View source for Netcat

{{Short description|Computer networking utility}}
{{Lowercase}}
{{Redirect|Ncat||NCAT (disambiguation){{!}}NCAT}}
{{Infobox software
| name = netcat
| logo = 
| screenshot = [[File:Netcat.png|frameless]]
| caption = The {{code|netcat}} command
| author = <nowiki>*Hobbit*</nowiki>
| developer = Avian Research
| released = {{Start date and age|1995|10|28}}<ref>{{cite web
| url=https://seclists.org/bugtraq/1995/Oct/28
| title=New tool available: Netcat
| author=Hobbit
| date=1995-10-28
| work=[[Bugtraq]] mailing list
| access-date=2019-06-05}}</ref>
| latest_release_version = 1.10
| latest_release_date = {{Start date and age|1996|03||df=yes}}
| operating_system = [[Unix]] and [[Unix-like]], [[DOS]], [[Microsoft Windows]], [[Windows CE]]
| genre = [[Computer network|Network]] [[Utility software|utility]]
| license = Original version: custom, [[Permissive free software licence|permissive]] license<br />GNU version: [[GNU General Public License|GPL]]<br />OpenBSD version: [[BSD licenses|BSD]]
| website = {{URL|nc110.sourceforge.io}}
| discontinued = yes
}}

'''netcat''' (often abbreviated to '''nc''') is a computer networking utility for reading from and writing to network connections using [[Transmission Control Protocol|TCP]] or [[User Datagram Protocol|UDP]]. The [[command (computing)|command]] is designed to be a dependable [[front and back ends|back-end]] that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of connection its user could need and has a number of built-in capabilities.

It is able to perform [[Port scanner|port scanning]], [[File transfer|file transferring]] and port listening.

==Features==
The original netcat's features include:<ref>{{Cite book |last=Vacca |first=John R. |title=Guide to Wireless Network Security |publisher=[[Springer Publishing]] |year=2006 |isbn=978-0387954257 |pages=266 |language=en}}</ref>

* Outbound or inbound connections, TCP or UDP, to or from any ports
* Full [[Reverse DNS lookup|DNS forward/reverse checking]], with appropriate warnings
* Ability to use any local source port
* Ability to use any locally configured network source address
* Built-in port-scanning capabilities, with randomization
* Built-in loose source-routing capability
* Can read command line arguments from standard input
* Slow-send mode, one line every N seconds
* [[Hex dump]] of transmitted and received data
* Optional ability to let another program service establish connections
* Optional [[telnet]]-options responder

Rewrites like GNU's and OpenBSD's support additional features. For example, OpenBSD's nc supports [[Transport Layer Security|TLS]], and GNU netcat natively supports a [[Tunneling protocol|tunneling]] mode supporting UDP and TCP (optionally allowing one to be tunneled over the other) in a single command,<ref>{{cite web
| url = http://netcat.sourceforge.net/
| title = The GNU Netcat project
| author = Giovanni Giacobbi
| date = 2006-11-01
| access-date = 2020-03-22}}</ref> where other versions may require piping data from one netcat instance to another.

==Ports and reimplementations==
The original version of netcat was a [[Unix]] program. The last version (1.10) was released in March 1996.<ref name="nc110">{{cite web |date=2008-02-14 |title=Netcat 1.10 |url=http://nc110.sourceforge.net/ |access-date=2019-06-05 |website=nc110.sourceforge.net}}</ref>

There are several implementations on [[POSIX]] systems, including rewrites from scratch like [[GNU]] netcat<ref>{{cite web
| url = http://netcat.sourceforge.net/
| title = The GNU Netcat project
| author = Giovanni Giacobbi
| date = 2006-11-01
| access-date = 2019-06-05}}</ref> or [[OpenBSD]] netcat,<ref>{{cite web
| url = https://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/nc/
| title = OpenBSD CVSWeb: /src/usr.bin/nc/
| publisher = [[OpenBSD]]
| access-date =  2019-06-05}}</ref>
the latter of which supports IPv6 and [[Transport Layer Security|TLS]]. The OpenBSD version has been ported to the [[FreeBSD]] base,<ref>{{cite web
| url = https://svnweb.freebsd.org/base/release/5.4.0/usr.bin/nc/Makefile?revision=141394&view=markup
| title = Contents of /release/5.4.0/usr.bin/nc/Makefile
| author = delphij
| date = 2005-02-06
| publisher=[[FreeBSD]]
| access-date = 2019-06-05}}</ref>
Windows/Cygwin,<ref>{{cite web
| url = https://www.daemon.de/Netcat
| title = Netcat OpenBSD Cygwin Port 1.10.2.3
| author = Thomas Linden
| website = Daemon.de
| date = 2011-03-02
| access-date = 2019-06-05}}</ref>
and Linux.<ref>[https://tracker.debian.org/pkg/netcat-openbsd Debian netcat-openbsd]</ref>
[[macOS|Mac OS X]] comes with netcat installed as of OSX 10.13 or users can use [[MacPorts]] to install a variant.<ref>{{cite web
| url = https://www.macports.org/ports.php?by=name&substr=netcat
| title = MacPorts Portfiles: netcat
| publisher = [[MacPorts]]
| access-date = 2019-06-05}}</ref>

A [[DOS]] version of ''netcat'' called ''NTOOL'' is included in the [[FreeDOS]] Package group ''Networking''.<ref>{{cite web
| url=https://www.ibiblio.org/pub/micro/pc-stuff/freedos/files/distributions/1.2/repos/pkg-html/group-net.html
| title=ibiblio.org FreeDOS Group -- Networking
| date=2019-03-03
| publisher=[[ibiblio]]
| access-date=2019-06-05}}</ref> It is based on the WatTCP stack and licensed under the [[European Union Public Licence]] Version 1.1.<ref>{{cite web
| url=https://www.bttr-software.de/products/jhoffmann/
| title=various tools
| author=Jürgen Hoffmann
| date=2018-11-03
| access-date=2019-06-05}}</ref>

Known ports for embedded systems includes versions for [[Windows CE]] (named "Netcat 4 wince"<ref>{{cite web
| url = https://www.dr-bischoff.de/wince/Andreas_WINCE_stuff.html#netcat
| title = Netcat 4 wince
| author = Andreas Bischoff
| date = 2010-06-07
| access-date = 2019-06-05}}</ref>) or for the [[iPhone]].<ref>{{cite web
| url = http://svn.telesphoreo.org/trunk/data/netcat/
| title = Revision 835: /trunk/data/netcat
| date = 2008-08-18
| access-date = 2019-06-05}}</ref>

[[BusyBox]] includes by default a lightweight version of netcat named mini-netcat.<ref>{{Cite web|url=https://git.busybox.net/busybox/tree/networking/nc.c?h=1_37_0|title=nc: mini-netcat - built from the ground up for LRP|website=git.busybox.net}}</ref> Another version with more features, named bloaty nc, is also available.<ref>{{Cite web|url=https://git.busybox.net/busybox/tree/networking/nc_bloaty.c?h=1_37_0|title=busybox - bloaty nc|website=git.busybox.net}}</ref>

[[Solaris (operating system)|Solaris]] 11 includes netcat implementation based on OpenBSD netcat.<ref>{{Cite web|url=https://docs.oracle.com/cd/E36784_01/html/E37476/gnqeb.html|title= Oracle Solaris 11.2 Information Library - Performing TCP and UDP Administration With the netcat Utility|website=docs.oracle.com}}</ref>

Socat<ref>{{cite web 
| url = http://www.dest-unreach.org/socat/
| title = socat - Multipurpose relay
| date = 2019-04-06
| access-date = 2019-06-05}}</ref> is a more complex variant of ''netcat''. It is larger and more flexible and has more options that must be configured for a given task.  On February 1, 2016, [[Santiago Zanella-Beguelin]] and [[Microsoft Vulnerability Research]] issued a security advisory regarding a [[composite number|composite]] [[Diffie-Hellman]] parameter which had been [[hard-coded]] into the [[OpenSSL]] implementation of ''socat''.<ref name="socat-secadv7">{{cite web
| url=http://www.dest-unreach.org/socat/contrib/socat-secadv7.html
| title=Socat security advisory 7
| date=2016-02-01
| access-date=2019-06-05}}</ref> The implausibility that a composite might have been unintentionally introduced where a [[prime number]] is required has led to the suspicion of [[sabotage]] to introduce a [[backdoor (computing)|backdoor]] [[software vulnerability]].<ref>{{cite web
| url=https://www.theregister.co.uk/2016/02/03/socat_backdoor_fix/
| title=Socat slams backdoor, sparks thrilling whodunit
| first=Richard
| last=Chirgwin
| date=2016-02-03
| publisher=[[The Register]]
| access-date=2019-06-05}}</ref> This socat bug affected version 1.7.3.0 and 2.0.0-b8 it was corrected in following releases from 1.7.3.1 and 2.0.0-b9.<ref name="socat-secadv7"/>

Cryptcat<ref>{{cite web
| url = http://cryptcat.sourceforge.net/
| title = CryptCat Project
| date = 2005-10-18
| access-date = 2019-06-05}}</ref> is a version of ''netcat'' with integrated transport encryption capabilities.

In the middle of 2005, [[Nmap]] announced another netcat incarnation called Ncat.<ref name="ncat">{{cite web
| url = https://nmap.org/ncat/
| title = Ncat - Netcat for the 21st Century
| publisher = [[Nmap]]
| date = 2009-07-08
| access-date = 2019-06-05}}</ref> It features new possibilities such as "Connection Brokering", TCP/UDP Redirection, SOCKS4 client and server support, ability to "Chain" Ncat processes, HTTP CONNECT proxying (and proxy chaining), SSL connect/listen support and IP address/connection filtering. Like [[Nmap]], Ncat is [[cross-platform]].

On some systems, modified versions or similar netcat utilities go by the command name(s) <code>nc</code>, <code>ncat</code>, <code>pnetcat</code>, <code>socat</code>, <code>sock</code>, <code>socket</code>, <code>sbd</code>.

===ncat===
Ncat is a similar tool to netcat provided by [[Nmap]] suite.<ref name="ncat"/> ''"While Ncat isn't built on any code from the “traditional” Netcat (or any other implementation), Ncat is most definitely based on Netcat in spirit and functionality."''<ref>{{Cite web
| url=https://manpages.debian.org/stretch/nmap/ncat.1.en.html
| title=ncat(1) - Ncat Reference Guide
| date=2016-12-18
| website=manpages.debian.org
| access-date=2019-06-05}}</ref>

Ncat features includes: ability to chain Ncats together, redirect both TCP and UDP ports to other sites, SSL support, and proxy connections via [[SOCKS4]] or HTTP (CONNECT method) proxies (with optional proxy authentication as well).<ref>{{Cite web|url=https://nmap.org/ncat/|title = Ncat - Netcat for the 21st Century}}</ref>

==See also==
{{Portal|Free and open-source software}}
* [[List of Unix commands]]
*{{anli|cat (Unix)}}
* [[Nmap]]
* [[OpenSSL]]
* [[Telnet]]
* [[Plink]]
* [[Packet Sender]]
* [[Banner grabbing]]

==References==
{{Reflist}}

==External links==
* {{official website|https://nc110.sourceforge.io}}
* {{man|1|nc|die.net}}

{{Unix commands}}

[[Category:Free network management software]]
[[Category:Network analyzers]]
[[Category:Port scanners]]
[[Category:Unix network-related software]]
[[Category:Windows network-related software]]
[[Category:Free software programmed in C]]
[[Program network station  program 4.2