Understanding "Connection Reset by Peer" in GoLang: A Troubleshooting Guide
Tags: go, networking, network, http
Published: 2023-06-02 03:16:32

## Introduction

When working with network connections in application, you may encounter the error message "Connection Reset by Peer". This error message can be confusing and frustrating, especially if you are not familiar with its meaning. In this troubleshooting guide, we will explore what "Connection Reset by Peer" means, what causes it, and how to fix it in your applications.

## What is "Connection Reset by Peer"?

"Connection Reset by Peer" is an error message that can occur when working with network connections. It indicates that the remote server or client closed the connection unexpectedly, resulting in the termination of the connection.

This error message can be caused by a variety of factors, such as network connectivity issues, misconfigured firewalls, or improper handling of network errors in your code. Some common causes of "Connection Reset by Peer" error messages include:

1. Network Issues: The client and server may be on different networks, and network-related problems such as firewalls, proxies, or routers could be interfering with the connection.
2. Server Overload: The server may be overwhelmed with requests, resulting in it terminating connections to maintain stability.
3. Incorrect Protocol Handling: If the client and server are using different protocols or versions, it can lead to unexpected behavior and connection resets.
4. Firewall or Security Configuration: A misconfigured firewall or security settings may reject or terminate connections, triggering the error.

Troubleshooting "Connection Reset by Peer":

1. Verify Connectivity: Ensure that both the client and server have a stable and reliable network connection. Check for any network-related issues or restrictions that might be causing the error.
2. Review Server Logs: Examine the server logs to identify any potential issues, such as server overload or resource exhaustion. These logs can provide valuable insights into the root cause of the connection reset.
3. Check Protocol Compatibility: Confirm that both the client and server are using the same protocol and compatible versions. Incompatible protocols can result in unexpected behavior and connection resets.
4. Modify Firewall or Security Settings: Temporarily disable firewalls or security measures to determine if they are causing the connection reset. If so, adjust the settings accordingly to allow the connection.
5. Implement Connection Retry Mechanism: To handle transient network issues or server overload situations, consider implementing a connection retry mechanism in your code. This allows the client to automatically reconnect and recover from connection resets.
6. Optimize Code and Resource Usage: Review your code to ensure that it is efficient and not unnecessarily consuming excessive resources. Optimize the code and resource usage to reduce the likelihood of encountering connection reset errors.

## Example

I was getting a code simple HTTP server and the client

```go
func client() {
	for i := 0; i < 100; i++ {
		res, err := http.Post("http://localhost:9000/test", "text/json", bytes.NewReader(nil))
		if err != nil {
			log.Printf("error sending: %s", err)
			continue
		}

		body, err := ioutil.ReadAll(res.Body)
		defer res.Body.Close()
		if err != nil {
			log.Printf("error reading: %s", err)
			continue
		}
		fmt.Printf("iteration #%0d read: %s", i, string(body))
		time.Sleep(time.Millisecond * 200)
	}
}

func srv() {
	mux := http.NewServeMux()
	mux.HandleFunc("/test", func(w http.ResponseWriter, req *http.Request) {
		fmt.Println(w, "test\n")
	})

	srv := &http.Server{
		Addr:        ":9000",
		Handler:     mux,
		IdleTimeout: time.Millisecond * 300,
	}
	srv.ListenAndServe()
}

func main() {
	go srv()
	time.Sleep(3 * time.Second)
	client()
}
```

When excusing this code, I get some errors:

```bash
2023/06/02 09:44:20 error sending: Post "http://localhost:9000/test": http: server closed idle connection
iteration #16 read: test
2023/06/02 09:44:20 error sending: Post "http://localhost:9000/test": EOF
iteration #18 read: test
2023/06/02 09:44:20 error sending: Post "http://localhost:9000/test": EOF
iteration #20 read: test
2023/06/02 09:44:20 error sending: Post "http://localhost:9000/test": read tcp 127.0.0.1:43258->127.0.0.1:9000: read: connection reset by peer
```

Cause****:**** 

HTTP 1.1 introduced the concept of persistent connections, enabling a client to reuse the same connection for multiple requests to a server. Instead of closing the connection immediately after receiving a response, the connection remains open, allowing subsequent requests to benefit from reduced latency and improved performance. This persistence is achieved by keeping the TCP connection alive.

Connection Closure and Non-Idempotent Requests:
In certain scenarios, a server may close the persistent connection unexpectedly, resulting in errors for the client. It is important to understand that non-idempotent requests, such as POST and PATCH, should not be retried when the connection is closed.

Idempotent requests are those that produce the same outcome regardless of how many times they are repeated. On the other hand, non-idempotent requests have the potential to cause side effects on the server with each execution. POST, for example, is commonly used to create new records, and multiple executions would result in the creation of multiple records.

Handling Connection Closure:
When a connection is closed, the client cannot be certain whether the request was successfully executed by the server. Hence, according to RFC 7230, which defines the HTTP 1.1 protocol, it is considered unsafe for the client to automatically retry non-idempotent requests such as POST. The client should instead treat the connection closure as a definitive response from the server and proceed accordingly.

> A request method is considered "idempotent" if the intended effect on the server of multiple identical requests with that method is the same as the effect for a single request [RFC 7230 4.2.2](https://datatracker.ietf.org/doc/html/rfc7231#section-4.2.2)
> 

Automatic retrying of non-idempotent requests could lead to unintended consequences, such as duplicate records being created on the server. Since the client cannot be certain whether the original request was processed by the server before the connection was closed, retrying the request would violate the idempotent nature of the operation.

My solution:

- Disable persistent connection on the client side
- Reduce connection timeout on the client side to be less than server’s

```go
  // Set Transport
	t := http.DefaultTransport.(*http.Transport).Clone()
	t.DisableKeepAlives = true
  // option 2
	// t.IdleConnTimeout = time.Millisecond * 100
	c := &http.Client{
		Transport: t,
	}
```

```go
res, err := c.Post("http://localhost:9000/test", "text/json", bytes.NewReader(nil))
```

Result:

```bash
iteration #0 read: test
iteration #1 read: test
....
iteration #98 read: test
iteration #99 read: test
```
[https://go.dev/play/p/Aw1PXsmm9lm](https://go.dev/play/p/Aw1PXsmm9lm)
## Conclusion

The "connection reset by peer" error is a common challenge faced by developers working with network programming. By understanding the causes, implementing proper troubleshooting techniques, and following preventive measures, you can effectively overcome this error and build robust and reliable network applications. Remember to leverage the power of GoLang's error-handling mechanisms and regularly monitor and optimize your server's performance to minimize the occurrence of connection reset errors.



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