Update 2009/05/15: The second part is not fully automatic. Sam did point his CrashPlan.app at my archives, but I didn't have to do anything on the client side. Still freaky!

Today I swapped backup drives with Sam. I have a Time Capsule (which hasn't failed much recently, to my surprise) for 'local' backups, and CrashPlan is for offsite backup. Sam discovered that CrashPlan is able to back up to a host behind a 'real' firewall. I'm not surprised that CrashPlan can use NAT-PMP to accept inbound backups, but I am surprised it apparently works through non-personal firewalls without manual configuration.

We believe Code42 must be running a heavy-duty proxying service, where each CrashPlan client connects to their servers, and their servers accept all the peer-to-peer backup traffic and dispatch it back down that 'inbound' connection. This is part of what makes IM work, but the traffic volume per user for CrashPlan is much higher than for something like AIM or Back to My Mac (screen sharing or individual file transfers). I hope this doesn't mean the software breaks if their server farm shuts down or is retired, and that they don't decide the intermediary service is too expensive (neither Sam nor I is paying for the service -- just for the CrashPlan+ licenses). If they really are retransmitting all p2p backups, code42 is passing double the non-local traffic of all their p2p users put together. They'd have to accept each byte from the client, then send it to the server. Backups across the local network don't need this. Perhaps CrashPlan is smart enough to only do this proxying trick for backup 'servers' behind uncooperative firewalls...

But tonight we got an even bigger surprise. When Sam plugged my 1tb drive into his Mac, both my Linux server and MBP immediately started backing up to the drive through Sam's computer. Note that I had not configured my systems to use his Mac. Apparently when he plugged the drive in, his Mac automatically registered the CrashPlan-created volume 'serial number' with the CrashPlan servers, and they automatically connected my clients to that drive (or perhaps it's all done via client serial numbers rather than volume numbers -- I'm just speculating here). Suddenly I had a new friend, 'Sanford' show up in both my CrashPlan clients. Freaky-deaky!