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How can I check if a specific UDP port is open in golang?

Until now I have tried many methods, but no one worked. Precisely, all of them, just tell if the server is responding, no matter what port I input.

METHOD ONE


func methodOne(ip string, ports []string) map[string]string {
    // check emqx 1883, 8083 port

    results := make(map[string]string)
    for _, port := range ports {
        address := net.JoinHostPort(ip, port)
        // 3 second timeout
        conn, err := net.DialTimeout("udp", address, 3*time.Second)
        if err != nil {
            results[port] = "failed"
            // todo log handler
        } else {
            if conn != nil {
                results[port] = "success"
                _ = conn.Close()
            } else {
                results[port] = "failed"
            }
        }
    }
    return results
}

METHOD TWO


func ping(host string, port string) error {
    address := net.JoinHostPort(host, port)
    conn, err := net.DialTimeout("udp", address, 1*time.Second)
    if conn != nil {
        fmt.Println(conn.LocalAddr())
        defer conn.Close()
    }
    return err
}

METHOD THREE

From this package: https://github.com/janosgyerik/portping


portping -c 3 -net udp 0.0.0.0.0 80
Matteo
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    UDP is connectionless, none of those methods will work. If the port is open, it's up to the application layer to answer (or not). If it's closed, you **might** get an ICMP error back. The best you can do with UDP pings is a lot of assumptions and treat some things as a success. – Marc Jun 27 '20 at 09:49
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    For a non-language-specific answer, try https://serverfault.com/questions/193425/verify-connectivity-to-a-server-on-a-udp-port or https://stackoverflow.com/questions/9265669/how-to-test-a-remote-udp-port – Marc Jun 27 '20 at 09:50

2 Answers2

3

You can't unless you know for sure that the server will send something back, then you can try to catch the response.

Check this link https://ops.tips/blog/udp-client-and-server-in-go/

Fahim Bagar
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2

Since UDP doesn't provide anything like "connection", it seems to me that the only way to check if remote UDP server is running - is to send a meaningful message, which is understandable by the server and requires "response" to that. Then we just have to wait for server reply and if there is one - the server is OK.

Again, for UDP we have to make several sendings, because if server is down, it won't receive the packet.

Following all that logic i've implemented GO library/cli utility supporting both UDP and TCP library and working the way described above.

Anton Elmanov
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