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Lossless Gigabit Remote Packet Capture With Linux

Original work from https://staff.washington.edu/corey/gulp/ and http://blog.crox.net/archives/72-gulp-tcpdump-alternative-for-lossless-capture-on-Linux.html. This repository is based on the latest patches from the original contributor (http://blog.crox.net/uploads/gulp-1.58-crox.tgz).

Features

• can run without root (see running gulp without root)
• rotate files using UTC timestamps for new file names
• separate reading and writing thread for increased performance

Getting started

Installation

From project root run ./scripts/install.sh (root required)

Building manually

Dependencies

• pcap.h
• cmake

Quick dependencies install command for Ubuntu (tested on Ubuntu 19.04)

sudo apt-get install libpcap-dev cmake

Build:

mkdir cmake-build-release && cd cmake-build-release
cmake .. -DCMAKE_BUILD_TYPE=Release
cmake --build .

Usage

Usage: ./gulp [--help | options]
    --help      prints this usage summary
    supported options include:
      -d        decapsulate Cisco ERSPAN GRE packets (sets -f value)
      -f "..."  specify a pcap filter - see manpage and -d
      -i eth#|- specify ethernet capture interface or '-' for stdin
      -s #      specify packet capture "snapshot" length limit
      -F        skip the interface type (Ethernet) check
      -r #      specify ring buffer size in megabytes (1-1024)
      -c        just buffer stdin to stdout (works with arbitrary data)
      -x        request exclusive lock (to be the only instance running)
      -X        run even when locking would forbid it
      -v        print program version and exit
      -Vx...x   display packet loss and buffer use - see manpage
      -p #      specify full/empty polling interval in microseconds
      -q        suppress buffer full warnings
      -z #      specify write blocksize (even power of 2, default 65536)
    for long-term capture
      -o dir    redirect pcap output to a collection of files in dir
      -n name   filename (default: pcap)
      -t        append UTC timestamp to the filename
      -C #      limit each pcap file in -o dir to # times the (-r #) size
      -G #      rotates the pcap file every # seconds
      -W #      overwrite pcap files in -o dir rather than start #+1 (max_files)
      -Z postrotate-command     run 'command file' after each rotation
    and some of academic interest only:
      -B        check if select(2) would ever have blocked on write
      -Y        avoid writes which would block

Examples

Assuming we already applied changes for running gulp without root otherwise we'll need to call sudo before each command.

Including UTC timestamp in file names

Save captured network traffic to a file with UTC timestamp in file name e.g. my_filename_20190821100215.pcap

mkdir -p savedir
gulp -i eth0 -t -o savedir/ -n my_filename

File rotation

Create a new file when the old grows over 100MB:

mkdir -p savedir
gulp -i eth0 -r 100 -C 1 -o savedir/ -n my_filename

Create a new file when the old grows over 100MB and include UTC timestamp in newly created file names:

mkdir -p savedir
gulp -i eth0 -r 100 -C 1 -o savedir/ -n my_filename -t

Compress rotated files

postrotate.sh

#!/usr/env bash

# gulp sends file name as an argumen to this script
IN_FN=$1
TMP_FN=$IN_FN.tmp
FIN_FN=$IN_FN.zst

zstd -q -19 --rm $IN_FN -o $TMP_FN
# signal with an atomic rename that the file is not being written to anymore
mv $TMP_FN $FIN_FN

We start gulp with the -Z flag:

mkdir -p savedir
gulp -i eth0 -t -r 100 -C 1 -n my_pcap_file -o savedir -Z postrotate.sh

Running without root

sudo groupadd pcap
sudo usermod -a -G pcap $USER
sudo mkdir -p /opt/gulp/bin
sudo cp build/bin/gulp /opt/gulp/bin
sudo chgrp pcap /opt/gulp/bin/gulp
sudo setcap cap_ipc_lock,cap_sys_nice,cap_net_raw,cap_net_admin=eip /opt/gulp/bin/gulp

Short explanation why we need these capabilities:

• cap_ipc_lock is required because we're calling mlock which guarantees us that the buffer in RAM will stay in RAM and will not be transferred to the SWAP area (in case another process would require more then available RAM)
• cap_sys_nice sets the reader thread to high CPU priority
• cap_net_raw and cap_net_admin allow us to capture on the network device without being root

Tunables

From Linux TCP tuning. Also see sysctl tweaks

Set the max OS send buffer size (wmem) and receive buffer size (rmem) to 12 MB for queues on all protocols. In other words set the amount of memory that is allocated for each TCP socket when it is opened or created while transferring files:

# echo 'net.core.wmem_max=12582912' >> /etc/sysctl.conf
# echo 'net.core.rmem_max=12582912' >> /etc/sysctl.conf

You also need to set minimum size, initial size, and maximum size in bytes:

# echo 'net.ipv4.tcp_rmem= 12582912 125829120 1258291200' >> /etc/sysctl.conf
# echo 'net.ipv4.tcp_wmem= 12582912 125829120 1258291200' >> /etc/sysctl.conf

Turn on window scaling which can be an option to enlarge the transfer window:

# echo 'net.ipv4.tcp_window_scaling = 1' >> /etc/sysctl.conf

Enable timestamps as defined in RFC1323:

# echo 'net.ipv4.tcp_timestamps = 1' >> /etc/sysctl.conf

Enable select acknowledgments:

# echo 'net.ipv4.tcp_sack = 1' >> /etc/sysctl.conf

Set maximum number of packets, queued on the INPUT side, when the interface receives packets faster than kernel can process them.

# echo 'net.core.netdev_max_backlog = 1000000' >> /etc/sysctl.conf