gso.go

package main

import (
	"encoding/binary"
	"log"
	"net"
	"syscall"

	"golang.org/x/sys/unix"
)

func main() {
	if err := run(); err != nil {
		log.Fatal(err)
	}
}

const size = 1337
const UDP_SEGMENT = 103

func run() error {
	addr, err := net.ResolveUDPAddr("udp", "0.0.0.0:1234")
	if err != nil {
		return err
	}
	conn, err := net.ListenUDP("udp", addr)
	if err != nil {
		return err
	}
	sconn, err := conn.SyscallConn()
	if err != nil {
		return err
	}
	if err := sconn.Control(func(fd uintptr) {
		unix.SetsockoptInt(int(fd), syscall.IPPROTO_UDP, UDP_SEGMENT, size)
	}); err != nil {
		return err
	}

	raddr, err := net.ResolveUDPAddr("udp", "8.8.8.8:53") // let's spam the Google DNS
	if err != nil {
		return err
	}

	// Write 50 kB with a single syscall.
	// The kernel / NIC will split this into UDP datagrams weighing 1337 bytes each.
	if _, _, err := conn.WriteMsgUDP(make([]byte, 50*1024), getCmsg(1337), raddr); err != nil {
		return err
	}
	return nil
}

// Generate a cmsg to set the GSO packet size.
// This only works on amd64.
// See https://github.com/golang/go/issues/59653.
//
// cm = CMSG_FIRSTHDR(&msg);
// cm->cmsg_level = SOL_UDP;
// cm->cmsg_type = UDP_SEGMENT;
// cm->cmsg_len = CMSG_LEN(sizeof(uint16_t));
// *((uint16_t *) CMSG_DATA(cm)) = gso_size;
func getCmsg(size int) []byte {
	b := make([]byte, 8+4+4+2)
	binary.LittleEndian.PutUint64(b[:8], 18)
	binary.LittleEndian.PutUint32(b[8:12], uint32(syscall.IPPROTO_UDP))
	binary.LittleEndian.PutUint32(b[12:16], uint32(UDP_SEGMENT))
	binary.LittleEndian.PutUint16(b[16:], uint16(size))
	return b
}