Added multicast DNS client

mdns_client.go

@@ -0,0 +1,117 @@
+package weavedns
+
+import (
+	"github.com/miekg/dns"
+	"log"
+	"net"
+	"sync"
+)
+
+// Portions of this code taken from github.com/armon/mdns
+
+const (
+	ipv4mdns = "224.0.0.251" // link-local multicast address
+	mdnsPort = 5353          // mDNS assigned port
+)
+
+var (
+	ipv4Addr = &net.UDPAddr{
+		IP:   net.ParseIP(ipv4mdns),
+		Port: mdnsPort,
+	}
+)
+
+type ResponseA struct {
+	Name string
+	Addr net.IP
+}
+
+type responseInfo struct {
+	ch chan<- *ResponseA
+}
+
+// Represents one query that we have sent for one name.
+// If we, internally, get several requests for the same name while we have
+// a query in flight, then we don't want to send more queries out.
+type inflightQuery struct {
+	name          string
+	Id            uint16 // the DNS message ID
+	responseInfos []*responseInfo
+	// add timeout here ?
+}
+
+type MDNSClient struct {
+	sync.RWMutex
+	conn     *net.UDPConn
+	addr     *net.UDPAddr
+	inflight map[string]*inflightQuery
+}
+
+func NewMDNSClient() (*MDNSClient, error) {
+	conn, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		log.Println(err)
+		return nil, err
+	}
+	retval := &MDNSClient{
+		conn:     conn,
+		addr:     ipv4Addr,
+		inflight: make(map[string]*inflightQuery)}
+	return retval, nil
+}
+
+func LinkLocalMulticastListener() (net.PacketConn, error) {
+	conn, err := net.ListenMulticastUDP("udp", nil, ipv4Addr)
+	return conn, err
+}
+
+func (c *MDNSClient) SendQuery(name string, querytype uint16, responseCh chan<- *ResponseA) error {
+	m := new(dns.Msg)
+	m.SetQuestion(name, querytype)
+	m.RecursionDesired = false
+
+	c.Lock()
+	query, found := c.inflight[name]
+	if !found {
+		query = &inflightQuery{name: name}
+		c.inflight[name] = query
+	}
+	info := &responseInfo{ch: responseCh}
+	query.responseInfos = append(query.responseInfos, info)
+	c.Unlock()
+
+	buf, err := m.Pack()
+	if err != nil {
+		return err
+	}
+	_, err = c.conn.WriteTo(buf, c.addr)
+	return err
+}
+
+func (c *MDNSClient) SendResponse(m *dns.Msg) error {
+	buf, err := m.Pack()
+	if err != nil {
+		return err
+	}
+	_, err = c.conn.WriteTo(buf, c.addr)
+	return err
+}
+
+func (c *MDNSClient) HandleResponse(r *dns.Msg) {
+	for _, answer := range r.Answer {
+		switch rr := answer.(type) {
+		case *dns.A:
+			c.Lock()
+			if query, found := c.inflight[rr.Hdr.Name]; found {
+				for _, resp := range query.responseInfos {
+					resp.ch <- &ResponseA{Name: rr.Hdr.Name, Addr: rr.A}
+				}
+				// To be simple for now, assume this is the only response coming
+				delete(c.inflight, rr.Hdr.Name)
+			} else {
+				log.Println("Response received that didn't match query", r)
+			}
+			c.Unlock()
+		}
+	}
+}