Fix DHCPv4 options encoding

aibor · aibor · commit 29bbec8fdbcd · 2023-07-26T19:50:15.000+02:00
For calculating the DHCPv4 length by the "Len" method there is always a
byte added for the end option. But the END option is only set when there
are any options at all. This results in a missing END option in case
there are no options present.

This commit moves the encoding of the END option so it is always set
unconditionally.
diff --git a/layers/dhcpv4.go b/layers/dhcpv4.go
@@ -222,8 +222,8 @@ func (d *DHCPv4) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.Serialize
 	copy(data[108:236], d.File)
 	binary.BigEndian.PutUint32(data[236:240], DHCPMagic)
 
+	offset := 240
 	if len(d.Options) > 0 {
-		offset := 240
 		for _, o := range d.Options {
 			if err := o.encode(data[offset:]); err != nil {
 				return err
@@ -235,10 +235,10 @@ func (d *DHCPv4) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.Serialize
 				offset += 2 + len(o.Data)
 			}
 		}
-		optend := NewDHCPOption(DHCPOptEnd, nil)
-		if err := optend.encode(data[offset:]); err != nil {
-			return err
-		}
+	}
+	optend := NewDHCPOption(DHCPOptEnd, nil)
+	if err := optend.encode(data[offset:]); err != nil {
+		return err
 	}
 	return nil
 }

Original file line number	Diff line number	Diff line change
`@@ -222,8 +222,8 @@ func (d *DHCPv4) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.Serialize`
`222`	`222`	`copy(data[108:236], d.File)`
`223`	`223`	`binary.BigEndian.PutUint32(data[236:240], DHCPMagic)`
`224`	`224`
	`225`	`+ offset := 240`
`225`	`226`	`if len(d.Options) > 0 {`
`226`		`- offset := 240`
`227`	`227`	`for _, o := range d.Options {`
`228`	`228`	`if err := o.encode(data[offset:]); err != nil {`
`229`	`229`	`return err`
`@@ -235,10 +235,10 @@ func (d *DHCPv4) SerializeTo(b gopacket.SerializeBuffer, opts gopacket.Serialize`
`235`	`235`	`offset += 2 + len(o.Data)`
`236`	`236`	`}`
`237`	`237`	`}`
`238`		`- optend := NewDHCPOption(DHCPOptEnd, nil)`
`239`		`- if err := optend.encode(data[offset:]); err != nil {`
`240`		`- return err`
`241`		`- }`
	`238`	`+ }`
	`239`	`+ optend := NewDHCPOption(DHCPOptEnd, nil)`
	`240`	`+ if err := optend.encode(data[offset:]); err != nil {`
	`241`	`+ return err`
`242`	`242`	`}`
`243`	`243`	`return nil`
`244`	`244`	`}`