fix(netxlite): consolidate IPv4/IPv6 checking code

Originally bassosimone/websteps-illustrated@966e7f7

See ooni/probe#2096

internal/netxlite/quirks.go

@@ -2,6 +2,7 @@ package netxlite
 
 import (
 	"errors"
+	"net"
 	"strings"
 )
 
@@ -52,26 +53,21 @@ func quirkReduceErrors(errorslist []error) error {
 // before IPv6. Dialers SHOULD call this code.
 //
 // It saddens me to have this quirk, but it is here to pair
-// with quirkReduceErrors, which assumes that <facepalm>.
+// with quirkReduceErrors, which assumes that IPv4 addrs
+// appear before IPv6 addrs <facepalm>.
+//
+// Note: this function will skip any input that is not not
+// a valid IPv4 or IPv6 address.
 //
 // See TODO(https://github.com/ooni/probe/issues/1779).
 func quirkSortIPAddrs(addrs []string) (out []string) {
-	isIPv6 := func(x string) bool {
-		// This check for identifying IPv6 is discussed
-		// at https://stackoverflow.com/questions/22751035
-		// and seems good-enough for our purposes.
-		return strings.Contains(x, ":")
-	}
-	isIPv4 := func(x string) bool {
-		return !isIPv6(x)
-	}
 	for _, addr := range addrs {
-		if isIPv4(addr) {
+		if net.ParseIP(addr) != nil && !isIPv6(addr) {
 			out = append(out, addr)
 		}
 	}
 	for _, addr := range addrs {
-		if isIPv6(addr) {
+		if net.ParseIP(addr) != nil && isIPv6(addr) {
 			out = append(out, addr)
 		}
 	}

internal/netxlite/quirks_test.go

@@ -57,7 +57,9 @@ func TestQuirkSortIPAddrs(t *testing.T) {
 		addrs := []string{
 			"::1",
 			"192.168.1.2",
+			"x.org", // ensure we skip non IP addrs
 			"2a00:1450:4002:404::2004",
+			"example.com", // ensure we skip non IP addrs
 			"142.250.184.36",
 			"2604:8800:5000:82:466:38ff:fecb:d46e",
 			"198.145.29.83",
@@ -83,4 +85,14 @@ func TestQuirkSortIPAddrs(t *testing.T) {
 			t.Fatal("expected nil output")
 		}
 	})
+
+	t.Run("with non-IP addrs", func(t *testing.T) {
+		addrs := []string{
+			"example.com",
+			"x.org",
+		}
+		if quirkSortIPAddrs(addrs) != nil {
+			t.Fatal("expected nil output")
+		}
+	})
 }