Apache Module mod_proxy

Forward Proxies and Reverse Proxies/Gateways

Apache HTTP Server can be configured in both a forward and reverse proxy (also known as gateway) mode.

An ordinary forward proxy is an intermediate server that sits between the client and the origin server. In order to get content from the origin server, the client sends a request to the proxy naming the origin server as the target. The proxy then requests the content from the origin server and returns it to the client. The client must be specially configured to use the forward proxy to access other sites.

A typical usage of a forward proxy is to provide Internet access to internal clients that are otherwise restricted by a firewall. The forward proxy can also use caching (as provided by mod_cache) to reduce network usage.

The forward proxy is activated using the ProxyRequests directive. Because forward proxies allow clients to access arbitrary sites through your server and to hide their true origin, it is essential that you secure your server so that only authorized clients can access the proxy before activating a forward proxy.

A reverse proxy (or gateway), by contrast, appears to the client just like an ordinary web server. No special configuration on the client is necessary. The client makes ordinary requests for content in the namespace of the reverse proxy. The reverse proxy then decides where to send those requests and returns the content as if it were itself the origin.

A typical usage of a reverse proxy is to provide Internet users access to a server that is behind a firewall. Reverse proxies can also be used to balance load among several back-end servers or to provide caching for a slower back-end server. In addition, reverse proxies can be used simply to bring several servers into the same URL space.

A reverse proxy is activated using the ProxyPass directive or the [P] flag to the RewriteRule directive. It is not necessary to turn ProxyRequests on in order to configure a reverse proxy.

Basic Examples

The examples below are only a very basic idea to help you get started. Please read the documentation on the individual directives.

In addition, if you wish to have caching enabled, consult the documentation from mod_cache.

ProxyPass "/foo" "http://foo.example.com/bar"
ProxyPassReverse "/foo" "http://foo.example.com/bar"

ProxyRequests On
ProxyVia On

<Proxy "*">
  Require host internal.example.com
</Proxy>

Access via Handler

You can also force a request to be handled as a reverse-proxy request, by creating a suitable Handler pass-through. The example configuration below will pass all requests for PHP scripts to the specified FastCGI server using reverse proxy:

<FilesMatch "\.php$">
    # Unix sockets require 2.4.7 or later
    SetHandler  "proxy:unix:/path/to/app.sock|fcgi://localhost/"
</FilesMatch>

This feature is available in Apache HTTP Server 2.4.10 and later.

Workers

The proxy manages the configuration of origin servers and their communication parameters in objects called workers. There are two built-in workers: the default forward proxy worker and the default reverse proxy worker. Additional workers can be configured explicitly.

The two default workers have a fixed configuration and will be used if no other worker matches the request. They do not use HTTP Keep-Alive or connection reuse. The TCP connections to the origin server will instead be opened and closed for each request.

Explicitly configured workers are identified by their URL. They are usually created and configured using ProxyPass or ProxyPassMatch when used for a reverse proxy:

ProxyPass "/example" "http://backend.example.com" connectiontimeout=5 timeout=30

This will create a worker associated with the origin server URL http://backend.example.com that will use the given timeout values. When used in a forward proxy, workers are usually defined via the ProxySet directive:

ProxySet "http://backend.example.com" connectiontimeout=5 timeout=30

or alternatively using Proxy and ProxySet:

<Proxy "http://backend.example.com">
  ProxySet connectiontimeout=5 timeout=30
</Proxy>

Using explicitly configured workers in the forward mode is not very common, because forward proxies usually communicate with many different origin servers. Creating explicit workers for some of the origin servers can still be useful if they are used very often. Explicitly configured workers have no concept of forward or reverse proxying by themselves. They encapsulate a common concept of communication with origin servers. A worker created by ProxyPass for use in a reverse proxy will also be used for forward proxy requests whenever the URL to the origin server matches the worker URL, and vice versa.

The URL identifying a direct worker is the URL of its origin server including any path components given:

ProxyPass "/examples" "http://backend.example.com/examples"
ProxyPass "/docs" "http://backend.example.com/docs"

This example defines two different workers, each using a separate connection pool and configuration.

ProxyPass "/apps" "http://backend.example.com/" timeout=60
ProxyPass "/examples" "http://backend.example.com/examples" timeout=10

Explicitly configured workers come in two flavors: direct workers and (load) balancer workers. They support many important configuration attributes which are described below in the ProxyPass directive. The same attributes can also be set using ProxySet.

The set of options available for a direct worker depends on the protocol which is specified in the origin server URL. Available protocols include ajp, fcgi, ftp, http and scgi.

Balancer workers are virtual workers that use direct workers known as their members to actually handle the requests. Each balancer can have multiple members. When it handles a request, it chooses a member based on the configured load balancing algorithm.

A balancer worker is created if its worker URL uses balancer as the protocol scheme. The balancer URL uniquely identifies the balancer worker. Members are added to a balancer using BalancerMember.

Controlling Access to Your Proxy

You can control who can access your proxy via the <Proxy> control block as in the following example:

<Proxy "*">
  Require ip 192.168.0
</Proxy>

For more information on access control directives, see mod_authz_host.

Strictly limiting access is essential if you are using a forward proxy (using the ProxyRequests directive). Otherwise, your server can be used by any client to access arbitrary hosts while hiding his or her true identity. This is dangerous both for your network and for the Internet at large. When using a reverse proxy (using the ProxyPass directive with ProxyRequests Off), access control is less critical because clients can only contact the hosts that you have specifically configured.

See Also the Proxy-Chain-Auth environment variable.

Slow Startup

If you're using the ProxyBlock directive, hostnames' IP addresses are looked up and cached during startup for later match test. This may take a few seconds (or more) depending on the speed with which the hostname lookups occur.

Intranet Proxy

An Apache httpd proxy server situated in an intranet needs to forward external requests through the company's firewall (for this, configure the ProxyRemote directive to forward the respective scheme to the firewall proxy). However, when it has to access resources within the intranet, it can bypass the firewall when accessing hosts. The NoProxy directive is useful for specifying which hosts belong to the intranet and should be accessed directly.

Users within an intranet tend to omit the local domain name from their WWW requests, thus requesting "http://somehost/" instead of http://somehost.example.com/. Some commercial proxy servers let them get away with this and simply serve the request, implying a configured local domain. When the ProxyDomain directive is used and the server is configured for proxy service, Apache httpd can return a redirect response and send the client to the correct, fully qualified, server address. This is the preferred method since the user's bookmark files will then contain fully qualified hosts.

Protocol Adjustments

For circumstances where mod_proxy is sending requests to an origin server that doesn't properly implement keepalives or HTTP/1.1, there are two environment variables that can force the request to use HTTP/1.0 with no keepalive. These are set via the SetEnv directive.

These are the force-proxy-request-1.0 and proxy-nokeepalive notes.

<Location "/buggyappserver/">
  ProxyPass "http://buggyappserver:7001/foo/"
  SetEnv force-proxy-request-1.0 1
  SetEnv proxy-nokeepalive 1
</Location>

In 2.4.26 and later, the "no-proxy" environment variable can be set to disable mod_proxy processing the current request. This variable should be set with SetEnvIf, as SetEnv is not evaluated early enough.

Request Bodies

Some request methods such as POST include a request body. The HTTP protocol requires that requests which include a body either use chunked transfer encoding or send a Content-Length request header. When passing these requests on to the origin server, mod_proxy_http will always attempt to send the Content-Length. But if the body is large and the original request used chunked encoding, then chunked encoding may also be used in the upstream request. You can control this selection using environment variables. Setting proxy-sendcl ensures maximum compatibility with upstream servers by always sending the Content-Length, while setting proxy-sendchunked minimizes resource usage by using chunked encoding.

Under some circumstances, the server must spool request bodies to disk to satisfy the requested handling of request bodies. For example, this spooling will occur if the original body was sent with chunked encoding (and is large), but the administrator has asked for backend requests to be sent with Content-Length or as HTTP/1.0. This spooling can also occur if the request body already has a Content-Length header, but the server is configured to filter incoming request bodies.

LimitRequestBody only applies to request bodies that the server will spool to disk

Reverse Proxy Request Headers

When acting in a reverse-proxy mode (using the ProxyPass directive, for example), mod_proxy_http adds several request headers in order to pass information to the origin server. These headers are:

X-Forwarded-For

The IP address of the client.

X-Forwarded-Host

The original host requested by the client in the Host HTTP request header.

X-Forwarded-Server

The hostname of the proxy server.

Be careful when using these headers on the origin server, since they will contain more than one (comma-separated) value if the original request already contained one of these headers. For example, you can use %{X-Forwarded-For}i in the log format string of the origin server to log the original clients IP address, but you may get more than one address if the request passes through several proxies.

See also the ProxyPreserveHost and ProxyVia directives, which control other request headers.

Note: If you need to specify custom request headers to be added to the forwarded request, use the RequestHeader directive.