1. Overview
Creating a TCP server application listening on a certain port is trivial when we use a high-level programming language. However, we might want to have a quick and simple socket server without learning a new programming language. Moreover, we might be unable to install or use the compiler or the interpreter on the system due to constrained privileges or resources.
In this tutorial, we’ll use the tools available in Bash to create a simple TCP socket server.
All commands and examples in this guide have been tested on Debian 12 running GNU Bash 5.2.15, netcat-openbsd 1.219, socat 1.7.4.4, and ncat 7.93.
2. Using netcat
To begin, we’ll use netcat. netcat is a flexible networking utility that we can use for a variety of purposes involving TCP and UDP protocols.
2.1. Installation
netcat is commonly available on most Unix-like systems. However, there are usually two packages of netcat already automatically installed on our system:
$ apt search netcat
...
netcat-openbsd/stable,now 1.219-1 amd64 [installed,automatic]
TCP/IP swiss army knife
netcat-traditional/stable,now 1.10-47 amd64 [installed]
TCP/IP swiss army knife
...netcat-traditional is the original netcat codebase. On the other hand, netcat-openbsd is more secure and feature-rich compared to the traditional version. netcat-openbsd derives from the OpenBSD project.
In case they’re not installed on our system, we can install the netcat-openbsd package with the apt command:
$ sudo apt install netcat-openbsd
$ nc -h
OpenBSD netcat (Debian patchlevel 1.219-1)
...After the installation finishes, we can run the nc binary with the -h option to check its version.
Both netcat-* packages contain the same binary – nc. However, on Debian 12, the default binary is the nc binary from the netcat-openbsd package:
$ whereis nc
nc: /usr/bin/nc /usr/share/man/man1/nc.1.gz
$ readlink -f /usr/bin/nc
/usr/bin/nc.openbsdReferring to the commands above, we used the whereis command to learn the location of the nc binary. Then, we used the readlink command to determine whether the path is an absolute path or a symbolic link (symlink) and retrieve the target of the symlink.
2.2. Creating a TCP Socket Server
Let’s use netcat to create a TCP socket server listening on port 8080:
$ nc -k -l 8080The -k option tells netcat to continue listening after the first connection completes rather than exiting the program. The nc command processes the incoming connections sequentially, not concurrently. In addition, the -l option instructs netcat to operate in listening mode.
While the server’s running, we open another terminal as a client to connect to the server by using the same command – nc:
$ nc localhost 8080Once the client is connected to the server, we can type random characters to send to the server, followed by pressing the Enter key. Subsequently, those characters should appear on the server-side terminal.
Furthermore, the socket connection is a two-way communication, meaning the server can also send messages to the client:
We can close the socket connection by simply pressing Ctrl + C on either the client- or server-side terminal.
We’ve successfully created a simple sequential TCP socket server using netcat in Bash.
3. Using socat
socat (SOcket CAT) is a feature-rich networking utility. It’s designed as a multi-purpose relay utility and offers many features, such as port forwarding and redirection, proxying/tunneling, and virtual terminal/serial port.
However, we can also it to create a simple TCP socket server.
3.1. Installation
The socat package is also available on most Unix-like systems under the canonical name socat:
$ apt search socat
socat/stable,now 1.7.4.4-2 amd64
multipurpose relay for bidirectional data transfer
$ sudo apt install socat
$ socat -V
...
socat version 1.7.4.4 on 06 Nov 2022 08:15:51 ...
...Once the installation is finished, we can check its version by passing the -V option.
3.2. Creating a TCP Socket Server
Let’s create a TCP socket server listening on port 8080:
$ socat TCP-LISTEN:8080,fork -Let’s review the options:
- TCP-LISTEN:8080: listen on port 8080
- fork: fork a new process for each incoming connection
- –: use the standard input and output for communication with the connected clients
By having the fork option, the TCP socket server is capable of handling concurrent requests. However, since each child process inherits the standard input/output descriptors from its parent process, this can lead to unexpected behavior when we try to send messages from the server.
Once the server is running, let’s open another terminal as the client and create a socket connection to the server:
$ socat STDIO TCP4:localhost:8080We also used the socat command to open the socket connection.
Let’s break down the command:
- STDIO: use the standard input/output (stdin/stdout) for communication
- TCP4:localhost:8080: connect to the localhost server on port 8080
Once the client is connected, we can simply type the message on the terminal, followed by pressing the Enter key. As a result, the message should appear on the server-side terminal.
Additionally, the socket connection enables two-way communication, so we can send and receive messages from both client- and server-side terminals:
If we want to close the socket connection, we can simply press Ctrl + C on either the client- or server-side terminal.
We’ve managed to create a simple concurrent TCP socket server using socat in a Bash environment.
4. Using ncat
ncat is a reimplementation or netcat. However, it’s designed with security in mind and offers broader features compared to netcat.
4.1. Installation
The ncat package is available on most Unix-like systems under the name ncat:
$ apt search ncat
...
ncat/stable 7.93+dfsg1-1 amd64
NMAP netcat reimplementation
...
$ sudo apt install ncat
$ ncat --version
Ncat: Version 7.93 ( https://nmap.org/ncat )After installing the package, we can verify the installation by checking its version, as shown in the output above.
4.2. Creating a TCP Socket Server
Since ncat is a reimplementation of netcat, they both share similar options.
Let’s create a TCP socket server:
$ ncat -k -l 8080The -k option means the ncat process keeps running after the client closes the connection. In addition, the -l option tells ncat to run in listening mode. Similar to netcat, the ncat command processes the incoming connections sequentially, not concurrently.
While the server’s running, we open another terminal to act as a client to connect to the server by using the same command – ncat:
$ ncat localhost 8080After the client connects to the server, we can send messages to the server by typing in the terminal and pressing the Enter key. Subsequently, those messages should appear on the server-side terminal.
Since the socket connection is a two-way communication, the server can also send messages to the client:
To close the socket connection, we can simply press Ctrl + C on either the client- or server-side terminal.
At this point, we’ve also successfully created a simple sequential TCP socket server using ncat in a Bash environment.
5. Conclusion
In this article, we learned how to create a simple TCP socket server in a Bash environment. We explored three networking utilities that are commonly available in most Linux official repositories: netcat, socat, and ncat. They can all be utilized to create a socket server, as well as to act as a socket client.
While all three are networking utilities, there are some differences. The netcat command is a simple networking utility to do basic networking tasks, while ncat is a reimplementation of the netcat codebase with security in mind. Therefore, both tools have many similar command options. On the other hand, socat is a more advanced version of the other two and offers even more features.
Additionally, both netcat and ncat process incoming connections sequentially, whereas socat supports concurrency, allowing it to handle multiple connections simultaneously.