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https://github.com/tevino/tcp-shaker

:heartbeat: Perform TCP handshake without ACK in Go, useful for health check, that is SYN, SYN-ACK, RST.
https://github.com/tevino/tcp-shaker

ack go golang golang-library golang-package handshake haproxy health-check linux network packets tcp

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:heartbeat: Perform TCP handshake without ACK in Go, useful for health check, that is SYN, SYN-ACK, RST.

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# TCP Checker :heartbeat:



[![Go Report Card](https://goreportcard.com/badge/github.com/tevino/tcp-shaker)](https://goreportcard.com/report/github.com/tevino/tcp-shaker)

[![GoDoc](https://godoc.org/github.com/tevino/tcp-shaker?status.svg)](https://godoc.org/github.com/tevino/tcp-shaker)

[![Build Status](https://travis-ci.org/tevino/tcp-shaker.svg?branch=master)](https://travis-ci.org/tevino/tcp-shaker)



This package is used to perform TCP handshake without ACK, which useful for TCP health checking.



HAProxy does this exactly the same, which is:



1. SYN

2. SYN-ACK

3. RST



This implementation has been running on tens of thousands of production servers for years.



## Why do I have to do this



In most cases when you establish a TCP connection(e.g. via `net.Dial`), these are the first three packets between the client and server([TCP three-way handshake][tcp-handshake]):



1. Client -> Server: SYN

2. Server -> Client: SYN-ACK

3. Client -> Server: ACK



**This package tries to avoid the last ACK when doing handshakes.**



By sending the last ACK, the connection is considered established.



However, as for TCP health checking the server could be considered alive right after it sends back SYN-ACK,



that renders the last ACK unnecessary or even harmful in some cases.



### Benefits



By avoiding the last ACK



1. Less packets better efficiency

2. The health checking is less obvious



The second one is essential because it bothers the server less.



This means the application level server will not notice the health checking traffic at all, **thus the act of health checking will not be

considered as some misbehavior of client.**



## Requirements



- Linux 2.4 or newer



There is a **fake implementation** for **non-Linux** platform which is equivalent to:



```go

conn, err := net.DialTimeout("tcp", addr, timeout)

conn.Close()

```



The reason for a fake implementation is that there is currently no way to perform an equivalent operation on certain platforms, specifically macOS. A fake implementation is a degradation on those platforms and ensures a successful compilation.



## Usage



```go

import "github.com/tevino/tcp-shaker"



// Initializing the checker

// It is expected to be shared among goroutines, only one instance is necessary.

c := NewChecker()



ctx, stopChecker := context.WithCancel(context.Background())

defer stopChecker()

go func() {

	if err := c.CheckingLoop(ctx); err != nil {

		fmt.Println("checking loop stopped due to fatal error: ", err)

	}

}()



<-c.WaitReady()



// Checking google.com



timeout := time.Second * 1

err := c.CheckAddr("google.com:80", timeout)

switch err {

case ErrTimeout:

	fmt.Println("Connect to Google timed out")

case nil:

	fmt.Println("Connect to Google succeeded")

default:

	fmt.Println("Error occurred while connecting: ", err)

}

```



## TODO



- [x] IPv6 support (Test environment needed, PRs are welcome)



## Special thanks to contributors



- @lujjjh Added zero linger support for non-Linux platform

- @jakubgs Fixed compatibility on Android

- @kirk91 Added support for IPv6



[tcp-handshake]: https://en.wikipedia.org/wiki/Handshaking#TCP_three-way_handshake