# Berkeley Packet Filter Question



## bzk (Dec 30, 2015)

Hello BSD folks,

I am developing a networking application in C and I have a question regarding BPF (Berkeley Packet Filters), I will give you an idea of the app first,  I need to send a packet from machine A to machine B (any kind of packet) so for this I wrote a packet generator application which will send a packet to machine B, but before sending the packet I need to append some metadata values at the end of the packet, already done, so in machine B I have a raw socket listener app ready to receive incoming packets from machine A, however I want to implement filtering with BPF on machine B, but as my metadata was appended at the end of the packet (have to be at the end), I need to read the packet length with(using) Berkeley Packet Filter to match a specific field to filter one of the bytes at the end of my packet (metadata appended), in other words I need to know the incoming packet length to filtered against one of the metadatas fields and be able to drop the packet before reaching user space applications(drop it in kernel space).

So my question is, Can I use BPF to read the packet length ?

TIA!


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## junovitch@ (Jan 1, 2016)

Did you get everything you needed from the mailing list thread? (https://lists.FreeBSD.org/pipermail/freebsd-net/2015-December/044160.html)


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## bzk (Jan 1, 2016)

Nop, Why?


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