April 27, 2017

mcblockd’s latest tricks: kill pf state, walk PCB list and kill TCP connections

Today I added a new feature to mcblockd to kill pf state for all hosts in a prefix when the prefix is added to one of my pf tables. This isn’t exactly what I want, but it’ll do for now.

mcblockd also now walks the PCB (protocol control block) list and drops TCP connections for hosts in a prefix I’ve just added to a pf table. Fortunately there was sample code in /usr/src/usr.sbin/tcpdrop/tcpdrop.c. The trick here is that I don’t currently have a means of mapping a pf table to where it’s applied (which ports, which interfaces). In the long term I might add code to figure that out, but in the interim I can configure ports and interfaces in mcblockd’s configuration file that will allow me to drop specific connections. For this first pass, I just toast all PCBs for a prefix.

The reason I added this feature: I occasionally see simultaneous login attempts from different IP addresses in the same prefix. If I’m going to block the prefix automatically, I want to cut off all of their connections right now, not after all of their connections have ended. Blowing away their pf state works, but leaves a hanging TCP connection in the ESTABLISHED state for a while. I want the PCBs to be cleaned up.

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