Why Identify the Failure Layer First?

A tunnel proxy request isn't just one connection. A single request involves at least the local program, proxy entry, proxy authentication, exit node, and target site. An anomaly at any layer can present as "proxy unavailable."

Use these symptoms for initial judgment:

Don't attribute every error to the proxy IP itself. For example, HTTP 407 usually means proxy authentication didn't pass; HTTP 403 requires further judgment on whether the proxy service returned it or the target site rejected the request.

What Common Configuration Errors Exist?

Config problems are the easiest to troubleshoot and most easily overlooked class of failure. Focus on:

1. Does the proxy protocol match
   HTTP proxy, HTTPS proxy, and SOCKS5 proxy have different integration methods. When the program is configured for SOCKS5, you can't just drop in an HTTP proxy address and expect auto-compatibility.
2. Is host address and port complete
   Check for missing ports, stray spaces, or mistaking the console-displayed API endpoint for the proxy entry address.
3. Is tunnel mode enabled
   Some clients require tunnel mode to be enabled separately — not just entering a proxy address. If the configuration UI has both "API extraction" and "fixed proxy" entries, confirm you're using the integration method the task actually needs.
4. Are HTTPS requests sent through the correct proxy method
   When accessing HTTPS targets, the client usually needs to establish a CONNECT tunnel through the proxy first. If the client only supports plain HTTP forwarding, it may fail during the handshake stage.
5. Do environment variables override program configuration
   Check HTTP_PROXY, HTTPS_PROXY, ALL_PROXY, and NO_PROXY. In command-line, container, or server environments, old config may take priority over new config in the program UI.

Delete unrelated parameters first — keep only proxy address, port, and authentication, and validate the connection with minimum config. The more config items, the easier it is for parameters to override each other.

What Do HTTP 407, 401, and 403 Each Mean?

All three status codes may be loosely called "authentication failures" by users, but troubleshooting directions differ.

On 407, re-copy proxy credentials first and confirm whether the password contains special characters that need escaping. If the service supports IP allowlist authentication, also confirm whether the current server's public exit address is in the allowlist.

On 401 or 403, don't repeatedly change the proxy password. Bypass the business code first — use the command line to access a specific test address, then check response headers and body to judge whether the rejection comes from the proxy layer or the target layer.

How to Judge Whether the Proxy Entry Is Really Reachable?

Use curl for a minimal test first — rule out business code, browser plugins, and complex request parameters:

curl -v \
  -x http://proxy_host:port \
  --proxy-user 'username:password' \
  https://target_domain/ \
  -o /dev/null

Watch for four categories of information during testing:

• Whether the proxy address resolves successfully
• Whether TCP connection to the proxy port completes
• Whether the proxy returns an authentication result
• Whether a CONNECT tunnel to the target site is established

If even the proxy port fails, check DNS, routing, firewall, and port policy first; if the proxy already returned 407, the network is basically reachable and the problem concentrates on authentication; if CONNECT is established but the target request fails, continue checking exit nodes and target site response.

You can also use nslookup or dig to check domain resolution, and telnet or nc to check port reachability. But port reachability only means a TCP connection was established — it doesn't guarantee proxy authentication and target access will succeed.

Where Should Proxy Timeouts Be Checked First?

Timeouts need to be distinguished by stage first:

1. Timeout connecting to the proxy entry
   May be unreachable proxy address, local network restrictions, blocked ports, or poor line quality from the current network to the proxy entry.
2. Timeout after proxy authentication
   May relate to exit node allocation, target region, target site response speed, or line congestion.
3. Target site response timeout
   Target site itself may respond slowly, or the target region may have unstable access quality on the current exit line.
4. Response body read timeout
   Connection is established but target returns data slowly. Don't simply judge this as proxy failure — check the client's read timeout and target endpoint processing time.

During troubleshooting, log DNS, TCP connection, TLS handshake, first byte, and full response time separately. If all requests time out before connecting to the proxy, the problem is closer to local network or proxy entry; if only specific target domains time out, the problem may concentrate on the target site or exit line.

Why Do Connection Resets or 502, 503, 504 Appear?

Connection reset usually means the connection was actively closed by one side during transmission. Common causes:

• Proxy entry detected an abnormal connection format
• Client reused a long-connection that had already expired
• Exit node temporarily unavailable
• Target site closed the connection early
• Concurrency or request frequency exceeded the stable carrying capacity of the current configuration

Validate in this order:

1. Disable connection pooling and long-connection reuse — test with single short connections
2. Lower concurrency — observe whether failure rate drops with concurrency
3. Set connection timeout, read timeout, and overall request timeout values separately
4. Use bounded exponential backoff retries for 502, 503, 504
5. Log the proxy entry, target domain, status code, and time for every request

If only one proxy entry keeps returning 502 while others are normal, preserve error time and request ID and submit to the service side for investigation. Don't retry infinitely in the program — otherwise a temporary failure can be amplified into higher request pressure.

Why Are High-Concurrency Scenarios More Prone to Failure?

A tunnel proxy doesn't mean you can increase concurrency without limit. Concurrent connection count, requests per second, requests per connection, and target site processing capacity all affect final success rate.

High-concurrency failures typically appear as:

• Normal at low concurrency, massive timeouts after raising concurrency
• First batch of requests succeeds, connections reset after sustained runtime
• Noticeably higher failure rate for the same exit or same target domain
• The program itself shows connection pool exhaustion, insufficient file descriptors, or thread blocking

Build a stepped stress-test flow:

1. Use single-thread or low concurrency to validate baseline connectivity
2. Raise concurrency in small steps each time, log success rate, average time, and timeout count
3. Find the range where failure rate clearly rises
4. Keep production concurrency within the stable range, set up queues for traffic bursts
5. Distinguish proxy service failures, target response failures, and local resource shortages

If only a specific target domain fails under high concurrency, don't directly conclude the proxy service is faulty overall — also check the target side's rate limits and response capacity.

How to Judge Whether the Problem Is Local Network, Proxy Service, or Target Site?

Use "three-group comparison":

Keep request method, headers, parameters, and timeout settings consistent during comparison. Otherwise too many variables make results hard to interpret.

Simplified judgment logic:

• Direct and proxy both fail: check target site, local network, or request itself first
• Direct succeeds, multiple proxies all fail: check proxy config, auth, or service status
• Only one proxy entry fails: log entry, time, and error code, submit for service-side investigation
• Only one target fails: check target access policy, DNS resolution, and regional lines
• Low concurrency succeeds, high concurrency fails: check concurrency, connection pool, and request frequency

What Information Should Be Logged During Troubleshooting?

Without logs, proxy failures often leave only guesswork. Log at minimum:

• Occurrence time and timezone
• Proxy entry, port, and protocol type (don't log sensitive credentials)
• Target domain and request method
• HTTP status code or client error code
• DNS, connection, handshake, first byte, and total time
• Concurrency count, retry count, and connection reuse state
• Whether the proxy entry was switched
• Whether failures only hit specific targets or time windows

Don't retain full username, password, token, or other sensitive information in logs. When submitting for service-side investigation, provide masked request IDs, time range, error codes, and reproduction conditions.

Tunnel Proxy Troubleshooting FAQ

Q: The proxy address opens in a browser but the business program still errors — why?

A browser being able to use the proxy doesn't mean the business program's protocol, auth format, and HTTPS tunneling behavior are identical. Validate on the same machine with the same proxy parameters via curl -v, then check whether the program has the wrong proxy type or environment variables set.

Q: HTTP 407 keeps appearing — re-entering the password doesn't help, what now?

Confirm you're using proxy authentication info, not the target site account. Then check password special characters, auth format, IP allowlist, and the server's public exit address. If multiple programs show 407 at once, prioritize checking credential status and allowlist config.

Q: Requests occasionally succeed, occasionally time out — is it an unstable proxy?

Not necessarily. It can also relate to exit nodes, target site response speed, concurrency, and connection reuse. Compare across different targets, different proxy entries, and different concurrency levels — judge the cause by the stage where errors cluster.

Q: Does a 403 return mean the proxy IP can no longer be used?

Can't be judged that directly. 403 may come from the target site or from the proxy service's access policy. Review response headers, response body, and request chain — and run a direct-connection vs. swap-proxy-entry comparison test on the same target.

Q: Can continuous retry solve timeouts?

Bounded retry may mitigate short network fluctuations, but infinite retry adds pressure on target and proxy. Set max retry count, exponential backoff, and failure logs — handle connection timeout and read timeout separately.

Q: When should you submit a ticket or contact the provider for investigation?

When a proxy entry keeps producing the same type of error, when multiple targets anomalize simultaneously, or after you've confirmed config, auth, and local network are normal — submit masked logs, occurrence time, error codes, request IDs, and reproduction steps so the service side can locate node and line issues.

Summary: when tunnel proxy errors, timeouts, or connection failures occur, first locate the failure layer by error code and failure stage, then check config, authentication, entry connectivity, exit lines, target response, and concurrency settings in order. Only after completing comparison testing can you judge whether the problem truly comes from the proxy service.