// overflowOrTruncated writes back an error to the client if the message does not fit.
// It updates prometheus metrics. If something has been written to the client, true
// will be returned.
func (s *server) overflowOrTruncated(w dns.ResponseWriter, m *dns.Msg, bufsize int, sy metrics.System) bool {
switch isTCP(w) {
case true:
if _, overflow := Fit(m, dns.MaxMsgSize, true); overflow {
metrics.ReportErrorCount(m, sy)
msgFail := s.ServerFailure(m)
w.WriteMsg(msgFail)
return true
}
case false:
// Overflow with udp always results in TC.
Fit(m, bufsize, false)
metrics.ReportErrorCount(m, sy)
if m.Truncated {
w.WriteMsg(m)
return true
}
}
return false
}
// ServeDNS is the handler for DNS requests, responsible for parsing DNS request, possibly forwarding
// it to a real dns server and returning a response.
func (s *server) ServeDNS(w dns.ResponseWriter, req *dns.Msg) {
m := new(dns.Msg)
m.SetReply(req)
m.Authoritative = true
m.RecursionAvailable = true
m.Compress = true
bufsize := uint16(512)
dnssec := false
tcp := false
start := time.Now()
q := req.Question[0]
name := strings.ToLower(q.Name)
if q.Qtype == dns.TypeANY {
m.Authoritative = false
m.Rcode = dns.RcodeRefused
m.RecursionAvailable = false
m.RecursionDesired = false
m.Compress = false
w.WriteMsg(m)
metrics.ReportRequestCount(m, metrics.Auth)
metrics.ReportDuration(m, start, metrics.Auth)
metrics.ReportErrorCount(m, metrics.Auth)
return
}
if o := req.IsEdns0(); o != nil {
bufsize = o.UDPSize()
dnssec = o.Do()
}
if bufsize < 512 {
bufsize = 512
}
// with TCP we can send 64K
if tcp = isTCP(w); tcp {
bufsize = dns.MaxMsgSize - 1
}
if s.config.Verbose {
logf("received DNS Request for %q from %q with type %d", q.Name, w.RemoteAddr(), q.Qtype)
}
// Check cache first.
m1 := s.rcache.Hit(q, dnssec, tcp, m.Id)
if m1 != nil {
metrics.ReportRequestCount(req, metrics.Cache)
if send := s.overflowOrTruncated(w, m1, int(bufsize), metrics.Cache); send {
return
}
// Still round-robin even with hits from the cache.
// Only shuffle A and AAAA records with each other.
if q.Qtype == dns.TypeA || q.Qtype == dns.TypeAAAA {
s.RoundRobin(m1.Answer)
}
if err := w.WriteMsg(m1); err != nil {
logf("failure to return reply %q", err)
}
metrics.ReportDuration(m1, start, metrics.Cache)
metrics.ReportErrorCount(m1, metrics.Cache)
return
}
for zone, ns := range *s.config.stub {
if strings.HasSuffix(name, zone) {
metrics.ReportRequestCount(req, metrics.Stub)
resp := s.ServeDNSStubForward(w, req, ns)
if resp != nil {
s.rcache.InsertMessage(cache.Key(q, dnssec, tcp), resp)
}
metrics.ReportDuration(resp, start, metrics.Stub)
metrics.ReportErrorCount(resp, metrics.Stub)
return
}
}
// If the qname is local.ds.skydns.local. and s.config.Local != "", substitute that name.
if s.config.Local != "" && name == s.config.localDomain {
name = s.config.Local
}
if q.Qtype == dns.TypePTR && strings.HasSuffix(name, ".in-addr.arpa.") || strings.HasSuffix(name, ".ip6.arpa.") {
metrics.ReportRequestCount(req, metrics.Reverse)
resp := s.ServeDNSReverse(w, req)
if resp != nil {
s.rcache.InsertMessage(cache.Key(q, dnssec, tcp), resp)
}
metrics.ReportDuration(resp, start, metrics.Reverse)
metrics.ReportErrorCount(resp, metrics.Reverse)
return
}
if q.Qclass != dns.ClassCHAOS && !strings.HasSuffix(name, s.config.Domain) {
metrics.ReportRequestCount(req, metrics.Rec)
resp := s.ServeDNSForward(w, req)
if resp != nil {
s.rcache.InsertMessage(cache.Key(q, dnssec, tcp), resp)
}
metrics.ReportDuration(resp, start, metrics.Rec)
metrics.ReportErrorCount(resp, metrics.Rec)
return
}
metrics.ReportCacheMiss(metrics.Response)
defer func() {
metrics.ReportDuration(m, start, metrics.Auth)
metrics.ReportErrorCount(m, metrics.Auth)
if m.Rcode == dns.RcodeServerFailure {
if err := w.WriteMsg(m); err != nil {
logf("failure to return reply %q", err)
}
return
}
// Set TTL to the minimum of the RRset and dedup the message, i.e. remove identical RRs.
m = s.dedup(m)
minttl := s.config.Ttl
if len(m.Answer) > 1 {
for _, r := range m.Answer {
if r.Header().Ttl < minttl {
minttl = r.Header().Ttl
}
}
for _, r := range m.Answer {
r.Header().Ttl = minttl
}
}
if dnssec {
if s.config.PubKey != nil {
m.AuthenticatedData = true
s.Denial(m)
s.Sign(m, bufsize)
}
}
if send := s.overflowOrTruncated(w, m, int(bufsize), metrics.Auth); send {
return
}
s.rcache.InsertMessage(cache.Key(q, dnssec, tcp), m)
if err := w.WriteMsg(m); err != nil {
logf("failure to return reply %q", err)
}
}()
if name == s.config.Domain {
if q.Qtype == dns.TypeSOA {
m.Answer = []dns.RR{s.NewSOA()}
return
}
if q.Qtype == dns.TypeDNSKEY {
if s.config.PubKey != nil {
m.Answer = []dns.RR{s.config.PubKey}
return
}
}
}
if q.Qclass == dns.ClassCHAOS {
if q.Qtype == dns.TypeTXT {
switch name {
case "authors.bind.":
fallthrough
case s.config.Domain:
hdr := dns.RR_Header{Name: q.Name, Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
authors := []string{"Erik St. Martin", "Brian Ketelsen", "Miek Gieben", "Michael Crosby"}
for _, a := range authors {
m.Answer = append(m.Answer, &dns.TXT{Hdr: hdr, Txt: []string{a}})
}
for j := 0; j < len(authors)*(int(dns.Id())%4+1); j++ {
q := int(dns.Id()) % len(authors)
p := int(dns.Id()) % len(authors)
if q == p {
p = (p + 1) % len(authors)
}
m.Answer[q], m.Answer[p] = m.Answer[p], m.Answer[q]
}
return
case "version.bind.":
fallthrough
case "version.server.":
hdr := dns.RR_Header{Name: q.Name, Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{Version}}}
return
case "hostname.bind.":
fallthrough
case "id.server.":
// TODO(miek): machine name to return
hdr := dns.RR_Header{Name: q.Name, Rrtype: dns.TypeTXT, Class: dns.ClassCHAOS, Ttl: 0}
m.Answer = []dns.RR{&dns.TXT{Hdr: hdr, Txt: []string{"localhost"}}}
return
}
}
// still here, fail
m.SetReply(req)
m.SetRcode(req, dns.RcodeServerFailure)
return
}
switch q.Qtype {
case dns.TypeNS:
if name != s.config.Domain {
break
}
// Lookup s.config.DnsDomain
records, extra, err := s.NSRecords(q, s.config.dnsDomain)
if isEtcdNameError(err, s) {
m = s.NameError(req)
return
}
m.Answer = append(m.Answer, records...)
m.Extra = append(m.Extra, extra...)
case dns.TypeA, dns.TypeAAAA:
records, err := s.AddressRecords(q, name, nil, bufsize, dnssec, false)
if isEtcdNameError(err, s) {
m = s.NameError(req)
return
}
m.Answer = append(m.Answer, records...)
case dns.TypeTXT:
records, err := s.TXTRecords(q, name)
if isEtcdNameError(err, s) {
m = s.NameError(req)
return
}
m.Answer = append(m.Answer, records...)
case dns.TypeCNAME:
records, err := s.CNAMERecords(q, name)
if isEtcdNameError(err, s) {
m = s.NameError(req)
return
}
m.Answer = append(m.Answer, records...)
case dns.TypeMX:
records, extra, err := s.MXRecords(q, name, bufsize, dnssec)
if isEtcdNameError(err, s) {
m = s.NameError(req)
return
}
m.Answer = append(m.Answer, records...)
m.Extra = append(m.Extra, extra...)
default:
fallthrough // also catch other types, so that they return NODATA
case dns.TypeSRV:
records, extra, err := s.SRVRecords(q, name, bufsize, dnssec)
if err != nil {
if isEtcdNameError(err, s) {
m = s.NameError(req)
return
}
logf("got error from backend: %s", err)
if q.Qtype == dns.TypeSRV { // Otherwise NODATA
m = s.ServerFailure(req)
return
}
}
// if we are here again, check the types, because an answer may only
// be given for SRV. All other types should return NODATA, the
// NXDOMAIN part is handled in the above code. TODO(miek): yes this
// can be done in a more elegant manor.
if q.Qtype == dns.TypeSRV {
m.Answer = append(m.Answer, records...)
m.Extra = append(m.Extra, extra...)
}
}
if len(m.Answer) == 0 { // NODATA response
m.Ns = []dns.RR{s.NewSOA()}
m.Ns[0].Header().Ttl = s.config.MinTtl
}
}