IFTOP(8) IFTOP(8)

NAME
iftop - display bandwidth usage on an interface by host

SYNOPSIS
iftop -h | [-nNpbBP] [-i interface] [-f filter code] [-F net/mask]

DESCRIPTION
iftop listens to network traffic on a named interface, or on the first interface
it can find which looks like an external interface if none is specified, and disâ
plays a table of current bandwidth usage by pairs of hosts. iftop must be run
with sufficient permissions to monitor all network traffic on the interface; see
pcap(3) for more information, but on most systems this means that it must be run
as root.

By default, iftop will look up the hostnames associated with addresses it finds in
packets. This can cause substantial traffic of itself, and may result in a confusâ
ing display. You may wish to suppress display of DNS traffic by using filter code
such as not port domain, or switch it off entirely, by using the -n option or by
pressing R when the program is running.

By default, iftop counts all IP packets that pass through the filter, and the
direction of the packet is determined according to the direction the packet is
moving across the interface. Using the -F option it is possible to get iftop to
show packets entering and leaving a given network. For example, iftop -F
10.0.0.0/255.0.0.0 will analyse packets flowing in and out of the 10.* network.

Some other filter ideas:

not ether host ff:ff:ff:ff:ff:ff
Ignore ethernet broadcast packets.

port http and not host webcache.example.com
Count web traffic only, unless it is being directed through a local web
cache.

icmp How much bandwidth are users wasting trying to figure out why the network
is slow?

OPTIONS
-h Print a summary of usage.

-n Donât do hostname lookups.

-N Do not resolve port number to service names

-p Run in promiscuous mode, so that traffic which does not pass directly
through the specified interface is also counted.

-P Turn on port display.

-b Donât display bar graphs of traffic.

-B Display bandwidth rates in bytes/sec rather than bits/sec.

-i interface
Listen to packets on interface.
-f filter code
Use filter code to select the packets to count. Only IP packets are ever
counted, so the specified code is evaluated as (filter code) and ip.

-F net/mask
Specifies a network for traffic analysis. If specified, iftop will only
include packets flowing in to or out of the given network, and packet
direction is determined relative to the network boundary, rather than to
the interface. You may specify mask as a dotted quad, such as
/255.255.255.0, or as a single number specifying the number of bits set in
the netmask, such as /24.

-c config file
Specifies an alternate config file. If not specified, iftop will use
~/.iftoprc if it exists. See below for a description of config files

DISPLAY
When running, iftop uses the whole screen to display network usage. At the top of
the display is a logarithmic scale for the bar graph which gives a visual indicaâ
tion of traffic.

The main part of the display lists, for each pair of hosts, the rate at which data
has been sent and received over the preceding 2, 10 and 40 second intervals. The
direction of data flow is indicated by arrows, <= and =>. For instance,

foo.example.com => bar.example.com 1Kb 500b 100b
<= 2Mb 2Mb 2Mb

shows, on the first line, traffic from foo.example.com to bar.example.com; in the
preceding 2 seconds, this averaged 1Kbit/s, around half that amount over the preâ
ceding 10s, and a fifth of that over the whole of the last 40s. During each of
those intervals, the data sent in the other direction was about 2Mbit/s. On the
actual display, part of each line is inverted to give a visual indication of the
10s average of traffic. You might expect to see something like this where host
foo is making repeated HTTP requests to bar, which is sending data back which satâ
urates a 2Mbit/s link.

By default, the pairs of hosts responsible for the most traffic (10 second averâ
age) are displayed at the top of the list.

At the bottom of the display, various totals are shown, including peak traffic
over the last 40s, total traffic transferred (after filtering), and total transfer
rates averaged over 2s, 10s and 40s.

SOURCE / DEST AGGREGATION
By pressing s or d while iftop is running, all traffic for each source or destinaâ
tion will be aggregated together. This is most useful when iftop is run in
promiscuous mode, or is run on a gateway machine.

PORT DISPLAY
S or D toggle the display of source and destination ports respectively. p will
toggle port display on/off.

DISPLAY TYPE
t cycles through the four line display modes; the default 2-line display, with
sent and received traffic on separate lines, and 3 1-line displays, with sent,
received, or total traffic shown.

DISPLAY ORDER
By default, the display is ordered according to the 10s average (2nd column). By
pressing 1, 2 or 3 it is possible to sort by the 1st, 2nd or 3rd column. By
pressing < or > the display will be sorted by source or destination hostname
respectively.

DISPLAY FILTERING
l allows you to enter a POSIX extended regular expression that will be used to
filter hostnames shown in the display. This is a good way to quickly limit what
is shown on the display. Note that this happens at a much later stage than filter
code, and does not affect what is actually captured. Display filters DO NOT
affect the totals at the bottom of the screen.

PAUSE DISPLAY / FREEZE ORDER
P will pause the current display.

o will freeze the current screen order. This has the side effect that traffic
between hosts not shown on the screen at the time will not be shown at all,
although it will be included in the totals at the bottom of the screen.

SCROLL DISPLAY
j and k will scroll the display of hosts. This feature is most useful when the
display order is frozen (see above).

FILTER CODE
f allows you to edit the filter code whilst iftop running. This can lead to some
unexpected behaviour.

CONFIG FILE
iftop can read its configuration from a config file. If the -c option is not
specified, iftop will attempt to read its configuration from ~/.iftoprc, if it
exists. Any command line options specified will override settings in the config
file.

The config file consists of one configuration directive per line. Each directive
is a name value pair, for example:

interface: eth0

sets the network interface. The following config directives are supported:

interface: if
Sets the network interface to if.

dns-resolution: (yes|no)
Controls reverse lookup of IP addresses.

port-resolution: (yes|no)
Controls conversion of port numbers to service names.

filter-code: bpf
Sets the filter code to bpf.

show-bars: (yes|no)
Controls display of bar graphs.

promiscuous: (yes|no)
Puts the interface into promiscuous mode.

port-display: (off|source-only|destination-only|on)
Controls display of port numbers.

hide-source: (yes|no)
Hides source host names.

hide-destination: (yes|no)
Hides destination host names.

use-bytes: (yes|no)
Use bytes for bandwidth display, rather than bits.

sort: (2s|10s|40s|source|destination)
Sets which column is used to sort the display.

line-display: (two-line|one-line-both|one-line-sent|one-line-received)
Controls the appearance of each item in the display.

show-totals: (yes|no)
Shows cumulative total for each item.

log-scale: (yes|no)
Use a logarithmic scale for bar graphs.

max-bandwidth: bw
Fixes the maximum for the bar graph scale to bw, e.g. "10M"

net-filter: net/mask
Defines an IP network boundary for determining packet direction.

screen-filter: regexp
Sets a regular expression to filter screen output.

QUIRKS (aka theyâre features, not bugs)
There are some circumstances in which iftop may not do what you expect. In most
cases what it is doing is logical, and we believe it is correct behaviour,
although Iâm happy to hear reasoned arguments for alternative behaviour.

Totals donât add up

There are several reasons why the totals may not appear to add up. The most obviâ
ous is having a screen filter in effect, or screen ordering frozen. In this case
some captured information is not being shown to you, but is included in the
totals.

A more subtle explanation comes about when running in promiscuous mode without
specifying a -F option. In this case there is no easy way to assign the direction
of traffic between two third parties. For the purposes of the main display this
is done in an arbitrary fashion (by ordering of IP addresses), but for the sake of
totals all traffic between other hosts is accounted as incoming, because thatâs
what it is from the point of view of your interface. The -F option allows you to
specify an arbitrary network boundary, and to show traffic flowing across it.

Peak totals donât add up

Again, this is a feature. The peak sent and peak received didnât necessarily hapâ
pen at the same time. The peak total is the maximum of sent plus received in each
captured time division.

Changing the filter code doesnât seem to work
cases what it is doing is logical, and we believe it is correct behaviour,
although Iâm happy to hear reasoned arguments for alternative behaviour.

Totals donât add up

There are several reasons why the totals may not appear to add up. The most obviâ
ous is having a screen filter in effect, or screen ordering frozen. In this case
some captured information is not being shown to you, but is included in the
totals.

A more subtle explanation comes about when running in promiscuous mode without
specifying a -F option. In this case there is no easy way to assign the direction
of traffic between two third parties. For the purposes of the main display this
is done in an arbitrary fashion (by ordering of IP addresses), but for the sake of
totals all traffic between other hosts is accounted as incoming, because thatâs
what it is from the point of view of your interface. The -F option allows you to
specify an arbitrary network boundary, and to show traffic flowing across it.

Peak totals donât add up

Again, this is a feature. The peak sent and peak received didnât necessarily hapâ
pen at the same time. The peak total is the maximum of sent plus received in each
captured time division.

Changing the filter code doesnât seem to work

Give it time. Changing the filter code affects what is captured from the time
that you entered it, but most of what is on the display is based on some fraction
of the last 40s window of capturing. After changing the filter there may be
entries on the display that are disallowed by the current filter for up to 40s.
DISPLAY FILTERING has immediate effect and does not affect what is captured.

FILES
~/.iftoprc
Configuration file for iftop.

SEE ALSO
tcpdump(8), pcap(3), driftnet(1).

AUTHOR
Paul Warren <[email protected]>

VERSION
$Id: iftop.8,v 1.25 2005/12/25 11:50:21 pdw Exp $

COPYING
This program is free software; you can redistribute it and/or modify it under the
terms of the GNU General Public License as published by the Free Software Foundaâ
tion; either version 2 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this
program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Camâ
bridge, MA 02139, USA.

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