Netcat
Rachele Weishaar
netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. The command is designed to be a dependable 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.
Rewrites like GNU's and OpenBSD's support additional features. For example, OpenBSD's nc supports TLS, and GNU netcat natively supports a tunneling mode supporting UDP and TCP (optionally allowing one to be tunneled over the other) in a single command,[3] where other versions may require piping data from one netcat instance to another.
There are several implementations on POSIX systems, including rewrites from scratch like GNU netcat[5] or OpenBSD netcat,[6]the latter of which supports IPv6 and TLS. The OpenBSD version has been ported to the FreeBSD base,[7]Windows/Cygwin,[8]and Linux.[9]Mac OS X comes with netcat installed as of OSX 10.13 or users can use MacPorts to install a variant.[10]
A DOS version of netcat called NTOOL is included in the FreeDOS Package group Networking.[11] It is based on the WatTCP stack and licensed under the European Union Public Licence Version 1.1.[12]
Socat[15] 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 Diffie-Hellman parameter which had been hard-coded into the OpenSSL implementation of socat.[16] 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 software vulnerability.[17] 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.[16]
In the middle of 2005, Nmap announced another netcat incarnation called Ncat.[19] 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.
I had to read this several times. WTF. Use netcat? Surely I was missing some wizardry in the FreeBSD kernel that configured a network printer as some local network target aliased to netlaser, which handled all the printing magic for me. But how? Years of arbitrarily picking from a list of similarly named print drivers in CUPS prevented my brain from accepting what was written. I was confused. After trying to decipher the other sections in the document, I decided I would go for it.
After 10+ years of Linux distributions pushing CUPS on me. Over a decade of this complex stack of drivers and daemons that I never quite trusted but \u201Ccouldn\u2019t live without.\u201D FreeBSD comes along and is like \u201Cyeah, just use netcat.\u201D And they\u2019re right. Just use netcat.
Verbose mode
In netcat, Verbose is a mode which can be initiated using [-v] parameter. Now verbose mode generates extended information. Basically, we will connect to a server using netcat two times to see the difference between normal and verbose modes.
The command is nc 192.168.17.43 21 -v
Historically, there is more than one implementation of netcat, most importantly the GNU and the OpenBSD one. I guess rootssh relies on the latter, which allows to use UNIX domain sockets via the -U option.
(FYI, @linev)
Pardon the noob question, but if I have pihole on a Raspbian (dietpi) system, do I need to build netcat-openbsd specially for my system now that it's an explicit dependency? There is "no installation candidate" in apt. Thanks in advance for your help.
I want to use netcat to dump a number of blocks of strings to a file. I've been running this in my linux terminal and everything works very well until I try to detach my command line. If I run my command as a detached session netcat stops after the first round of strings.
netcat wants to send its stdin across the socket and print out the response, so it expects to have them both connected and active. I don't know a good way of creating a command that waits forever, but you could try
Netcat should be available on almost any modern Linux distribution. Ubuntu ships with the BSD variant of netcat, and this is what we will be using in this guide. Other versions may operate differently or provide other options.
Netcat is not restricted to sending TCP and UDP packets. It also can listen on a port for connections and packets. This gives us the opportunity to connect two instances of netcat in a client-server relationship.
Because we are establishing a regular TCP connection, we can transmit just about any kind of information over that connection. It is not limited to chat messages that are typed in by a user. We can use this knowledge to turn netcat into a file transfer program.
You should now have a pretty good idea as to what netcat can be used for. It is a versatile tool that can be useful to diagnose problems and verify that base-level functionality is working correctly with TCP/UDP connections.
Using netcat, you can communicate between different computers very easily for quick interactions. Netcat attempts to make network interactions transparent between computers by taking the complexity out of forming connections.
Obligatory vendor-bash: If "nc" had become a standard utility years ago, the commercial vendors would have likely packaged it setuid root and with -DGAPING_SECURITY_HOLE turned on but not documented. It is hoped that netcat will aid people in finding and fixing the no-brainer holes of this sort that keep appearing, by allowing easier experimentation with the "bare metal" of the network layer.
I can use "netcat -e" to run other application, for example imap or pop3. section "#ifdef GAPING_SECURITY_HOLE" may contain a bug that allows you to run a shell, which is why it's ifdefed out by default. ifdef wraping security-critical section.
On some occasions, like zfs send/recv, there might be IO bottlenecks, on receiver side mainly.Replacing netcat with mbuffer might be helpful here, as it will buffer IO instead of letting sender wait.Here's an example of using mbuffer with a 1GB data buffer and 128k send/receive buffer:
By itself netcat doesn't have encryption or authentication controls so while the traffic could be encrypted via OpenSSL or GnuPG with some clever piping and a bit if loops on the listening side, if you're on an untrustworthy network you might lose the race-condition to read using it and netcat doesn't allow multiple clients without clever scripting... but I'm not here to bash a tool but instead notify you of a Bash tool ;-)
To use netcat to learn more, you need to pass it the correct data to elicit a valid response. That means you have to understand http if you want to find out if it's running a web server, smtp if it's running a mail sender, etc. You have to know what port a web server runs on, the name of the server, the protocol, everything.
"Port-scanning is a popular method for exploring what's out there. Netcat accepts its commands with options first, then the target host, and everything thereafter is interpreted as port names or numbers, or ranges of ports in M-N syntax. CAVEAT: some port names in /etc/services contain hyphens -- netcat currently will not correctly parse those, so specify ranges using numbers if you can. If more than one port is thus specified, netcat connects to all of them, sending the same batch of data from standard input [up to 8K worth] to each one that is successfully connected to. Specifying multiple ports also suppresses diagnostic messages about refused connections, unless -v is specified twice for "more verbosity". This way you normally get notified only about genuinely open connections. Example: nc -v -w 2 -z target 20-30 will try connecting to every port between 20 and 30 [inclusive] at the target, and will likely inform you about an FTP server, telnet server, and mailer along the way. The -z switch prevents sending any data to a TCP connection and very limited probe data to a UDP connection, and is thus useful as a fast scanning mode just to see what ports the target is listening on. To limit scanning speed if desired, -i will insert a delay between each port probe. There are some pitfalls with regard to UDP scanning, described later, but in general it works well."
For example, the spark streaming example uses the netcat utility (for example nc -lk 5005). However, I have another service that listens for incoming messages and then spit out a message. So I need some kind of server that can listen to messages from service A and pass them to spark.
Unix like systems allow us to pipeline the output of one command to the input of another, which is what makes this possible. We take the output of the tar archiving utility and push it into netcat to move over the network.
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