行108-132是运行TCP/IP侦听器的主函数。行109使Go运行环境使用所有物理可用的CPU。
108 func main() {
109 runtime.GOMAXPROCS(runtime.NumCPU())
110 flag.Parse()
111 if flag.NFlag() != 3 {
112 fmt.Printf("usage: gotcpspy -host target_host -port target_port
-listen_post=local_port\n")
113 flag.PrintDefaults()
114 os.Exit(1)
115 }
116 target := net.JoinHostPort(*host, *port)
117 fmt.Printf("Start listening on port %s and
forwarding data to %s\n",
*listen_port, target)
118 ln, err := net.Listen("tcp", ":"+*listen_port)
119 if err != nil {
120 fmt.Printf("Unable to start listener, %v\n", err)
121 os.Exit(1)
122 }
123 conn_n := 1
124 for {
125 if conn, err := ln.Accept(); err == nil {
126 go process_connection(conn, conn_n, target)
127 conn_n += 1
128 } else {
129 fmt.Printf("Accept failed, %v\n", err)
130 }
131 }
132 }
就这些，只有132行。请注意：程序仅使用了Go语言自身的标准库。
现在准备运行：
go run gotcpspy.go -host pop.yandex.ru -port 110 -local_port 8080
输出应为：
Start listening on port 8080 and forwarding data to pop.yandex.ru:110
然后在另一个窗口运行：
telnet localhost 8080
然后回车，比如：USER test [ENTER] 和 PASS none [ENTER]。三个日志文件将被创建（当你运行时间戳会不同）。
双向十六进制导出日志文件 log-2012.04.20-19.55.17-0001-192.168.1.41 -49544-213.180.204.37-110.log:
Connected to pop.yandex.ru:110 at 2012.04.20-19.55.17
Received (#0, 00000000) 38 bytes from 192.168.1.41-49544
00000000 2b 4f 4b 20 50 4f 50 20 59 61 21 20 76 31 2e 30
|+OK POP Ya! v1.0|
00000010 2e 30 6e 61 40 32 36 20 48 74 6a 4a 69 74 63 50
|.0na@26 HtjJitcP|
00000020 52 75 51 31 0d 0a
|RuQ1..|
Sent (#0) to [--1]-8080
Received (#0, 00000000) 11 bytes from [--1]-8080
00000000 55 53 45 52 20 74 65 73 74 0d 0a
|USER test..|
Sent (#0) to 192.168.1.41-49544
Received (#1, 00000026) 23 bytes from 192.168.1.41-49544
00000000 2b 4f 4b 20 70 61 73 73 77 6f 72 64 2c 20 70 6c
|+OK password, pl|
00000010 65 61 73 65 2e 0d 0a
|ease...|
Sent (#1) to [--1]-8080
Received (#1, 0000000B) 11 bytes from [--1]-8080
00000000 50 41 53 53 20 6e 6f 6e 65 0d 0a
|PASS none..|
Sent (#1) to 192.168.1.41-49544
Received (#2, 0000003D) 72 bytes from 192.168.1.41-49544
00000000 2d 45 52 52 20 5b 41 55 54 48 5d 20 6c 6f 67 69
|-ERR [AUTH] logi|
00000010 6e 20 66 61 69 6c 75 72 65 20 6f 72 20 50 4f 50
|n failure or POP|
00000020 33 20 64 69 73 61 62 6c 65 64 2c 20 74 72 79 20
|3 disabled, try |
00000030 6c 61 74 65 72 2e 20 73 63 3d 48 74 6a 4a 69 74
|later. sc=HtjJit|
00000040 63 50 52 75 51 31 0d 0a
|cPRuQ1..|
Sent (#2) to [--1]-8080
Disconnected from 192.168.1.41-49544
Disconnected from [--1]-8080
Finished at 2012.04.20-19.55.17, duration 5.253979s