flowtop { [options] }


flowtop is a top-like connection tracking tool that can run on an end host or small home router. It is able to present TCP, UDP/UDP-lite, SCTP, DCCP, and ICMP(v6) flows that have been collected by the kernel's netfilter connection tracking framework, thus no packet capturing in user space needs to be done.

flowtop is able to give you a quick overview of current connections on your local system, e.g. for debugging purposes or to answer questions like:

    * If you access website X, what other connections are being opened in
      the background that I'm not aware of?
    * What connections are active that pass one's router?
    * I have this proprietary binary Y, to where does it connect?
    * To which countries am I sending data?
    * Are there any suspicious background connections on my machine?
    * How many active connections does binary Y have?

The following information will be presented in flowtop's output:

    * Application name and PID when run on local machine
    * Reverse DNS for source and destination
    * Geo-location information (country, city)
    * Used protocols (IPv4, IPv6, TCP, UDP, SCTP, ICMP, ...)
    * Flow port's service name heuristic
    * Transport protocol state machine information

In order for flowtop to work, netfilter must be active and running on your machine, thus kernel-side connection tracking is active.

flowtop's intention is just to get a quick look over your active connections. If you want logging support, have a look at netfilter's conntrack(8) tools instead.


-4, --ipv4

Display IPv4 flows. That is the default when flowtop is started without any arguments.

-6, --ipv6

Display IPv6 flows. That is the default when flowtop is started without any arguments.

-T, --tcp

Display TCP flows. That is the default when flowtop is started without any arguments.

-U, --udp

Display UDP and UDP-lite flows.

-D, --dccp

Display DCCP flows.

-I, --icmp

Display ICMP version 4 and version 6 flows.

-S, --sctp

Display SCTP flows.

-s, --show-src

Also show source information of the flow, not only destination information.

-u, --update

The built-in database update mechanism will be invoked to get Maxmind's latest database. To configure search locations for databases, the file /etc/netsniff-ng/geoip.conf contains possible addresses. Thus, to save bandwidth or for mirroring Maxmind's databases (to bypass their traffic limit policy), different hosts or IP addresses can be placed into geoip.conf, separated by a newline.

-v, --version

Show version information and exit.

-h, --help

Show user help and exit.



Default ncurses output for flowtop that tracks IPv4, IPv6 flows for TCP.

flowtop -46UTDISs

This example enables the maximum display options for flowtop.


Files under /etc/netsniff-ng/ can be modified to extend flowtop's service resolution and lookup information.

    * tcp.conf - TCP port/services map
    * udp.conf - UDP port/services map
    * geoip.conf - GeoIP database mirrors


With a fairly high rate of connection tracking updates, flowtop can become unresponsive for short periods of time while scrolling. The right fix would be to replace flowtop's connection management backend with a better design with respect to the locking approach. This is still on the "todo" list.


flowtop is licensed under the GNU GPL version 2.0.


flowtop was originally written for the netsniff-ng toolkit by Daniel Borkmann. It is currently maintained by Tobias Klauser <[email protected]> and Daniel Borkmann <[email protected]>.

RELATED TO flowtop…


Manpage was written by Daniel Borkmann.


This page is part of the Linux netsniff-ng toolkit project. A description of the project, and information about reporting bugs, can be found at