Adding a simple client

client.go

@@ -0,0 +1,233 @@
+package mdns
+
+import (
+	"fmt"
+	"github.com/miekg/dns"
+	"log"
+	"net"
+	"strings"
+	"sync"
+	"time"
+)
+
+// ServiceEntry is returned after we query for a service
+type ServiceEntry struct {
+	Name string
+	Addr net.IP
+	Port int
+	Info string
+
+	hasTXT bool
+	sent   bool
+}
+
+// complete is used to check if we have all the info we need
+func (s *ServiceEntry) complete() bool {
+	return s.Addr != nil && s.Port != 0 && s.hasTXT
+}
+
+// LookupDomain looks up a given service, in a domain, waiting at most
+// for a timeout before finishing the query. The results are streamed
+// to a channel. Sends will not block, so clients should make sure to
+// either read or buffer.
+func LookupDomain(service, domain string, timeout time.Duration, entries chan<- *ServiceEntry) error {
+	// Create a new client
+	client, err := newClient()
+	if err != nil {
+		return err
+	}
+	defer client.Close()
+
+	// Create the query name
+	serviceAddr := fmt.Sprintf("%s.%s.", trimDot(service), trimDot(domain))
+
+	// Run the query
+	return client.query(serviceAddr, timeout, entries)
+}
+
+// Lookup is the same as LookupDomain, however it only searches in the "local"
+// domain, and uses a one second lookup timeout.
+func Lookup(service string, entries chan<- *ServiceEntry) error {
+	return LookupDomain(service, "local", time.Second, entries)
+}
+
+// Client provides a query interface that can be used to
+// search for service providers using mDNS
+type client struct {
+	ipv4List *net.UDPConn
+	ipv6List *net.UDPConn
+
+	closed    bool
+	closedCh  chan struct{}
+	closeLock sync.Mutex
+}
+
+// NewClient creates a new mdns Client that can be used to query
+// for records
+func newClient() (*client, error) {
+	// Create a IPv4 listener
+	ipv4, err := net.ListenUDP("udp4", &net.UDPAddr{IP: net.IPv4zero, Port: 0})
+	if err != nil {
+		log.Printf("[ERR] mdns: Failed to bind to udp4 port: %v", err)
+	}
+	ipv6, err := net.ListenUDP("udp6", &net.UDPAddr{IP: net.IPv6zero, Port: 0})
+	if err != nil {
+		log.Printf("[ERR] mdns: Failed to bind to udp6 port: %v", err)
+	}
+
+	if ipv4 == nil && ipv6 == nil {
+		return nil, fmt.Errorf("Failed to bind to any udp port!")
+	}
+
+	c := &client{
+		ipv4List: ipv4,
+		ipv6List: ipv6,
+		closedCh: make(chan struct{}),
+	}
+	return c, nil
+}
+
+// Close is used to cleanup the client
+func (c *client) Close() error {
+	c.closeLock.Lock()
+	defer c.closeLock.Unlock()
+
+	if c.closed {
+		return nil
+	}
+	c.closed = true
+	close(c.closedCh)
+
+	if c.ipv4List != nil {
+		c.ipv4List.Close()
+	}
+	if c.ipv6List != nil {
+		c.ipv6List.Close()
+	}
+	return nil
+}
+
+// query is used to perform a lookup and stream results
+func (c *client) query(service string, timeout time.Duration, entries chan<- *ServiceEntry) error {
+	// Start listening for response packets
+	msgCh := make(chan *dns.Msg, 32)
+	go c.recv(c.ipv4List, msgCh)
+	go c.recv(c.ipv6List, msgCh)
+
+	// Send the query
+	m := new(dns.Msg)
+	m.SetQuestion(service, dns.TypeANY)
+	if err := c.sendQuery(m); err != nil {
+		return nil
+	}
+
+	// Map the in-progress responses
+	inprogress := make(map[string]*ServiceEntry)
+
+	// Listen until we reach the timeout
+	finish := time.After(timeout)
+	for {
+		select {
+		case resp := <-msgCh:
+			var inp *ServiceEntry
+			for _, answer := range resp.Answer {
+				switch rr := answer.(type) {
+				case *dns.PTR:
+					// Create new entry for this
+					inp = ensureName(inprogress, rr.Ptr)
+
+				case *dns.SRV:
+					// Get the port
+					inp = ensureName(inprogress, rr.Target)
+					inp.Port = int(rr.Port)
+
+				case *dns.TXT:
+					// Pull out the txt
+					inp = ensureName(inprogress, rr.Hdr.Name)
+					inp.Info = strings.Join(rr.Txt, "|")
+					inp.hasTXT = true
+
+				case *dns.A:
+					// Pull out the IP
+					inp = ensureName(inprogress, rr.Hdr.Name)
+					inp.Addr = rr.A
+
+				case *dns.AAAA:
+					// Pull out the IP
+					inp = ensureName(inprogress, rr.Hdr.Name)
+					inp.Addr = rr.AAAA
+				}
+			}
+
+			// Check if this entry is complete
+			if inp.complete() && !inp.sent {
+				inp.sent = true
+				select {
+				case entries <- inp:
+				default:
+				}
+			} else {
+				// Fire off a node specific query
+				m := new(dns.Msg)
+				m.SetQuestion(inp.Name, dns.TypeANY)
+				if err := c.sendQuery(m); err != nil {
+					log.Printf("[ERR] mdns: Failed to query instance %s: %v", inp.Name, err)
+				}
+			}
+		case <-finish:
+			return nil
+		}
+	}
+	return nil
+}
+
+// sendQuery is used to multicast a query out
+func (c *client) sendQuery(q *dns.Msg) error {
+	buf, err := q.Pack()
+	if err != nil {
+		return err
+	}
+	if c.ipv4List != nil {
+		c.ipv4List.WriteTo(buf, ipv4Addr)
+	}
+	if c.ipv6List != nil {
+		c.ipv6List.WriteTo(buf, ipv6Addr)
+	}
+	return nil
+}
+
+// recv is used to receive until we get a shutdown
+func (c *client) recv(l *net.UDPConn, msgCh chan *dns.Msg) {
+	if l == nil {
+		return
+	}
+	buf := make([]byte, 65536)
+	for !c.closed {
+		n, err := l.Read(buf)
+		if err != nil {
+			continue
+		}
+		msg := new(dns.Msg)
+		if err := msg.Unpack(buf[:n]); err != nil {
+			log.Printf("[ERR] mdns: Failed to unpack packet: %v", err)
+			continue
+		}
+		select {
+		case msgCh <- msg:
+		case <-c.closedCh:
+			return
+		}
+	}
+}
+
+// ensureName is used to ensure the named node is in progress
+func ensureName(inprogress map[string]*ServiceEntry, name string) *ServiceEntry {
+	if inp, ok := inprogress[name]; ok {
+		return inp
+	}
+	inp := &ServiceEntry{
+		Name: name,
+	}
+	inprogress[name] = inp
+	return inp
+}