How to write a DNS server in Go
Here’s a DNS server written in Go.
It uses the package github.com/miekg/dns,
which is an alternative to the standard library’s DNS stuff,
and also lets you implement a DNS server rather than just a client.
The example DNS server below only knows how to answer A queries,
and for these, it only knows the IP addresses of two domains.
package main
import (
"net"
"strconv"
"log"
"github.com/miekg/dns"
)
var domainsToAddresses map[string]string = map[string]string{
"google.com.": "1.2.3.4",
"jameshfisher.com.": "104.198.14.52",
}
type handler struct{}
func (this *handler) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
msg := dns.Msg{}
msg.SetReply(r)
switch r.Question[0].Qtype {
case dns.TypeA:
msg.Authoritative = true
domain := msg.Question[0].Name
address, ok := domainsToAddresses[domain]
if ok {
msg.Answer = append(msg.Answer, &dns.A{
Hdr: dns.RR_Header{ Name: domain, Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 60 },
A: net.ParseIP(address),
})
}
}
w.WriteMsg(&msg)
}
func main() {
srv := &dns.Server{Addr: ":" + strconv.Itoa(53), Net: "udp"}
srv.Handler = &handler{}
if err := srv.ListenAndServe(); err != nil {
log.Fatalf("Failed to set udp listener %s\n", err.Error())
}
}
Example usage:
$ sudo go run main.go &
$ dig @127.0.0.1 jameshfisher.com
; <<>> DiG 9.8.3-P1 <<>> @127.0.0.1 jameshfisher.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 25014
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available
;; QUESTION SECTION:
;jameshfisher.com. IN A
;; ANSWER SECTION:
jameshfisher.com. 60 IN A 104.198.14.52
;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Sun Aug 27 00:27:06 2017
;; MSG SIZE rcvd: 66
This program could be extended with more custom logic. For example, at Pusher (the company I work for), we could operate a DNS server which resolves a Pusher app’s domain to the cluster that the app is on.
Tagged . All content copyright James Fisher 2017. This post is not associated with my employer.
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