GitHub was essentially down for about an hour today, starting at around 11 am Eastern (1500 UTC) due to a reported DDoS attack. From their status page:
15:05 UTC We’re currently experiencing a large DDoS attack. The site is experiencing major packet loss and is mostly unavailable. We’re working to further mitigate the attack.
16:10 UTC We’ve mitigated the DDoS attack and the site should responding normally. We’re still investigating the cause of the small increase in exceptions when accessing the GitHub API.
