A 502 bad gateway error occurs when a web server acts as a gateway or proxy and receives an invalid response from the upstream server it accessed. This often prevents the web server from sending a response back to the client. Some common causes and fixes for a 502 bad gateway error are:
What is a 502 Bad Gateway Error?
The 502 bad gateway error is an HTTP status code that means the server acting as a gateway or proxy received an invalid response from the upstream server it accessed to handle the HTTP request. As a result, the gateway server cannot process the request properly and send back a response to the client.
The 502 status code comes from the Hypertext Transfer Protocol (HTTP) used on the web and defined in RFC 7231. The code stands for “Bad Gateway” and is categorized as a server error, meaning the source of the problem is the server, not the client.
Some key things to know about 502 errors:
- They occur when a server is acting as a gateway or proxy, not when processing requests directly.
- They indicate the gateway got an invalid or empty response from the upstream server.
- They prevent the gateway from generating a response to send to the requesting client.
- They are often intermittent, can come and go.
Common Causes of 502 Errors
There are a few common causes and scenarios that typically lead to 502 bad gateway errors occurring:
1. Problems with Upstream Server
The most common trigger for 502 errors is that the upstream server the gateway connected to sent an invalid response or no response at all. This could happen because of any of these issues with the upstream server:
- Overloaded and unable to respond
- Crashed, frozen, or hung
- Rebooted during the request
- Firewalled, blocking access
- Misconfigured, blocking or rejecting requests
If the upstream server fails to send back a proper response for any reason, it leads to a 502 from the gateway server.
2. Connectivity Issues
Problems with the network connection between the gateway server and upstream server can also easily cause 502 errors. Some examples include:
- Unstable connections or intermittent loss of connectivity
- High packet loss and latency
- Network equipment failures like router, switch, or firewall issues
- DNS failures leading to inability to resolve upstream host
- TCP/IP failures leading to inability to communicate
If the gateway becomes disconnected from the upstream server while handling the request, it will often result in a 502 error.
3. Server Misconfigurations
In some cases, 502 errors occur due to misconfigurations on the gateway or proxy server itself. Some misconfigurations that can trigger 502 errors include:
- Upstream server IP or hostname is misconfigured
- Upstream server port is incorrect
- TLS/SSL settings are wrong, preventing connectivity
- Timeout values are set too low
- Resources like memory, CPU are exhausted
Double checking the gateway server’s configuration against the upstream server can uncover the issue.
4. Application Bugs and Errors
Software bugs and errors in the web app code itself can also lead to invalid or empty responses being generated upstream:
- Upstream app crashing or aborting response
- Code bugs leading to blank or partial responses
- Errors in business logic or database queries
If the application running on the upstream server has bugs that prevent it from sending back a valid response, it can result in 502 errors on the gateway.
How to Fix 502 Bad Gateway Errors
Fixing 502 errors generally involves troubleshooting and resolving the underlying problem preventing proper responses from the upstream server. Here are some troubleshooting tips:
1. Check Upstream Server Health
First step is verifying the upstream server is running properly and not overloaded, crashed, or having connectivity issues. Try direct connections to the upstream host and monitor its logs:
- Ping upstream host to check if reachable
- Telnet to upstream ports to check for connectivity issues
- Check upstream server logs for crashes, failures, or overload conditions
- Monitor upstream health metrics like memory, CPU, disk, network
Resolve any upstream health or load issues to see if it fixes the 502 errors.
2. Verify Configurations Match
Double check the gateway server’s configurations match how the upstream server is listening:
- Confirm upstream host IP/hostname is correct
- Check gateway server is connecting to right upstream port
- Verify any TLS/SSL settings needed to connect
- Check timeout values aren’t too short
Correct any mismatched configurations between the gateway and upstream server.
3. Check Connectivity and Network
Troubleshoot the network path between the two servers:
- Trace the route and check for packet loss
- Monitor network equipment like routers, switches, firewalls
- Confirm DNS resolves upstream server name properly
- Check for high latency, jitter, or congestion
Resolve any network-related connectivity or latency issues.
4. Tune Gateway Server Settings
If the gateway server is being overloaded with requests, tune its settings to handle more load:
- Scale up gateway server with more CPUs, memory, bandwidth
- Increase limits on concurrent connections
- Add retry/queueing capabilities
- Load balance traffic across multiple gateway servers
Alleviating load and resource constraints on the gateway can help.
5. Check for Code Bugs and Errors
Thoroughly test the application code running on the upstream server:
- Load test upstream app and watch for failures or degraded performance
- Review upstream app logs for exceptions and errors
- Debug code, inspecting business logic and database access
- Check for scenarios not handling errors properly
Fix any upstream application bugs or errors that could lead to failed responses.
6. Monitor and Address Recurring Issues
Keep a close eye on recurring 502 errors and try to identify the root cause:
- Graph trends over time to see if spikes match a pattern
- Log additional debug-level detail for troubleshooting
- Monitor memory, CPU, and disk usage for exhaustion issues
- Watch for individual IP addresses or endpoints that repeatedly error
Continuously monitoring 502 errors can often uncover the underlying fault.
Best Practices for Preventing 502 Errors
In addition to troubleshooting and fixing 502 errors, some best practices can help avoid and minimize them:
Monitor Server Health Proactively
Keep a close eye on upstream server health metrics and get alerts on warning signs like high load, failed health checks, and resource exhaustion. Quickly address any upstream problems before they cascading failures.
Load Test Upstream Servers
Execute load and stress tests against the upstream app servers to uncover any weaknesses under heavy load. Fix issues discovered under high traffic before they cause widespread 502 failures.
Use Redundancy and Retries
Implement redundancy and retries in both the gateway and upstream tiers to minimize failed or lost requests. For example, deploy multiple upstream app servers behind a load balancer.
Tune Timeouts and Queueing
Configure timeouts and queueing on the gateway tier to handle spikes in traffic and upstream issues gracefully. Avoid overwhelming the upstream tier.
Follow Best Practices
Follow coding best practices, review logs, monitor key performance metrics, and run in a supported environment. Keep the gateway and upstream servers optimized.
Test Configurations and Deployments
Thoroughly test configuration changes as well as upstream server and application code deployments. Roll back immediately on any connectivity or responsiveness issues.
Common 502 Troubleshooting Questions
Here are some common questions and issues when troubleshooting and resolving 502 bad gateway errors:
Why am I Getting Intermittent 502 Errors?
Intermittent or occasional 502 errors often stem from transient network connectivity issues, brief spikes in traffic overloading the upstream server, or restarting the upstream server. Check connectivity, upstream server load, and logs for clues.
502 Errors with Specific Upstream Hosts Only?
If 502 errors occur for some upstream hosts but not others, focus troubleshooting on those specific upstream servers. They likely have an issue or misconfiguration.
502 Errors when Upstream Server Restarts?
If you only see 502 errors around upstream server restarts, increase timeouts and add queuing/retries on the gateway. The gateway should handle transient upstream unavailability during restarts.
Why am I Getting 502s for Some Requests Only?
502 errors for some requests but not others can indicate a code issue on the upstream server. Inspect upstream app logs for failures on specific endpoints or use cases.
Upstream Server Logs Show Timeouts?
If the upstream server logs show gateway connection timeouts, adjust configurations on both sides to allow adequate time for responses during peaks.
Conclusion
502 bad gateway errors indicate the server acting as a gateway or proxy got an invalid response from the upstream server, preventing it from responding properly to client requests. Fixing 502 errors involves troubleshooting issues like upstream outages or overload, network connectivity, server configuration mismatches, and software bugs. Following best practices like redundancy, monitoring, and testing can help minimize these errors.