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 displays 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 confusing 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 n 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 Don’t 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 Don’t 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 indication 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 preceding 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 saturates a 2Mbit/s link.
By default, the pairs of hosts responsible for the most traffic (10
second average) 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 destination 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".
Note that the value has to always be in bits, regardless if the
option to display in bytes has been chosen.
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 they’re 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 I’m happy to hear reasoned arguments for
alternative behaviour.
Totals don’t add up
There are several reasons why the totals may not appear to add up. The
most obvious 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 that’s 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 don’t add up
Again, this is a feature. The peak sent and peak received didn’t
necessarily happen at the same time. The peak total is the maximum of
sent plus received in each captured time division.
Changing the filter code doesn’t 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 <pdw@ex-parrot.com>
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 Foundation; 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, Cambridge, MA 02139, USA.