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CrashPlan pier-to-pier works with no open inbound TCP ports. How?

Last Modified: 2015-03-16
We've configured all office laptops to backup via CrashPlan to our secondary office server. My partner and I are fuzzy on how CrashPlan pier-to-pier traffic works given the fact that we haven't to poke any holes in the firewall. Here's our theory:

The CrashPlan service on the client machine (containing files that require backup) launches a session with a CrashPlan server. The firewall associated with the client machine is cool with a session that was launched via an outbound packet, and everything works.
The CrashPlan service on our secondary server works in a similar fashion, launching a session with a CrashPlan server. Again, because the initial packet was outbound, the firewall allows the session.
This is where we get fuzzy, does the CrashPlan server continue to act as an intermediary for the backup? (That seems like a hell of a lot of traffic considering all the CrashPlan backups going at any given time.)
Or, does the CrashPlan server somehow "handoff" the session between the two piers? (This seems like it would cause the firewalls to balk.)
Or, is our theory completely off-base?
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Aaron TomoskyDirector, SD-WAN Solutions

is upnp enabled on your routers? That would explain the magic.

Otherwise you do need to poke holes in the firewall at both sites
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Perfect. Thanks.
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