PROXY protocol support for our, err, proxy

We’re increasingly using our IPv4 to IPv6 reverse proxy to host websites on IPv6-only virtual machines. One of the downsides of proxying is that your server doesn’t get to see the client’s real IP address. For non-SSL connections, the proxy can insert an “X-Forwarded-For” header, but SSL is increasingly becoming the norm, and one of the nice things about an SNI-aware reverse proxy is that it doesn’t need to do SSL off load: we don’t need your certificates on our proxy and your traffic stays encrypted until it hits your server. Of course, this means that we can’t go inserting any headers into your connection either.

Fortunately, there is a solution: PROXY protocol. This is a protocol-agnostic mechanism for passing information from a reverse proxy to a server, including the client IP address.

We’ve just added support for PROXY protocol to our reverse proxy:

Turning this on allows your server to get the client IP address, but as it’s an additional protocol, not part of HTTP, your server must be expecting it: turning this on and pointing it at a standard HTTP server will result in a broken website.

Most web servers have support for this. NGINX has support built in, and just needs “proxy_protocol” adding after the listen directive:

server {
    listen 80   proxy_protocol;
    listen 443  ssl proxy_protocol;
    ...
}

You will probably also want some additional configuration to actually set the IP address that gets used for logs etc., and also to ensure that you only trust proxy information from the real proxy servers.

For Apache, support is provided by mod_proxy_protocol, which needs to be installed manually. Once done, configuration is easy:

<VirtualHost *:443>
  ...
  ProxyProtocol On

  CustomLog ${APACHE_LOG_DIR}/access.log "%a %l %u %t \"%r\" %>s %b \"%{Referer}i\" \"%{User-agent}i\""

The CustomLog line instructs Apache to use the real client IP rather than the proxy. You should now see v4 addresses being happily logged on your IPv6 server:

root@vm1:~# tail -n 1 /var/log/apache2/access.log
93.93.130.44 - - [29/Apr/2016:14:05:32 +0100] "GET / HTTP/1.1" 200 321 "-" "curl/7.26.0"

Unfortunately the module doesn’t currently provide a way to restrict enablement to trusted proxies only. As such, you’ll probably want to install a firewall to restrict HTTP/HTTPS traffic to only come from our proxies, as otherwise clients could easily fake their IP address.

One thing to watch out for is that although this is applied within a VirtualHost configuration, it’ll actually apply to all virtual hosts on the same IP address and port. This is an unavoidable side effect of the fact that the proxy information is sent before we start talking HTTP. Of course, with IPv6, throwing another IP address at the problem isn’t an issue.