Apache Module mod_proxy

AllowCONNECT Directive

The AllowCONNECT directive specifies a list of port numbers to which the proxy CONNECT method may connect. Today's browsers use this method when a https connection is requested and proxy tunneling over HTTP is in effect.

By default, only the default https port (443) and the default snews port (563) are enabled. Use the AllowCONNECT directive to override this default and allow connections to the listed ports only.

Note that you'll need to have mod_proxy_connect present in the server in order to get the support for the CONNECT at all.

BalancerMember Directive

This directive adds a member to a load balancing group. It could be used within a <Proxy balancer://...> container directive, and can take any of the key value pairs available to ProxyPass directives.

The balancerurl is only needed when not in <Proxy balancer://...> container directive. It corresponds to the url of a balancer defined in ProxyPass directive.

NoProxy Directive

This directive is only useful for Apache proxy servers within intranets. The NoProxy directive specifies a list of subnets, IP addresses, hosts and/or domains, separated by spaces. A request to a host which matches one or more of these is always served directly, without forwarding to the configured ProxyRemote proxy server(s).

The host arguments to the NoProxy directive are one of the following type list:

Domain

A Domain is a partially qualified DNS domain name, preceded by a period. It represents a list of hosts which logically belong to the same DNS domain or zone (i.e., the suffixes of the hostnames are all ending in Domain).

Examples

.com .apache.org.

To distinguish Domains from Hostnames (both syntactically and semantically; a DNS domain can have a DNS A record, too!), Domains are always written with a leading period.

Note

Domain name comparisons are done without regard to the case, and Domains are always assumed to be anchored in the root of the DNS tree, therefore two domains .ExAmple.com and .example.com. (note the trailing period) are considered equal. Since a domain comparison does not involve a DNS lookup, it is much more efficient than subnet comparison.

SubNet

A SubNet is a partially qualified internet address in numeric (dotted quad) form, optionally followed by a slash and the netmask, specified as the number of significant bits in the SubNet. It is used to represent a subnet of hosts which can be reached over a common network interface. In the absence of the explicit net mask it is assumed that omitted (or zero valued) trailing digits specify the mask. (In this case, the netmask can only be multiples of 8 bits wide.) Examples:

192.168 or 192.168.0.0

the subnet 192.168.0.0 with an implied netmask of 16 valid bits (sometimes used in the netmask form 255.255.0.0)

192.168.112.0/21

the subnet 192.168.112.0/21 with a netmask of 21 valid bits (also used in the form 255.255.248.0)

As a degenerate case, a SubNet with 32 valid bits is the equivalent to an IPAddr, while a SubNet with zero valid bits (e.g., 0.0.0.0/0) is the same as the constant _Default_, matching any IP address.

IPAddr

A IPAddr represents a fully qualified internet address in numeric (dotted quad) form. Usually, this address represents a host, but there need not necessarily be a DNS domain name connected with the address.

Example

192.168.123.7

Note

An IPAddr does not need to be resolved by the DNS system, so it can result in more effective apache performance.

Hostname

A Hostname is a fully qualified DNS domain name which can be resolved to one or more IPAddrs via the DNS domain name service. It represents a logical host (in contrast to Domains, see above) and must be resolvable to at least one IPAddr (or often to a list of hosts with different IPAddrs).

Examples

prep.ai.example.com
www.apache.org

Note

In many situations, it is more effective to specify an IPAddr in place of a Hostname since a DNS lookup can be avoided. Name resolution in Apache can take a remarkable deal of time when the connection to the name server uses a slow PPP link.

Hostname comparisons are done without regard to the case, and Hostnames are always assumed to be anchored in the root of the DNS tree, therefore two hosts WWW.ExAmple.com and www.example.com. (note the trailing period) are considered equal.

See also

• DNS Issues

<Proxy> Directive

Directives placed in <Proxy> sections apply only to matching proxied content. Shell-style wildcards are allowed.

For example, the following will allow only hosts in yournetwork.example.com to access content via your proxy server:

The following example will process all files in the foo directory of example.com through the INCLUDES filter when they are sent through the proxy server:

See also

• <ProxyMatch>

ProxyBadHeader Directive

The ProxyBadHeader directive determines the behaviour of mod_proxy if it receives syntactically invalid response header lines (i.e. containing no colon) from the origin server. The following arguments are possible:

IsError

Abort the request and end up with a 502 (Bad Gateway) response. This is the default behaviour.

Ignore

Treat bad header lines as if they weren't sent.

StartBody

When receiving the first bad header line, finish reading the headers and treat the remainder as body. This helps to work around buggy backend servers which forget to insert an empty line between the headers and the body.

ProxyBlock Directive

The ProxyBlock directive specifies a list of words, hosts and/or domains, separated by spaces. HTTP, HTTPS, and FTP document requests to sites whose names contain matched words, hosts or domains are blocked by the proxy server. The proxy module will also attempt to determine IP addresses of list items which may be hostnames during startup, and cache them for match test as well. That may slow down the startup time of the server.

rocky.wotsamattau.edu would also be matched if referenced by IP address.

Note that wotsamattau would also be sufficient to match wotsamattau.edu.

Note also that

blocks connections to all sites.

ProxyDomain Directive

This directive is only useful for Apache proxy servers within intranets. The ProxyDomain directive specifies the default domain which the apache proxy server will belong to. If a request to a host without a domain name is encountered, a redirection response to the same host with the configured Domain appended will be generated.

ProxyErrorOverride Directive

This directive is useful for reverse-proxy setups, where you want to have a common look and feel on the error pages seen by the end user. This also allows for included files (via mod_include's SSI) to get the error code and act accordingly (default behavior would display the error page of the proxied server, turning this on shows the SSI Error message).

This directive does not affect the processing of informational (1xx), normal success (2xx), or redirect (3xx) responses.

ProxyFtpDirCharset Directive

The ProxyFtpDirCharset directive defines the character set to be set for FTP directory listings in HTML generated by mod_proxy_ftp.

ProxyIOBufferSize Directive

The ProxyIOBufferSize directive adjusts the size of the internal buffer, which is used as a scratchpad for the data between input and output. The size must be at least 8192.

When the mod_proxy_ajp module is used, this value is aligned to a 1024 byte boundary, and values larger than 65536 are set to 65536 in accordance with the AJP protocol.

In almost every case there's no reason to change that value.

<ProxyMatch> Directive

The <ProxyMatch> directive is identical to the <Proxy> directive, except it matches URLs using regular expressions.

See also

• <Proxy>

ProxyMaxForwards Directive

The ProxyMaxForwards directive specifies the maximum number of proxies through which a request may pass, if there's no Max-Forwards header supplied with the request. This may be set to prevent infinite proxy loops, or a DoS attack.

Note that setting ProxyMaxForwards is a violation of the HTTP/1.1 protocol (RFC2616), which forbids a Proxy setting Max-Forwards if the Client didn't set it. Earlier Apache versions would always set it. A negative ProxyMaxForwards value, including the default -1, gives you protocol-compliant behaviour, but may leave you open to loops.

ProxyPass Directive

This directive allows remote servers to be mapped into the space of the local server; the local server does not act as a proxy in the conventional sense, but appears to be a mirror of the remote server. The local server is often called a reverse proxy or gateway. The path is the name of a local virtual path; url is a partial URL for the remote server and cannot include a query string.

Suppose the local server has address http://example.com/; then

will cause a local request for http://example.com/mirror/foo/bar to be internally converted into a proxy request to http://backend.example.com/bar.

The ! directive is useful in situations where you don't want to reverse-proxy a subdirectory, e.g.

will proxy all requests to /mirror/foo to backend.example.com except requests made to /mirror/foo/i.

In Apache HTTP Server 2.1 and later, mod_proxy supports pooled connections to a backend server. Connections created on demand can be retained in a pool for future use. Limits on the pool size and other settings can be coded on the ProxyPass directive using key=value parameters, described in the table below.

By default, mod_proxy will allow and retain the maximum number of connections that could be used simultaneously by that web server child process. Use the max parameter to reduce the number from the default. Use the ttl parameter to set an optional time to live; connections which have been unused for at least ttl seconds will be closed. ttl can be used to avoid using a connection which is subject to closing because of the backend server's keep-alive timeout.

The pool of connections is maintained per web server child process, and max and other settings are not coordinated among all child processes, except when only one child process is allowed by configuration or MPM design.

If the ProxyPass directive scheme starts with the balancer:// (eg: balancer://cluster/, any path information is ignored) then a virtual worker that does not really communicate with the backend server will be created. Instead it is responsible for the management of several "real" workers. In that case the special set of parameters can be add to this virtual worker. See mod_proxy_balancer for more information about how the balancer works.

A sample balancer setup

Setting up a hot-standby, that will only be used if no other members are available

Normally, mod_proxy will canonicalise ProxyPassed URLs. But this may be incompatible with some backends, particularly those that make use of PATH_INFO. The optional nocanon keyword suppresses this, and passes the URL path "raw" to the backend. Note that may affect the security of your backend, as it removes the normal limited protection against URL-based attacks provided by the proxy.

When used inside a <Location> section, the first argument is omitted and the local directory is obtained from the <Location>. The same will occur inside a <LocationMatch> section, however ProxyPass does not interpret the regexp as such, so it is necessary to use ProxyPassMatch in this situation instead.

This directive is not supported in <Directory> or <Files> sections.

If you require a more flexible reverse-proxy configuration, see the RewriteRule directive with the [P] flag.

The optional interpolate keyword (available in httpd 2.2.9 and later), in combination with ProxyPassInterpolateEnv causes the ProxyPass to interpolate environment variables, using the syntax ${VARNAME}. Note that many of the standard CGI-derived environment variables will not exist when this interpolation happens, so you may still have to resort to mod_rewrite for complex rules. Also note that interpolation is not supported within the scheme portion of a URL. Dynamic determination of the scheme can be accomplished with mod_rewrite as in the following example.

RewriteEngine On

RewriteCond %{HTTPS} =off
RewriteRule . - [E=protocol:http]
RewriteCond %{HTTPS} =on
RewriteRule . - [E=protocol:https]

RewriteRule ^/mirror/foo/(.*) %{ENV:protocol}://backend.example.com/$1 [P]
ProxyPassReverse  /mirror/foo/ http://backend.example.com/
ProxyPassReverse  /mirror/foo/ https://backend.example.com/

ProxyPassInterpolateEnv Directive

This directive, together with the interpolate argument to ProxyPass, ProxyPassReverse, ProxyPassReverseCookieDomain and ProxyPassReverseCookiePath enables reverse proxies to be dynamically configured using environment variables, which may be set by another module such as mod_rewrite. It affects the ProxyPass, ProxyPassReverse, ProxyPassReverseCookieDomain, and ProxyPassReverseCookiePath directives, and causes them to substitute the value of an environment variable varname for the string ${varname} in configuration directives (if the interpolate option is set).

Keep this turned off (for server performance) unless you need it!

ProxyPassMatch Directive

This directive is equivalent to ProxyPass, but makes use of regular expressions, instead of simple prefix matching. The supplied regular expression is matched against the url, and if it matches, the server will substitute any parenthesized matches into the given string and use it as a new url.

Suppose the local server has address http://example.com/; then

will cause a local request for http://example.com/foo/bar.gif to be internally converted into a proxy request to http://backend.example.com/foo/bar.gif.

The ! directive is useful in situations where you don't want to reverse-proxy a subdirectory.

When used inside a <LocationMatch> section, the first argument is omitted and the regexp is obtained from the <LocationMatch>.

If you require a more flexible reverse-proxy configuration, see the RewriteRule directive with the [P] flag.

ProxyPassReverse Directive

This directive lets Apache adjust the URL in the Location, Content-Location and URI headers on HTTP redirect responses. This is essential when Apache is used as a reverse proxy (or gateway) to avoid by-passing the reverse proxy because of HTTP redirects on the backend servers which stay behind the reverse proxy.

Only the HTTP response headers specifically mentioned above will be rewritten. Apache will not rewrite other response headers, nor will it rewrite URL references inside HTML pages. This means that if the proxied content contains absolute URL references, they will by-pass the proxy. A third-party module that will look inside the HTML and rewrite URL references is Nick Kew's mod_proxy_html.

path is the name of a local virtual path. url is a partial URL for the remote server - the same way they are used for the ProxyPass directive.

For example, suppose the local server has address http://example.com/; then

will not only cause a local request for the http://example.com/mirror/foo/bar to be internally converted into a proxy request to http://backend.example.com/bar (the functionality ProxyPass provides here). It also takes care of redirects the server backend.example.com sends: when http://backend.example.com/bar is redirected by him to http://backend.example.com/quux Apache adjusts this to http://example.com/mirror/foo/quux before forwarding the HTTP redirect response to the client. Note that the hostname used for constructing the URL is chosen in respect to the setting of the UseCanonicalName directive.

Note that this ProxyPassReverse directive can also be used in conjunction with the proxy pass-through feature (RewriteRule ... [P]) from mod_rewrite because it doesn't depend on a corresponding ProxyPass directive.

The optional interpolate keyword (available in httpd 2.2.9 and later), used together with ProxyPassInterpolateEnv, enables interpolation of environment variables specified using the format ${VARNAME}. Note that interpolation is not supported within the scheme portion of a URL.

When used inside a <Location> section, the first argument is omitted and the local directory is obtained from the <Location>. The same occurs inside a <LocationMatch> section, but will probably not work as intended, as ProxyPassReverse will interpret the regexp literally as a path; if needed in this situation, specify the ProxyPassReverse outside the section, or in a separate <Location> section.

This directive is not supported in <Directory> or <Files> sections.

ProxyPassReverseCookieDomain Directive

Usage is basically similar to ProxyPassReverse, but instead of rewriting headers that are a URL, this rewrites the domain string in Set-Cookie headers.

ProxyPassReverseCookiePath Directive

Useful in conjunction with ProxyPassReverse in situations where backend URL paths are mapped to public paths on the reverse proxy. This directive rewrites the path string in Set-Cookie headers. If the beginning of the cookie path matches internal-path, the cookie path will be replaced with public-path.

In the example given with ProxyPassReverse, the directive:

will rewrite a cookie with backend path / (or /example or, in fact, anything) to /mirror/foo/.

ProxyPreserveHost Directive

When enabled, this option will pass the Host: line from the incoming request to the proxied host, instead of the hostname specified in the ProxyPass line.

This option should normally be turned Off. It is mostly useful in special configurations like proxied mass name-based virtual hosting, where the original Host header needs to be evaluated by the backend server.

ProxyReceiveBufferSize Directive

The ProxyReceiveBufferSize directive specifies an explicit (TCP/IP) network buffer size for proxied HTTP and FTP connections, for increased throughput. It has to be greater than 512 or set to 0 to indicate that the system's default buffer size should be used.

ProxyRemote Directive

This defines remote proxies to this proxy. match is either the name of a URL-scheme that the remote server supports, or a partial URL for which the remote server should be used, or * to indicate the server should be contacted for all requests. remote-server is a partial URL for the remote server. Syntax:

scheme is effectively the protocol that should be used to communicate with the remote server; only http and https are supported by this module. When using https, the requests are forwarded through the remote proxy using the HTTP CONNECT method.

In the last example, the proxy will forward FTP requests, encapsulated as yet another HTTP proxy request, to another proxy which can handle them.

This option also supports reverse proxy configuration - a backend webserver can be embedded within a virtualhost URL space even if that server is hidden by another forward proxy.

ProxyRemoteMatch Directive

The ProxyRemoteMatch is identical to the ProxyRemote directive, except the first argument is a regular expression match against the requested URL.

ProxyRequests Directive

This allows or prevents Apache from functioning as a forward proxy server. (Setting ProxyRequests to Off does not disable use of the ProxyPass directive.)

In a typical reverse proxy or gateway configuration, this option should be set to Off.

In order to get the functionality of proxying HTTP or FTP sites, you need also mod_proxy_http or mod_proxy_ftp (or both) present in the server.

In order to get the functionality of (forward) proxying HTTPS sites, you need mod_proxy_connect enabled in the server.

See also

• Forward and Reverse Proxies/Gateways

ProxySet Directive

This directive is used as an alternate method of setting any of the parameters available to Proxy balancers and workers normally done via the ProxyPass directive. If used within a <Proxy balancer url|worker url> container directive, the url argument is not required. As a side effect the respective balancer or worker gets created. This can be useful when doing reverse proxying via a RewriteRule instead of a ProxyPass directive.

ProxyStatus Directive

This directive determines whether or not proxy loadbalancer status data is displayed via the mod_status server-status page.

ProxyTimeout Directive

This directive allows a user to specify a timeout on proxy requests. This is useful when you have a slow/buggy appserver which hangs, and you would rather just return a timeout and fail gracefully instead of waiting however long it takes the server to return.

ProxyVia Directive

This directive controls the use of the Via: HTTP header by the proxy. Its intended use is to control the flow of proxy requests along a chain of proxy servers. See RFC 2616 (HTTP/1.1), section 14.45 for an explanation of Via: header lines.

• If set to Off, which is the default, no special processing is performed. If a request or reply contains a Via: header, it is passed through unchanged.
• If set to On, each request and reply will get a Via: header line added for the current host.
• If set to Full, each generated Via: header line will additionally have the Apache server version shown as a Via: comment field.
• If set to Block, every proxy request will have all its Via: header lines removed. No new Via: header will be generated.