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The Click Modular Router

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Click is a modular router toolkit. To use it you'll need to know how to compile and install the software, how to write router configurations, and how to write new elements. Our ACM Transactions on Computer Systems paper will give you a feeling for what Click can do. Using the optimization tools under CLICKDIR/tools, you can get even better performance than that paper describes.


Subdirectory Description
CLICKDIR/apps Click-related applications
CLICKDIR/apps/clicky GTK+ program for displaying configurations and interacting with drivers
CLICKDIR/apps/csclient Command-line program for interacting with drivers
CLICKDIR/apps/ClickController Java program for interacting with drivers
CLICKDIR/conf example configuration files
CLICKDIR/doc documentation
CLICKDIR/elements element source code
CLICKDIR/elements/analysis …for trace analysis and manipulation
CLICKDIR/elements/app …for application-level protocols (e.g. FTP)
CLICKDIR/elements/aqm …for active queue management (e.g. RED)
CLICKDIR/elements/ethernet …for Ethernet
CLICKDIR/elements/etherswitch …for an Ethernet switch
CLICKDIR/elements/grid …for the Grid mobile ad-hoc wireless network protocols
CLICKDIR/elements/icmp …for ICMP
CLICKDIR/elements/ip …for IPv4
CLICKDIR/elements/ip6 …for IPv6
CLICKDIR/elements/ipsec …for IPsec
CLICKDIR/elements/linuxmodule …for the Linux kernel driver
CLICKDIR/elements/local …for your own elements (empty)
CLICKDIR/elements/ns …for the NS network simulator driver
CLICKDIR/elements/radio …for communicating with wireless radios
CLICKDIR/elements/standard …for simple protocol-generic elements
CLICKDIR/elements/tcpudp …for TCP and UDP
CLICKDIR/elements/test …for regression tests
CLICKDIR/elements/threads …for thread management
CLICKDIR/elements/userlevel …for the user-level driver
CLICKDIR/elements/wifi …for 802.11
CLICKDIR/etc/samplepackage sample source code for Click element package
CLICKDIR/etc/samplellrpc sample source code for reading Click LLRPCs
CLICKDIR/etc/diagrams files for drawing Click diagrams
CLICKDIR/etc/libclick files for standalone user-level Click library
CLICKDIR/include/click common header files
CLICKDIR/include/clicknet header files defining network headers
CLICKDIR/lib common non-element source code
CLICKDIR/linuxmodule Linux kernel module driver
CLICKDIR/ns NS driver (integrates with the NS simulator)
CLICKDIR/test regression tests
CLICKDIR/tools Click tools
CLICKDIR/tools/lib …common code for tools
CLICKDIR/tools/click-align …enforces alignment for non-x86 machines
CLICKDIR/tools/click-combine …merges routers into combined configuration
CLICKDIR/tools/click-devirtualize …removes virtual functions from source
CLICKDIR/tools/click-fastclassifier …specializes Classifiers into C++ code
CLICKDIR/tools/click-mkmindriver …build environments for minimal drivers
CLICKDIR/tools/click-install …installs configuration into kernel module
CLICKDIR/tools/click-pretty …pretty-prints Click configuration as HTML
CLICKDIR/tools/click-undead …removes dead code from configurations
CLICKDIR/tools/click-xform …pattern-based configuration optimizer
CLICKDIR/tools/click2xml …convert Click language <-> XML
CLICKDIR/userlevel user-level driver


The file in this directory contains installation instructions. User documentation is in the doc subdirectory, which contains manual pages for the Click language, the Linux kernel module, and several tools; it also has a script that generates manual pages for many of the elements distributed in this package. To install these manual pages so you can read them, follow the instructions, but make install-man instead of make install.

Running a Click Router

Before playing with a Click router, you should get familiar with the Click configuration language. You use this to tell Click how to process packets. The language describes a graph of “elements,” or packet processing modules. See the doc/click.5 manual page for a detailed description, or check the conf directory for some simple examples.

Click can be compiled as a user-level program or as a kernel module for Linux. Either driver can receive and send packets; the kernel module directly interacts with device drivers, while the user-level driver uses packet sockets (on Linux) or the pcap library (everywhere else).

User-Level Program

Run the user-level program by giving it the name of a configuration file: click CONFIGFILE.

Linux Kernel Module

See the doc/click.o.8 manual page for a detailed description. To summarize, install a configuration by running click-install CONFIGFILE. This will also install the kernel module if necessary and report any errors to standard error. (You must run make install before click-install will work.)

NS-3 Simulator

See for more information. Further information on NS-3 and Click is available in the NS-3 manual.

NS-2 Simulator

See for more information. Once a Click-enabled version of NS-2 is installed, the 'ns' command is able to run Click scripts as part of a normal NS-2 simulation.


Click’s user-level driver supports DPDK. Before running in DPDK mode, the DPDK must be set up properly as per the DPDK documentation. This mainly involves setting up huge pages and binding some NIC to the DPDK userspace driver. E.g., to set up huge pages:

echo 1024 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages
mkdir -p /mnt/huge
mount -t hugetlbfs nodev /mnt/huge

On x86_64 you might achieve better performances with 1G huge pages, which must be enabled through the kernel cmdline.

Intel NICs use entierly userspace drivers and needs to be bound to DPDK. Eg., to bind eth0 to DPDK:

modprobe uio_pci_generic
dpdk/tools/ --bind=uio_pci_generic eth0

Refer to the DPDK documentation for more details about huge pages and binding devices.

Unlike most other DPDK applications, you have to pass DPDK EAL arguments between --dpdk and --, then pass Click arguments. As the DPDK EAL will handle thread management instead of Click, Click's -j/--threads argument will be disabled when --dpdk is active. You should give at least the following two EAL arguments for best practice. This is required with older versions of DPDK, even if running on a single core:

  • -c COREMASK: hexadecimal bitmask of cores to run on
  • -n NUM: number of memory channels

If -c or -l is not provided, DPDK will use all available cores.

A sample command to run a click configuration on 4 cores on a computer with 4 memory channels and listen for control connections on TCP port 8080 would be:

click --dpdk -c 0xf -n 4 -- -p 8080 configfile

If Click is launched without --dpdk, it will run in normal userlevel mode without involving DPDK EAL, meaning that any DPDK element will not work.


Some sample configurations are included in the conf directory, including a Perl script that generated the IP router configurations used in our TOCS paper (conf/ and a set of patterns for the click-xform pattern optimizer (conf/ip.clickpat).

Adding Your Own Elements

Please see the FAQ in this directory to learn how to add elements to Click.

Copyright and License

Most of Click is distributed under the Click license, a version of the MIT License. See the LICENSE file for details. Each source file should identify its license. Source files that do not identify a specific license are covered by the Click license.

Parts of Click are distributed under different licenses. The specific licenses are listed below.

  • drivers/e1000*, etc/linux-*-patch, linuxmodule/proclikefs.c: These portions of the Click software are derived from the Linux kernel, and are thus distributed under the GNU General Public License, version 2. The GNU General Public License is available via the Web and in etc/COPYING.

  • include/click/bigint.hh: This portion of the Click software derives from the GNU Multiple Precision Arithmetic Library, and is thus distributed under the GNU Lesser General Public License, version 3. This license is available via the Web and in etc/COPYING.lgpl.

Element code that uses only Click’s interfaces will not be derived from the Linux kernel. (For instance, those interfaces have multiple implementations, including some that run at user level.) Thus, for element code that uses only Click’s interfaces, the BSD-like Click license applies, not the GPL or the LGPL.

Bugs, Questions, etc.

We welcome bug reports, questions, comments, code, whatever you'd like to give us. GitHub issues are the best way to stay in touch.