Unable to config NGINX to re‑resolves a domain name for UDP / TCP

[jmcewan127@…]

According the docs, ​https://www.nginx.com/blog/dns-service-discovery-nginx-plus/ this is possible in the open source version.

The example is for the http block,

resolver 10.0.0.2 valid=10s;
server {

location / {

set $backend_servers backends.example.com;
proxy_pass ​http://$backend_servers:8080;

}

}

And according to the note following:

"Note: For TCP/UDP load balancing, this method of using a variable in the proxy_pass directive is supported in NGINX 1.11.3 and later, and NGINX Plus R10 and later."

But clearly not, as in 1.11.10 using set is not possible in a top level stream block. For example:

resolver 10.0.0.2 valid=10s;
server {

set $backend_servers backends.example.com;
listen 12345 udp;
proxy_pass $backend_servers:12345;

}

set will error as [emerg] "set" directive is not allowed here in /etc/nginx/nginx.conf:15

So either the document is wrong, or there's a bug where set should be supported but isn't.

Either way need to be able have the DNS re-resolve for TCP/UDP forwarding [given the dynamic nature of the destination IP].

[Maxim Dounin]

There is no set directive in stream. You may provide variables for proxy_pass by other means though, see ​docs.

If you think that the blog post needs better clarification for the note in question, consider writing a comment to the blog post. If you need help with configuring nginx, consider using ​mailing list.

[jmcewan127@…]

Your answer is what I am telling you.
The doc's indicate you support a mechanism re-resolving DNS for UDP/TCP.
Looking in the mail list, there is the exact same question with the exact same problem.
[Oct 2016, no reasonable answer].
Clearly you do not support set.
Perhaps consider it as a feature request, if this is not possible.

[Maxim Dounin]

You are wrong, dynamic name resolution (including re-resolving) is supported by the stream module when you use variables in proxy_pass. It is to be configured differently though, as there is no set directive. For example, you can use ​map to provide appropriate backend name:

stream {
    resolver 127.0.0.1;

    map $remote_addr $backend {
        default foo.example.com;
    }

    server {
        listen 12345;
        proxy_pass $backend:12345;
    }
}

As previously suggested, if you need help with configuring nginx, please consider ​support options available. No further clarification will be given here, sorry.

[jmcewan127@…]

Im happy to be wrong. However the map directive is only available in the http block.
ergo its scope doesn't apply.

hmm seems the docs are mix matched. I am wrong awesome.

​http://nginx.org/en/docs/http/ngx_http_map_module.html

Syntax: map string $variable { ... }
Default: —
Context: http

thx.

[Maxim Dounin]

As previously said, no further clarification will be given here. Please use the ​support options available.