dhcpv4: Also write out 0-length options

When there was a 0-length generic option, it would be omitted.
Rapid commit (option 80) is a 0-length option with a length tag.
There could also be 0-length options in the vendor extensions, and the
RFC states that they should be written out:

> Any options defined subsequent to this document should contain a
> length octet even if the length is fixed or zero.

Signed-off-by: Anatole Denis <natolumin@unverle.fr>

2 files changed, 26 insertions, 2 deletions

diff --git a/dhcpv4/options.go b/dhcpv4/options.go
index ea902f6..058c4ad 100644
--- a/dhcpv4/options.go
+++ b/dhcpv4/options.go
@@ -185,12 +185,20 @@ func (o Options) Marshal(b *uio.Lexer) {
 		code := uint8(c)
 		// Even if the End option is in there, don't marshal it until
 		// the end.
-		if code == optEnd {
+		// Don't write padding either, since the options are sorted
+		// it would always be written first which isn't useful
+		if code == optEnd || code == optPad {
 			continue
 		}
 
 		data := o[code]
 
+		// Ensure even 0-length options are written out
+		if len(data) == 0 {
+			b.Write8(code)
+			b.Write8(0)
+			continue
+		}
 		// RFC 3396: If more than 256 bytes of data are given, the
 		// option is simply listed multiple times.
 		for len(data) > 0 {
diff --git a/dhcpv4/options_test.go b/dhcpv4/options_test.go
index 6c5393c..3850d2d 100644
--- a/dhcpv4/options_test.go
+++ b/dhcpv4/options_test.go
@@ -194,9 +194,25 @@ func TestOptionsMarshal(t *testing.T) {
 				5, 1, 10,
 			),
 		},
+		{
+			// Test 0-length options
+			opts: Options{
+				80: []byte{},
+			},
+			want: []byte{80, 0},
+		},
+		{
+			// Test special options, handled by the message marshalling code
+			// and ignored by the options marshalling code
+			opts: Options{
+				0:   []byte{}, // Padding
+				255: []byte{}, // End of options
+			},
+			want: nil, // not written out
+		},
 	} {
 		t.Run(fmt.Sprintf("Test %02d", i), func(t *testing.T) {
-			require.Equal(t, uio.ToBigEndian(tt.opts), tt.want)
+			require.Equal(t, tt.want, uio.ToBigEndian(tt.opts))
 		})
 	}
 }