Why are my proxy requests failing on some domains but not others?

Proxy requests that work for some domains but fail for others usually come down to one of these causes:

1) The target site blocks or filters proxies

Many sites detect:

• datacenter IPs
• known proxy/VPN IP ranges
• unusual request patterns
• missing/odd headers

Symptoms: 403, CAPTCHA pages, connection resets, or “Access denied”.

2) DNS resolution differs through the proxy

Some proxies resolve the domain themselves, others expect the client to resolve it first.

Symptoms:

• one domain resolves fine, another gives Could not resolve host
• works with IP but not hostname

3) TLS/SSL handshake issues

Some domains require newer TLS versions, specific ciphers, or proper SNI.

Symptoms:

• SSL errors
• handshake failures
• connection closes right after CONNECT

4) The proxy doesn’t support the request type

Not all proxies support:

• HTTPS CONNECT
• HTTP/2
• WebSockets
• large request bodies
• certain methods like PUT, PATCH, DELETE

5) Authentication or IP allowlisting issues

Some domains may require:

• source IP allowlisting
• proxy user/pass auth
• region-specific access

6) Routing or network path problems

The proxy may have trouble reaching certain ASN/regions or specific CDNs.

Symptoms:

• timeouts on only some domains
• works intermittently
• failures from certain proxy endpoints only

7) Headers/cookies/redirect behavior

Some sites require:

• Host header exactly matching the domain
• User-Agent
• Accept-Language
• cookies from prior requests

8) Proxy configuration differences by scheme

You may be using:

• http://proxy for HTTP targets
• https://proxy incorrectly
• SOCKS proxy when the client expects HTTP proxy

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Quick ways to isolate the issue

Test from the proxy itself

Try:

curl -v -x http://PROXY_HOST:PORT https://example.com

Compare a working vs failing domain

Look at:

• status code
• TLS errors
• DNS errors
• timeout vs immediate failure

Try IP vs hostname

If IP works but hostname fails, it’s likely DNS/SNI/Host-header related.

Check proxy logs

See whether the request reaches the proxy and what it returns.

Try a clean browser-like request

Use full headers and a common User-Agent.

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Most common pattern

If some domains work and others fail, the most common causes are:

1. target site blocks proxy IPs
2. TLS/SNI issues
3. proxy DNS or routing limitations

If you want, paste:

• the proxy type
• one working domain
• one failing domain
• the exact error message or curl output

and I can help narrow it down quickly.