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dns.go
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dns.go
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package main
import (
"context"
"fmt"
"log"
"net"
"strings"
"time"
"github.com/miekg/dns"
)
const SOAString string = "@ SOA prisoner.iana.org. hostmaster.root-servers.org. 2002040800 1800 900 0604800 604800"
func isIPv4(addr net.IP) bool {
return strings.Contains(addr.String(), ".")
}
func getDomain(domain string) string {
if dns.IsFqdn(domain) {
domain = domain[0 : len(domain)-1]
}
return strings.ToLower(domain)
}
func getARecords(fqdn string, recordTTL int, ipaddrs []net.IP) ([]dns.RR, error) {
var records []dns.RR
for _, ip := range ipaddrs {
if !isIPv4(ip) {
// We just skip all the IPv6 addresses
continue
}
str := fmt.Sprintf("%s %d IN A %s", fqdn, recordTTL, ip.String())
rr, err := dns.NewRR(str)
if err != nil {
return nil, err
}
records = append(records, rr)
}
return records, nil
}
func getAAAARecords(fqdn string, recordTTL int, ipaddrs []net.IP) ([]dns.RR, error) {
var records []dns.RR
for _, ip := range ipaddrs {
if isIPv4(ip) {
// We just skip all the IPv4 addresses
continue
}
str := fmt.Sprintf("%s %d IN AAAA %s", fqdn, recordTTL, ip.String())
rr, err := dns.NewRR(str)
if err != nil {
return nil, err
}
records = append(records, rr)
}
return records, nil
}
func getTXTRecords(fqdn string, recordTTL int, values []string) ([]dns.RR, error) {
var records []dns.RR
for _, val := range values {
str := fmt.Sprintf("%s %d IN TXT %s", fqdn, recordTTL, val)
rr, err := dns.NewRR(str)
if err != nil {
return nil, err
}
records = append(records, rr)
}
return records, nil
}
func processQuery(db Database, msg *dns.Msg, soa dns.RR, ns []dns.RR, config dnsConfig) error {
var (
answer []dns.RR
)
// Multiple questions are never used in practice
q := msg.Question[0]
// Use 1 second timeout for the database queries to avoid stalling
ctx, cancel := context.WithTimeout(context.Background(), time.Second)
defer cancel()
domain := getDomain(q.Name)
domainExists, err := db.DoesDomainExist(ctx, domain)
if err != nil {
return err
}
recordTTL := config.RecordTTL
if q.Qtype == dns.TypeA || q.Qtype == dns.TypeAAAA {
ipaddrs, err := db.GetIPAddresses(ctx, domain)
if err != nil {
return err
}
if q.Qtype == dns.TypeA {
answer, err = getARecords(q.Name, recordTTL, ipaddrs)
} else {
answer, err = getAAAARecords(q.Name, recordTTL, ipaddrs)
}
if err != nil {
return err
}
} else if q.Qtype == dns.TypeTXT {
txtvals, err := db.GetTXTValues(ctx, domain)
if err != nil {
return err
}
answer, err = getTXTRecords(q.Name, recordTTL, txtvals)
if err != nil {
return err
}
}
if len(answer) == 0 {
// Default response is authoritative with SOA
msg.Authoritative = true
msg.Ns = []dns.RR{soa}
if !domainExists {
// No records for the whole domain
msg.Rcode = dns.RcodeNameError
}
return nil
}
// Send a successful response with an answer
msg.Authoritative = true
msg.Ns = ns
msg.Answer = answer
return nil
}
func getHandler(db Database, domain string, nameservers []string, config dnsConfig) func(dns.ResponseWriter, *dns.Msg) {
nshdr := dns.RR_Header{Name: domain, Rrtype: dns.TypeNS, Class: dns.ClassINET, Ttl: 3600}
var nsrr []dns.RR
for _, ns := range nameservers {
rr := new(dns.NS)
rr.Hdr = nshdr
rr.Ns = dns.Fqdn(ns)
nsrr = append(nsrr, rr)
}
SOAFormat := fmt.Sprintf("%s SOA %s %s %%s 28800 7200 604800 86400", strings.ToLower(domain), strings.ToLower(nameservers[0]), "admin.domain.foo")
return func(w dns.ResponseWriter, req *dns.Msg) {
serial := time.Now().Format("2006010215")
SOAString := fmt.Sprintf(SOAFormat, serial)
SOA, err := dns.NewRR(SOAString)
if err != nil {
// FIXME: Handle error, should not happen
}
msg := new(dns.Msg)
msg.SetReply(req)
if req.Opcode == dns.OpcodeQuery {
if err := processQuery(db, msg, SOA, nsrr, config); err != nil {
// FIXME: Handle ServFail
}
}
w.WriteMsg(msg)
}
}
func startDNS(db Database, config dnsConfig) {
domain := dns.Fqdn(config.Domain)
var nsfqdns []string
for _, nsstr := range config.NameServers {
nsfqdns = append(nsfqdns, dns.Fqdn(nsstr))
}
dns.HandleFunc(domain, getHandler(db, domain, nsfqdns, config))
server := &dns.Server{Addr: ":53", Net: "udp"}
fmt.Printf("Starting DNS server at localhost:53\n")
log.Fatal(server.ListenAndServe())
}