What causes "Unable to connect to the remote server"

I've got a downloader app. It works fine for me, but one user send me bug report saying that System.Net.WebException is throwed with 'Unable to connect to the remote server', while it works for the link good on my pc. It works with other links but wit开发者_运维问答h this particular, don't. What causes this exception? And what can be the difference between my pc configuration and his.

The program can't find the server in a resonable timeframe. As the link works for you, this is probably a local problem. Maybe blocked by a filter, dns resolve doesn't work, long route through servers, problem with a backbone on the route.

Please ask your user to confirm wether he can open the link regulary outside your program. If this doesn't work either, the fault isn't on your side. If he should be able to, you'll need to provide extensive logging code which will help you pin the issue. However, it is very much possible that you'll never find the bug unless you are able to debug the application on his machine with his environment.