Set REMOTE_ADDR to X-Forwarded-For in apache

Question

In a situation where Apache is sitting behind a reverse proxy (such as Squid), the cgi environment variable REMOTE_ADDR gets the address of the proxy rather than the client.

However, the proxy will set a header called X-Forwarded-For to contain the original IP address of the client so that Apache can see it.

The question is, how do we get Apache to replace REMOTE_ADDR with the value in the X-Forwarded-For header so that all of the web applications will transparently see the correct address?

Answer 1

You can use mod_rpaf for that. http://stderr.net/apache/rpaf/

Answer 2

Note that the X-Forwarded-For header may contain a list of IP addresses if the request has traversed more than one proxy. In this case, you usually want the leftmost IP. You can extract this with a SetEnvIf:

SetEnvIf X-Forwarded-For "^(\d{1,3}+\.\d{1,3}+\.\d{1,3}+\.\d{1,3}+).*" XFFCLIENTIP=$1

Note the use of $1 to set the XFFCLIENTIP environment variable to hold the contents of the first group in the regex (in the parentheses).

Then you can use the value of the environment variable to set headers (or use it in Apache log formats so that the logs contain the actual client IP).

Answer 3

In addition to mod_rpaf as mentioned before, it appears that mod_extract_forwarded will perform this function as well.

One advantage to mod_extract_forwarded is that it is available from EPEL for RHEL/CentOS servers whereas mod_rpaf is not.

It appears that neither of these two modules allow you to whitelist an entire subnet of proxy servers, which is why the CloudFlare folks created their own plugin: mod_cloudflare which, it should be noted, is not a general-purpose tool like the other two; it contains a hardcoded list of CloudFlare subnets.

Answer 4

Yes, we can do this.

Just add a auto_prepend_file in your PHP.ini like auto_prepend_file = "c:/prepend.php" and in this file add this:

if (isset($_SERVER['HTTP_X_FORWARDED_FOR'])) {
    $_SERVER['REMOTE_ADDR'] = $_SERVER['HTTP_X_FORWARDED_FOR'];
}

You need the MOD_REMOTEIP in apache width RemoteIPHeader X-Real-IP.

Cheers,

Guiremach

Answer 5

Unfortunately,

at the time of this writing, none of the backports and forks at freshports.org, people.apache.org or gist.github.com worked. They were all based on an early alpha version of apache httpd 2.3 which was neither compatible with current versions of 2.2 nor 2.4.

So after hours of wasting time while trying to adjust the backports to create a real working one for httpd 2.2, I decided to move to httpd 2.4. Within httpd 2.4, mod_remoteip works smoothly, even if a load balancer has permanent keepalive connections which it uses to proxy requests from different actual client ip addresses to the backend. I'm not sure if the other modules can handle this situation (changing client ip addresses on each request within the same connection).

Answer 6

Currently apache module mod_remoteip is the recommended way to do this; rpaf hasn't been reliably maintained, and can cause problems.

Answer 7

Remember that this value can be spoofed. See http://blog.c22.cc/2011/04/22/surveymonkey-ip-spoofing/ for a real-life example with Cross-site Scripting consequences.

Answer 8

You can install the module mod_extract_forwarded and set MEFaccept parameter to all.