mod_remoteip
Replaces the apparent client remote IP address and hostname for the request with the IP address list presented by a proxies or a load balancer via the request headers.
Base
mod_remoteip.c
remoteip_module

This module is used to treat the remote host which initiated the request as the originating remote host as identified by httpd for the purposes of authorization and logging, even where that remote host is behind a load balancer, front end server, or proxy server.

The module replaces the apparent remote (client) IP/hostname for the request with the IP address reported in the request header configured with the RemoteIPHeader directive.

Once replaced as instructed, this apparent IP address is then used for mod_authz_host features Require host and Require ip, is reported by mod_status, and is recorded by mod_log_config `%a` and `%h` directives. It also determines the machine probed for an inetd identity by mod_ident based on the IdentityCheck configuration.

It is critical to only enable this behavior from

intermediate hosts (proxies, etc) which are trusted by this server, since it is trivial for the remote client to impersonate another client.

mod_authz_host
mod_status
mod_log_config
mod_ident

# Remote IP Processing

Apache identifies the client with the connection's remote_ip value, and the connection remote_host and remote_logname are derived from this value. These fields play a role in authentication, authorization and logging and other purposes by other loadable modules.

mod_remoteip replaces the true remote_ip with the advertised remote_ip as provided by a proxy, for every evaluation of the client that occurs in the server, and resets the remote_host and remote_logname values to trigger a fresh dns or ident query of the remote IP address.

When multiple, comma delimited remote IP addresses are listed in the header value, they are processed in Right-to-Left order. Processing halts when a given remote IP address is not trusted to present the preceeding IP address. The header field is updated to this remaining list of unconfirmed IP addresses, or if all IP addresses were trusted, this header is removed from the request altogether.

In replacing the remote_ip, the module stores the list of intermediate hosts in a remoteip-proxy-ip-list note, which mod_log_config can record using the `%{remoteip-proxy-ip-list}n` format token. If the administrator needs to store this as an additional header, this same value can also be recording as a header using the directive RemoteIPProxiesHeader.

## IPv4-over-IPv6 Mapped Addresses

As with httpd in general, any IPv4-over-IPv6 mapped addresses are recorded in their IPv4 representation.

## Internal (Private) Addresses

All internal addresses 10/8, 172.16/12, 192.168/16, 169.254/16 and 127/8 blocks (and IPv6 addresses outside of the public 2000::/3 block) are only evaluated by mod_remoteip when RemoteIPInternalProxy internal (intranet) proxies are registered.

RemoteIPHeader
Declare the header field which should be parsed for client IP addresses
RemoteIPHeader header-field
server config
virtual host

The RemoteIPHeader directive triggers mod_remoteip to treat the value of the specified header-field header as the client IP address, or list of intermediate client IP addresses, subject to further configuration of the RemoteIPInternalProxy and RemoteIPTrustedProxy directives. Unless these other directives are used, mod_remoteip will trust all hosts presenting a RemoteIPHeader IP value.

# Internal (Load Balancer) Example

RemoteIPHeader X-Client-IP

# Proxy Example

RemoteIPHeader X-Forwarded-For
RemoteIPInternalProxy
Declare client intranet IP addresses trusted to present the RemoteIPHeader value
RemoteIPInternalProxy proxy-ip|proxy-ip/subnet|hostname ...
server config
virtual host

The RemoteIPInternalProxy directive adds one or more addresses (or address blocks) to trust as presenting a valid RemoteIPHeader value of the client IP. Unlike the RemoteIPTrustedProxy directive, any IP address presented in this header, including private intranet addresses, are trusted when passed from these proxies.

# Internal (Load Balancer) Example

RemoteIPHeader X-Client-IP
RemoteIPTrustedProxy 10.0.2.0/24
RemoteIPTrustedProxy gateway.localdomain
RemoteIPInternalProxyList
Declare client intranet IP addresses trusted to present the RemoteIPHeader value
RemoteIPInternalProxyList filename
server config
virtual host

The RemoteIPInternalProxyList directive specifies a file parsed at startup, and builds a list of addresses (or address blocks) to trust as presenting a valid RemoteIPHeader value of the client IP.

The '`#`' hash character designates a comment line, otherwise each whitespace or newline separated entry is processed identically to the RemoteIPInternalProxy directive.

# Internal (Load Balancer) Example

RemoteIPHeader X-Client-IP
RemoteIPTrustedProxyList conf/trusted-proxies.lst

# conf/trusted-proxies.lst contents

# Our internally trusted proxies;
10.0.2.0/24 #Everyone in the testing group
gateway.localdomain #The front end balancer
RemoteIPProxiesHeader
Declare the header field which will record all intermediate IP addresses
RemoteIPProxiesHeader HeaderFieldName
server config
virtual host

The RemoteIPProxiesHeader directive specifies a header into which mod_remoteip will collect a list of all of the intermediate client IP addresses trusted to resolve the actual remote IP. Note that intermediate RemoteIPTrustedProxy addresses are recorded in this header, while any intermediate RemoteIPInternalProxy addresses are discarded.

# Example

RemoteIPHeader X-Forwarded-For
RemoteIPProxiesHeader X-Forwarded-By
RemoteIPTrustedProxy
Declare client intranet IP addresses trusted to present the RemoteIPHeader value
RemoteIPTrustedProxy proxy-ip|proxy-ip/subnet|hostname ...
server config
virtual host

The RemoteIPTrustedProxy directive adds one or more addresses (or address blocks) to trust as presenting a valid RemoteIPHeader value of the client IP. Unlike the RemoteIPInternalProxy directive, any intranet or private IP address reported by such proxies, including the 10/8, 172.16/12, 192.168/16, 169.254/16 and 127/8 blocks (or outside of the IPv6 public 2000::/3 block) are not trusted as the remote IP, and are left in the RemoteIPHeader header's value.

# Trusted (Load Balancer) Example

RemoteIPHeader X-Forwarded-For
RemoteIPTrustedProxy 10.0.2.16/28
RemoteIPTrustedProxy proxy.example.com
RemoteIPTrustedProxyList
Declare client intranet IP addresses trusted to present the RemoteIPHeader value
RemoteIPTrustedProxyList filename
server config
virtual host

The RemoteIPTrustedProxyList directive specifies a file parsed at startup, and builds a list of addresses (or address blocks) to trust as presenting a valid RemoteIPHeader value of the client IP.

The '`#`' hash character designates a comment line, otherwise each whitespace or newline seperated entry is processed identically to the RemoteIPTrustedProxy directive.

# Trusted (Load Balancer) Example

RemoteIPHeader X-Forwarded-For
RemoteIPTrustedProxyList conf/trusted-proxies.lst

# conf/trusted-proxies.lst contents

# Identified external proxies;
192.0.2.16/28 #wap phone group of proxies
proxy.isp.example.com #some well known ISP