Why would a “java.net.ConnectException: Connection timed out” exception occur when URL is up?

Question

I\'m getting a ConnectException: Connection timed out with some frequency from my code. The URL I am trying to hit is up. The same code works for some users,

Answer 1

This can be a IPv6 problem (the host publishes an IPv6 AAAA-Address and the users host thinks it is configured for IPv6 but it is actually not correctly connected). This can also be a network MTU problem, a firewall block, or the target host might publish different IP addresses (randomly or based on originators country) which are not all reachable. Or similliar network problems.

You cant do much besides setting a timeout and adding good error messages (especially printing out the hosts' resolved address). If you want to make it more robust add retry, parallel trying of all addresses and also look into name resolution caching (positive and negative) on the Java platform.