This article describes how to correctly specify the local IP address when using the net.DialTCP function of the Go language. When trying to specify a local IP address, you may encounter a "dial tcp [IP address]:[port number]: An invalid argument was supplied" error. This article will explain the cause of this problem and provide the correct solution.

Use net.DialTCP to specify the local IP address

When using the net.DialTCP function, the first parameter is the network type (eg "tcp"), the second parameter is the local address (laddr), and the third parameter is the remote address (raddr). In the problem description, I tried to use the net.DialTCP function to specify the local IP address, but encountered the "An invalid argument was supplied" error.

 package main

import (
    "fmt"
    "net"
)

func main() {
    var localaddr net.TCPAddr
    var remoteaddr net.TCPAddr
    localaddr.IP = net.ParseIP("192.168.1.104")
    localaddr.Port = 6000
    remoteaddr.IP = net.ParseIP("192.168.1.104")
    remoteaddr.Port = 5000

    if localaddr.IP == nil || remoteaddr.IP == nil {
        fmt.Println("error")
    }

    if _, err := net.DialTCP("tcp", &localaddr, &remoteaddr); err != nil {
        fmt.Println(err)
    }

    fmt.Println("End")
}

The above code snippet will report errors in some cases because Go's net package handles local addresses differently than some other implementations.

solution

When connecting to the local host, the net package allows the use of port numbers to simplify the specification of addresses. The following two ways of writing are equivalent:

• :5000
• :5000

Therefore, when the destination address is a local address, the port number can be used directly instead of the full IP address and port number. However, it should be noted that in the net.DialTCP function, the laddr parameter still requires a net.TCPAddr structure.

Here is the corrected code example:

 package main

import (
    "fmt"
    "net"
)

func main() {
    remoteaddr, err := net.ResolveTCPAddr("tcp", "192.168.1.104:5000")
    if err != nil {
        fmt.Println("ResolveTCPAddr error:", err)
        return
    }

    // If you need to specify the local address and port, you can create a TCPAddr structure localaddr, err := net.ResolveTCPAddr("tcp", "192.168.1.104:6000")
    if err != nil {
        fmt.Println("ResolveTCPAddr error:", err)
        return
    }

    conn, err := net.DialTCP("tcp", localaddr, remoteaddr)
    if err != nil {
        fmt.Println("DialTCP error:", err)
        return
    }
    defer conn.Close()

    fmt.Println("Connected to:", conn.RemoteAddr())
}

In this revised example, we use the net.ResolveTCPAddr function to parse the address string and convert it into a net.TCPAddr structure. If you need to specify a local address, use the same method to resolve it.

Things to note

• Make sure that the local IP address specified is a valid IP address on the machine.
• If you do not need to specify a local IP address, you can set the laddr parameter to nil and let the system automatically select it.
• In practical applications, errors need to be handled according to specific circumstances, such as network connection errors, address resolution errors, etc.

Summarize

This article describes how to correctly specify the local IP address when using the net.DialTCP function of the Go language. You can avoid "An invalid argument was supplied" errors by using the net.ResolveTCPAddr function to resolve the address string. At the same time, attention should also be paid to handling possible errors to ensure the robustness of the program.

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