nginx resolver -- dns

Resolve means which DNS server should nginx refer to when it has to resolve a external url. If you have a config like below

location / {    proxy_pass http://www.example.com/abc/def; }

Now by default nginx will pick your resolver from the host /etc/resolv.conf and it may not be what you need. May be you want to use google DNS resolver for just this case. Then you will update your nginx config to below

location / {    resolver 8.8.8.8;    proxy_pass http://www.example.com/abc/def; }

May be you are using a local DNS resolver to route within you local network then you may use something like below

location / {    resolver 192.168.11.10;    proxy_pass http://machineabc/abc/def; }

"Resolver" parameter defines the location of DNS server that nginx must use in order to resolve the IP of the URL passed under proxy_pass;

As explained by Tarun, by default nginx will pick your resolver from the host /etc/resolv.conf and once resolved, it will cache the IP. Resolver is mostly used in two cases:
1. Either in a private network, to resolve the IP's that exist in your network.
2. Or used at a place where the IP of your proxy_pass or upstream location changes very frequently and you cannot rely upon nginx cached IP.

In the example you specified, the resolver will be the IP of the DNS server that could resolve your location. This could be either of :

1) 127.0.0.1 : If the web server itself is a DNS server, for this you need to setup DNS server on port 53(default) of this server.

2) x.x.x.x : The IP of the DNS server hosted in, either in your private network or any public DNS server if your URL's are publicly accessible. One may use 8.8.8.8 (Google's public DNS server).

3) You specified 10.x.x.x : Assuming that you were referring the AWS documentation. If not, in general, 10.x.x.x again needs to be a DNS server IP, which in case of AWS is 10.0.0.2. AWS reserves a few IP's of its VPC's and the second IP x.x.x.2 is reserved for DNS server. Note that in case your VPC is not 10.0.0.0/16, this IP will change accordingly. Eg: Lets say your VPC is 10.192.0.0/16, then you will be using 10.192.0.2 as resolver.

For above ref to https://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Subnets.html

https://www.jethrocarr.com/2013/11/02/nginx-reverse-proxies-and-dns-resolution/

There is another way to make this, if you want to set manually the resolution, without using external tools like bind9 or dnsmasq

location / {    set $upstream 12.34.56.78;  # desired IP resolution    proxy_pass  http://$upstream:8080; # desired port    proxy_set_header Host example.com; # desired host}