Golang read system dns configuration

LICENSE

Copyright (c) 2024 Brian Wojtczak.

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showdns.go

// @author: Brian Wojtczak
// @copyright: 2025 by K42 Software OÜ
// @license: BSD-style license found in the LICENSE file

package main

import (
	"context"
	"fmt"
	"net"
	"os"
	"slices"
)

// GetSystemConfig retrieves the system's DNS configuration.
func GetSystemConfig() (servers []string) {
	//
	// The Go standard library has a multi-platform DNS resolver which is able
	// to read the system's DNS configuration. Unfortunately, it does not
	// provide a method to retrieve the DNS servers directly.
	//
	// However, by injecting a dialer (which always fails), we can capture the
	// DNS servers that the resolver would have used to resolve a domain name.
	// In this way we learn the system configuration, without having to
	// replicate the complex configuration lookup logic.
	//
	resolver := net.Resolver{
		PreferGo:     true,
		StrictErrors: true,
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			if !slices.Contains(servers, address) {
				servers = append(servers, address)
			}
			return nil, os.ErrDeadlineExceeded
		},
	}
	_, _ = resolver.LookupIPAddr(context.Background(), "example.com")
	return servers
}

func main() {
	servers := GetSystemConfig()
	for i, server := range servers {
		fmt.Printf("Server %d: %s\n", i+1, server)
	}
}