.

This value will be used for proxy.timeout.connect and proxy.time-
out.first_byte as well, unless these parameters are explicitely set.

proxy.timeout.keepalive
Sets the upstream timeout for idle connections in milliseconds.

Upstream connection becomes non-persistent if the value is set to zero.
The value should be set to something smaller than that being set at the
upstream server.

proxy.tunnel
A boolean flag (ON or OFF) indicating whether or not to allow tun-
nelling to the backend server.

When set to ON, CONNECT requests and WebSocket handshakes are forwarded
to the backend server. Then, if the backend server accepts those re-
quests, H2O forwards the HTTP response to the client and acts as a bi-
directional tunnel.

Timeouts are governed by properties proxy.timeout.connect and
proxy.timeout.io.

proxy.zerocopy
Sets the use of zerocopy operations for forwarding the response body.

By default, this flag is set to OFF, in which case the response bytes
are read from the upstream socket to an internal buffer as they arrive,
then shipped to the client. Maximum size of this buffer is controlled
by proxy.max-buffer-size. The drawback of this approach is that it
causes pressure on memory bandwidth.

This knob provides two alternative modes to remedy the pressure:

When set to enabled, if zerocopy operation in supported by the down-
stream connection (i.e., downstream connection being cleoartext or en-
crypted using kernel TLS), h2o uses a pipe as an internal buffer in-
stead of using userspace memory. Data is moved to the pipe using the
splice system call, then shipped to the downstream connection by an-
other call to splice. Pressure to memory bandwidth is eliminated, as
the splice system call merely moves the references to kernel memory be-
tween file descriptors.

When set to always, data from upstream is spliced into a pipe regard-
less of downstream connection providing support for zerocopy. When the
downstream connection does not support zerocopy, data is intially moved
into the pipe, then gets read and written to the socket (as well as be-
ing encrypted, if necessary) as late as it becomes possible to send the
data. This approach does not reduce the total amount of bytes flowing
through the CPU, but reduces the amount of userspace memory used by h2o
by delaying the reads, thereby reducing cache spills.

See also: proxy.max-spare-pipes, ssl-offload

REDIRECT DIRECTIVES
This document describes the configuration directives of the redirect
handler.

redirect
Redirects the requests to given URL.

The directive rewrites the URL by replacing the host and path part of
the URL at which the directive is used with the given URL. For exam-
ple, when using the configuration below, requests to http://exam-
ple.com/abc.html will be redirected to https://example.com/abc.html.

If the argument is a scalar, the value is considered as the URL to
where the requests should be redirected.

Following properties are recognized if the argument is a mapping.

url URL to redirect to status the three-digit status code to use (e.g.
301) internal either YES or NO (default); if set to YES, then the
server performs an internal redirect and return the content at the
redirected URL

Example: Redirect all HTTP to HTTPS permanently (except for the files
under RSS)

hosts:
"example.com:80":
paths:
"/":
redirect:
status: 301
url: "https://example.com/"
"/rss":
file.dir: /path/to/rss

REPROXY DIRECTIVES
This document describes the configuration directives of the reproxy
handler.

reproxy
A boolean flag (ON or OFF) indicating if the server should recognize
the X-Reproxy-URL header sent from upstream servers.

If H2O recognizes the header, it fetches the contents of the resource
specified by the header, and sends the contents as the response to the
client. If the status code associated with the X-Reproxy-URL header is
307 or 308, then the method of the original request is used to obtain
the specified resource. Otherwise, the request method is changed to
GET.

For example, an upstream server may send an URL pointing to a large im-
age using the X-Reproxy-URL header stored on a distributed file system,
and let H2O fetch and return the content to the client, instead of
fetching the image by itself. Doing so would reduce the load on the
application server.

STATUS DIRECTIVES
The status handler exposes the current states of the HTTP server. This
document describes the configuration directives of the handler.

status
(since v2.0) If the argument is ON, the directive registers the status
handler to the current path.

Access to the handler should be restricted, considering the fact that
the status includes the details of in-flight HTTP requests. The exam-
ple below uses Basic authentication.

Example: Exposing status with Basic authentication

paths:
/server-status:
mruby.handler: |
require "htpasswd.rb"
Htpasswd.new("/path/to/.htpasswd", "status")
status: ON

The information returned by the /json handler can be filtered out using
the optional show=module1,module2 parameter. There are currently three
modules defined:

requests: displays the requests currently in-flight. durations: dis-
plays durations statistics for requests since server start time in sec-
onds (returns all zeros unless duration-stats is ON). errors: displays
counters for internally generated errors. main: displays general dae-
mon-wide stats.

duration-stats
(since v2.1) Gather timing stats for requests.

If the argument is ON, this directive populates duration statistics in
seconds, to be consumed by status handlers. Enabling this feature has
a noticeable CPU and memory impact.

Note that the time spent while processing a request in a blocking man-
ner (such as opening a file or a mruby handler that does invoke a net-
work operation) will not be reflected to the process_time element of
the duration stats due to the fact that the timer being used for mea-
suring the time spent is updated only once per loop.

THROTTLE RESPONSE DIRECTIVES
The throttle response handler performs per response traffic throttling,
when an X-Traffic header exists in the response headers.

The value of X-Traffic header should be an integer that represents the
speed you want in bytes per second. This header CAN be set with
header.add so that traffic for static assets can also be easily throt-
tled.

The following are the configuration directives recognized by the han-
dler.

throttle-response
(since v2.1) Enables traffic throttle per HTTP response.

If the argument is ON, the traffic per response is throttled as long as
a legal X-Traffic header exists. If the argument is OFF, traffic
throttle per response is disabled.

Example: Enabling traffic throttle per response with static file con-
figuration

# enable throttle
throttle-response: ON

# an example host configuration that throttle traffic to ~100KB/s
hosts:
default:
paths:
/:
file.dir: /path/to/assets
header.add: "X-Traffic: 100000"

USING BASIC AUTHENTICATION
Starting from version 1.7, H2O comes with a mruby script named ht-
passwd.rb that implements Basic Authentication. The script provides a
Rack handler that implements Basic Authentication using password files
generated by the htpasswd command.

Below example uses the mruby script to restrict access to the path. If
authentication fails, the mruby handler returns a 401 Unauthorized re-
sponse. If authentication succeeds, the handler returns a 399 re-
sponse, and the request is delegated internally to the next handler
(i.e. file.dir).

Example: Configuring HTTP authentication using htpasswd.rb

paths:
"/":
mruby.handler: |
require "htpasswd.rb"
Htpasswd.new("/path/to/.htpasswd", "realm-name")
file.dir: /path/to/doc_root

In H2O versions prior to 2.0, you should specify
"#{$H2O_ROOT}/share/h2o/mruby/htpasswd.rb" as the argument to require,
since the directory is not registered as part of $LOAD_PATH.

For convenience, the mruby script also forbids access to files or di-
rectories that start with .ht.

USING CGI
Starting from version 1.7, H2O comes with a FastCGI-to-CGI gateway
(fastcgi-cgi), which can be found under share/h2o directory of the in-
stallation path. The gateway can be used for running CGI scripts
through the FastCGI handler.

The example below maps .cgi files to be executed by the gateway. It is
also possible to run CGI scripts under different privileges by specify-
ing the user attribute of the directive.

Example: Execute .cgi files using FastCGI-to-CGI gateway

file.custom-handler:
extension: .cgi
fastcgi.spawn:
command: "exec $H2O_ROOT/share/h2o/fastcgi-cgi"

The gateway also provides options to for tuning the behavior. A full
list of options can be obtained by running the gateway directly with
--help option.

Example: Output of share/h2o/fastcgi-cgi --help

$ share/h2o/fastcgi-cgi --help
Usage:
share/h2o/fastcgi-cgi [options]

Options:
--listen=sockfn path to the UNIX socket. If specified, the program will
create a UNIX socket at given path replacing the existing
file (should it exist). If not, file descriptor zero (0)
will be used as the UNIX socket for accepting new
connections.
--max-workers=nnn maximum number of CGI processes (default: unlimited)
--pass-authz if set, preserves HTTP_AUTHORIZATION parameter
--verbose verbose mode

USING MRUBY
mruby is a lightweight implementation of the Ruby programming language.
With H2O, users can implement their own request handling logic using
mruby, either to generate responses or to fix-up the request / re-
sponse.

Rack-based Programming Interface

The interface between the mruby program and the H2O server is based on
Rack interface specification. Below is a simple configuration that re-
turns hello world.

Example: Hello-world in mruby

paths:
"/":
mruby.handler: |
Proc.new do |env|
[200, {'content-type' => 'text/plain'}, ["Hello world0]]
end

It should be noted that as of H2O version 1.7.0, there are limitations
when compared to ordinary web application server with support for Rack
such as Unicorn:

no libraries provided as part of Rack is available (only the interface
is compatible)

In addition to the Rack interface specification, H2O recognizes status
code 399 which can be used to delegate request to the next handler.
The feature can be used to implement access control and response header
modifiers.

Access Control

By using the 399 status code, it is possible to implement access con-
trol using mruby. The example below restricts access to requests from
192.168. private address.

Example: Restricting access to 192.168.

paths:
"/":
mruby.handler: |
lambda do |env|
if /168./.match(env["REMOTE_ADDR"])
return [399, {}, []]
end
[403, {'content-type' => 'text/plain'}, ["access forbidden0]]
end

Support for Basic Authentication is also provided by an mruby script.

Delegating the Request

When enabled using the reproxy directive, it is possible to delegate
the request from the mruby handler to any other handler.

Example: Rewriting URL with delegation

paths:
"/":
mruby.handler: |
lambda do |env|
if /user([^]+)/.match(env["PATH_INFO"])
return [307, {"x-reproxy-url" => "/user.php?user=#{$1}"}, []]
end
return [399, {}, []]
end

Modifying the Response

When the mruby handler returns status code 399, H2O delegates the re-
quest to the next handler while preserving the headers emitted by the
handler. The feature can be used to add extra headers to the response.

For example, the following example sets cache-control header for re-
quests against .css and .js files.

Example: Setting cache-control header for certain types of files

paths:
"/":
mruby.handler: |
Proc.new do |env|
headers = {}
if /.(css|js).match(env["PATH_INFO"])
headers["cache-control"] = "max-age=86400"
end
[399, headers, []]
end
file.dir: /path/to/doc-root

Or in the example below, the handler triggers HTTP/2 server push with
the use of Link: rel=preload headers, and then requests a FastCGI ap-
plication to process the request.

Example: Pushing asset files

paths:
"/":
mruby.handler: |
Proc.new do |env|
push_paths = []
# push css and js when request is to dir root or HTML
if /(|.html).match(env["PATH_INFO"])
push_paths << ["/css/style.css", "style"]
push_paths << ["/js/app.js", "script"]
end
[399, push_paths.empty? ? {} : {"link" => push_paths.map{|p| "<#{p[0]}>; rel=preload; as=#{p[1]}"}.join("0)}, []]
end
fastcgi.connect: ...

Using the HTTP Client

Starting from version 1.7, a HTTP client API is provided. HTTP re-
quests issued through the API will be handled asynchronously; the
client does not block the event loop of the HTTP server.

Example: Mruby handler returning the response of http://example.com

paths:
"/":
mruby.handler: |
Proc.new do |env|
req = http_request("http://example.com")
status, headers, body = req.join
[status, headers, body]
end

http_request is the method that issues a HTTP request.

The method takes two arguments. First argument is the target URI.
Second argument is an optional hash; method (defaults to GET), header,
body attributes are recognized.

The method returns a promise object. When #join method of the promise
is invoked, a three-argument array containing the status code, response
headers, and the body is returned. The response body is also a
promise. Applications can choose from three ways when dealing with the
body: a) call #each method to receive the contents, b) call #join to
retrieve the body as a string, c) return the object as the response
body of the mruby handler.

The header and the body object passed to http_request should conform to
the requirements laid out by the Rack specification for request header
and request body. The response header and the response body object re-
turned by the #join method of the promise returned by http_request con-
forms to the requirements of the Rack specification.

Since the API provides an asynchronous HTTP client, it is possible to
effectively issue multiple HTTP requests concurrently and merge them
into a single response.

When HTTPS is used, servers are verified using the properties of
proxy.ssl.cafile and proxy.ssl.verify-peer specified at the global
level.

Timeouts defined for the proxy handler (proxy.timeout.*) are applied to
the requests that are issued by the http_request method.

Logging Arbitrary Variable

In version 2.3, it is possible from mruby to set and log an arbitrary-
named variable that is associated to a HTTP request. A HTTP response
header that starts with x-fallthru-set- is handled specially by the H2O
server. Instead of sending the header downstream, the server accepts
the value as a request environment variable, taking the suffix of the
header name as the name of the variable.

This example shows how to read request data, parse json and then log
data from mruby.

Example: Logging the content of a POST request via request environment
variable

paths:
"/":
mruby.handler: |
Proc.new do |env|
input = env["rack.input"] ? env["rack.input"].read : '{"default": "true"}'
parsed_json = JSON.parse(input)
parsed_json["time"] = Time.now.to_i
logdata = parsed_json.to_s
[204, {"x-fallthru-set-POSTDATA" => logdata}, []]
end
access-log:
path: /path/to/access-log.json
escape: json
format: '{"POST": %{POSTDATA}e}'

USING DOS DETECTION
Starting from version 2.1, H2O comes with a mruby script named dos_de-
tector.rb that implements DoS Detection feature. The script provides a
Rack handler that detects HTTP flooding attacks based on the client's
IP address.

Basic Usage

Below example uses the mruby script to detect DoS attacks. The default
detecting strategy is simply counting requests within configured pe-
riod. If the count exceeds configured threshold, the handler returns a
403 Forbidden response. Otherwise, the handler returns a 399 response,
and the request is delegated internally to the next handler.

Example: Configuring DoS Detection

paths:
"/":
mruby.handler: |
require "dos_detector.rb"
DoSDetector.new({
:strategy => DoSDetector::CountingStrategy.new({
:period => 10, # default
:threshold => 100, # default
:ban_period => 300, # default
}),
})
file.dir: /path/to/doc_root

In the example above, the handler countup the requests within 10 sec-
onds for each IP address, and when the count exceeds 100, it returns a
403 Forbidden response for the request and marks the client as "Banned"
for 300 seconds. While marked as "Banned", the handler returns a 403
Forbidden to all requests from the same IP address.

Configuring Details

You can pass the following parameters to DoSDetector.new .

:strategy
The algorithm to detect DoS attacks. You can write and pass your own
strategies if needed. The default strategy is DoSDetector.Count-
ingStrategy which takes the following parameters:

:period
Time window in seconds to count requests. The default value is
10.

:threshold
Threshold count of request. The default value is 100.

:ban_period
Duration in seconds in which "Banned" client continues to be re-
stricted. The default value is 300.

:callback
The callback which is called by the handler with detecting result.
You can define your own callback to return arbitrary response, set re-
sponse headers, etc. The default callback returns 403 Forbidden if DoS
detected, otherwise delegate the request to the next handler.

:forwarded

If set true, the handler uses X-HTTP-Forwarded-For header to get
client's IP address if the header exists. The default value is true.

:cache_size

The capacity of the LRU cache which preserves client's IP address
and associated request count. The default value is 128.

Example: Configuring Details

paths:
"/":
mruby.handler: |
require "dos_detector.rb"
DoSDetector.new({
:strategy => DoSDetector::CountingStrategy.new,
:forwarded => false,
:cache_size => 2048,
:callback => proc {|env, detected, ip|
if detected && ! ip.start_with?("192.168.")
[503, {}, ["Service Unavailable"]]
else
[399, {}, []]
end
}
})
file.dir: /path/to/doc_root

Points to Notice

For now, counting requests is "per-thread" and not shared between
multiple threads.

ACCESS CONTROL
Starting from version 2.1, H2O comes with a DSL-like mruby library
which makes it easy to write access control list (ACL).

Example

Below example uses this Access Control feature to write various access
control.

Example: Access Control

paths:
"/":
mruby.handler: |
acl {
allow { addr == "127.0.0.1" }
deny { user_agent.match(/curl/i) && ! addr.start_with?("192.168.") }
respond(503, {}, ["Service Unavailable"]) { addr == malicious_ip }
redirect("https://example.com/", 301) { path =~ /moved/ }
use Htpasswd.new("/path/to/.htpasswd", "realm") { path.start_with?("/admin") }
}
file.dir: /path/to/doc_root

In the example, the handler you get by calling acl method will do the
following:

if the remote IP address is exactly equal to "127.0.0.1", the re-
quest will be delegated to the next handler (i.e. serve files under
/path/to/doc_root) and all following acl settings are ignored

otherwise, if the user agent string includes "curl" and the remote
IP address doesn't start with "192.168.", this handler immediately re-
turns 403 Forbidden response

otherwise, if the remote IP address is exactly equal to the mali-
cious_ip variable, this handler immediately returns 503 Service Un-
available response

otherwise, if the request path matches with the pattern /moved/i,
this handler immediately redirects the client to "https://example.com"
with 301 status code

otherwise, if the request path starts with /admin, apply Basic Au-
thentication to the request (for details of Basic Authentication, see
here).

otherwise, the request will be delegated to the next handler (i.e.
serve files under /path/to/doc_root)

ACL Methods

An ACL handler is built by calling ACL methods, which can be used like
directives. ACL methods can only be used in acl block.

Each ACL method adds a filter to the handler, which checks whether the
request matches the provided condition or not. Every ACL method can be
accompanied by a condition block, which should return boolean value.

The filter defined by the method that first matched the accompanying
condition gets applied (e.g. response 403 Forbidden, redirect to some-
where). If a condition block is omitted, all requests matches. If
none of the conditions matches the request, the handler returns 399 and
the request will be delegated to the next handler.

allow
Adds a filter which delegates the request to the next handler if the
request matches the provided condition.

allow { ..condition.. }

deny
Adds a filter which returns 403 Forbidden if the request matches the
provided condition.

deny { ..condition.. }

redirect
Adds a filter which redirects the client if the request matches the
provided condition.

redirect(location, status) { ..condition.. }

Parameters:
location: Location to which the client will be redirected. Re-
quired.
status: Status code of the response. Default value: 302

respond
Adds a filter which returns arbitrary response if the request matches
the provided condition.

respond(status, header, body) { ..condition.. }

Parameters:
status: Status code of the response. Required.
header: Header key-value pairs of the response. Default value:
{}
body: Body array of the response. Default value: []

use
Adds a filter which applies the provided handler (callable object) if
the request matches the provided condition.

use(proc) { ..condition.. }

Parameters:
proc: Callable object that should be applied

Matching Methods

In a condition block, you can use helpful methods which return particu-
lar properties of the request as string values. Matching methods can
only be used in a condition block of the ACL methods.

addr
Returns the remote IP address of the request.

addr(forwarded)

Parameters:
forwarded: If true, returns the value of X-Forwarded-For header
if it exists. Default value: true

path
Returns the requested path string of the request.

path()

method
Returns the HTTP method of the request.

method()

header
Returns the header value of the request associated with the provided
name.

header(name)

Parameters:
name: Case-insensitive header name. Required.

user_agent
Shortcut for header("user-agent").

user_agent()

Caution

Several restrictions are introduced to avoid misconfiguration when us-
ing acl method.

acl method can be called only once in each handler configuration If acl
method is used, the handler returned by the configuration directive
must be the one returned by the acl method

If a configuration violates these restrictions, the server will detect
it and refuse to launch with error message.

For example, both of the following examples violate the restrictions
above, so the server will refuse to start up.

Example: Misconfiguration Example 1

paths:
"/":
mruby.handler: |
acl { # this block will be ignored!
allow { addr == "127.0.0.1" }
}
acl {
deny
}
file.dir: /path/to/doc_root

Example: Misconfiguration Example 2

paths:
"/":
mruby.handler: |
acl { # this block will be ignored!
allow { addr == "127.0.0.1" }
deny
}
proc {|env| [399, {}, []}
file.dir: /path/to/doc_root

You can correct these like the following:

Example: Valid Configuration Example

paths:
"/":
mruby.handler: |
acl {
allow { addr == "127.0.0.1" }
deny
}
file.dir: /path/to/doc_root

How-To

Matching IP Address Blocks

You can match an IP address against predefined list of address blocks
using a script named trie_addr.rb.

Below is an example.

Example: Address Block Matching Example

paths:
"/":
mruby.handler: |
require "trie_addr.rb"
trie = TrieAddr.new.add(["192.168.0.0/16", "172.16.0.0/12"])
acl {
allow { trie.match?(addr) }
deny
}
file.dir: /path/to/doc_root

This library currently supports only IPv4 addresses. TrieAddr#match?
returns false when it receives an invalid IPv4 address (including an
IPv6 address) as an argument..

USING H2OLOG FOR TRACING
h2olog is an experimental BPF (kernel doc) backed tracing tool for the
H2O server. It can be used for tracing quicly and h2o USDT probes.

Since h2olog is an experimental program, its command-line interface
might change without notice.

Installing from Source

See requirements for build prerequisites. If dependencies are satis-
fied, h2olog is built automatically. It is possible to manually turn on
/ off the build of h2olog by using the -DWITH_H2OLOG option. This op-
tion takes either ON> or OFF as the argument. If you have BCC in-
stalled to a non-standard path, use pkg-config for cmake.

$ PKG_CONFIG_PATH=/path/to/bcc/lib/pkgconfig cmake [options]

Requirements

For building h2olog

C++11 compiler CMake for generating the build files pkg-config for de-
tecting dependencies Python 3 for the code generator BCC (BPF compiler
collection, a.k.a. bpfcc; >= 0.11.0) installed on your system

For Ubuntu 20.04 or later, you can install dependencies with:

$ sudo apt install clang cmake python3 libbpfcc-dev linux-headers-$(uname -r)

For running h2olog

Root privilege to execute h2olog Linux kernel (>= 4.10)

Quickstart h2olog -H -p $H2O_PID shows varnishlog-like tracing.

$ sudo h2olog -H -p $(pgrep -o h2o)

11 0 RxProtocol HTTP/3.0
11 0 RxHeader :authority torumk.com
11 0 RxHeader :method GET
11 0 RxHeader :path /
11 0 RxHeader :scheme https
11 0 TxStatus 200
11 0 TxHeader content-length 123
11 0 TxHeader content-type text/html

Tracing USDT events Server-side QUIC events can be traced using the
quic subcommand. Events are rendered in JSON Lines format.

$ sudo h2olog -p $(pgrep -o h2o)

Heres an example trace.

{"time":1584380825832,"type":"accept","conn":1,"dcid":"f8aa2066e9c3b3cf"}
{"time":1584380825835,"type":"crypto-decrypt","conn":1,"pn":0,"len":1236}
{"time":1584380825832,"type":"quictrace-recv","conn":1,"pn":0}
{"time":1584380825836,"type":"crypto-handshake","conn":1,"ret":0}

H2O.CONF(5)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=h2o.conf&sektion=5&manpath=FreeBSD+Ports+15.1.quarterly>

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