From 81b142925fac8c717f8488d1c05dbc0116dac752 Mon Sep 17 00:00:00 2001
From: Brian Cunnie <brian.cunnie@gmail.com>
Date: Sun, 24 Sep 2023 10:07:12 +0200
Subject: [PATCH] Clarify UDP binding code

In preparation for TCP binding, I re-worked the UDP binding process so
that it could be more understandable and more easily replicated.

I don't know that it's more understandable. I may have failed.
---
 src/sslip.io-dns-server/main.go | 87 ++++++++++++++++++---------------
 1 file changed, 48 insertions(+), 39 deletions(-)

diff --git a/src/sslip.io-dns-server/main.go b/src/sslip.io-dns-server/main.go
index 2236d3f..21a4768 100644
--- a/src/sslip.io-dns-server/main.go
+++ b/src/sslip.io-dns-server/main.go
@@ -40,55 +40,42 @@ func main() {
 		log.Println(logmessage)
 	}
 
-	connUdp, err := net.ListenUDP("udp", &net.UDPAddr{Port: *bindPort})
-	//  common err hierarchy: net.OpError → os.SyscallError → syscall.Errno
-	switch {
-	case err == nil:
-		log.Printf("Successfully bound via UDP to all IPs, port %d.\n", *bindPort)
-		wg.Add(1)
-		go readFrom(connUdp, &wg, x, *quiet)
-	case isErrorPermissionsError(err):
-		log.Printf("Try invoking me with `sudo` because I don't have permission to bind to port %d.\n", *bindPort)
+	var udpConns []*net.UDPConn
+	var unboundIPs []string
+	udpConn, err := net.ListenUDP("udp", &net.UDPAddr{Port: *bindPort})
+	if err == nil {
+		udpConns = append(udpConns, udpConn)
+	}
+	if isErrorPermissionsError(err) {
+		log.Printf("Try invoking me with `sudo` because I don't have permission to bind to UDP port %d.\n", *bindPort)
 		log.Fatal(err.Error())
-	case isErrorAddressAlreadyInUse(err):
-		log.Printf("I couldn't bind via UDP to \"0.0.0.0:%d\" (INADDR_ANY, all interfaces), so I'll try to bind to each address individually.\n", *bindPort)
-		ipCIDRs := listLocalIPCIDRs()
-		var boundIPsPorts, unboundIPs []string
-		for _, ipCIDR := range ipCIDRs {
-			ip, _, err := net.ParseCIDR(ipCIDR)
-			if err != nil {
-				log.Printf(`I couldn't parse the local interface "%s".`, ipCIDR)
-				continue
-			}
-			connUdp, err = net.ListenUDP("udp", &net.UDPAddr{
-				IP:   ip,
-				Port: *bindPort,
-				Zone: "",
-			})
-			if err != nil {
-				unboundIPs = append(unboundIPs, ip.String())
-			} else {
-				wg.Add(1)
-				boundIPsPorts = append(boundIPsPorts, connUdp.LocalAddr().String())
-				go readFrom(connUdp, &wg, x, *quiet)
-			}
-		}
-		if len(boundIPsPorts) == 0 {
-			log.Fatalf("I couldn't bind via UDP to any IPs on port %d, so I'm exiting", *bindPort)
-		}
-		log.Printf(`I bound via UDP to the following IPs: "%s"`, strings.Join(boundIPsPorts, `", "`))
+	}
+	if isErrorAddressAlreadyInUse(err) {
+		// do some stuff
+		log.Printf("I couldn't bind via UDP to \"[::]:%d\" (INADDR_ANY, all interfaces), so I'll try to bind to each address individually.\n", *bindPort)
+		udpConns, unboundIPs = bindUDPAddressesIndividually(*bindPort)
 		if len(unboundIPs) > 0 {
 			log.Printf(`I couldn't bind via UDP to the following IPs: "%s"`, strings.Join(unboundIPs, `", "`))
 		}
-	default:
+		if len(udpConns) == 0 {
+			log.Fatalf("I couldn't bind via UDP to any IPs on port %d, so I'm exiting", *bindPort)
+		}
+	}
+	if err != nil {
 		log.Fatal(err.Error())
 	}
+	var boundIPs []string
+	for _, udpConn := range udpConns {
+		boundIPs = append(boundIPs, udpConn.LocalAddr().String())
+		go readFrom(udpConn, x, *quiet)
+		wg.Add(1)
+	}
+	log.Printf(`I bound via UDP to the following IPs: "%s"`, strings.Join(boundIPs, `", "`))
 	log.Printf("Ready to answer queries")
 	wg.Wait()
 }
 
-func readFrom(conn *net.UDPConn, wg *sync.WaitGroup, x *xip.Xip, quiet bool) {
-	defer wg.Done()
+func readFrom(conn *net.UDPConn, x *xip.Xip, quiet bool) {
 	for {
 		query := make([]byte, 512)
 		_, addr, err := conn.ReadFromUDP(query)
@@ -110,6 +97,28 @@ func readFrom(conn *net.UDPConn, wg *sync.WaitGroup, x *xip.Xip, quiet bool) {
 	}
 }
 
+func bindUDPAddressesIndividually(bindPort int) (udpConns []*net.UDPConn, unboundIPs []string) {
+	ipCIDRs := listLocalIPCIDRs()
+	for _, ipCIDR := range ipCIDRs {
+		ip, _, err := net.ParseCIDR(ipCIDR)
+		if err != nil {
+			log.Printf(`I couldn't parse the local interface "%s".`, ipCIDR)
+			continue
+		}
+		udpConn, err := net.ListenUDP("udp", &net.UDPAddr{
+			IP:   ip,
+			Port: bindPort,
+			Zone: "",
+		})
+		if err != nil {
+			unboundIPs = append(unboundIPs, ip.String())
+		} else {
+			udpConns = append(udpConns, udpConn)
+		}
+	}
+	return udpConns, unboundIPs
+}
+
 func listLocalIPCIDRs() []string {
 	var ifaces []net.Interface
 	var cidrStrings []string