Re: tracing mysterious rebooting on centos 5

["Robert G. (Doc) Savage" <dsavage@peaknet.net>]

On Thu, 2009-10-29 at 01:29 -0500, Brandon Joseph Adams wrote:
> Hi
> 
> So I'm back doing linux stuff after a few years and I ran into a rather 
> signficant hangup. The system I'm working on is a CentOS 5.4 machine 
> keeps rebooting at semi-random intervals whenever the iptables rules 
> change. I'm not currently sure if this is a fault in my iptables rules, a 
> hardware problem, or the result of a rogue legacy script finding something 
> amiss and rebooting. The solution I came up with is as follows (with auditd 
> enabled):
> 
> auditctl -a entry,always -S reboot
> 
> My only concern is that there might be another way to reboot the system 
> that isn't using reboot(2). init 6 seems to use reboot(2) and the userspace 
> reboot command seems to use it as well. Is there another way (outside of 
> magic sysrq) that the system might be rebooting via software?
> 
> Also, is there a better (maybe more obvious) way to be checking for this. 
> The logs in /var/log aren't very helpful and last & friends just tell me that 
> a reboot happened, not what caused it.

Brandon,

Welcome back to the "big leagues". I'm running RHEL5.4 Server
(supplemented by some CentOS workstation packages) on a SuperMicro
dual-socket Opteron server. I've never seen a spontaneous reboot problem
like you describe, so it's probably not due to any underlying software
bug or misconfiguration.

What's your hardware? More specifically, how old is the power supply?
The last time I had spontaneous reboots I traced it to broken-down
electrolytic filter capacitors in the power supply that was allowing AC
spiking onto the +5V and +12V power lines. Electrolytics are the big
beer-can size capacitors filled with a moist paste (the electrolyte).
With time and temperature, that paste eventually dries out and the
capacitor loses its ability to filter AC voltages applied across its
terminals. Any power supply older than 4-5 years should be considered
suspect.

It's easy enough to diagnose this. Take a volt-ohm meter (VOM) set to
measure AC volts and measure between +5V or+12V leads and ground on any
unused Molex power connector. It should read zero on all scale settings.
If you have access to an oscilloscope, you should see no spikes riding
on the DC voltages.

--Doc


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