Nginx resolver address from /etc/resolv.conf

Solution 1:

Unfortunately there's no easy way to do this because nginx use it's own resolver implementation. The two solutions I see are :

1) You generate the resolver list from a script and include it, e.g. :

echo resolver $(awk 'BEGIN{ORS=" "} $1=="nameserver" {print $2}' /etc/resolv.conf) ";" > /etc/nginx/resolvers.conf

http {

    include resolvers.conf;

}

2) You recompile nginx with a third party module like the (very) experimental perl module and write a variable handler :

http {

    perl_modules perl/lib;
    perl_set $resolvers '

        sub {
            return system("awk BEGIN{ORS=\" \"} /nameserver/{print \$2}" /etc/resolv.conf");
        };

    resolver "$resolvers";
}

Now, if you are a hell of a C coder (prepare your eyes for some blood), you can still write an alternative patch or module to make it work this way.

Solution 2:

If you're using the Openresty version of nginx then you can use their special local argument to the resolver directive which when set to local=on, means the standard path of /etc/resolv.conf will be used by the resolver (for more details see Openresty resolver docs):

resolver local=on;

---

Solution 3:

For Docker users, solution found here:

Heres a workaround for people using Docker.

export NAMESERVER=`cat /etc/resolv.conf | grep "nameserver" | awk '{print $2}' | tr '\n' ' '`

What this does is take all the nameserver entries from /etc/resolv.conf and print them in a line, so you can use them with nginx's resolver directive. Your Dockerfile will need to have a custom script for the entrypoint that generates the config file and then starts nginx. Lets say you have a file called nginx.conf.template that looks something like:

...snip...
http {
  server {

    resolver $NAMESERVER valid=10s;

    ...snip....  
    }
  }
}

Your startup script can then use the envsubst program to generate an nginx.conf and then start nginx. eg:

#!/bin/bash
if [ "$NAMESERVER" == "" ]; then
    export NAMESERVER=`cat /etc/resolv.conf | grep "nameserver" | awk '{print $2}' | tr '\n' ' '`
fi

echo "Nameserver is: $NAMESERVER"

echo "Copying nginx config"
envsubst '$NAMESERVER' < /nginx.conf.template > /nginx.conf

echo "Using nginx config:"
cat /nginx.conf

echo "Starting nginx"
nginx -c /nginx.conf -g "daemon off;"

NOTE that in docker this tend to result in the same file, as by default the docker embedded DNS server is 127.0.0.11, see this answer to Docker Network Nginx Resolver.