mod_proxy - Apache HTTP Server

Apache HTTP Server Version 2.0

Apache > HTTP Server > Documentation > Version 2.0 > Modules

# Apache Module mod_proxy

Available Languages: en

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.

### Directives

### Topics

- Forward and Reverse Proxies
- Basic Examples
- Controlling access to your proxy
- FTP Proxy
- Slow Startup
- Intranet Proxy
- Protocol Adjustments

### See also

- `mod_cache`
- `mod_proxy_http`
- `mod_proxy_ftp`
- `mod_proxy_connect`
- `mod_ssl`

## 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
<Proxy *>
Order deny,allow
Deny from all
Allow from internal.example.com
</Proxy>`

### Reverse Proxy

`ProxyRequests Off
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
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 `<Proxy>` control block as in the following example:

`<Proxy *>
Order Deny,Allow
Deny from all
Allow from 192.168.0
</Proxy>`

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.

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

## 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

## <Proxy> Directive

Description: | Container for directives applied to proxied resources
Syntax: | `<Proxy wildcard-url> ...</Proxy>`
Context: | server config, virtual host
Status: | Extension
Module: | mod_proxy

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:

`<Proxy *>
Order Deny,Allow
Deny from all
Allow from yournetwork.example.com
</Proxy>`

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:

`<Proxy http://example.com/foo/*>
SetOutputFilter INCLUDES
</Proxy>`

## 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).

## 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.

## <ProxyMatch> Directive

Description: | Container for directives applied to regular-expression-matched proxied resources
Syntax: | `<ProxyMatch regex> ...</ProxyMatch>`
Context: | server config, virtual host
Status: | Extension
Module: | mod_proxy

The `<ProxyMatch>` directive is identical to the `<Proxy>` 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 `<Location>` section, the first argument is omitted and the local directory is obtained from the `<Location>`.

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 `<Location>` section, the first argument is omitted and the local directory is obtained from the `<Location>`.

## 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.

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