Alright, you guys are gonna love this. I have kismet installed and set-up in server mode, using the wireless card in monitor mode. I'm using sysv, and /etc/rc.local is configured as so:
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# Start the wifi card in monitor mode for Kismet reasons
airmon-ng start wlan0
/usr/bin/sudo -u pi "/usr/bin/kismet_server" -n --daemonize -p /home/pi/kismet_server
At boot, I watched the startup messages (I don't know the proper term for those), and noticed that, when it runs this command, it complains that dhclient and dhclient script might interfere -- which is strange, since when I run the airmon command as root (sudo su) within a console, it doesn't produce that error. Also, "~# killall -9 dhclient" solved the original issue, albeit superficially. So, I think, I'll just comment-out those two lines from rc.local. Sure enough, after a reboot, my cpu usage runs exactly as expected.
Now, fully intrigued, I uncomment those lines, reboot, and double-check htop. That screenshot I sent in my previous post, it shows a dhclient command constantly running, around 4%, but without those lines in rc.local it doesn't show any dhclient at all, and neither do any of my other Debian systems. I suddenly have an epiphany: why is it running the entire time? So, I try a simple "~# dhclient eth0", and in a normal system, it would show a simple, one time "RTNETLINK answers: File Exists". However, when I run the same command in my server, it gives me two messages, ad infinitum:
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...
/sbin/dhclient-script: 28: .: Can't open /usr/share/sendmail/dynamic
RTNETLINK answers: File exists
/sbin/dhclient-script: 28: .: Can't open /usr/share/sendmail/dynamic
RTNETLINK answers: File exists
...
until I CTRL+C. So, it appears the issue was that I had daemonized an error message, and the little atom processor wasn't happy with that.
This was, however, a nice tidbit of information.
I ran across a thread which articulated solutions to the same issue. Sendmail was already removed through apt-get, but I guess a lingering file interfered with dhclient's normal behavior, so I went ahead and removed it:
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rm /etc/dhcp/dhclient-exit-hooks.d/sendmail
And now everything is up to normal. Thanks to everyone for helping out, however, I'm still curious as to why htop didn't show anything abnormal in the process list; and if that's normal to not show, I still wonder if there would have been a better way to narrow that without crapshooting through programs and bootscreens. Ah, well...