I haven't written here much lately. I've been swamped with work and real life.
Recently, my wife wanted me to clone one of my VMs so she could play around with running a MUD for some friends. Yes, my wife's a nerd like me. As anyone who's ever run a game server can guess, it didn't take long for the griefers to show up. She asked me to log everything going to her VM. I could have probably compiled DaemonLogger or something similar, but I decided tcpdump was more than capable enough for us.
sudo tcpdump -i eth0 -wPacketLog -W10 -C100M
Throw that in the background (or in a tmux/screen session) and enjoy a 1GB looped recording of everything you can see on the network, broken into 100MB chunks (named PacketLog0 - PacketLog9), overwriting old files as it goes. You can also add typical tcpdump filters (e.g. "tcp port 80") to the end if you want. Adjust -W to increase/decrease the number of files it saves and -C to adjust the number of MB of data kept in each file. File prefixes, as you guessed, are controlled by the -w option.
If you want to monitor your whole network, this works best if you have a span/mirror port set up, or you can make a passive network tap.
To review the contents of the saved file, use tcpdump -nvXs0 -r PacketLogN (displays the contents in Hex/Ascii side-by-side format) on the file you want. You can also use tcpdump filters here to watch specific connections, protocols and/or hosts.