Example Test for DNS is not helpful

[lcrilly]

Describe the bug

On this page, https://docs.nginx.com/nginx/admin-guide/load-balancer/udp-health-check/#example-test-for-dns
The send and expect parameters are confusing and misleading. They cannot (and should not) be used to create a working DNS health check.

To reproduce

Steps to reproduce the behavior:

1. Configure NGINX Plus with the example match configuration.
2. Watch all health checks fail
3. Notice that NGINX Plus is requesting a DNS lookup for *stlu.msl.edu which is bad for the servers in the upstream group and bad for any subsequent DNS server that will handle this request. And it's bad for Massachusetts School of Law.

Expected behavior

1. Docs should explain that DNS health checks require configuration to be made on the DNS server.
2. Docs should provide a means for creating the hex string to send a DNS lookup
3. Docs should explain what the expect value should be based on 1 & 2

[mjang]

I think https://docs.nginx.com/nginx/admin-guide/load-balancer/udp-health-check/ is most relevant. That page suggests that send and receive can be used to create a working DNS health check

which we now know is wrong.

[lcrilly]

Apologies for not adding the URI. You're right @mjang , I am reffering to https://docs.nginx.com/nginx/admin-guide/load-balancer/udp-health-check/#example-test-for-dns (will edit the issue).

[y82]

#442