参考： https://serverfault.com/questions/240476/how-to-force-nginx-to-resolve-dns-of-a-dynamic-hostname-everytime-when-doing-p/593003#593003

Using a variable in proxy_pass forces re-resolution of the DNS names because NGINX treats variables differently to static configuration. From the NGINX proxy_pass documentation:

Parameter value can contain variables. In this case, if an address is specified as a domain name, the name is searched among the described server groups, and, if not found, is determined using a resolver.

For example:

server {
    ...
    resolver 127.0.0.1;
    set $backend "http://dynamic.example.com:80";
    proxy_pass $backend;
    ...
}

Note: A resolver (i.e. the name server to use) MUST be available and configured for this to work (and entries inside a /etc/hosts file won’t be used in a lookup).

By default, version 1.1.9 or later versions of NGINX cache answers using the TTL value of a response and an optional valid parameter allows the cache time to be overridden:

resolver 127.0.0.1 [::1]:5353 valid=30s;

Before version 1.1.9, tuning of caching time was not possible, and nginx always cached answers for the duration of 5 minutes…