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man pages section 8: System Administration Commands

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Updated: Wednesday, February 9, 2022

iftop (8)


iftop - display bandwidth usage on an interface by host


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


IFTOP(8)                    System Manager's Manual                   IFTOP(8)

       iftop - display bandwidth usage on an interface by host

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

       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.

       Solaris version of iftop support Solaris features like zone and  vanity
       naming.  For  example,  you can specify the name of a renamed interface
       generated by dladm(8) for iftop.  Additionally iftop can also  be  exe-
       cuted  in  exclusive IP stack zones, for it only use IP interfaces, the
       function and usage of iftop in exclusive IP stack zones are the same as
       it  in  global zone. As the isolation feature of Solaris zones, you can
       not see the network traffic of a non-global zone from  global  zone  or
       other non-global zones by iftop.

       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 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 direc-
       tion 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 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 bandwith are users wasting trying to figure out why the
              network is slow?

       -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 net-
              work, 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 /, 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

       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.exam-
       ple.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.

       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.

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

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

       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.

       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  actu-
       ally  captured.  Display filters DO NOT affect the totals at the bottom
       of the screen.

       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.

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

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

       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 sup-

       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 cummulative 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 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  alterna-
       tive 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  nec-
       essarily  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 disal-
       lowed by the current filter for up to 40s.  DISPLAY FILTERING has imme-
       diate effect and does not affect what is captured.

              Configuration file for iftop.

       This  program  does not have IPv6 support yet, this is an architectural
       bug that will be fixed.

       For Solaris version of iftop, it can not be executed properly in shared
       IP stack zones, this is a limitation bug of this program.

       See attributes(7) for descriptions of the following attributes:

       |Availability   | diagnostic/iftop |
       |Stability      | Uncommitted      |

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

       Paul Warren <pdw@ex-parrot.com>

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

       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 MER-
       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.

       Source  code  for open source software components in Oracle Solaris can
       be found at https://www.oracle.com/downloads/opensource/solaris-source-

       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source  was  downloaded from  http://www.ex-parrot.com/~pdw/iftop/down-

       Further information about this software can be found on the open source
       community website at http://www.ex-parrot.com/~pdw/iftop/.