commit 4024bcf42fac1e5c2fad4058f281a72c88b872ae
Author: Zack Buhman <zack@buhman.org>
Date:   Tue Aug 18 23:53:39 2026 -0500

    initial

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..d866725
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,3 @@
+*.o
+*.out
+.gdb_history
\ No newline at end of file
diff --git a/bpf.asm b/bpf.asm
new file mode 100644
index 0000000..38d9e9d
--- /dev/null
+++ b/bpf.asm
@@ -0,0 +1,11 @@
+l0:     ldh [12]              /* eth.type */
+l1:     jeq #0x800, l2, l10   /* if ipv4, l3 */
+l2:     ldb [23]              /* ip.proto */
+l3:     jeq #0x11, l4, l10    /* if UDP, l5 */
+l4:     ldh [20]              /* ip.frag_offset */
+l5:     jset #0x1fff, l10, l6 /* if not fragmented, l7 */
+l6:     ldxb 4*([14]&0xf)     /* ip.hdr_len * 4 -> X */
+l7:     ldh [x + 16]          /* 36: UDP destination port */
+l8:     jeq #0x44, l9, l10    /* if equal to 68, l10 */
+l9:     ret #0x40000          /* keep 262144 bytes */
+l10:    ret #0                /* discard the packet */
diff --git a/client.cpp b/client.cpp
new file mode 100644
index 0000000..b47151d
--- /dev/null
+++ b/client.cpp
@@ -0,0 +1,233 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <time.h>
+
+#include <arpa/inet.h>
+#include <sys/socket.h>
+#include <sys/ioctl.h>
+#include <linux/if_packet.h>
+#include <linux/if_ether.h>
+#include <linux/if.h>
+#include <linux/sockios.h>
+#include <linux/filter.h>
+
+#include "ipv4.h"
+#include "udp4.h"
+#include "dhcp4.h"
+#include "eth.h"
+
+int make_packet(uint8_t * buf,
+                uint8_t const * eth_source_address,
+                uint8_t const * eth_destination_address,
+                uint32_t const ipv4_source_address,
+                uint32_t const ipv4_destination_address,
+                uint32_t const dhcp_transaction_id,
+                uint32_t const dhcp_message_type)
+{
+  //////////////////////////////////////////////////////////////////////
+  // dhcp4
+  //////////////////////////////////////////////////////////////////////
+
+  int dhcp4_offset = (sizeof (eth::header)) + (sizeof (ipv4::header)) + (sizeof (udp4::header));
+  int dhcp4_length = dhcp4::make_message(reinterpret_cast<void *>(&buf[dhcp4_offset]),
+                                         dhcp4::op::bootrequest,
+                                         dhcp_transaction_id,
+                                         eth_source_address,
+                                         dhcp_message_type);
+
+  //////////////////////////////////////////////////////////////////////
+  // eth
+  //////////////////////////////////////////////////////////////////////
+
+  int eth_offset = 0;
+  eth::header * eth_header = reinterpret_cast<eth::header *>(&buf[eth_offset]);
+  memcpy(eth_header->destination_address, eth_destination_address, 6);
+  memcpy(eth_header->source_address, eth_source_address, 6);
+  eth_header->ether_type = htons(0x0800);
+
+  //////////////////////////////////////////////////////////////////////
+  // ipv4
+  //////////////////////////////////////////////////////////////////////
+
+  int ipv4_offset = (sizeof (eth::header));
+  int ipv4_payload_length = dhcp4_length + (sizeof (udp4::header));
+  int ipv4_identification = 55179;
+  ipv4::make_header(reinterpret_cast<void *>(&buf[ipv4_offset]),
+                    ipv4_payload_length,
+                    ipv4_identification,
+                    ipv4_source_address,
+                    ipv4_destination_address);
+
+  //////////////////////////////////////////////////////////////////////
+  // udp4
+  //////////////////////////////////////////////////////////////////////
+
+  int udp4_offset = (sizeof (eth::header)) + (sizeof (ipv4::header));
+  udp4::make_header(reinterpret_cast<void *>(&buf[udp4_offset]),
+                    htons(68),
+                    htons(67),
+                    ipv4_source_address,
+                    ipv4_destination_address,
+                    &buf[dhcp4_offset],
+                    dhcp4_length);
+
+  return dhcp4_offset + dhcp4_length;
+}
+
+void interface_send(int sock,
+                    int interface_index,
+                    void * buf,
+                    int length,
+                    uint8_t const * eth_destination_address)
+{
+  struct sockaddr_ll socket_address;
+  socket_address.sll_ifindex = interface_index;
+  socket_address.sll_halen = ETH_ALEN;
+  memcpy(socket_address.sll_addr, eth_destination_address, 6);
+  int flags = 0;
+  int sret = sendto(sock, buf, length, flags,
+                    (struct sockaddr *)&socket_address, sizeof(struct sockaddr_ll));
+  if (sret < 0) {
+    perror("sendto");
+    exit(EXIT_FAILURE);
+  }
+}
+
+void attach_dhcp_filter(int sock)
+{
+  struct sock_filter filter[] = {
+    { 0x28,  0,  0, 0x0000000c },
+    { 0x15,  0,  8, 0x00000800 },
+    { 0x30,  0,  0, 0x00000017 },
+    { 0x15,  0,  6, 0x00000011 },
+    { 0x28,  0,  0, 0x00000014 },
+    { 0x45,  4,  0, 0x00001fff },
+    { 0xb1,  0,  0, 0x0000000e },
+    { 0x48,  0,  0, 0x00000010 },
+    { 0x15,  0,  1, 0x00000044 },
+    { 0x06,  0,  0, 0x00040000 },
+    { 0x06,  0,  0, 0000000000 },
+  };
+
+  struct sock_fprog fprog = {
+    .len = sizeof(filter) / sizeof(filter[0]),
+    .filter = filter,
+  };
+  int ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &fprog, (sizeof (fprog)));
+  if (ret < 0) {
+    perror("setsockopt(SO_ATTACH_FILTER)");
+    exit(EXIT_FAILURE);
+  }
+}
+
+int main()
+{
+  srand(time(NULL));
+
+  uint8_t buf[16384];
+
+  //////////////////////////////////////////////////////////////////////
+  // socket
+  //////////////////////////////////////////////////////////////////////
+
+  int s = socket(AF_PACKET, SOCK_RAW, IPPROTO_RAW);
+  if (s < 0) {
+    perror("socket");
+    exit(EXIT_FAILURE);
+  }
+
+  {
+    int enable = 1;
+    int ret = setsockopt(s, SOL_SOCKET, SO_BROADCAST, &enable, (sizeof (int)));
+    if (ret < 0) {
+      perror("setsockopt(SO_BROADCAST)");
+      exit(EXIT_FAILURE);
+    }
+  }
+
+  //////////////////////////////////////////////////////////////////////
+  // interface index
+  //////////////////////////////////////////////////////////////////////
+
+  char const * interface_name = "wlp174s0";
+
+  struct ifreq if_index;
+  memset(&if_index, 0, (sizeof (struct ifreq)));
+  strncpy(if_index.ifr_name, interface_name, IFNAMSIZ - 1);
+  int iret = ioctl(s, SIOCGIFINDEX, &if_index);
+  if (iret < 0) {
+    perror("ioctl(SIOCGIFINDEX)");
+    exit(EXIT_FAILURE);
+  }
+  printf("interface index %d\n", if_index.ifr_ifindex);
+
+  //////////////////////////////////////////////////////////////////////
+  // interface address
+  //////////////////////////////////////////////////////////////////////
+
+  struct ifreq if_hwaddr;
+  memset(&if_hwaddr, 0, sizeof(struct ifreq));
+  strncpy(if_hwaddr.ifr_name, interface_name, IFNAMSIZ - 1);
+  int hret = ioctl(s, SIOCGIFHWADDR, &if_hwaddr);
+  if (hret < 0) {
+    perror("ioctl(SIOCGIFHWADDR)");
+    exit(EXIT_FAILURE);
+  }
+  printf("hardware address %02x:%02x:%02x:%02x:%02x:%02x\n",
+         (uint8_t)if_hwaddr.ifr_hwaddr.sa_data[0], (uint8_t)if_hwaddr.ifr_hwaddr.sa_data[1],
+         (uint8_t)if_hwaddr.ifr_hwaddr.sa_data[2], (uint8_t)if_hwaddr.ifr_hwaddr.sa_data[3],
+         (uint8_t)if_hwaddr.ifr_hwaddr.sa_data[4], (uint8_t)if_hwaddr.ifr_hwaddr.sa_data[5]);
+
+  //////////////////////////////////////////////////////////////////////
+  // bind
+  //////////////////////////////////////////////////////////////////////
+
+  struct sockaddr_ll bind_addr = {
+    .sll_family   = AF_PACKET,
+    .sll_protocol = htons(ETH_P_ALL),
+    .sll_ifindex  = if_index.ifr_ifindex,
+  };
+  int bret = bind(s, (struct sockaddr *)&bind_addr, (sizeof (struct sockaddr_ll)));
+  if (bret < 0) {
+    perror("bind");
+    exit(EXIT_FAILURE);
+  }
+
+  attach_dhcp_filter(s);
+
+  //////////////////////////////////////////////////////////////////////
+  // send
+  //////////////////////////////////////////////////////////////////////
+
+  uint32_t dhcp_transaction_id = rand();
+
+  uint8_t eth_source_address[6] = {};
+  memcpy(eth_source_address, if_hwaddr.ifr_hwaddr.sa_data, 6);
+  uint8_t eth_destination_address[] = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff};
+
+  uint32_t ipv4_source_address = 0x00000000;
+  uint32_t ipv4_destination_address = 0xffffffff;
+
+  int length = make_packet(buf,
+                           eth_source_address,
+                           eth_destination_address,
+                           ipv4_source_address,
+                           ipv4_destination_address,
+                           dhcp_transaction_id,
+                           dhcp4::message_type::dhcpdiscover);
+
+  interface_send(s, if_index.ifr_ifindex, buf, length, eth_destination_address);
+
+  uint8_t recvbuf[65536];
+  struct sockaddr_ll recv_address;
+  int flags = 0;
+  socklen_t socklen = (sizeof (struct sockaddr_ll));
+  ssize_t ret = recvfrom(s, recvbuf, 16384, flags,
+                         (struct sockaddr *)&recv_address, &socklen);
+  printf("recv ret %ld\n", ret);
+
+  close(s);
+  exit(EXIT_SUCCESS);
+}
diff --git a/dhcp4.cpp b/dhcp4.cpp
new file mode 100644
index 0000000..5359230
--- /dev/null
+++ b/dhcp4.cpp
@@ -0,0 +1,67 @@
+#include <string.h>
+
+#include <arpa/inet.h>
+
+#include "dhcp4.h"
+
+namespace dhcp4 {
+
+  int make_message(void * buf,
+                   uint32_t bootp_message_type,
+                   uint32_t transaction_id,
+                   uint8_t const * chaddr,
+                   uint32_t dhcp_message_type)
+  {
+    memset(buf, 0, (sizeof (message)));
+    message * msg = reinterpret_cast<message *>(buf);
+
+    msg->op = bootp_message_type;
+    msg->htype = htype::ethernet;
+    msg->hlen = 6;
+    msg->xid = htonl(transaction_id);
+    memcpy(msg->chaddr, chaddr, 6);
+
+    int index = 0;
+
+    uint8_t * cookie = &msg->options[index];
+    index += 4;
+    cookie[0] = 0x63;
+    cookie[1] = 0x82;
+    cookie[2] = 0x53;
+    cookie[3] = 0x63;
+
+    uint8_t * message_type = &msg->options[index];
+    index += 2 + 1;
+    message_type[0] = option::message_type;
+    message_type[1] = 1; // length
+    message_type[2] = dhcp_message_type;
+
+    {
+      uint8_t * prl = &msg->options[index];
+      int length = 4;
+      index += 2 + length;
+      prl[0] = option::parameter_request_list;
+      prl[1] = length;
+      prl[2] = 1; // subnet mask
+      prl[3] = 3; // router
+      prl[4] = 6; // domain name server
+      prl[5] = 28; // broadcast address
+    }
+
+    if (0) {
+      uint8_t * message_size = &msg->options[index];
+      int length = 2;
+      index += 2 + length;
+      message_size[0] = option::maximum_message_size;
+      message_size[1] = length;
+      message_size[2] = 0x05;
+      message_size[3] = 0xc0;
+    }
+
+    uint8_t * end = &msg->options[index];
+    index += 1;
+    end[0] = 255;
+
+    return (sizeof (message)) + index;
+  }
+}
diff --git a/dhcp4.h b/dhcp4.h
new file mode 100644
index 0000000..fd4ef45
--- /dev/null
+++ b/dhcp4.h
@@ -0,0 +1,71 @@
+#pragma once
+
+#include <stdint.h>
+#include <assert.h>
+
+namespace dhcp4 {
+
+  struct __attribute__((packed)) message {
+    uint8_t op; // message type
+    uint8_t htype; // hardware type
+    uint8_t hlen; // hardware address length
+    uint8_t hops;
+    uint32_t xid; // transaction id
+    uint16_t secs; // seconds elapsed
+    uint16_t flags;
+    uint32_t ciaddr; // client IP address
+    uint32_t yiaddr; // your (client) IP address
+    uint32_t siaddr; // next server IP address
+    uint32_t giaddr; // relay agent IP address
+    uint8_t chaddr[16]; // client hardware address
+    uint8_t sname[64];
+    uint8_t file[128];
+    uint8_t options[];
+  };
+
+  struct op {
+    enum {
+      bootrequest = 1,
+      bootreply = 2,
+    };
+  };
+
+  struct htype {
+    enum {
+      ethernet = 1,
+    };
+  };
+
+  static_assert((sizeof (message)) == 236);
+
+  struct option {
+    enum {
+      ip_address_lease_time = 51,
+      option_overload = 52,
+      message_type = 53,
+      server_identifier = 54,
+      parameter_request_list = 55,
+      maximum_message_size = 57,
+      client_identifier = 61,
+    };
+  };
+
+  struct message_type {
+    enum {
+      dhcpdiscover = 1,
+      dhcpoffer = 2,
+      dhcprequest = 3,
+      dhcpdecline = 4,
+      dhcpack = 5,
+      dhcpnak = 6,
+      dhcprelease = 7,
+      dhcpinform = 8,
+    };
+  };
+
+  int make_message(void * buf,
+                   uint32_t bootp_message_type,
+                   uint32_t transaction_id,
+                   uint8_t const * chaddr,
+                   uint32_t dhcp_message_type);
+}
diff --git a/eth.h b/eth.h
new file mode 100644
index 0000000..5fc2a40
--- /dev/null
+++ b/eth.h
@@ -0,0 +1,11 @@
+#pragma once
+
+#include <stdint.h>
+
+namespace eth {
+  struct header {
+    uint8_t destination_address[6];
+    uint8_t source_address[6];
+    uint16_t ether_type;
+  };
+}
diff --git a/ipv4.cpp b/ipv4.cpp
new file mode 100644
index 0000000..7b7bc02
--- /dev/null
+++ b/ipv4.cpp
@@ -0,0 +1,70 @@
+#include <stdint.h>
+#include <stddef.h>
+#include <string.h>
+
+#include <arpa/inet.h>
+
+#include "ipv4.h"
+
+namespace ipv4 {
+
+  uint32_t checksum_partial(uint32_t sum, void const * src, int length)
+  {
+    uint8_t const * addr = reinterpret_cast<uint8_t const *>(src);
+
+    ptrdiff_t i = 0;
+    while (length >= 2) {
+      sum += (addr[i+0] << 8) | (addr[i+1] << 0);
+      i += 2;
+      length -= 2;
+    }
+
+    if (length > 0) {
+      sum += (addr[i+0] << 8);
+    }
+
+    return sum;
+  }
+
+  uint32_t checksum_finish(uint32_t sum)
+  {
+    while (sum >> 16) {
+      sum = (sum & 0xffff) + (sum >> 16);
+    }
+
+    return static_cast<uint16_t>(~sum);
+  }
+
+  uint32_t checksum(void const * src, int length)
+  {
+    uint32_t sum = checksum_partial(0, src, length);
+    return checksum_finish(sum);
+  }
+
+  header * make_header(void * buf,
+                       uint16_t payload_length,
+                       uint16_t identification,
+                       uint32_t source_address, // big endian
+                       uint32_t destination_address) // big endian
+  {
+    memset(buf, 0, (sizeof (header)));
+    header * hdr = reinterpret_cast<header *>(buf);
+
+    const int version = 4;
+    const int ihl = (sizeof (header)) / 4;
+    const int total_length = payload_length + (sizeof (header));
+
+    hdr->version_ihl = (version << 4) | (ihl << 0);
+    //hdr->type_of_service = 0xb8;
+    hdr->total_length = htons(total_length);
+    hdr->identification = htons(identification);
+    hdr->time_to_live = 64;
+    hdr->protocol = protocol::udp;
+    hdr->source_address = source_address;
+    hdr->destination_address = destination_address;
+
+    hdr->header_checksum = htons(checksum(buf, (sizeof (header))));
+
+    return hdr;
+  }
+}
diff --git a/ipv4.h b/ipv4.h
new file mode 100644
index 0000000..bf52273
--- /dev/null
+++ b/ipv4.h
@@ -0,0 +1,37 @@
+#pragma once
+
+#include <stdint.h>
+
+namespace ipv4 {
+
+  struct header {
+    uint8_t version_ihl;
+    uint8_t type_of_service;
+    uint16_t total_length;
+    uint16_t identification;
+    uint16_t fragment_offset;
+    uint8_t time_to_live;
+    uint8_t protocol;
+    uint16_t header_checksum;
+    uint32_t source_address;
+    uint32_t destination_address;
+  };
+
+  static_assert((sizeof (header)) == 20);
+
+  uint32_t checksum_partial(uint32_t sum, void const * src, int length);
+  uint32_t checksum_finish(uint32_t sum);
+  uint32_t checksum(void const * src, int length);
+
+  header * make_header(void * buf,
+                       uint16_t payload_length,
+                       uint16_t identification,
+                       uint32_t source_address, // big endian
+                       uint32_t destination_address); // big endian
+
+  struct protocol {
+    enum {
+      udp = 17,
+    };
+  };
+}
diff --git a/test.cpp b/test.cpp
new file mode 100644
index 0000000..b6c5184
--- /dev/null
+++ b/test.cpp
@@ -0,0 +1,64 @@
+#include <sys/socket.h>
+#include <netinet/in.h>
+#include <arpa/inet.h>
+
+#include <stdio.h>
+
+#include "udp4.h"
+#include "ipv4.h"
+#include "dhcp4.h"
+
+void dump_hex(void const * src, int length)
+{
+  uint8_t const * u8 = reinterpret_cast<uint8_t const *>(src);
+  for (int i = 0; i < length; i++) {
+    printf("%02x ", u8[i]);
+    if (i % 16 == 15)
+      printf("\n");
+  }
+  printf("\n");
+}
+
+int main()
+{
+  int udp_header = 8 + 339;
+
+  unsigned char payload[] = {0x2, 0x1, 0x6, 0x0, 0x12, 0x27, 0x67, 0xa8, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc0, 0xa8, 0x1, 0xf0, 0xc0, 0xa8, 0x1, 0x1, 0x0, 0x0, 0x0, 0x0, 0xf4, 0x7b, 0x9, 0x99, 0x3e, 0x28, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x63, 0x82, 0x53, 0x63, 0x35, 0x1, 0x5, 0x36, 0x4, 0xc0, 0xa8, 0x1, 0x1, 0x33, 0x4, 0x0, 0x1, 0x51, 0x80, 0x3a, 0x4, 0x0, 0x0, 0xa8, 0xc0, 0x3b, 0x4, 0x0, 0x1, 0x27, 0x50, 0x1c, 0x4, 0xc0, 0xa8, 0x1, 0xff, 0x3, 0x4, 0xc0, 0xa8, 0x1, 0x1, 0xf, 0x3, 0x6c, 0x61, 0x6e, 0x1, 0x4, 0xff, 0xff, 0xff, 0x0, 0x6, 0x4, 0xc0, 0xa8, 0x1, 0x1, 0x7d, 0x28, 0x0, 0x0, 0xd, 0xe9, 0x23, 0x6, 0x8, 0x47, 0x52, 0x42, 0x45, 0x33, 0x33, 0x31, 0x43, 0x5, 0xf, 0x52, 0x30, 0x31, 0x43, 0x31, 0x4d, 0x32, 0x35, 0x34, 0x36, 0x30, 0x30, 0x32, 0x48, 0x45, 0x4, 0x6, 0x30, 0x30, 0x30, 0x46, 0x42, 0x33, 0xff};
+
+  uint8_t buf[16384];
+
+  struct in_addr srcA;
+  struct in_addr dstA;
+  inet_aton("192.168.1.1", &srcA);
+  inet_aton("192.168.1.240", &dstA);
+
+  int ipv4_payload_length = (sizeof (payload)) + (sizeof (udp4::header));
+  int ipv4_identification = 55179;
+  int offset = 0;
+  ipv4::header * ipv4_header = ipv4::make_header(reinterpret_cast<void *>(&buf[offset]),
+                                                 ipv4_payload_length,
+                                                 ipv4_identification,
+                                                 srcA.s_addr,
+                                                 dstA.s_addr);
+  offset += (sizeof (ipv4::header));
+  dump_hex(ipv4_header, (sizeof (ipv4::header)));
+
+  udp4::header * udp4_header = udp4::make_header(reinterpret_cast<void *>(&buf[offset]),
+                                                 htons(67),
+                                                 htons(68),
+                                                 srcA.s_addr,
+                                                 dstA.s_addr,
+                                                 payload,
+                                                 (sizeof (payload)));
+  offset += (sizeof (udp4::header));
+
+  dump_hex(udp4_header, (sizeof (udp4::header)));
+
+  uint32_t transaction_id = 0x122767a8;
+  int dhcp4_length = dhcp4::make_message(reinterpret_cast<void *>(&buf[offset]),
+                                         dhcp4::op::bootrequest,
+                                         transaction_id,
+                                         dhcp4::message_type::dhcprequest);
+  dump_hex(&buf[offset], dhcp4_length);
+  offset += dhcp4_length;
+}
diff --git a/udp4.cpp b/udp4.cpp
new file mode 100644
index 0000000..8a38d03
--- /dev/null
+++ b/udp4.cpp
@@ -0,0 +1,41 @@
+#include <stdio.h>
+#include <string.h>
+
+#include <arpa/inet.h>
+
+#include "ipv4.h"
+#include "udp4.h"
+
+namespace udp4 {
+  header * make_header(void * buf,
+                       uint32_t source_port,
+                       uint32_t destination_port,
+                       uint32_t source_address,
+                       uint32_t destination_address,
+                       void const * payload,
+                       int payload_length)
+  {
+    memset(buf, 0, (sizeof (header)));
+    header * hdr = reinterpret_cast<header *>(buf);
+
+    hdr->source_port = source_port;
+    hdr->destination_port = destination_port;
+    hdr->length = htons(payload_length + (sizeof (header)));
+
+    ipv4_psuedo_header phdr;
+    phdr.source_address = source_address;
+    phdr.destination_address = destination_address;
+    phdr.zeros = 0;
+    phdr.protocol = ipv4::protocol::udp;
+    phdr.udp_length = hdr->length; // already big endian
+
+    uint32_t sum = 0;
+    sum = ipv4::checksum_partial(sum, reinterpret_cast<void const *>(&phdr), (sizeof (ipv4_psuedo_header)));
+    sum = ipv4::checksum_partial(sum, reinterpret_cast<void const *>(hdr), (sizeof (header)));
+    sum = ipv4::checksum_partial(sum, reinterpret_cast<void const *>(payload), payload_length);
+
+    hdr->checksum = htons(ipv4::checksum_finish(sum));
+
+    return hdr;
+  }
+}
diff --git a/udp4.h b/udp4.h
new file mode 100644
index 0000000..628ee9f
--- /dev/null
+++ b/udp4.h
@@ -0,0 +1,32 @@
+#pragma once
+
+#include <stdint.h>
+
+#include "ipv4.h"
+
+namespace udp4 {
+  struct header {
+    uint16_t source_port;
+    uint16_t destination_port;
+    uint16_t length;
+    uint16_t checksum;
+  };
+
+  static_assert((sizeof (header)) == 8);
+
+  struct ipv4_psuedo_header {
+    uint32_t source_address;
+    uint32_t destination_address;
+    uint8_t zeros;
+    uint8_t protocol;
+    uint16_t udp_length;
+  };
+
+  header * make_header(void * buf,
+                       uint32_t source_port,
+                       uint32_t destination_port,
+                       uint32_t source_address,
+                       uint32_t destination_address,
+                       void const * payload,
+                       int payload_length);
+}