NGINX -- show cached IPs for host names in config files? - nginx

Question

[SHORT VERSION] I understand when NGINX looks at a config file, it does DNS lookups on the hostnames in it, and then stores the results (IP addresses the hostnames should resolve to) somewhere and uses them until the next time it looks at a config file (which, to my understanding, is not until the next restart by default). Is there a way to see this hostnames-to-ips mapping that my currently-running NGINX service has? I am aware there are ways to configure my NGINX to account for changes in IPs for a hostname. I wish to see what my NGINX currently thinks it should resolve my hostname to.
[Elaborated] I'm using the DNS name of an AWS ELB (classic) as the hostname for a proxy_pass. And since both the public and private IPs of an AWS ELB can change (without notice), whatever IP(s) NGINX has mapped for that hostname at the start of its service will become outdated upon such change. I believe the IP-change just happened for me, as my NGINX service is forwarding traffic to a cluster different than what is specified in its config. Restarting the NGINX service fixes the problem. But, again, I'm looking to SEE where NGINX currently thinks it should send the traffic to, not how to fix it or prevent it (plenty of resources online for working with dynamic upstreams, which I evidently should have consumed prior to deploying my NGINX services...).
Thank you in advance!

Answer 1

All you need is the resolver option.
http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver
With this option nginx will lookup DNS changes without restarting. But only for proxy_pass directive. This wont work, if you are using upstream. DNS resolve of upstream servers supported only in Nginx PLUS version.
If you want to know IP of upstream server, there is few ways:
- in PLUS version you can use status module or upstream_conf module, but PLUS version is not free
- some 3rd party status modules
- write this IP to log with each request, just add $upstream_addr variable to your custom access log. $upstream_addr contains IP address of backend server used in current request. Example of config:
log_format upstreamlog '$remote_addr - $remote_user [$time_local] '
'"$request" $status $bytes_sent $upstream_addr';
server {
...
access_log /tmp/test_access_log upstreamlog;
resolver ip.of.local.resolver;
location / {
set $pass dns_name.of.backend;
proxy_pass http://$pass;
}
}
Note: always use variable for proxy_pass - only in this case resolver will be used. Example of log:
127.0.0.1 - - [10/Jan/2017:02:12:15 +0300] "GET / HTTP/1.1" 200 503 213.180.193.3:80
127.0.0.1 - - [10/Jan/2017:02:12:25 +0300] "GET / HTTP/1.1" 200 503 213.180.193.3:80
.... IP address changed, nginx wasn't restarted ...
127.0.0.1 - - [10/Jan/2017:02:13:55 +0300] "GET / HTTP/1.1" 200 503 93.158.134.3:80
127.0.0.1 - - [10/Jan/2017:02:13:59 +0300] "GET / HTTP/1.1" 200 503 93.158.134.3:80