Connection Timeout

A connection timeout happens when a client sends a TCP connection request (SYN) and receives no reply before its time limit expires. The request never reaches the stage of sending HTTP data, so the failure is below the application layer.

Why it matters

It points to a different set of causes than a slow application. The server may be offline, a firewall may be silently dropping packets, the route may be broken, or the port may not be open. A refused connection (instant reset) means something answered and said no; a timeout means nothing answered at all.

Example

$ curl -sS --connect-timeout 5 https://example.com
curl: (28) Failed to connect to example.com port 443 after 5001 ms: Timeout was reached

Check in this order:

1. Does the name resolve to the correct IP? Use the DNS checker.
2. Is the port reachable from outside? Use the port checker.
3. Is a firewall or security group dropping traffic, or is the server's backlog full?
4. Does the problem affect all regions or one network? Compare locations.

Typical client defaults are 5 to 30 seconds. See also read timeout.

Common mistakes

• Setting the connect timeout equal to the total request timeout.
• Retrying instantly without backoff against a host that is down.
• Confusing it with a 504, which means a gateway connected but the upstream was too slow.

Related

• Read timeout
• Three-way handshake
• Connection time
• Retry with backoff

This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.