> For the complete documentation index, see [llms.txt](https://packetgen.gitbook.io/wiki/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://packetgen.gitbook.io/wiki/protocols/udp.md). # UDP A UDP header consists of: ``` 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-------------------------------+-------------------------------+ | Source Port | Destination Port | +-------------------------------+-------------------------------+ | Length | Checksum | +-------------------------------+-------------------------------+ ``` * 16-bit source port (`UDP#sport`), * 16-bit destination port (`UDP#dport`), * 16-bit length (`UDP#length`), * 16-bit checksum (`UDP#checksum`), * and a body (`UDP#body`). A UDP header may be built this way: ``` pg> PacketGen::Header::UDP.new(dport: 53, sport: 65053) => ---- PacketGen::Header::UDP ------------------------------------------ Int16 sport: 65053 (0xfe1d) Int16 dport: 53 (0x0035) Int16 length: 8 (0x0008) Int16 checksum: 0 (0x0000) ``` A UDP over IP packet may be created this way: ```ruby pkt = PacketGen.gen('IP', src: '127.0.0.1', dst: '127.0.0.1'). add('UDP', dport: 80, sport: 44158, body: 'abcd') pkt.is?('IP') # => true pkt.is?('UDP') # => true pkt.udp # => PacketGen::Header::UDP pkt.udp.sport # => 65053 pkt.udp.dport # => 53 pkt.udp.length # => Integer pkt.udp.checksum # => Integer pkt.udp.body # => "abcd" pkt.body # => "abcd" pkt.udp.sport = 44444 ``` `checksum` and `length` fields may be computed by `Header::UDP#calc_sum` and `Header::UDP#calc_length` respectively. All checksum and length fields from this packet may by computed at once using `pkt.calc`: ``` pg> pkt.calc pg> pkt => -- PacketGen::Packet ------------------------------------------------- ---- PacketGen::Header::IP ------------------------------------------- Int8 u8: 69 (0x45) version: 4 ihl: 5 Int8 tos: 0 (0x00) Int16 length: 32 (0x0020) Int16 id: 39945 (0x9c09) Int16 frag: 0 (0x0000) flags: none frag_offset: 0 (0x0000) Int8 ttl: 64 (0x40) Int8 protocol: 17 (0x11) Int16 checksum: 57537 (0xe0c1) Addr src: 127.0.0.1 Addr dst: 127.0.0.1 String options: "" ---- PacketGen::Header::UDP ------------------------------------------ Int16 sport: 44444 (0xad9c) Int16 dport: 80 (0x0050) Int16 length: 12 (0x000c) Int16 checksum: 36640 (0x8f20) ---- Body ------------------------------------------------------------ 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 ---------------------------------------------------------------------- 61 62 63 64 abcd ---------------------------------------------------------------------- ``` See also .