After installing Asterisk, I noticed a large amount of unexpected Internet traffic. It seems that the Asterisk box was spewing packets to 192.168.0.24.
(Since I don't use NAT, these were going straight out to the ISP who was rejecting them as unroutable):

10:53:51.533338 IP 90.155.92.244.58133 > 192.168.0.24.pktcable-cops: Flags [S], seq 3310868053, win 14600, options [mss 1460,sackOK,TS val 144030427 ecr 0,nop,wscale 4], length 0
10:53:51.557719 IP 90.155.53.6 > 90.155.92.244: ICMP net 192.168.0.24 unreachable, length 36
10:53:51.559294 IP 90.155.92.244.58134 > 192.168.0.24.pktcable-cops: Flags [S], seq 133054872, win 14600, options [mss 1460,sackOK,TS val 144030453 ecr 0,nop,wscale 4], length 0
10:53:51.583881 IP 90.155.53.6 > 90.155.92.244: ICMP net 192.168.0.24 unreachable, length 36
10:53:51.585418 IP 90.155.92.244.58135 > 192.168.0.24.pktcable-cops: Flags [S], seq 2670163269, win 14600, options [mss 1460,sackOK,TS val 144030479 ecr 0,nop,wscale 4], length 0
10:53:51.608970 IP 90.155.53.6 > 90.155.92.244: ICMP net 192.168.0.24 unreachable, length 36
10:53:51.610451 IP 90.155.92.244.58136 > 192.168.0.24.pktcable-cops: Flags [S], seq 3941291750, win 14600, options [mss 1460,sackOK,TS val 144030504 ecr 0,nop,wscale 4], length 0
10:53:51.633890 IP 90.155.53.6 > 90.155.92.244: ICMP net 192.168.0.24 unreachable, length 36
10:53:51.635443 IP 90.155.92.244.58137 > 192.168.0.24.pktcable-cops: Flags [S], seq 2254839251, win 14600, options [mss 1460,sackOK,TS val 144030529 ecr 0,nop,wscale 4], length 0
10:53:51.659223 IP 90.155.53.6 > 90.155.92.244: ICMP net 192.168.0.24 unreachable, length 36
10:53:51.660665 IP 90.155.92.244.58138 > 192.168.0.24.pktcable-cops: Flags [S], seq 1080245882, win 14600, options [mss 1460,sackOK,TS val 144030554 ecr 0,nop,wscale 4], length 0

It goes on.... it seems to be attempting a SYN-flood attack. What the hell is it doing?

I found 192.168.0.24 listed in res_pktccops.conf, and unloading the res_pktccops.so seems to have fixed the problem.

This is *not* something that a default install should be doing out of the box.