bpf - all your packets belong to me

BPFAll your Packets belong to Me

@_xhr_

xhr@giessen.ccc.de

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BPF ?

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tcpdump ?

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NIC¯

Link-Layer Driver¯

Protocol Stack¯

Userland

Packet Flow

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Smart Idea

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NIC¯

Link-Layer Driver¯

Filter¯

Buffer¯

Userland

Packet Flow

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BPF is rather old...McCanne. Jacobson.The BSD Packet Filter: A New Architecture for User-level Packet Capture. in USENIX. 1993.

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Have you met ...

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tcpdump -i eth0 ip6

That's the filter

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0 ldh [12]1 jeq #0x86dd jt 2 jf 32 ret #655353 ret #0

Ethernet Protocol Type

0x86dd == IPv6

Accept Packet

Drop Packet

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Linux got a BPF JIT in 2011

Check net/core/filter.c

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Packet Filter only for Packets?

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seccomp?

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So, how does this work?

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Attach a filter to a socket

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[...]

struct sock_filter code[] = { { 0x28, 0, 0, 0x0000000c }, [...]};

struct sock_fprog bpf = {.len = ARRAY_SIZE(code),.filter = code,

};

sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));

ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));

[...]

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So, how can I use this?

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Need for Space

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A 32 bit wide accumulator

X 32 bit wide X register

M[] 16 x 32 bit "scratch memory"

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Some Instructions

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ld*

st*

j*

ret

$alu

Load Instructions

Store Instructions

Jumps

Return

ALU instructions

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Hmm … k. IDE anyone?

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tools/net/

bpf_asm

bpf_dbg

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Demo

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What now?

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Packet Filtering

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Can I haz xt_bpf, plz?

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iptables -A <CHAIN> \-m bpf \--bytecode "…" \-j <TARGET>

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And Why?

Because we can!!1

Full packet control

Fine grained filters

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Q & Axhr

xhr@giessen.ccc.de@_xhr_