netadd: split ValidatenodeAnn into sig and field checks

Check that the node ann doesnt contain more than 1 DNS addr.
This will ensure that we now start rejecting new node announcements
with multiple DNS addrs since this check is called in the gossiper
before persisting a node ann to our local graph.

It also validates the DNS fields according to BOLT #7 specs.

lnwire/dns_addr_test.go

@@ -0,0 +1,87 @@
+package lnwire
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestValidateDNSAddr tests hostname and port validation per BOLT #7.
+func TestValidateDNSAddr(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name     string
+		hostname string
+		port     uint16
+		err      error
+	}{
+		{
+			name:     "empty hostname",
+			hostname: "",
+			port:     9735,
+			err:      ErrEmptyDNSHostname,
+		},
+		{
+			name:     "zero port",
+			hostname: "example.com",
+			port:     0,
+			err:      ErrZeroPort,
+		},
+		{
+			name:     "hostname too long",
+			hostname: strings.Repeat("a", 256),
+			port:     9735,
+			err: fmt.Errorf("%w: DNS hostname length 256",
+				ErrHostnameTooLong),
+		},
+		{
+			name:     "hostname with invalid ASCII space",
+			hostname: "exa mple.com",
+			port:     9735,
+			err: fmt.Errorf("%w: hostname 'exa mple.com' contains "+
+				"invalid character ' ' at position 3",
+				ErrInvalidHostnameCharacter),
+		},
+		{
+			name:     "hostname with invalid ASCII underscore",
+			hostname: "example_node.com",
+			port:     9735,
+			err: fmt.Errorf("%w: hostname 'example_node.com' "+
+				"contains invalid character '_' at position 7",
+				ErrInvalidHostnameCharacter),
+		},
+		{
+			name:     "hostname with non-ASCII character",
+			hostname: "example❄️.com",
+			port:     9735,
+			err: fmt.Errorf("%w: hostname 'example❄️.com' "+
+				"contains invalid character '❄' at position 7",
+				ErrInvalidHostnameCharacter),
+		},
+		{
+			name:     "valid hostname",
+			hostname: "example.com",
+			port:     9735,
+		},
+		{
+			name:     "valid hostname with numbers",
+			hostname: "node101.example.com",
+			port:     9735,
+		},
+		{
+			name:     "valid hostname with hyphens",
+			hostname: "my-node.example-domain.com",
+			port:     9735,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := ValidateDNSAddr(tc.hostname, tc.port)
+			require.Equal(t, err, tc.err)
+		})
+	}
+}