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.