Add home server from JID lookup helper.

src/xmpp/dns.go

@@ -0,0 +1,34 @@
+package xmpp
+
+import (
+	"fmt"
+	"net"
+)
+
+const (
+	XMPP_CLIENT_PORT = 5222
+)
+
+// Perform a DNS SRV lookup and return an ordered list of "host:port" TCP
+// addresses for the JID's home server.
+func HomeServerAddrs(jid JID) (addr []string, err error) {
+
+	// DNS lookup.
+	_, addrs, err := net.LookupSRV("xmpp-client", "tcp", jid.Domain)
+	if err != nil {
+		return
+	}
+
+	// If there's nothing in DNS then assume the JID's domain and the standard
+	// port will work.
+	if len(addrs) == 0 {
+		addr = []string{fmt.Sprintf("%s:%d", jid.Domain, XMPP_CLIENT_PORT)}
+		return
+	}
+
+	// Build list of "host:port" strings.
+	for _, a := range addrs {
+		addr = append(addr, fmt.Sprintf("%s:%d", a.Target, a.Port))
+	}
+	return
+}