Databases, Systems & Networks

There are many traffic monitoring tools available on Linux, which can monitor/classify network traffic, and report real-time traffic statistics in fancy user interfaces. Most of these tools (e.g., ntopng, iftop) are powered by libpcap, which is a packet capture library used to monitor network traffic in user space. Despite their versatility, however, libpcap-based network monitoring tools cannot scale to handle traffic on multi Gigabit rate network interfaces, due to the overhead associated with user-space packet capture.

In this tutorial, I will present simple shell scripts that can monitor network traffic on per-interface basis, without relying on slow libpcap library. These scripts are fast enough to support multi Gigabit rates, but only suitable if you are interested in « aggregate » network statistics on per interface basis.

The secret for the scripts lies in sysfs virtual filesystem which is used by the kernel to export device- or driver-related information to user space. Network interface related statistics are exported via /sys/class/net/<ethX>/statistics.

For example, the statistics on eth0 interface are found in these files:

• /sys/class/net/eth0/statistics/rx_packets: number of packets received
• /sys/class/net/eth0/statistics/tx_packets: number of packets transmitted
• /sys/class/net/eth0/statistics/rx_bytes: number of bytes received
• /sys/class/net/eth0/statistics/tx_bytes: number of bytes transmitted
• /sys/class/net/eth0/statistics/rx_dropped: number of packets dropped while received
• /sys/class/net/eth0/statistics/tx_dropped: number of packets dropped while transmitted

The numbers stored in the files are automatically refreshed in real-time by the kernel. Therefore, you can write scripts that calculate traffic statistics based on these files.

The following are two such scripts (thanks to joemiller). The first script counts the number of packets per second, received (RX) or sent (TX) on an interface, while the latter scripts measures the network bandwidth of incoming (RX) and outgoing (TX) traffic on an interface. For these scripts to work, you do not need to install anything.

Measure Packets per Second on an Interface

#!/bin/bash
INTERVAL="1"  # update interval in seconds
if [ -z "$1" ]; then
        echo
        echo usage: $0 [network-interface]
        echo
        echo e.g. $0 eth0
        echo
        echo shows packets-per-second
        exit
fi

IF=$1

while true
do
        R1=`cat /sys/class/net/$1/statistics/rx_packets`
        T1=`cat /sys/class/net/$1/statistics/tx_packets`
        sleep $INTERVAL
        R2=`cat /sys/class/net/$1/statistics/rx_packets`
        T2=`cat /sys/class/net/$1/statistics/tx_packets`
        TXPPS=`expr $T2 - $T1`
        RXPPS=`expr $R2 - $R1`
        echo "TX $1: $TXPPS pkts/s RX $1: $RXPPS pkts/s"
done

Measure Network Bandwidth on an Interface

#!/bin/bash
 
INTERVAL="1" # update interval in seconds
 
if [ -z "$1" ]; then
 echo
 echo usage: $0 [network-interface]
 echo
 echo e.g. $0 eth0
 echo
 exit
fi
 
IF=$1
 
while true
do
 R1=`cat /sys/class/net/$1/statistics/rx_bytes`
 T1=`cat /sys/class/net/$1/statistics/tx_bytes`
 sleep $INTERVAL
 R2=`cat /sys/class/net/$1/statistics/rx_bytes`
 T2=`cat /sys/class/net/$1/statistics/tx_bytes`
 TBPS=`expr $T2 - $T1`
 RBPS=`expr $R2 - $R1`
 TKBPS=`expr $TBPS / 1024`
 RKBPS=`expr $RBPS / 1024`
 echo "TX $1: $TKBPS kB/s RX $1: $RKBPS kB/s"
done