Listening to Random Available Port in Go

tech · Jan 4, 2022 · ~1 min
Photo by @mbaumi on Unsplash
Photo by @mbaumi on Unsplash

To use a random available port in Golang, you can use :0. I believe the port 0 would works for another language as well as I tried in python.

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$ python -m SimpleHTTPServer 0
Serving HTTP on 0.0.0.0 port 43481 ...

According to lifewire, port 0 is a non-ephemeral port that works as a wildcard that tells the system to find any available ports particularly in the Unix OS.

The Go Code

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package main

import (
    "log"
    "net"
    "net/http"
)

func createListener() (l net.Listener, close func()) {
    l, err := net.Listen("tcp", ":0")
    if err != nil {
        panic(err)
    }

    return l, func() {
        _ = l.Close()
    }
}

func main() {
    l, close := createListener()
    defer close()

    http.Handle("/", http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
        // handle like normal
    }))

    log.Println("listening at", l.Addr().(*net.TCPAddr).Port)
    http.Serve(l, nil)
}

Execute it:

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$ go run main.go 
2022/01/04 17:40:16 listening at 33845

Thank you for reading!

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