Please define exactly what you mean by "hang". Is it:
1) Hard hang - kernel unresponsive, no traffic flowing, interfaces do not answer ping, console port stuck. Generally caused by a hardware interrupt or other atomic kernel operation never completing or bad hardware, must power cycle to recover
2) Process hang - Kernel is consuming all available CPU and/or memory, interfaces still answer pings, basic firewall traffic not requiring process space interaction still flows through the firewall, and the console is stuck as the servicing process cannot get any CPU slices or sufficient memory. Will probably require a power cycle to recover from, or when enough processes die off from lack of memory the console port may become available to initiate a reboot. Incidentally if the hard drive on a security gateway permanently dies (or just stops responding until the whole system is power cycled), the gateway will exhibit all of these "process hang" attributes. The kernel preloads all needed code and data into RAM at startup and can continue to operate, while most processes require interaction with the hard drive for paging and such and will eventually die.
3) NIC hang - All NIC's appear to be operating normally based on the LEDs, but no network traffic of any kind (including pinging firewall interfaces) functions. Console port is still responsive to initiate a reboot.
4) Interaction hang - System has crashed and is trying to boot back up, but is waiting for some user interaction on the console before proceeding. Typically this is to approve some action such as fsck repairing disk corruption, or acknowledging some kind of failure. Console is responsive, but network interfaces may not respond at all depending on how far the system has managed to boot up.
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