FYI: upstream cannot be forced - requires nginx plus

[darylteo]

Based on my own research, unfortunately it does not seem to be possible to force dns lookup for upstream

https://nginx.org/en/docs/http/ngx_http_upstream_module.html

Dynamically configurable group with periodic health checks is available as part of our commercial subscription:

Which allows

upstream dynamic {
    zone upstream_dynamic 64k;

    server backend1.example.com      weight=5;
    server backend2.example.com:8080 fail_timeout=5s slow_start=30s;
    server 192.0.2.1                 max_fails=3;
    server backend3.example.com      **resolve**;
    server backend4.example.com      service=http resolve;

    server backup1.example.com:8080  backup;
    server backup2.example.com:8080  backup;
}

Additionally, the following parameters are available as part of our commercial subscription:

resolve
monitors changes of the IP addresses that correspond to a domain name of the server, and automatically modifies the upstream configuration without the need of restarting nginx (1.5.12). The server group must reside in the shared memory.