TCP Connections in Go

Question

Here is my code:

package main                                                                       

import (                                                                           
    "fmt"                                                                          
    "net"                                                                          
)                                                                                  

func main() {                                                                      
    addr, _ := net.ResolveTCPAddr("tcp", "127.0.0.1:8081")                         
    listener, _ := net.ListenTCP("tcp", addr)                                      
    fmt.Printf("listener addr: %s\n", listener.Addr().String())                    
    for {                                                                          
        conn, err := listener.AcceptTCP()                                          
        if err != nil {                                                            
            // handle error                                                        
            fmt.Println("err")                                                     
            return                                                                 
        }                                                                          
        go handleConnection(conn)                                                  
    }                                                                              
}                                                                                  

func handleConnection(conn *net.TCPConn) {                                         
    fmt.Printf("conn addr: %s\n", conn.LocalAddr().String())                       
    fmt.Printf("conn remote addr: %s\n", conn.RemoteAddr().String())               
}

Output

listener addr: 127.0.0.1:8081
conn addr: 127.0.0.1:8081
conn remote addr: 127.0.0.1:1234

Why do listener and conn have the same address? In TCP, I thought a new socket was created for new connections.

score 0 · Answer 1 · answered Mar 01 '15 at 21:35

0

No, your Listener is accepting connections in port 8081, so the LocalAddr is going to have that port. If you dial out to another server, you usually use a different port each time, though that's not required either.

answered Mar 01 '15 at 21:35

JimB

104,193
13
262
255

score 0 · Accepted Answer · answered Mar 01 '15 at 22:52

It got me puzzled for a second, but this is correct. A new socket is indeed created (with a unique local+remote address tuple). This quote from wikipedia describes it pretty well:

A server may create several concurrently established TCP sockets with the same local port number and local IP address, each mapped to its own server-child process, serving its own client process. They are treated as different sockets by the operating system, since the remote socket address (the client IP address and/or port number) are different; i.e. since they have different socket pair tuples.

If you think about it other way around, i.e. outgoing connection, you don't find it strange seeing remote address being the same across many sockets (e.g. google.com:80), so the same holds for incoming connections.

Probably a nice side effect of this is that tools like netstat, when inspecting sockets, are nicely showing the source port, instead of random pairs.

TCP Connections in Go

2 Answers2

Linked