Get the PID of a SSH controlmaster process

controlmaster_pid.go

package main

import (
	"bytes"
	"encoding/binary"
	"io"
	"log"
	"net"
	"os"
	"time"
)

var helloHandshake = []byte{
	// Length
	0, 0, 0, 8,
	// MSG_HELLO
	0, 0, 0, 1,
	// Protocol Version
	0, 0, 0, 4,
}

var aliveCheck = []byte{
	// Length
	0, 0, 0, 8,
	// MUX_C_ALIVE_CHECK
	16, 0, 0, 4,
	// Request ID
	1, 2, 3, 4,
}

var aliveResponse = []byte{
	// Length
	0, 0, 0, 12,
	// MUX_S_ALIVE
	128, 0, 0, 5,
	// Request ID
	1, 2, 3, 4,
	// PID
	0, 0, 0, 0,
}

func main() {
	path := os.Args[1]
	conn, err := net.DialTimeout("unix", path, 3*time.Second)
	if err != nil {
		log.Fatalf("%s: failed to dial ControlMaster: %v", path, err)
	}
	defer conn.Close()
	// Hello handshake
	if _, err := conn.Write(helloHandshake); err != nil {
		log.Fatalf("%s: failed to write hello handshake: %v", path, err)
	}
	// Read the response to our handshake and ensure it is same version
	helloReply := make([]byte, len(helloHandshake))
	if _, err := io.ReadFull(conn, helloReply); err != nil {
		log.Fatalf("%s: failed to read handshake response: %v", path, err)
	}
	if !bytes.Equal(helloHandshake, helloReply) {
		log.Fatalf("%s: did not recieve expected handshake response", path)
	}
	// Send the alive check
	if _, err := conn.Write(aliveCheck); err != nil {
		log.Fatalf("%s: failed to write alive check: %v", path, err)
	}
	// Read the alive response
	aliveReply := make([]byte, len(aliveResponse))
	if _, err := io.ReadFull(conn, aliveReply); err != nil {
		log.Fatalf("%s: failed to read alive response: %v", path, err)
	}
	if !bytes.Equal(aliveResponse[:12], aliveReply[:12]) {
		log.Fatalf("%s: did not recieve expected alive response", path)
	}
	pid := binary.BigEndian.Uint32(aliveReply[12:16])
	log.Printf("PID: %d", pid)
}