Linux iptraf and iftop: Monitor,Analyse Network Traffic and Bandwidth

Sarath Pillai's picture
Network Traffic and bandwidth

Analyzing and monitoring network traffic of an entire network infrastructure can be done by plotting graphs based on any RRD tool. However monitoring traffic (both outgoing and incoming) on a Linux machine becomes a priority during crisis. There are pretty good number of open source tools available for Linux, that does this job.

We will be discussing two such tools in this post. The tools discussed in this post presents various important pieces of information about the traffic send and received by the network interfaces in Linux,which can be very useful in finding bottlenecks in network bandwidth.

The information that's made available to the user by these tools are as below.

  • PID(Process ID) sending maximum network traffic
  • Interface with high network bandwidth usage
  • TCP,UDP,ICMP,IP traffic summery per interface
  • Packet counts per interface in Linux
  • Error counts of packets send and recieved

The tools that we will be discussing in the posts are:

  1. Iftop
  2. Iptraf


Network Bandwidth Monitoring in Linux with Iftop command

One heavily used command in Linux by all system administrators is the "ps" command. This command outputs the process table and complete details of all the process running on a Linux system. Another such tool with a more interactive interface for process monitoring is "top".

The main reason why i mentioned "ps" and "top" command is the fact that iftop command is very much analogous to the very famous "top" command, with the only difference that iftop command gives you a complete network usage statistics instead of process.

System Libraries used by iftop command are as below.

  • Libpcap for capturing packets.
  • Libcurses for operations on terminals.

I will recommend reading the below post for libpcap & tcpdump packet analysis.

Read: Traffic analysis with Libpcap in Linux


Points to note about iftop command

  • It displays per host bandwidth usage
  • Hostname resolution is performed by default in the output
  • You need to run it as root user
  • View traffic to and from a specific network of your interest


How to install iftop in centos/rhel

If you have epel yum reposotory enabled then you can simply do a "yum install iftop".

Otherwise you can download the source package tar file from iftop website. Simply unpack the tar file and run the below commands inside the iftop directory to install it.

# ./configure
# make
# make install

iftop command usage and examples

Simply running the iftop command will give you an output something like the below.

                12.5kb          25.0kb          37.5kb          50.0kb    62.5kb
└───────────────┴───────────────┴───────────────┴───────────────┴───────────────           =>               944b   1.02kb  1.46kb
                           <=                             160b    160b    200b           =>                 0b      0b     72b
                           <=                               0b      0b    149b
TX:             cum:   3.05kB   peak:   3.22kb  rates:    944b   1.02kb  1.53kb
RX:                     698B            1.48kb            160b    160b    349b
TOTAL:                 3.74kB           4.70kb           1.08kb  1.18kb  1.87kb

In the above shown example output of iftop command, we can see traffic send to and from the host "myvm1", the direction of traffic send and received are shown by "=>" and "<="

By default the output is sorted from highest using to lowest using. So its quite easy to determine the host using highest traffic from one look at the output.

the last three columns you see is the output for bandwidth usage in the last 2,10 and 40 seconds.

One major fact to note in the output is that, it will show you the traffic from the first network interface card. For displaying traffic summery for network interface card of your interest you need to pass interface name with "-i" option in the command line as shown below.


[root@myvm1 ~]# iftop -i eth1

As i mentioned before, iftop by default will display host names in the output. If your server is a high traffic server, doing a name resolution for each and every ip iftop finds, will itself generate a large amount of packets, which can make the output slightly deviated(as iftop will count each and every packets through an interface). So its always better to suppress hostname resolution from the command line itself, which can be done by the following method.

[root@myvm1 ~]# iftop -n

Using source and destination port options is always helpful in finding the process using the source port on the server.You can ask iftop to display source and destination port with "SHIFT + s" and "SHIFT + d" interactively.

                12.5kb          25.0kb          37.5kb          50.0kb    62.5kb
└───────────────┴───────────────┴───────────────┴───────────────┴───────────────        =>        1.80kb  2.64kb  2.64kb
                           <=                             160b    853b    853b        =>        1.34kb  1.12kb  1.12kb
                           <=                             320b    320b    320b

You can clearly see that the source port is "ssh" and the destination is an arbitrary port on the gateway server.

You can always find the process using a port with the help of lsof command in Linux as shown below.

[root@myvm1 ~]# lsof -i :111
portmap 3861  rpc    3u  IPv4  10265       UDP *:sunrpc

The last row of the iftop output shown the received and transmitted traffic(RX and TX) for 2,10,and 40 seconds, with the grand total of the both.

Further details of command line options for iftop can be found from its man page.

[root@myvm1 ~]# man iftop
[root@myvm1 ~]#


Network traffic monitoring in Linux using iptraf command

Iptraf is much more detailed in its output compared to iftop command. Iptraf shows traffic summery based on the protocol of the traffic.

It shows the summery for tcp,udp,icmp,ip traffic in its output. It also comes handy to find out checksum errors in the packet send and received.

You can download the source package from iptraf website

Otherwise you can get the RPM package from here and install it using "rpm -ivh" command.

Simply running "iptraf" will show you a menu interface exactly like in the below shown picture.

iptraf command interface

  • Selecting the first option of "IP traffic monitor" will take you to an interface with all ip connections established with source and destination port with number of packets send,received in real time. In the resulting output you can use "SHIFT + M" option to see packet & window size in real time
  • Selecting "general interface statistics" Will give you statistics about ip,non-ip,and bad ip packets on all interfaces and also the transfer rate per interface
  • Selecting "detailed interface statistics" will show you complete details of number of packets per protocol and checksum errors transmission rate and receive rate etc.The output is quite elaborate as shown below.
┌ Statistics for eth0 ─────────────────────────────────────────────────────────┐
│                                                                              │
│               Total      Total    Incoming   Incoming    Outgoing   Outgoing │
│             Packets      Bytes     Packets      Bytes     Packets      Bytes │
│ Total:         1032     200746         353      22884         679     177862 │
│ IP:            1032     184348         353      15992         679     168356 │
│ TCP:           1032     184348         353      15992         679     168356 │
│ UDP:              0          0           0          0           0          0 │
│ ICMP:             0          0           0          0           0          0 │
│ Other IP:         0          0           0          0           0          0 │
│ Non-IP:           0          0           0          0           0          0 │
│                                                                              │
│                                                                              │
│ Total rates:         47.0 kbits/sec        Broadcast packets:            0   │
│                      31.2 packets/sec      Broadcast bytes:              0   │
│                                                                              │
│ Incoming rates:       5.6 kbits/sec                                          │
│                      10.6 packets/sec                                        │
│                                            IP checksum errors:           0   │
│ Outgoing rates:      41.4 kbits/sec                                          │
│                      20.6 packets/sec                                        │
└ Elapsed time:   0:00 ────────────────────────────────────────────────────────┘
  • Selecting the option "Statistical break downs"  will allow you to get a sorted output based on "packet size", as well as "TCP & UDP ports". This is a nice method, to determine if you are hit by an attack, because most of the times, attackers send large number of packets with large packet size.

A complete list of command line options for iptraf can be found from the iptraf man page.

[root@myvm1 ~]# man iptraf
[root@myvm1 ~]#

Hope this article was helpful for inbound & outbound traffic and bandwidth monitoring in Linux on an interface basis

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Average: 3.8 (140 votes)


Very helpful!

Could anyone advise on the method used by iptraf to calculate Peak values?
Iptraf minimum measurement interval is 60 sec. It prints out Average and Peak kbps values.
So I assume iptraf would measure kbps value per ~5 sec intervals and then print out the max as a the Peak value.

Very good explanation of the tool. Very helpful.

Its very help full for has .

Thanks, helped me to undersand this awesome tool.

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