Nginx proxy_pass with $remote_addr

If the proxy_pass statement has no variables in it, then it will use the "gethostbyaddr" system call during start-up or reload and will cache that value permanently.

if there are any variables, such as using either of the following:

set $originaddr http://origin.example.com;
proxy_pass $originaddr;
# or even
proxy_pass http://origin.example.com$request_uri;

Then nginx will use a built-in resolver, and the "resolver" directive must be present. "resolver" is probably a misnomer; think of it as "what DNS server will the built-in resolver use". Since nginx 1.1.9 the built-in resolver will honour DNS TTL values. Before then it used a fixed value of 5 minutes.

It seems a bit strange that nginx is failing to resolve the domain name at runtime rather than at configuration time (since the domain name is hard coded). Adding a resolver declaration to the location block usually fixes dns issues experienced at runtime. So your location block might look like:

location ^~ /freegeoip/ {
  #use google as dns
  resolver 8.8.8.8;
  proxy_pass http://freegeoip.net/json/$remote_addr;
}

This solution is based on an article I read a while back - Proxy pass and resolver. Would be worth a read.