Last active
February 20, 2024 05:57
-
-
Save marten-seemann/a549773b53f30960b966a9f4068b6e48 to your computer and use it in GitHub Desktop.
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
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 | |
} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment