Usually, when you make outgoing TCP connections, you don't care about the source address, and you can just get a free ephemeral one.
Sometimes you do, and you want all your connections to reuse the same source ip:port, after all, TCP connections only need to be a unique 4 tuple.
Sometimes you want even more, you want to accept incoming connections on a port, and also make outgoing connections from the same port.
The long way round, with syscalls
socket: to get a new file descriptor, fdbind: to associate fd with local addressconnect: to associate fd with remote address 1func dial(la *syscall.SockaddrInet4) net.Conn {
2 syscall.ForkLock.Lock()
3 fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_STREAM, 0)
4 if err != nil {
5 panic(err)
6 }
7 syscall.ForkLock.Unlock()
8
9 err = syscall.Bind(fd, la)
10 if err != nil {
11 panic(err)
12 }
13
14 ra := &syscall.SockaddrInet4{Port: 9999}
15 copy(ra.Addr[:], net.IPv4(127, 0, 0, 1))
16
17 err := syscall.Connect(fd, ra)
18 if err != nil {
19 panic(err)
20 }
21
22 conn, err := net.FileConn(os.NewFile(uintptr(fd), ""))
23 if err != nil {
24 panic(err)
25 }
26
27 conn.Write([]byte("foo bar"))
28}
socket: to get a new file descriptorbind: to associate fd with local addresslisten: to allow incoming connectionsaccept: (?) 1func listen(sa *syscall.SockaddrInet4) {
2 syscall.ForkLock.Lock()
3 fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_STREAM, 0)
4 if err != nil {
5 panic(err)
6 }
7 syscall.ForkLock.Unlock()
8
9 err = syscall.Bind(fd, la)
10 if err != nil {
11 panic(err)
12 }
13
14 err = syscall.Listen(fd, 10)
15 if err != nil {
16 panic(err)
17 }
18
19 l, err := net.FileListener(os.NewFile(uintptr(fd), ""))
20 if err != nil {
21 panic(err)
22 }
23
24 for {
25 conn, err := l.Accept()
26 if err != nil {
27 panic(err)
28 }
29 conn.Write([]byte("hello world"))
30 }
31}
setsockopt: modify how the socket will behave, use before bind
there are 2 options we care about:
unix.SO_REUSEADDR: allows bind to reuse addresses for outgoing requests, also skips wait for timeoutunix.SO_REUSEPORT: allows multiple sockets to use the same address1err = syscall.SetsockoptInt(fd, syscall.SOL_SOCKET, unix.SO_REUSEADDR|unix.SO_REUSEPORT, 1)
2if err != nil {
3 panic(err)
4}
Thankfully net is flexible enough that
we don't need to do everything with syscalls
1func dial() {
2 la, err := net.ResolveTCPAddr("tcp", "127.0.0.1:9999")
3 if err != nil {
4 panic(err)
5 }
6
7 d := &net.Dialer{
8 LocalAddr: la,
9 Control: func(network, address string, c syscall.RawConn) error {
10 var err error
11 c.Control(func(fd uintptr) {
12 err = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, unix.SO_REUSEADDR|unix.SO_REUSEPORT, 1)
13 })
14 return err
15 },
16 }
17
18 conn, err := d.Dial("tcp", "127.0.0.1:10000")
19 if err != nil {
20 panic(err)
21 }
22
23 conn.Write([]byte("foo bar"))
24}
1func listen() {
2 lc := &net.ListenConfig{
3 Control: func(network, address string, c syscall.RawConn) error {
4 var err error
5 c.Control(func(fd uintptr) {
6 err = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, unix.SO_REUSEADDR|unix.SO_REUSEPORT, 1)
7 })
8 return err
9 },
10 }
11
12 l, err := lc.Listen(context.Background(), "tcp", "127.0.0.1:9999")
13 if err != nil {
14 panic(err)
15 }
16
17 for {
18 conn, err := l.Accept()
19 if err != nil {
20 panic(err)
21 }
22 conn.Write([]byte("hello world"))
23 }
24}