How do I troubleshoot "failed to connect" messages?
So, I load up Burp Pro, restore defaults (latest versions of burp and JRE), enable the proxy and then in Chrome, with FoxyProxy browse to the site I want to scan/spider.
I add said site to scope, and then start spidering. In the alerts tab there will be an increasing number of alerts saying that I failed to connect to the site name, and the same thing happens when I use the scanner.
Burp is not using an upstream proxy, and I have no idea why this is happening. The alerts tab doesn't give a lot of information.
What can I do to troubleshoot?
It’s possible that your spidering and scanning are overloading some component resulting in the connection failures – this could be your own machine (the OS won’t open network connections fast enough) or the target application (if it can’t scale to support the rate of requests) or something in between.
In terms of diagnosing the cause of the issue, if Burp isn’t displaying any other alerts that point towards the cause of the problem, then you could try using Wireskark to monitor the traffic at the network level.
In terms of resolving this so you can continue testing, you could try tweaking the settings for the Spider and Scanner engines, to reduce the thread count, add a short throttle between requests, or increase the “pause before retry” when a network error occurs: