flowtop man page

flowtop — top-like netfilter TCP/UDP/SCTP/DCCP/ICMP(v6) flow tracking


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?
* How long are connections active already?
* At which rate am I sending/receiving data?

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
* Byte/packet counters (if they are enabled)
* Connection duration (if timestamping is enabled)
* Flow send/receive rate (if byte/packet counters are enabled)

In order for flowtop to work, netfilter must be active and running on your machine, thus kernel-side connection tracking is active. If netfilter is not running, you can activate it with iptables(8):

iptables -A INPUT -p tcp -m state --state ESTABLISHED -j ACCEPT

iptables -A OUTPUT -p tcp -m state --state NEW,ESTABLISHED -j ACCEPT

To dump byte/packet counters flowtop enables the sysctl(8) parameter “net.netfilter.nf_conntrack_acct” via:

echo 1 > /proc/sys/net/netfilter/nf_conntrack_acct

and resets it to the previously set value on exit. These counters will only be active on connections which were created after accounting was enabled. Thus, to have these counters be active all the time the parameter should be enabled after the system is up. To automatically enable it, sysctl.conf(8) or sysctl.d(8) might be used.

To calculate the connection duration flowtop enables the sysctl(8) parameter “net.netfilter.nf_conntrack_timestamp” via:

echo 1 > /proc/sys/net/netfilter/nf_conntrack_timestamp

and resets it to the previously set value on exit.

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.

-n, --no-dns

Don't perform hostname lookup. Only numeric addresses will be shown for flow endpoints.

-G, --no-geoip

Don't perform GeoIP lookup. No geographical information will be shown for flow endpoints.

-s, --show-src

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

-b, --bits

Show flow rates in bits per second instead of bytes per second.

-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.

-t <time>, --interval <time>

Flow info refresh interval in seconds, default is 1s.

-v, --version

Show version information and exit.

-h, --help

Show user help and exit.

Usage Example


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

flowtop -46UTDISs

This example enables the maximum display options for flowtop.

Config Files

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 was originally written for the netsniff-ng toolkit by Daniel Borkmann. It is currently maintained by Tobias Klauser <tklauser@distanz.ch> and Daniel Borkmann <dborkma@tik.ee.ethz.ch>.

See Also

netsniff-ng(8), trafgen(8), mausezahn(8), ifpps(8), bpfc(8), astraceroute(8), curvetun(8), iptables(8), sysctl(8), sysctl.conf(8), sysctl.d(8)


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 http://netsniff-ng.org/.

Referenced By

astraceroute(8), bpfc(8), curvetun(8), ifpps(8), mausezahn(8), netsniff-ng(8), trafgen(8).

Explore man page connections for flowtop(8).

netsniff-ng toolkit 03 March 2013