skbdump's Introduction

skbdump

skbdump takes advantages of tc-bpf(8) to dump traffic on a network.

This tool is created by the following motives:

tcpdump(8) is bypassed when a bpf program on a device redirects the skb to another device;
tcpdump(8) works slowly and affects network performance;
tcpdump(8) -i any relies on Linux cooked-mode capture (SLL) and the link layer header isn't available;
tcpdump(8) doesn't reflect the information of direction: egress or ingress;
tcpdump(8) can't capture the skb metadata in the struct __sk_buff;

However, tcpdump(8) does have something I really appriciate, such as pcap-filter(8) for packet filtering, and I want to make sure my tool can still leverage the power of that.

Usage

Usage of skbdump:
  -i, --interface string       interface to capture (default "lo")
  -w, --pcap-filename string   output pcap filename (default "skbdump.pcap")
  -p, --priority uint32        filter priority (default 1)
  -s, --skb-filename string    output skb filename (default "skbdump.skb")

Please be aware that every capture will dump two files, one is pcap file which I recommand you open it by wireshark, and the other is skb text file just simply recording skb metadata in JSON.

Some examples:

skbdump -i eth0 port 80
skbdump -i eth0 udp
skbdump -i eth0 host 10.10.1.1
skbdump -i any dst 10.10.1.20

Known Issues

There are some bugs of transforming cbpf to ebpf, and now the tool will break for skbdump -i lo arp. To workaround the issue you can use the equivalent filter expression such as skbdump -i lo ether proto arp.
Some complicated filter expression doesn't work as expected, such as 'tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420'.
Currently the tool only supports capturing packets with maximum 1024 bytes.

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