parse server records

parseSRV.go

package main

import (
	"fmt"

	"github.com/miekg/dns"
)

func main() {
	fmt.Println("vim-go")
	rr, _ := dns.NewRR("_sip._tcp.example.com. 86400 IN SRV 0 5 5060 sipserver.example.com.")
	println(parseSRV(rr))
	rr, _ = dns.NewRR("miek.nl. 86400 IN SRV 0 5 5060 sipserver.example.com.")
	println(parseSRV(rr))
	rr, _ = dns.NewRR("_si\\.p._tcp.example.com. 86400 IN SRV 0 5 5060 sipserver.example.com.")
	println(parseSRV(rr))
}

// ParseSRV parses a server records and removed the _http._tcp labels to leave the
// actual domain name. If the srv record's ownername does not follow this scheme
// then whole name it returned.
// The function checks if the first label start with an "_" and the second
// label is either "_tcp" or "_udp".
func parseSRV(srv dns.RR) string {
	name := srv.Header().Name

	// Does it start with a "_" ?
	if len(name) > 0 && name[0] != '_' {
		return name
	}

	// First label
	first, end := dns.NextLabel(name, 0)
	if end {
		return name
	}
	// Second label
	off, end := dns.NextLabel(name, first)
	if end {
		return name
	}

	// first:off has captured _tcp. or _udp. (if present)
	second := name[first:off]
	if len(second) > 0 && second[0] != '_' {
		return name
	}
	if len(second) != 5 {
		return name
	}

	// bit convoluted to avoid strings.ToLower

	// matches _tcp
	if (second[1] == 't' || second[1] == 'T') && (second[2] == 'c' || second[2] == 'C') &&
		(second[3] == 'p' || second[3] == 'P') {
		return name[off:]
	}
	// matches _udp
	if (second[1] == 'u' || second[1] == 'U') && (second[2] == 'd' || second[2] == 'D') &&
		(second[3] == 'p' || second[3] == 'P') {
		return name[off:]
	}

	return name
}