Chinese in Canada

Prepared By
Visolve Squid Team

Introduction
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This document describes reverse proxies, and how they are used to improve Web server performance. Section 1 gives an introduction to reverse proxies, describing what they are and what they are used for. Section 2 compares reverse proxy caches with standard and transparent proxy caches, explaining the different functionality each provides. Section 3 illustrates how the reverse proxy actually caches the content and delivers it to the client. Section 4 describes how to configure Squid as a reverse proxy cache.

What is Reverse Proxy Cache
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Reverse proxy cache, also known as Web Server Acceleration, is a method of reducing the load on a busy web server by using a web cache between the server and the internet. Another benefit that can be gained is improved security. It's one of many ways to improve scalability without increasing the complexity of maintenence too much. A good use of a reverse proxy is to ease the burden on a web server that provides both static and dynamic content. The static content can be cached on the reverse proxy while the web server will be freed up to better handle the dynamic content.

By deploying Reverse Proxy Server alongside web servers, sites will:



Avoid the capital expense of purchasing additional web servers by increasing the capacity of existing servers.
Serve more requests for static content from web servers.
Serve more requests for dynamic content from web servers.
Increase profitability of the business by reducing operating expenses including the cost of bandwidth required to serve content.
Accelerate the response time of web and accelerate page download times to end users, delivering a faster, better experience to site visitors. When planning a Reverse-Proxy implementation the origin server's content should be written with the proxy server in mind, i.e. it should be "Cache Friendly". If the origin server's content is not "cache aware", it will not be able to take full advantage of the reverse proxy cache.


In Reverse Proxy mode, the Proxy Server functions more like a web server with respect to the clients it services. Unlike internal clients, external clients are not reconfigured to access the proxy server. Instead, the site URL routes the client to the proxy as if it were a web server. Replicated content is delivered from the proxy cache to the external client without exposing the origin server or the private network residing safely behind the firewall. Multiple reverse proxy servers can be used to balance the load on an over-taxed web server in much the same way.

The objective of this white paper is to explain the implementation of Squid as a Reverse-proxy also known as Web Server-accelerator. The basic concept of caching is explained followed by the actual implementation and testing of the reverse-proxy mode of squid.

About Squid
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Squid is an Open source high-performance Proxy caching server designed to run on Unix systems. National Science Foundation funds squid project, Squid has its presence in numerous ISP's and corporate around the globe. Squid can do much more than what most of the proxy servers around can do.

Reverse Proxy compared with other Proxy caches

There are three main ways that proxy caches can be configured on a network :

Standard Proxy Cache

A standard proxy cache is used to cache static web pages (html and images) to a machine on the local network. When the page is requested a second time, the browser returns the data from the local proxy instead of the origin web server. The browser is explicitly configured to direct all HTTP requests to the proxy cache, rather than the target web server. The cache then either satisfies the request itself or passes on the request to the target server.

Transparent Cache

A transparent cache achieves the same goal as a standard proxy cache, but operates transparently to the browser. The browser does not need to be explicitly configured to access the cache. Instead, the transparent cache intercepts network traffic, filters HTTP traffic (on port 80), and handles the request if the item is in the cache. If the item is not in the cache, the packets are forwarded to the origin web server. For Linux, the transparent cache uses iptables or ipchains to intercept and filter the network traffic. Transparent caches are especially useful to ISPs, because they require no browser setup modification. Transparent caches are also the simplest way to use a cache internally on a network (at peering-hand off points between an ISP and a larger network, for example), because they don't require explicit coordination with other caches.

Reverse Proxy Cache

A reverse proxy cache differs from standard and transparent caches, in that it reduces load on the origin web server, rather than reducing upstream network bandwidth on the client side. Reverse Proxy Caches offload client requests for static content from the web server, preventing unforeseen traffic surges from overloading the origin server. The proxy server sits between the Internet and the Web site and handles all traffic before it can reach the Web server. A reverse proxy server intercepts requests to the Web server and instead responds to the request out of a store of cached pages. This method improves the performance by reducing the amount of pages actually created "fresh" by the Web server.

How Reverse proxy caches work
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A reverse proxy is positioned between the internet and the web server

When a client browser makes an HTTP request, the DNS will route the request to the reverse proxy machine, not the actual web server. The reverse proxy will check its cache to see if it contains the requested item. If not, it connects to the real web server and downloads the requested item to its disk cache. The reverse proxy can only server cacheable URLs (such as html pages and images).

Dynamic content such as cgi scripts and Active Server Pages cannot be cached. The proxy caches static pages based on HTTP header tags that are returned from the web page.




The four most important header tags are:

Last-Modified: Tells the proxy when the page was last modified.
Expires: Tells the proxy when to drop the page from the cache.
Cache-Control: Tells the proxy if the page should be cached.
Pragma: Also tells the proxy if the page should be cached.
For example, by default all Active Server Pages return "Cache-control: private." Therefore, no Active Server Pages will be cached on a reverse proxy server.

Configuring Squid as Reverse Proxy (http accelerator)
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To set up Squid as an httpd accelerator, you simply configure the squid.conf file. Usually it is found in either /usr/local/squid/etc, when installed directly from source code, or /etc/squid when pre-installed on Red Hat Linux systems. The squid.conf file is used to set and configure all the different options for the Squid proxy server. As root open the squid.conf file in your favorite text editor. If the real web server runs on a separate machine than the Squid reverse proxy, edit the following options in the squid.conf file :

http_port 80 # Port of Squid proxy
httpd_accel_host 172.16.1.115 # IP address of web server
httpd_accel_port 80 # Port of web server
httpd_accel_single_host on # Forward uncached requests to single host
httpd_accel_with_proxy on #
httpd_accel_uses_host_header off

If the web server runs on the same machine where Squid is running, the web server daemon must be set to run on port 81 (or any other port than 80). With the Apache web server, it can done by assigning the line "Port 80" to "Port 81" in its httpd.conf file. The Squid.conf must also be modified to redirect missed requests to port 81 of the local machine :

http_port 80 # Port of Squid proxy
httpd_accel_host localhost # IP address of web server
httpd_accel_port 81 # Port of web server
httpd_accel_single_host on # Forward uncached requests to single host
httpd_accel_with_proxy on #
httpd_accel_uses_host_header off

We describe these options in greater detail.

http_port 80

The option http_port specifies the port number where Squid will listen for HTTP client requests. If this option is set to port 80, the client will have the illusion of being connected to the actual web server. This options should always be port 80.

httpd_accel_host 172.16.1.115 and httpd_accel_port 80

The options httpd_accel_host and httpd_accel_port specify the IP address and port number of the real HTTP Server, such as Apache. In our configuration, the real HTTP Web Server is on the IP address 172.16.1.115 and on port 80.

If we are using the reverse proxy for more than one web server, then we must use the word virtual as the httpd_accel_host. Uncached requests can only be forwarded to one port. There is no table that associates accelerated hosts and a destination port. When the web server is running on the same machine as Squid, set the web server to listen for connections on a different port (8000, for example), and set the httpd_accel_port option to the same value.

httpd_accel_single_host on

To run Squid with a single back end web server, set httpd_accel_single_host option to on. Squid will forward all uncached requests to this web server regardless of what any redirectors or Host headers says. If the Squid reverse proxy must support multiple back end web servers, set this option to off, and use a redirector (or host table or private DNS) to map the requests to the appropriate back end servers. Note that the mapping needs to be a 1-1 mapping between requested and backend (from redirector) domain names or caching will fail, as caching is performed using the URL returned from the redirector. See also rewrites_host_header.

httpd_accel_with_proxy on


If one wants to use Squid as both an httpd accelerator and as a proxy for local client machines, set the httpd_accel_with_proxy to on. By default, it is off. Note however that your proxy users may have trouble reaching the accelerated domains, unless their browsers are configured not to use the Squid proxy for those domains. The no_proxy option can be used to direct clients not to use the proxy for certain domains.

httpd_accel_uses_host_header off

Requests in HTTP version 1.1 include a Host header, specifying the host name (or IP address) of the URL. This option should remain off in reverse proxy mode. The only time this option must be set to on is when Squid is configured as a Transparent proxy.

It's important to note that acls (access control lists) are checked before this translation. You must combine this option with strict source-address checks, so you cannot use this option to accelerate multiple back end servers.

Configuring Squid as Reverse Proxy for Multiple Domains (http accelerator)
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You can configure squid in an accelerator mode for multiple domains also. For example you can configure single squid machine for www.abc.com, www.xyz.com, www.lmn.com.



Squid configuration

httpd_accel_host virtual

httpd_accel_port 80 (the web server port)

httpd_accel_single_host off (It should be disabled when we are going to the reverse proxy for multiple servers)

httpd_accel_uses_host_header on

Note: When you compile Squid enable the Internal DNS option

To set the reverse proxy for the domain

www.abc.com 192.168.1.2 www.xyz.com 192.168.1.2 www.lmn.com 192.168.1.2

Let reverse_proxy server ip be 192.168.1.2

DNS entry in the reverse proxy server

You need to configure Intranet DNS and Internet DNS. You can configure split DNS if you want both the DNS in same machine. Instead of Intranet DNS you can have domain entries in /etc/hosts. You have to configure squid with --disable-internal-dns to use /etc/hosts file lookup.

Internal DNS entry


www.abc.com IN A 172.16.1.2 www.xyz.com IN A 172.16.1.3 www.lmn.com IN A 172.16.1.4

Note: If you have compiled with disable internal dns, then add the entry in the /etc/hosts like

172.16.1.2 www.abc.com 172.16.1.3 www.xyz.com 172.16.1.4 www.lmn.com

Internal DNS entry

www.abc.com 192.168.1.2 www.xyz.com 192.168.1.2 www.lmn.com 192.168.1.2

References
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ViSolve Squid Configuration Manual 2.4
ViSolve Squid Configuration Manual 3.0

Conclusion
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Reverse proxying is a special proxy deployment used to reduce load on a web server. The reverse proxy server is placed outside the firewall, acting as the web server to external clients. Cached requests are sent back directly to the clients without any computation from the actual web server. Uncached requests must be forwarded to the backend web server, and the response from the web server is then cached in the reverse proxy.

http://www.visolve.com/squid/whitepapers/reverseproxy.php

Description: HTTP/1.1 proxy/gateway server
Status: Extension
Module Identifier: proxy_module
Source File: mod_proxy.c
Summary
Warning

Do not enable proxying with ProxyRequests until you have secured your server. Open proxy servers are dangerous both to your network and to the Internet at large.
This module implements a proxy/gateway for Apache. It implements proxying capability for FTP, CONNECT (for SSL), HTTP/0.9, HTTP/1.0, and HTTP/1.1. The module can be configured to connect to other proxy modules for these and other protocols.

Apache's proxy features are divided into several modules in addition to mod_proxy: mod_proxy_http, mod_proxy_ftp and mod_proxy_connect. Thus, if you want to use one or more of the particular proxy functions, load mod_proxy and the appropriate module(s) into the server (either statically at compile-time or dynamically via the LoadModule directive).

In addition, extended features are provided by other modules. Caching is provided by mod_cache and related modules. The ability to contact remote servers using the SSL/TLS protocol is provided by the SSLProxy* directives of mod_ssl. These additional modules will need to be loaded and configured to take advantage of these features.

Forward and Reverse Proxies

Apache can be configured in both a forward and reverse proxy 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 and 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 proxys 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, 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 name-space of the reverse proxy. The reverse proxy then decides where to send those requests, and returns the content as if it was 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.

Forward Proxy

ProxyRequests On
ProxyVia On


Order deny,allow
Deny from all
Allow from internal.example.com

Reverse Proxy

ProxyRequests Off


Order deny,allow
Allow from all


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

Controlling access to your proxy

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


Order Deny,Allow
Deny from all
Allow from 192.168.0

For more information on access control directives, see mod_access.

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.


FTP Proxy

Why doesn't file type xxx download via FTP?
You probably don't have that particular file type defined as application/octet-stream in your proxy's mime.types configuration file. A useful line can be

application/octet-stream bin dms lha lzh exe class tgz taz
How can I force an FTP ASCII download of File xxx?
In the rare situation where you must download a specific file using the FTP ASCII transfer method (while the default transfer is in binary mode), you can override mod_proxy's default by suffixing the request with ;type=a to force an ASCII transfer. (FTP Directory listings are always executed in ASCII mode, however.)

How can I access FTP files outside of my home directory?
An FTP URI is interpreted relative to the home directory of the user who is logging in. Alas, to reach higher directory levels you cannot use /../, as the dots are interpreted by the browser and not actually sent to the FTP server. To address this problem, the so called Squid %2f hack was implemented in the Apache FTP proxy; it is a solution which is also used by other popular proxy servers like the Squid Proxy Cache. By prepending /%2f to the path of your request, you can make such a proxy change the FTP starting directory to / (instead of the home directory). For example, to retrieve the file /etc/motd, you would use the URL:

ftp://user@host/%2f/etc/motd
How can I hide the FTP cleartext password in my browser's URL line?
To log in to an FTP server by username and password, Apache uses different strategies. In absense of a user name and password in the URL altogether, Apache sends an anonymous login to the FTP server, i.e.,

user: anonymous
password: apache_proxy@
This works for all popular FTP servers which are configured for anonymous access.

For a personal login with a specific username, you can embed the user name into the URL, like in:

ftp://username@host/myfile
If the FTP server asks for a password when given this username (which it should), then Apache will reply with a 401 (Authorization required) response, which causes the Browser to pop up the username/password dialog. Upon entering the password, the connection attempt is retried, and if successful, the requested resource is presented. The advantage of this procedure is that your browser does not display the password in cleartext (which it would if you had used

ftp://username:password@host/myfile
in the first place).

Note

The password which is transmitted in such a way is not encrypted on its way. It travels between your browser and the Apache proxy server in a base64-encoded cleartext string, and between the Apache proxy and the FTP server as plaintext. You should therefore think twice before accessing your FTP server via HTTP (or before accessing your personal files via FTP at all!) When using unsecure channels, an eavesdropper might intercept your password on its way.

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 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 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 you have a application server which doesn't implement keepalives or HTTP/1.1 properly, there are 2 environment variables which when set send a 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.


ProxyPass http://buggyappserver:7001/foo/
SetEnv force-proxy-request-1.0 1
SetEnv proxy-nokeepalive 1


AllowCONNECT Directive

Description: Ports that are allowed to CONNECT through the proxy
Syntax: AllowCONNECT port [port] ...
Default: AllowCONNECT 443 563
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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.


NoProxy Directive

Description: Hosts, domains, or networks that will be connected to directly
Syntax: NoProxy host [host] ...
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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).

Example

ProxyRemote * http://firewall.mycompany.com:81
NoProxy .mycompany.com 192.168.112.0/21
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 .MyDomain.com and .mydomain.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.mit.edu
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.MyDomain.com and www.mydomain.com. (note the trailing period) are considered equal.
See also
DNS Issues

Directive

Description: Container for directives applied to proxied resources
Syntax: ...
Context: server config, virtual host
Status: Extension
Module: mod_proxy
Directives placed in 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:


Order Deny,Allow
Deny from all
Allow from yournetwork.example.com

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:


SetOutputFilter INCLUDES


ProxyBadHeader Directive

Description: Determines how to handle bad header lines in a response
Syntax: ProxyBadHeader IsError|Ignore|StartBody
Default: ProxyBadHeader IsError
Context: server config, virtual host
Status: Extension
Module: mod_proxy
Compatibility: Available in Apache 2.0.44 and later
The ProxyBadHeader directive determines the behaviour of mod_proxy if it receives syntactically invalid header lines (i.e. containing no colon). 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

Description: Words, hosts, or domains that are banned from being proxied
Syntax: ProxyBlock *|word|host|domain [word|host|domain] ...
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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.

Example

ProxyBlock joes-garage.com some-host.co.uk rocky.wotsamattau.edu
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

ProxyBlock *
blocks connections to all sites.


ProxyDomain Directive

Description: Default domain name for proxied requests
Syntax: ProxyDomain Domain
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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.

Example

ProxyRemote * http://firewall.mycompany.com:81
NoProxy .mycompany.com 192.168.112.0/21
ProxyDomain .mycompany.com

ProxyErrorOverride Directive

Description: Override error pages for proxied content
Syntax: ProxyErrorOverride On|Off
Default: ProxyErrorOverride Off
Context: server config, virtual host
Status: Extension
Module: mod_proxy
Compatibility: Available in version 2.0 and later
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).


ProxyFtpDirCharset Directive

Description: Define the character set for proxied FTP listings
Syntax: ProxyFtpDirCharset character set
Default: ProxyFtpDirCharset ISO-8859-1
Context: server config, virtual host, directory
Status: Extension
Module: mod_proxy
Compatibility: Available in Apache 2.0.62 and later
The ProxyFtpDirCharset directive defines the character set to be set for FTP directory listings in HTML generated by mod_proxy_ftp.


ProxyIOBufferSize Directive

Description: Determine size of internal data throughput buffer
Syntax: ProxyIOBufferSize bytes
Default: ProxyIOBufferSize 8192
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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 less or equal 8192.

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


Directive

Description: Container for directives applied to regular-expression-matched proxied resources
Syntax: ...
Context: server config, virtual host
Status: Extension
Module: mod_proxy
The directive is identical to the directive, except it matches URLs using regular expressions.


ProxyMaxForwards Directive

Description: Maximium number of proxies that a request can be forwarded through
Syntax: ProxyMaxForwards number
Default: ProxyMaxForwards 10
Context: server config, virtual host
Status: Extension
Module: mod_proxy
Compatibility: Available in Apache 2.0 and later
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 is set to prevent infinite proxy loops, or a DoS attack.

Example

ProxyMaxForwards 15

ProxyPass Directive

Description: Maps remote servers into the local server URL-space
Syntax: ProxyPass [path] !|url
Context: server config, virtual host, directory
Status: Extension
Module: mod_proxy
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. 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

ProxyPass /mirror/foo/ http://backend.example.com/
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.

ProxyPass /mirror/foo/i !
ProxyPass /mirror/foo http://backend.example.com
will proxy all requests to /mirror/foo to backend.example.com except requests made to /mirror/foo/i.

Note

Order is important. you need to put the exclusions before the general proxypass directive.
When used inside a section, the first argument is omitted and the local directory is obtained from the .

The ProxyRequests directive should usually be set off when using ProxyPass.
If you require a more flexible reverse-proxy configuration, see the RewriteRule directive with the [P] flag.


ProxyPassReverse Directive

Description: Adjusts the URL in HTTP response headers sent from a reverse proxied server
Syntax: ProxyPassReverse [path] url
Context: server config, virtual host, directory
Status: Extension
Module: mod_proxy
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 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

ProxyPass /mirror/foo/ http://backend.example.com/
ProxyPassReverse /mirror/foo/ http://backend.example.com/
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 its doesn't depend on a corresponding ProxyPass directive.

When used inside a section, the first argument is omitted and the local directory is obtained from the .


ProxyPreserveHost Directive

Description: Use incoming Host HTTP request header for proxy request
Syntax: ProxyPreserveHost On|Off
Default: ProxyPreserveHost Off
Context: server config, virtual host
Status: Extension
Module: mod_proxy
Compatibility: Available in Apache 2.0.31 and later.
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

Description: Network buffer size for proxied HTTP and FTP connections
Syntax: ProxyReceiveBufferSize bytes
Default: ProxyReceiveBufferSize 0
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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.

Example

ProxyReceiveBufferSize 2048

ProxyRemote Directive

Description: Remote proxy used to handle certain requests
Syntax: ProxyRemote match remote-server
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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:

remote-server = scheme://hostname[:port]
scheme is effectively the protocol that should be used to communicate with the remote server; only http is supported by this module.

Example

ProxyRemote http://goodguys.com/ http://mirrorguys.com:8000
ProxyRemote * http://cleversite.com
ProxyRemote ftp http://ftpproxy.mydomain.com:8080
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

Description: Remote proxy used to handle requests matched by regular expressions
Syntax: ProxyRemoteMatch regex remote-server
Context: server config, virtual host
Status: Extension
Module: mod_proxy
The ProxyRemoteMatch is identical to the ProxyRemote directive, except the first argument is a regular expression match against the requested URL.


ProxyRequests Directive

Description: Enables forward (standard) proxy requests
Syntax: ProxyRequests On|Off
Default: ProxyRequests Off
Context: server config, virtual host
Status: Extension
Module: mod_proxy
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 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.

Warning

Do not enable proxying with ProxyRequests until you have secured your server. Open proxy servers are dangerous both to your network and to the Internet at large.

ProxyTimeout Directive

Description: Network timeout for proxied requests
Syntax: ProxyTimeout seconds
Default: ProxyTimeout 300
Context: server config, virtual host
Status: Extension
Module: mod_proxy
Compatibility: Available in Apache 2.0.31 and later
This directive allows a user to specifiy 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

Description: Information provided in the Via HTTP response header for proxied requests
Syntax: ProxyVia On|Off|Full|Block
Default: ProxyVia Off
Context: server config, virtual host
Status: Extension
Module: mod_proxy
This directive controls the use of the Via: HTTP header by the proxy. Its intended use is to control the flow of 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.

http://httpd.apache.org/docs/2.0/mod/mod_proxy.html

http://compnetworking.about.com/od/workingwithipaddresses/qt/ipaddresslookup.htm