Essential Guide to Troubleshooting Operational Issues for Full-Stack Developers
Published: 2024-06-12 08:57:22

In the ever-evolving landscape of software development, back-end development and operation related issues are inevitable and can seriously disrupt your workflow. Working within the complex infrastructure of modern systems often leads to unexpected problems that require prompt attention and resolution. As a full-stack developer, your ability to quickly diagnose and fix these issues is crucial. This guide offers practical insights into identifying and resolving a range of common errors. You'll find sample code and step-by-step debugging tips to help you swiftly and effectively get back on track.

## Networking Errors

### 1. Connection Reset by Peer

**Diagram:**

```plaintext
Client ---------------> Server
       Connection Reset by Peer
```

**Example Command:**

```sh
# Using curl to simulate connection reset
curl -kvvv http://example.com
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
* Recv failure: Connection reset by peer
* Closing connection 0
```

**Possible Causes:**
- Server crash or restart
- Network instability
- Timeout settings
- Proxy or firewall interference

**Remedies:**
1. **Check Server Logs**: Look for crashes or restarts.
2. **Network Stability**: Ensure there are no network disruptions.
3. **Timeout Settings**: Verify and adjust connection timeouts.
4. **Check Proxy and Firewall**: Ensure proxies and firewalls are not interfering.

### 2. Connection Refused

**Diagram:**

```plaintext
Client --------------X Server
       Connection Refused
```

**Example Command:**

```sh
# Using telnet to check connection
telnet example.com 80
```

**Possible Response:**

```plaintext
Trying 93.184.216.34...
telnet: Unable to connect to remote host: Connection refused
```

**Possible Causes:**
- Server not running
- Port not open
- Firewall blocking the connection
- Proxy server misconfiguration

**Remedies:**
1. **Server Running**: Ensure the server is running.
2. **Port Open**: Verify the server is listening on the correct port.
3. **Firewall Rules**: Check for firewall rules blocking the connection.
4. **Proxy Settings**: Verify proxy server settings.

### 3. Timeout Error (ETIMEOUT)

**Diagram:**

```plaintext
Client ---------------> Server
       Timeout
```

**Example Command:**

```sh
# Using curl with timeout
curl -kvvv http://example.com --max-time 5
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
* Operation timed out after 5001 milliseconds with 0 bytes received
* Closing connection 0
```

**Possible Causes:**
- Network latency
- Server overload
- Incorrect timeout settings
- Proxy server delays

**Remedies:**
1. **Network Latency**: Check network latency and bandwidth.
2. **Server Load**: Ensure the server is not overloaded.
3. **Adjust Timeouts**: Configure appropriate timeout settings.
4. **Check Proxy Server**: Ensure the proxy server is not causing delays.

### 4. Host Unreachable

**Diagram:**

```plaintext
Client ---------------> Network
       Host Unreachable
```

**Example Command:**

```sh
# Using ping to check host reachability
ping -c 4 unreachable-host
```

**Possible Response:**

```plaintext
ping: unreachable-host: Name or service not known
```

**Possible Causes:**
- Host is down
- Incorrect routing or firewall rules
- Network configuration issues
- Proxy server issues

**Remedies:**
1. **Check Host Availability**: Ensure the host is up and reachable.
2. **Routing/Firewall Rules**: Verify routing and firewall rules.
3. **Network Configuration**: Check for network configuration issues.
4. **Proxy Server**: Check proxy server settings.

### 5. Network Unreachable

**Diagram:**

```plaintext
Client ---------------> Network
       Network Unreachable
```

**Example Command:**

```sh
# Using ping to check network reachability
ping -c 4 example.com
```

**Possible Response:**

```plaintext
ping: connect: Network is unreachable
```

**Possible Causes:**
- Network configuration issues
- Physical network problems
- Network device failure
- Proxy server issues

**Remedies:**
1. **Network Configuration**: Check network settings and configurations.
2. **Physical Connections**: Inspect physical network connections.
3. **Network Devices**: Ensure network devices are functioning properly.
4. **Proxy Server**: Verify proxy server settings.

### 6. No Route to Host

**Diagram:**

```plaintext
Client ---------------> Network
       No Route to Host
```

**Example Command:**

```sh
# Using traceroute to check routing
traceroute no-route-host
```

**Possible Response:**

```plaintext
traceroute to no-route-host (93.184.216.34), 30 hops max, 60 byte packets
 1  * * *
 2  * * *
 ...
30  * * *
```

**Possible Causes:**
- Incorrect network configuration
- Network partitioning
- Routing issues
- Proxy server misconfiguration

**Remedies:**
1. **Network Configuration**: Verify and correct network configuration.
2. **Routing Table**: Ensure correct routing table entries.
3. **Network Segmentation**: Resolve any network segmentation issues.
4. **Proxy Server**: Check proxy server configuration.

### 7. Connection Aborted

**Diagram:**

```plaintext
Client ---------------> Server
       Connection Aborted
```

**Example Command:**

```sh
# Using curl to simulate connection
curl -kvvv http://example.com
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
* Recv failure: Connection reset by peer
* Closing connection 0
```

**Possible Causes:**
- Server error
- Network instability
- Server resource limits
- Proxy or firewall interference

**Remedies:**
1. **Server Logs**: Investigate server logs for error causes.
2. **Network Stability**: Check network stability and connectivity.
3. **Server Resources**: Ensure server resources are adequate.
4. **Proxy and Firewall**: Verify proxy and firewall settings.

### 8. Connection Reset

**Diagram:**

```plaintext
Client ---------------> Network
       Connection Reset
```

**Example Command:**

```sh
# Using curl to check connection
curl -kvvv http://example.com
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
* Recv failure: Connection reset by peer
* Closing connection 0
```

**Possible Causes:**
- Network device reset
- Network instability
- Connection settings
- Proxy server issues

**Remedies:**
1. **Network Devices**: Check network devices and their configurations.
2. **Network Stability**: Ensure network stability.
3. **Connection Settings**: Verify connection settings and protocols.
4. **Proxy Server**: Check proxy server settings.

### 9. Broken Pipe

**Diagram:**

```plaintext
Client ---------------> Server
       Broken Pipe
```

**Example Command:**

```sh
# Using curl to check connection
curl -kvvv http://example.com
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
* Recv failure: Connection reset by peer
* Closing connection 0
```

**Possible Causes:**
- Remote host closed connection
- Network issues
- Resource limits
- Proxy or firewall interference

**Remedies:**
1. **Remote Host Availability**: Ensure the remote host is available.
2. **Network Stability**: Check for network stability and connectivity.
3. **Resource Limits**: Ensure resource limits are not exceeded.
4. **Proxy and Firewall**: Verify proxy and firewall settings.

### 10. Protocol Error



**Diagram:**

```plaintext
Client ---------------> Server
       Protocol Error
```

**Example Command:**

```sh
# Using curl with invalid protocol
curl -kvvv http://example.com --http2
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
< HTTP/1.1 400 Bad Request
< Content-Type: text/html; charset=UTF-8
< Content-Length: 155
< Connection: close
< 
<html><head><title>400 Bad Request</title></head><body>400 Bad Request</body></html>
```

**Possible Causes:**
- Incompatible protocol versions
- Corrupted data packets
- Incorrect protocol configuration

**Remedies:**
1. **Protocol Versions**: Ensure compatible protocol versions.
2. **Data Integrity**: Verify data integrity during transmission.
3. **Configuration**: Check for proper protocol configuration.

### 11. SSL/TLS Handshake Failure

**Diagram:**

```plaintext
Client ---------------> Server
       SSL/TLS Handshake Failure
```

**Example Command:**

```sh
# Using curl to check SSL handshake
curl -kvvv https://example.com
```

**Possible Response:**

```plaintext
* Rebuilt URL to: https://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS alert, Server hello (2):
* error:14077410:SSL routines:SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure
* Closing connection 0
```

**Possible Causes:**
- Certificate mismatch/expiration
- Incompatible SSL/TLS versions
- Incorrect SSL/TLS configuration

**Remedies:**
1. **Certificate Validity**: Ensure the certificate is valid and not expired.
2. **Protocol Compatibility**: Verify that both client and server support the same SSL/TLS protocols.
3. **Check Configuration**: Ensure correct SSL/TLS configuration on the server.

### 12. OpenSSL Error

**Diagram:**

```plaintext
Client ---------------> Server
       OpenSSL Error
```

**Example Command:**

```sh
# Using openssl to test SSL connection
openssl s_client -connect example.com:443
```

**Possible Response:**

```plaintext
CONNECTED(00000003)
140735194809440:error:14077410:SSL routines:SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure:s23_clnt.c:586:
---
no peer certificate available
---
No client certificate CA names sent
---
SSL handshake has read 7 bytes and written 289 bytes
---
New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported
Compression: NONE
Expansion: NONE
No ALPN negotiated
SSL-Session:
    Protocol  : TLSv1.2
    Cipher    : 0000
    Session-ID: 
    Session-ID-ctx: 
    Master-Key: 
    Key-Arg   : None
    PSK identity: None
    PSK identity hint: None
    SRP username: None
    Start Time: 1615393671
    Timeout   : 7200 (sec)
    Verify return code: 0 (ok)
---
```

**Possible Causes:**
- Invalid certificates
- Unsupported protocols
- Incorrect SSL/TLS configuration

**Remedies:**
1. **Valid Certificates**: Ensure valid and compatible certificates.
2. **Supported Protocols**: Verify support for required SSL/TLS protocols.
3. **Configuration**: Check SSL/TLS configuration on both client and server.

### 13. SIGTERM (Signal Termination)

**Diagram:**

```plaintext
Process -------------> SIGTERM
       Graceful Termination
```

**Example Command:**

```sh
# Using kill to send SIGTERM
kill -SIGTERM <process_id>
```

**Possible Causes:**
- Manual/system shutdown
- Restart initiated

**Remedies:**
1. **Graceful Handling**: Ensure the application handles SIGTERM gracefully.
2. **Investigate Termination**: Determine why the SIGTERM signal was sent.
3. **Perform Cleanup**: Ensure proper cleanup before termination.

### 14. SIGKILL (Signal Kill)

**Diagram:**

```plaintext
Process -------------> SIGKILL
       Immediate Termination
```

**Example Command:**

```sh
# Using kill to send SIGKILL
kill -SIGKILL <process_id>
```

**Possible Causes:**
- Manual kill command
- Out-of-memory (OOM) killer

**Remedies:**
1. **Avoid SIGKILL**: Use SIGTERM instead to allow graceful shutdown.
2. **Optimize Memory Usage**: Ensure the application does not exhaust memory.
3. **Investigate OOM Killer**: Check if the Out-of-Memory killer terminated the process.

### 15. Exited with Code 0

**Diagram:**

```plaintext
Process ---------------> Exit
       Code 0 (Success)
```

**Example Command:**

```sh
# Using shell script to exit with code 0
echo "Task completed successfully"
exit 0
```

**Possible Causes:**
- Normal completion of the process

**Remedies:**
1. **Normal Completion**: Verify that the process completed as expected.
2. **No Action Needed**: No further action required for code 0.

### 16. Exited with Code 1

**Diagram:**

```plaintext
Process ---------------> Exit
       Code 1 (Error)
```

**Example Command:**

```sh
# Using shell script to exit with code 1
echo "An error occurred"
exit 1
```

**Possible Causes:**
- Unhandled exception
- Incorrect configuration

**Remedies:**
1. **Review Logs**: Check application logs for error details.
2. **Implement Error Handling**: Ensure proper error handling and logging.
3. **Verify Configuration**: Check application configuration for issues.

### 17. Exited with Code 137

**Diagram:**

```plaintext
Process ---------------> Exit
       Code 137 (SIGKILL)
```

**Example Command:**

```sh
# Using shell script to simulate SIGKILL
kill -SIGKILL $$
```

**Possible Causes:**
- Manual kill command
- OOM killer

**Remedies:**
1. **Investigate Logs**: Check for logs indicating the reason for SIGKILL.
2. **Optimize Memory Usage**: Prevent the Out-of-Memory killer from terminating the process.
3. **Avoid SIGKILL**: Use SIGTERM for graceful shutdown.

### 18. Exited with Code 143

**Diagram:**

```plaintext
Process ---------------> Exit
       Code 143 (SIGTERM)
```

**Example Command:**

```sh
# Using shell script to simulate SIGTERM
kill -SIGTERM $$
```

**Possible Causes:**
- Graceful termination
- Manual or system shutdown

**Remedies:**
1. **Graceful Handling**: Ensure the application handles SIGTERM gracefully.
2. **Investigate Termination**: Determine why the SIGTERM signal was sent.
3. **Perform Cleanup**: Ensure proper cleanup before termination.

### 19. ETIMEOUT

**Diagram:**

```plaintext
Client ---------------> Server
       ETIMEOUT
```

**Example Command:**

```sh
# Using curl to simulate timeout
curl -kvvv http://example.com --max-time 5
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
* Operation timed out after 5001 milliseconds with 0 bytes received
* Closing connection 0
```

**Possible Causes:**
- Slow network
- Server delay
- Incorrect timeout settings

**Remedies:**
1. **Network Performance**: Check network latency and performance.
2. **Optimize Server Response**: Ensure the server responds promptly.
3. **Adjust Timeouts**: Configure appropriate timeout settings.

### 20. Socket Connection Error

**Diagram:**

```plaintext
Client ---------------> Server
       Socket Connection Error
```

**Example Command:**

```sh
# Using curl to check connection
curl -kvvv http://example.com
```

**Possible Response:**

```plaintext
* Rebuilt URL to: http://example.com/
*   Trying 93.184.216.34...
* TCP_NODELAY set
* Connected to example.com (93.184.216.34) port 80 (#0)
> GET / HTTP/1

.1
> Host: example.com
> User-Agent: curl/7.68.0
> Accept: */*
* Recv failure: Connection reset by peer
* Closing connection 0
```

**Possible Causes:**
- Network problems
- Server issues
- Socket handling errors

**Remedies:**
1. **Network and Server Logs**: Check logs for error details.
2. **Socket Handling**: Ensure proper socket handling in the application.
3. **Network Stability**: Verify network stability and connectivity.

### 21. OCP Container CrashLoopBackOff

**Diagram:**

```plaintext
Client ---------------> OpenShift
       CrashLoopBackOff
```

**Example Code:**

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: crash-loop-pod
spec:
  containers:
  - name: crash-loop-container
    image: busybox
    command: ["sh", "-c", "exit 1"]
```

**Possible Causes:**
- Application error
- Misconfiguration

**Remedies:**
1. **Application Logs**: Check application logs for error details.
2. **Configuration**: Verify container configuration.
3. **Resource Limits**: Ensure adequate resources are allocated.

### 22. OCP Container ImagePullBackOff

**Diagram:**

```plaintext
Client ---------------> OpenShift
       ImagePullBackOff
```

**Example Code:**

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: image-pull-backoff-pod
spec:
  containers:
  - name: missing-image-container
    image: non-existent-image:latest
```

**Possible Causes:**
- Image not found
- Network issues
- Registry access problems

**Remedies:**
1. **Image Availability**: Verify image availability in the registry.
2. **Network Access**: Ensure network access to the registry.
3. **Registry Credentials**: Check registry credentials for authentication.

### 23. OCP Container Not Ready

**Diagram:**

```plaintext
Client ---------------> OpenShift
       Container Not Ready
```

**Example Code:**

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: not-ready-pod
spec:
  containers:
  - name: ready-check-container
    image: busybox
    readinessProbe:
      exec:
        command:
        - cat
        - /tmp/healthy
      initialDelaySeconds: 5
      periodSeconds: 5
```

**Possible Causes:**
- Initialization issues
- Readiness probe failures
- Dependency problems

**Remedies:**
1. **Initialization Logs**: Check container initialization logs.
2. **Readiness Probes**: Verify readiness probe configurations.
3. **Dependencies**: Ensure all dependencies are available and accessible.

### 24. Docker Daemon Not Running

**Diagram:**

```plaintext
Client ---------------> Docker
       Docker Daemon Not Running
```

**Example Command:**

```sh
# Check Docker daemon status
sudo systemctl status docker
```

**Possible Causes:**
- Docker service not started
- Docker crashes

**Remedies:**
1. **Start Docker Service**: `sudo systemctl start docker`
2. **Check Docker Logs**: `sudo journalctl -u docker.service`
3. **Restart Docker Service**: `sudo systemctl restart docker`

### 25. Image Not Found

**Diagram:**

```plaintext
Client ---------------> Docker Hub
       Image Not Found
```

**Example Command:**

```sh
# Using docker to pull an image
docker pull non-existent-image:latest
```

**Possible Response:**

```plaintext
Error response from daemon: manifest for non-existent-image:latest not found: manifest unknown: manifest unknown
```

**Possible Causes:**
- Image not available in registry
- Incorrect image name or tag

**Remedies:**
1. **Verify Image Name**: Ensure the image name and tag are correct.
2. **Check Registry**: Confirm the image exists in the Docker registry.
3. **Authenticate**: If the image is private, ensure proper authentication.

### 26. Container Not Starting

**Diagram:**

```plaintext
Client ---------------> Docker
       Container Not Starting
```

**Example Command:**

```sh
# Using docker logs to check container output
docker logs <container_id>
```

**Possible Response:**

```plaintext
Error response from daemon: Container <container_id> is not running
```

**Possible Causes:**
- Configuration errors
- Missing dependencies
- Insufficient resources

**Remedies:**
1. **Check Container Logs**: `docker logs <container_id>` for error details.
2. **Verify Configuration**: Ensure correct container configuration.
3. **Dependencies**: Verify all dependencies are available.

### 27. Volume Mount Error

**Diagram:**

```plaintext
Client ---------------> Docker
       Volume Mount Error
```

**Example Command:**

```sh
# Using docker to run a container with volume
docker run -v /invalid/host/path:/container/path busybox
```

**Possible Response:**

```plaintext
docker: Error response from daemon: invalid volume specification: '/invalid/host/path:/container/path'.
```

**Possible Causes:**
- Incorrect volume syntax
- Permission issues
- Invalid paths

**Remedies:**
1. **Verify Volume Syntax**: Ensure correct volume mount syntax.
2. **Check Permissions**: Verify file and directory permissions.
3. **Correct Paths**: Ensure host paths exist and are accessible.

### 28. Network Issues

**Diagram:**

```plaintext
Client ---------------> Docker
       Network Issues
```

**Example Command:**

```sh
# Using docker to inspect network
docker network inspect <network_name>
```

**Possible Response:**

```plaintext
[
    {
        "Name": "<network_name>",
        "Id": "9d8e4d8a4f8a4e8b4d8e4a",
        "Created": "2021-03-10T10:00:00.000000000Z",
        "Scope": "local",
        "Driver": "bridge",
        ...
    }
]
```

**Possible Causes:**
- Network misconfiguration
- Conflicting network names
- Network device issues

**Remedies:**
1. **Check Network Settings**: Verify Docker network settings.
2. **Resolve Conflicts**: Address any conflicting network names.
3. **Inspect Network**: Use `docker network inspect` for details.

### 29. Permission Denied

**Diagram:**

```plaintext
Client ---------------> Docker
       Permission Denied
```

**Example Command:**

```sh
# Using docker to run a container with volume
docker run -v /restricted/host/path:/container/path busybox
```

**Possible Response:**

```plaintext
docker: Error response from daemon: error while creating mount source path '/restricted/host/path': mkdir /restricted/host/path: permission denied.
```

**Possible Causes:**
- Incorrect user permissions
- SELinux/AppArmor restrictions
- Invalid path permissions

**Remedies:**
1. **Adjust Permissions**: Ensure proper file and directory permissions.
2. **Configure SELinux/AppArmor**: Properly configure security settings.
3. **Verify User Access**: Ensure the Docker user has necessary permissions.

### 30. Out of Memory (OOM) Error

**Diagram:**

```plaintext
Client ---------------> Docker
       Out of Memory (OOM)
```

**Example Command:**

```sh
# Using docker to run a memory-intensive container
docker run -m 512m --memory-swap 512m memory-hog
```

**Possible Response:**

```plaintext
docker: Error response from daemon: container memory limit exceeded.
```

**Possible Causes:**
- Insufficient memory allocation
- Memory leaks
- Resource limits exceeded

**Remedies:**
1. **Increase Memory Limits**: Allocate more memory to the container.
2. **Optimize Memory Usage**: Ensure the application uses memory efficiently.
3. **Monitor Resources**: Use monitoring tools to track memory usage.

### 31. Port Already in Use

**Diagram:**

```plaintext
Client ---------------> Docker
       Port Already in Use
```

**Example Command:**

```sh
# Using docker to run a container with port mapping
docker run -p 80:80 nginx
```

**Possible Response:**

```plaintext
docker: Error response from daemon: driver failed programming external connectivity on endpoint <container_name> (e6c8e6c8e6c8): Bind for 0.0.0.0:80 failed: port is already allocated.
```

**Possible Causes:**
- Port conflict with another service or container
- Port already in use
- Incorrect port mapping

**Remedies:**
1. **Use Different Ports**: Change to a different port if a conflict exists.
2. **Stop Conflicting Services**: Stop other services using the same port.
3. **Check Port Bindings**: Verify current port bindings.

### 32. DNS Resolution Failure

**Diagram:**

```plaintext
Client ---------------> Docker
       DNS Resolution Failure
```

**Example Command:**

```sh
# Using docker to run a container and check DNS resolution
docker run busybox nslookup google.com
```

**Possible Response:**

```plaintext
Server:    8.8.8.8
Address 1: 8.8.8.8 dns.google

Name:      google.com
Address 1: 172.217.16.206 fra16s29-in-f14.1

e100.net
```

**Possible Causes:**
- Incorrect DNS settings
- Network issues
- DNS server problems

**Remedies:**
1. **Configure DNS Settings**: Ensure correct DNS configuration.
2. **Check Network Connectivity**: Verify network connectivity.
3. **Inspect DNS Logs**: Check DNS server logs for issues.

### 33. Image Pull Rate Limit

**Diagram:**

```plaintext
Client ---------------> Docker Hub
       Image Pull Rate Limit
```

**Example Command:**

```sh
# Using docker to pull an image
docker pull busybox
```

**Possible Response:**

```plaintext
Error response from daemon: toomanyrequests: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit
```

**Possible Causes:**
- Exceeding the free tier limit on Docker Hub

**Remedies:**
1. **Authenticate to Docker Hub**: Use `docker login` for authentication.
2. **Consider Paid Plan**: Upgrade to a paid plan for higher rate limits.
3. **Use Private Registry**: Host images on a private Docker registry.

### 34. `deb.debian.org` Timeout Error

**Diagram:**

```plaintext
Client ---------------> deb.debian.org
       Timeout
```

**Example Dockerfile:**

```Dockerfile
# Dockerfile
FROM node:10
RUN apt-get update && apt-get install -y curl
```

**Possible Response:**

```plaintext
Err:1 http://deb.debian.org/debian buster InRelease
  Could not connect to deb.debian.org:80 (151.101.0.204), connection timed out
```

**Possible Causes:**
- Network latency
- Server issues at `deb.debian.org`
- Outdated image base

**Remedies:**
1. **Update Base Image**: Use a newer Node.js image version.
2. **Use Alternative Mirrors**: Modify `/etc/apt/sources.list` to use different mirrors.
3. **Increase Timeout**: Adjust timeout settings in the Dockerfile or build script.

**Example Solution:**

```Dockerfile
# Updated Dockerfile
FROM node:14-buster

RUN sed -i 's/deb.debian.org/ftp.debian.org/' /etc/apt/sources.list

RUN apt-get update && apt-get install -y curl
```

### 35. Load Balancer 502 Bad Gateway

**Diagram:**

```plaintext
Client ---------------> Load Balancer ---------------> Server
       502 Bad Gateway
```

**Example Command:**

```sh
# Using curl to check load balancer status
curl -kvvv http://loadbalancer.example.com
```

**Possible Causes:**
- Backend server down
- Incorrect backend configuration
- Timeout issues

**Remedies:**
1. **Check Backend Servers**: Ensure backend servers are running and reachable.
2. **Verify Configuration**: Check load balancer and backend server configurations.
3. **Timeout Settings**: Adjust timeout settings on the load balancer.

### 36. Proxy Server 504 Gateway Timeout

**Diagram:**

```plaintext
Client ---------------> Proxy Server ---------------> Server
       504 Gateway Timeout
```

**Example Command:**

```sh
# Using curl to check proxy server status
curl -kvvv http://proxy.example.com
```

**Possible Causes:**
- Slow backend server response
- Proxy server timeout settings
- Network latency

**Remedies:**
1. **Backend Response**: Ensure backend servers respond promptly.
2. **Adjust Timeout**: Increase timeout settings on the proxy server.
3. **Network Latency**: Check and optimize network latency.

### 37. Proxy Server Authentication Error

**Diagram:**

```plaintext
Client ---------------> Proxy Server
       Authentication Error
```

**Example Command:**

```sh
# Using curl to test proxy authentication
curl -U user:password -kvvv http://proxy.example.com
```

**Possible Response:**

```plaintext
* Proxy auth using Basic with user 'user'
* Proxy error: "HTTP/1.1 407 Proxy Authentication Required"
```

**Possible Causes:**
- Incorrect credentials
- Proxy server misconfiguration
- Authentication method mismatch

**Remedies:**
1. **Verify Credentials**: Ensure correct username and password.
2. **Check Proxy Configuration**: Verify proxy server settings.
3. **Authentication Method**: Ensure the client and proxy support the same authentication method.

## Summary Table

Here's the summarized table of errors with proper formatting:

| Error                           | Description                         | Possible Causes                                                                                | Remedies                                                                                                           |
|---------------------------------|-------------------------------------|------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------|
| Connection Reset by Peer        | Remote server closed connection     | Server crash, timeout, network instability, proxy or firewall interference                      | Ensure server stability, adjust timeout settings, check network stability, verify proxy and firewall settings     |
| Connection Refused              | Connection attempt refused          | Server not running, port not open, firewall blocking, proxy server misconfiguration              | Start the server, open the port, check firewall settings, verify proxy server settings                            |
| Timeout Error (ETIMEOUT)        | Operation exceeded timeout duration | Slow network, server overload, incorrect timeout settings, proxy server delays                   | Check network latency, optimize server, adjust timeout settings, check proxy server                               |
| Host Unreachable                | Cannot reach specified host         | Host down, incorrect routing/firewall rules, network configuration issues, proxy server issues   | Verify host availability, check routing and firewall rules, check network configuration, verify proxy server      |
| Network Unreachable             | Network unreachable from client     | Network configuration issues, physical network problems, network device failure, proxy server issues | Check network configuration, inspect physical connections, ensure network devices function properly, check proxy server |
| No Route to Host                | No route to specified host          | Incorrect network configuration, network partitioning, routing issues, proxy server misconfiguration | Correct network configuration, resolve segmentation issues, check proxy server configuration                      |
| Connection Aborted              | Connection aborted by host          | Server error, network instability, server resource limits, proxy or firewall interference         | Investigate server logs, check network stability, ensure server resources are adequate, verify proxy and firewall settings |
| Connection Reset                | Connection reset by network         | Network device reset, instability, connection settings, proxy server issues                      | Check network devices, ensure stability, verify connection settings and protocols, check proxy server settings    |
| Broken Pipe                     | Connection broken while writing     | Remote host closed connection, network issues, resource limits, proxy or firewall interference    | Ensure remote host availability, check network stability, ensure resource limits are not exceeded, verify proxy and firewall settings |
| Protocol Error                  | Network protocol mismatch/issue     | Incompatible protocol versions, corrupted data packets, incorrect protocol configuration         | Use compatible protocols, check data integrity, verify protocol configuration                                    |
| SSL/TLS Handshake Failure       | SSL/TLS handshake failed            | Certificate mismatch/expiration, incompatible SSL/TLS versions, incorrect SSL/TLS configuration  | Ensure valid certificates, verify SSL/TLS configurations                                                          |
| OpenSSL Error                   | General OpenSSL error               | Invalid certificates, unsupported protocols, incorrect SSL/TLS configuration                      | Use valid certificates, ensure protocol support, check SSL/TLS configuration                                     |
| SIGTERM (Signal Termination)    | Graceful termination of process     | Manual/system shutdown, restart                                                                  | Ensure graceful handling, investigate cause of termination, perform cleanup                                      |
| SIGKILL (Signal Kill)           | Immediate termination of process    | Manual kill, Out-of-memory (OOM) killer                                                          | Avoid SIGKILL, optimize memory usage, investigate OOM killer                                                     |
| Exited with Code 0              | Process terminated successfully     | Normal completion                                                                                | No action needed                                                                                                  |
| Exited with Code 1              | Process terminated with error       | Unhandled exception, incorrect configuration                                                     | Review logs, implement error handling, verify configuration                                                      |
| Exited with Code 137            | Process terminated by SIGKILL       | Manual kill, OOM killer                                                                          | Investigate logs, optimize memory usage, avoid SIGKILL                                                           |
| Exited with Code 143            | Process terminated by SIGTERM       | Graceful termination                                                                             | Ensure proper handling and cleanup                                                                               |
| ETIMEOUT                        | Operation timed out                 | Slow network, server delay                                                                       | Check network performance, optimize server response, adjust timeout settings                                     |
| Socket Connection Error         | General socket connection issue     | Network problems, server issues, socket handling errors                                          | Check network and server logs, ensure proper socket handling, verify network stability                           |
| OCP Container CrashLoopBackOff  | Container continuously crashing     | Application error, misconfiguration                                                              | Check application logs, correct configuration, ensure adequate resources                                         |
| OCP Container ImagePullBackOff  | Failure to pull container image     | Image not found, network issues, registry access problems                                        | Verify image availability, check network/registry access, verify registry credentials                            |
| OCP Container Not Ready         | Container not ready for traffic     | Initialization issues, readiness probe failures, dependency problems                             | Investigate initialization logs, check readiness probe settings, ensure dependencies are available               |
| Docker Daemon Not Running       | Docker commands fail                | Docker service not started, crashes                                                              | Start Docker service, check daemon logs, restart Docker service                                                  |
| Image Not Found                 | Docker cannot find specified image  | Image not available in registry, incorrect name/tag                                              | Verify image name and tag, ensure image exists in registry, ensure proper authentication                         |
| Container Not Starting          | Container fails to start            | Configuration errors, missing dependencies, insufficient resources                                | Check container logs (`docker logs <container_id>`), verify configuration, ensure dependencies are available    |
| Volume Mount Error              | Error mounting volumes in Docker    | Incorrect volume syntax, permission issues, invalid paths                                         | Verify volume syntax, check file and directory permissions, ensure host paths exist and are accessible            |
| Network Issues                  | Docker container network issues     | Network misconfiguration, conflicting network names, network device issues                        | Check network settings, resolve conflicts, inspect network (`docker network inspect <network_name>`)              |
| Permission Denied               | Permission issues accessing files   | Incorrect user permissions, SELinux/AppArmor restrictions, invalid path permissions               | Adjust file permissions, configure SELinux/AppArmor properly, verify user access                                 |
| Out of Memory (OOM) Error       | Container killed due to memory      | Insufficient memory allocation, memory leaks, resource limits exceeded                            | Increase memory limits, optimize application memory usage, monitor resources                                     |
| Port Already in Use             | Port conflict                       | Port conflict with another service or container, port already in use, incorrect port mapping      | Use different ports, stop conflicting services, verify current port bindings                                     |
| DNS Resolution Failure          | Cannot resolve DNS names            | Incorrect DNS settings, network issues, DNS server problems                                       | Configure DNS settings, ensure network connectivity, check DNS server logs                                       |
| Image Pull Rate Limit           | Docker Hub rate limits              | Exceeding the free tier limit on Docker Hub                                                       | Authenticate to Docker Hub, consider a paid plan for higher limits, use private registry                         |
| `deb.debian.org` Timeout Error  | Timeout pulling packages            | Network latency, server issues at `deb.debian.org`, outdated image base                           | Update Node.js image, use alternative mirrors, increase timeout settings                                         |
| Load Balancer 502 Bad Gateway   | Bad gateway error from load balancer| Backend server down, incorrect backend configuration, timeout issues                              | Ensure backend servers are running and reachable, check load balancer and backend configurations, adjust timeout settings |
| Proxy Server 504 Gateway Timeout| Gateway timeout error from proxy    | Slow backend server response, proxy server timeout settings, network latency                      | Ensure backend servers respond promptly, increase timeout settings on proxy server, optimize network latency      |
| Proxy Server Authentication Error | Authentication failure with proxy | Incorrect credentials, proxy server misconfiguration, authentication method mismatch              | Verify credentials, check proxy server settings, ensure client and proxy support the same authentication method |

## Conclusion

By following the outlined steps, diagrams, and example code, you can quickly diagnose issues and implement effective solutions. This comprehensive guide provides a valuable reference for improving application reliability and performance.

Feel free to share your thoughts and experiences in the comments below!