fix: smarter host IP determination for mDNS (#38)

Serving `.local` aliases over mDNS requires broadcasting the IP address
of the host machine, so that other devices on the network can connect to
it. Previous to this PR, localias would do lookups by trying to lookup
`{hostname}.local`. As reported in
#34, and previously in
#20 and
#21, this had a variety of
issues and did not work consistently.

This PR attemtps to fix this problem once and for all by finding the
host's IP by iterating through the network interfaces and looking for
non-loopback IPV4 addresses. I tested that this fixes the problem
reported in #34 by manually setting my hostname to
`something.fritz.box`, seeing that `localias` would fail to determine
the IP address, then running this code and seeing that it worked
correctly.

In addition to the IPV4 addresses that are found, this code also
broadcasts `127.0.0.1` and `::1` so that DNS requests on macOS respond
instantly. For some reason, if an IPV6 address is not included in the
mDNS response, running `curl myhost.local` will always have a 5-6 second
delay on macOS. I can't explain it but I have a workaround.

pkg/server/mdns.go

@@ -47,10 +47,12 @@ func newMDNSServer(entries []config.Entry) (*mdns.Server, error) {
 	// it will run on. If the hostname doesn't have a `.local` suffix, we need
 	// to add one for some reason based on the example mDNS code I've found.
 	// If the hostname already has a `.local` suffix, we should keep it.
-	localhost := ensureSuffix(hostname, ".local")
-	baseIPs, err := net.LookupIP(localhost)
+	baseIPs, err := getIPAddressesForMDNS()
 	if err != nil {
-		return nil, fmt.Errorf("could not determine host IP for .local domains: %w", err)
+		return nil, err
+	}
+	if baseIPs == nil {
+		return nil, fmt.Errorf("unable to serve mDNS: could not determine a local IP for host %s", hostname)
 	}
 	var ms multiservice
 	for _, entry := range localEntries {
@@ -90,12 +92,13 @@ func newMDNSServer(entries []config.Entry) (*mdns.Server, error) {
 			// actually running.
 			baseIPs,
 			// Just for fun, include a TXT record giving Localias credit.
-			[]string{ehost + " @ " + localhost + " via localias"},
+			[]string{ehost + " @ " + hostname + " via localias"},
+			// nil,
 		)
 		if err != nil {
 			return nil, err
 		}
-		fmt.Printf("mDNS: serving %s\n", entry.Host())
+		fmt.Printf("mDNS: serving %s (%s)\n", entry.Host(), ipStrings(baseIPs))
 		ms = append(ms, service)
 	}
 	return mdns.NewServer(&mdns.Config{Zone: ms})
@@ -116,6 +119,54 @@ func caddyPort(entry config.Entry) int {
 	return 80
 }
 
-func ensureSuffix(s, suffix string) string { //nolint:unparam // ignore
+func ensureSuffix(s, suffix string) string {
 	return strings.TrimSuffix(s, suffix) + suffix
 }
+
+func getIPAddressesForMDNS() ([]net.IP, error) {
+	// Get a list of all network interfaces
+	interfaces, err := net.Interfaces()
+	if err != nil {
+		return nil, err
+	}
+
+	var ipAddresses []net.IP
+	for _, iface := range interfaces {
+		// Get a list of all unicast IP addresses associated with the interface
+		addrs, err := iface.Addrs()
+		if err != nil {
+			return nil, err
+		}
+
+		for _, addr := range addrs {
+			var ip net.IP
+			switch v := addr.(type) {
+			case *net.IPNet:
+				ip = v.IP
+			case *net.IPAddr:
+				ip = v.IP
+			}
+			// Exclude local loopback addresses and V6 addresses,
+			if ip == nil || ip.IsLoopback() || ip.To4() == nil {
+				continue
+			}
+			ipAddresses = append(ipAddresses, ip)
+		}
+	}
+	// On MacOS Sonoma 14.4.1, if you don't include the IPV6 link-local IP in
+	// the broadcast, DNS lookups will hang for like 5-6 seconds each time, even
+	// with the IPV4 address in /etc/hosts. I can't explain it but including
+	// fe80::1 makes the lookups instant.
+	if len(ipAddresses) != 0 {
+		ipAddresses = append(ipAddresses, net.ParseIP("::1"), net.ParseIP("127.0.0.1"))
+	}
+	return ipAddresses, nil
+}
+
+func ipStrings(ips []net.IP) string {
+	var ipStrings []string
+	for _, ip := range ips {
+		ipStrings = append(ipStrings, ip.String())
+	}
+	return strings.Join(ipStrings, ",")
+}