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dns.go
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dns.go
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package nameserver
import (
"bytes"
"fmt"
"math/rand"
"net"
"os"
"sort"
"strings"
"sync"
"time"
"github.com/miekg/dns"
"github.com/weaveworks/weave/net/address"
)
const (
topDomain = "."
reverseDNSdomain = "in-addr.arpa."
udpBuffSize = uint16(4096)
minUDPSize = 512
DefaultListenAddress = "0.0.0.0:53"
DefaultTTL = 1
DefaultClientTimeout = 5 * time.Second
)
type Upstream interface {
Config() *dns.ClientConfig
}
func NewUpstream(resolvConf, filterAddress string) Upstream {
return &upstream{
resolvConf: resolvConf,
filterAddress: filterAddress,
cachedConfig: &dns.ClientConfig{}}
}
type upstream struct {
sync.Mutex
resolvConf string
filterAddress string
cachedConfig *dns.ClientConfig
lastStat time.Time
lastModified time.Time
}
func (u *upstream) Config() *dns.ClientConfig {
u.Lock()
defer u.Unlock()
// If we checked less than five seconds ago return what we already have
now := time.Now()
if now.Sub(u.lastStat) < 5*time.Second {
return u.cachedConfig
}
u.lastStat = now
// Reread file if it has changed
if fi, err := os.Stat(u.resolvConf); err == nil && fi.ModTime() != u.lastModified {
if config, err := dns.ClientConfigFromFile(u.resolvConf); err == nil {
config.Servers = filter(config.Servers, u.filterAddress)
u.lastModified = fi.ModTime()
u.cachedConfig = config
}
}
return u.cachedConfig
}
func filter(ss []string, s string) []string {
for i := 0; i < len(ss); {
if ss[i] == s {
ss = append(ss[:i], ss[i+1:]...)
continue
}
i++
}
return ss
}
type DNSServer struct {
ns *Nameserver
domain string
ttl uint32
address string
servers []*dns.Server
upstream Upstream
tcpClient *dns.Client
udpClient *dns.Client
}
func NewDNSServer(ns *Nameserver, domain, address string, upstream Upstream, ttl uint32, clientTimeout time.Duration) (*DNSServer, error) {
s := &DNSServer{
ns: ns,
domain: dns.Fqdn(domain),
ttl: ttl,
address: address,
upstream: upstream,
tcpClient: &dns.Client{Net: "tcp", ReadTimeout: clientTimeout},
udpClient: &dns.Client{Net: "udp", ReadTimeout: clientTimeout, UDPSize: udpBuffSize},
}
err := s.listen(address)
return s, err
}
func (d *DNSServer) String() string {
var buf bytes.Buffer
fmt.Fprintf(&buf, "WeaveDNS (%s)\n", d.ns.ourName)
fmt.Fprintf(&buf, " listening on %s, for domain %s\n", d.address, d.domain)
fmt.Fprintf(&buf, " response ttl %d\n", d.ttl)
return buf.String()
}
func (d *DNSServer) listen(address string) error {
udpListener, err := net.ListenPacket("udp", address)
if err != nil {
return err
}
udpServer := &dns.Server{PacketConn: udpListener, Handler: d.createMux(d.udpClient, minUDPSize)}
tcpListener, err := net.Listen("tcp", address)
if err != nil {
udpServer.Shutdown()
return err
}
tcpServer := &dns.Server{Listener: tcpListener, Handler: d.createMux(d.tcpClient, -1)}
d.servers = []*dns.Server{udpServer, tcpServer}
return nil
}
func (d *DNSServer) ActivateAndServe() {
for _, server := range d.servers {
go func(server *dns.Server) {
server.ActivateAndServe()
}(server)
}
}
func (d *DNSServer) Stop() error {
for _, server := range d.servers {
if err := server.Shutdown(); err != nil {
return err
}
}
return nil
}
type handler struct {
*DNSServer
maxResponseSize int
client *dns.Client
}
func (d *DNSServer) createMux(client *dns.Client, defaultMaxResponseSize int) *dns.ServeMux {
m := dns.NewServeMux()
h := &handler{
DNSServer: d,
maxResponseSize: defaultMaxResponseSize,
client: client,
}
m.HandleFunc(d.domain, h.handleLocal)
m.HandleFunc(reverseDNSdomain, h.handleReverse)
m.HandleFunc(topDomain, h.handleRecursive)
return m
}
func (h *handler) handleLocal(w dns.ResponseWriter, req *dns.Msg) {
h.ns.debugf("local request: %+v", *req)
if len(req.Question) != 1 {
h.nameError(w, req)
return
}
hostname := dns.Fqdn(req.Question[0].Name)
if strings.Count(hostname, ".") == 1 {
hostname = hostname + h.domain
}
addrs := h.ns.Lookup(hostname)
if len(addrs) == 0 {
h.nameError(w, req)
return
}
// Per RFC4074, if we have an A but another type was requested,
// return 'no error' with empty answer section
if req.Question[0].Qtype != dns.TypeA {
h.respond(w, h.makeResponse(req, nil))
return
}
header := dns.RR_Header{
Name: req.Question[0].Name,
Rrtype: dns.TypeA,
Class: dns.ClassINET,
Ttl: h.ttl,
}
answers := make([]dns.RR, len(addrs))
for i, addr := range addrs {
ip := addr.IP4()
answers[i] = &dns.A{Hdr: header, A: ip}
}
shuffleAnswers(&answers)
h.respond(w, h.makeResponse(req, answers))
}
func (h *handler) handleReverse(w dns.ResponseWriter, req *dns.Msg) {
h.ns.debugf("reverse request: %+v", *req)
if len(req.Question) != 1 || req.Question[0].Qtype != dns.TypePTR {
h.nameError(w, req)
return
}
ipStr := strings.TrimSuffix(req.Question[0].Name, "."+reverseDNSdomain)
ip, err := address.ParseIP(ipStr)
if err != nil {
h.nameError(w, req)
return
}
hostname, err := h.ns.ReverseLookup(ip.Reverse())
if err != nil {
h.handleRecursive(w, req)
return
}
header := dns.RR_Header{
Name: req.Question[0].Name,
Rrtype: dns.TypePTR,
Class: dns.ClassINET,
Ttl: h.ttl,
}
answers := []dns.RR{&dns.PTR{
Hdr: header,
Ptr: hostname,
}}
h.respond(w, h.makeResponse(req, answers))
}
func (h *handler) handleRecursive(w dns.ResponseWriter, req *dns.Msg) {
h.ns.debugf("recursive request: %+v", *req)
// Resolve unqualified names locally
if len(req.Question) == 1 {
hostname := dns.Fqdn(req.Question[0].Name)
if strings.Count(hostname, ".") == 1 {
h.handleLocal(w, req)
return
}
}
upstreamConfig := h.upstream.Config()
for _, server := range upstreamConfig.Servers {
reqCopy := req.Copy()
reqCopy.Id = dns.Id()
response, _, err := h.client.Exchange(reqCopy, fmt.Sprintf("%s:%s", server, upstreamConfig.Port))
if (err != nil && err != dns.ErrTruncated) || response == nil {
h.ns.debugf("error trying %s: %v", server, err)
continue
}
response.Id = req.Id
if h.responseTooBig(req, response) {
response.Compress = true
}
h.respond(w, response)
return
}
h.respond(w, h.makeErrorResponse(req, dns.RcodeServerFailure))
}
func (h *handler) makeResponse(req *dns.Msg, answers []dns.RR) *dns.Msg {
response := &dns.Msg{}
response.SetReply(req)
response.RecursionAvailable = true
response.Authoritative = true
response.Answer = answers
if !h.responseTooBig(req, response) {
return response
}
// search for smallest i that is too big
maxSize := h.getMaxResponseSize(req)
i := sort.Search(len(answers), func(i int) bool {
// return true if too big
response.Answer = answers[:i+1]
return response.Len() > maxSize
})
response.Answer = answers[:i]
if i < len(answers) {
response.Truncated = true
}
return response
}
func (h *handler) makeErrorResponse(req *dns.Msg, code int) *dns.Msg {
response := &dns.Msg{}
response.SetReply(req)
response.RecursionAvailable = true
response.Rcode = code
return response
}
func (h *handler) responseTooBig(req, response *dns.Msg) bool {
return len(response.Answer) > 1 && h.maxResponseSize > 0 && response.Len() > h.getMaxResponseSize(req)
}
func (h *handler) respond(w dns.ResponseWriter, response *dns.Msg) {
h.ns.debugf("response: %+v", response)
if err := w.WriteMsg(response); err != nil {
h.ns.infof("error responding: %v", err)
}
}
func (h *handler) nameError(w dns.ResponseWriter, req *dns.Msg) {
h.respond(w, h.makeErrorResponse(req, dns.RcodeNameError))
}
func (h *handler) getMaxResponseSize(req *dns.Msg) int {
if opt := req.IsEdns0(); opt != nil {
return int(opt.UDPSize())
}
return h.maxResponseSize
}
func shuffleAnswers(answers *[]dns.RR) {
if len(*answers) <= 1 {
return
}
for i := range *answers {
j := rand.Intn(i + 1)
(*answers)[i], (*answers)[j] = (*answers)[j], (*answers)[i]
}
}