A few interesting things I've stumbled across while finalizing some of my migration preparations from my previous server to the new one
- If you have multiple network adapters, your default route might not be the one you want. At least in CentOS, you can configure that in /etc/sysconfig/network with GATEWAY and GATEWAYDEV. This could be the problem if you're finding that you can't connect to your system (ping, ssh, etc.) even when it's capable of reaching the outside world... and even more, that if you disable your internal network adapter, everything "magically works." netstat -r/route -e might reveal that your default gateway isn't the one you think it is.
- Brute force ssh attacks aren't fun, and they've been pretty common for a while. True, they're probably not going to hack into your box (You have disabled root ssh access, right? And you don't have any of the standard accounts they try to hack in with?), but they can be rather annoying. Fortunately, it's actually pretty easy to use iptables to limit the number of ssh connections per minute.