Some tech documentation and snippets, finally organized.
Posts tagged as tcpdump

Wireshark-readable tcpdump to remote disk

This line captures all packets on the network interface eth0 with source or destination TARGET_IP. To obtain a format readable by wireshark, tcpdump must be configured to not truncate packets. This is achieved by setting the -s flag to 0 or 65535 (maximum packet length in bytes).

The output is piped through ssh to OUTFILE on a remote host.

$ tcpdump -i eth0 -w - -s 0 host TARGET_IP |\
    ssh USER@HOST "cat > dump.pcap"

[Ctrl]+[c] to stop the capture...

To avoid packet dropping at high traffic volume, it may be reasonable to put the capturing computer as transparent bridge into the IP stream and use a logging host on an independent network, connected to an additional (third) network interface ethZ.

In one go:




#iptables-restore < /etc/iptables/bridge.v4

ifdown "$NIC1"
ifdown "$NIC2"
brctl addbr br0
brctl addif br0 "$NIC1"
brctl addif br0 "$NIC2"
ifconfig "$NIC1"
ifconfig "$NIC2"
ifconfig br0 "$BRIDGE_IP" netmask "$MASK" up

tcpdump -i "$NIC1" -w - -s 65535 host "$TARGET_IP" |\
    ssh "$SSHD" "cat > dump_`date +%H.%M.%S`.pcap"

Note: When running this script on a headless/remote machine (e.g. an OpenWRT router), double check its iptables rules and possible sshd restrictions to not lock you out or even brick the box.