Create a TCP and UDP Client and Server using Go

Go is a compiled, statically typed programming language developed by Google. Many modern applications, including Docker, Kubernetes, and Terraform, are written in Go. Go packages allow developers to organize and reuse Go code in a simple and maintainable manner.

In this guide, you will use the net package, which is a part of Go’s standard library, to create TCP and UDP servers and clients. This guide is meant to provide instructional examples to help you become more familiar with the Go programming language.

Throughout this guide you will create the following:

• A TCP server and client. The TCP server accepts incoming messages from a TCP client and responds with the current date and time.
• A UDP server and client. The UDP server accepts incoming messages from a UDP client and responds with a random number.
• A concurrent TCP server that accepts incoming messages from several TCP clients and responds with the number of clients currently connected to it.

1. If you are not familiar with using Go packages, review the Getting Started with Go Packages guide.

2. Install Go on your computer if it is not already installed. You can follow our guide How to Install Go on Ubuntu for installation steps.

   This guide requires Go version 1.8 or higher. It is considered good practice to have the latest version of Go installed. You can check your Go version by executing the following command:

    go version
   

Note

This guide is written for a non-root user. Depending on the TCP/IP port number that you use when running the TCP and UDP servers, you may need to prefix commands with sudo. If you are not familiar with the sudo command, see the Users and Groups guide.

Go’s net package provides a portable interface for network I/O, including TCP/IP, UDP, domain name resolution, and Unix domain sockets. You will use this package to create TCP and UDP servers and clients in this guide.

Use the table below as a quick reference for some of the net package functions used throughout this guide. To view all types and functions included in the net package, see Golang’s official documentation.

Note

All versions of net.Dial() and net.Listen() return data types that implement the io.Reader and io.Writer interfaces. This means that you can use regular File I/O functions to send and receive data from a TCP/IP connection.

In this section, you will create a generic TCP client and server using Go. After creating the client and server, you will run them to test their connection with each other.

Note

The netcat command line utility can be used to test TCP/IP client and server connections.

The TCP client that you will create in this section will allow you to interact with any TCP server.

1. In your current working directory, create a file named tcpC.go with the following content:

   File: ./tcpC.go

   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

   package main import ( "bufio" "fmt" "net" "os" "strings" ) func main() { arguments := os.Args if len(arguments) == 1 { fmt.Println("Please provide host:port.") return } CONNECT := arguments[1] c, err := net.Dial("tcp", CONNECT) if err != nil { fmt.Println(err) return } for { reader := bufio.NewReader(os.Stdin) fmt.Print(">> ") text, _ := reader.ReadString('\n') fmt.Fprintf(c, text+"\n") message, _ := bufio.NewReader(c).ReadString('\n') fmt.Print("->: " + message) if strings.TrimSpace(string(text)) == "STOP" { fmt.Println("TCP client exiting...") return } } }

• This file creates the main package, which declares the main() function. The function will use the imported packages to create a TCP client.
• The main() function gathers command line arguments in the arguments variable and makes sure that a value for host:port was sent.
• The CONNECT variable stores the value of arguments[1]to be used in the net.Dial() call.
• A call to net.Dial() begins the implementation of the TCP client and will connect you to the desired TCP server. The second parameter of net.Dial() has two parts; the first is the hostname or the IP address of the TCP server and the second is the port number the TCP server listens on.
• bufio.NewReader(os.Stdin) and ReadString() is used to read user input. Any user input is sent to the TCP server over the network using Fprintf().
• bufio reader and the bufio.NewReader(c).ReadString('\n') statement read the TCP server’s response. The error variable is ignored here for simplicity.
• The entire for loop that is used to read user input will only terminate when you send the STOP command to the TCP server.

You are now ready to create the TCP server. The TCP server will return the current date and time to the TCP client using a single network packet.

1. In your current working directory, create a file named tcpS.go with the following content:

   File: ./tcpS.go

   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

   package main import ( "bufio" "fmt" "net" "os" "strings" "time" ) func main() { arguments := os.Args if len(arguments) == 1 { fmt.Println("Please provide port number") return } PORT := ":" + arguments[1] l, err := net.Listen("tcp", PORT) if err != nil { fmt.Println(err) return } defer l.Close() c, err := l.Accept() if err != nil { fmt.Println(err) return } for { netData, err := bufio.NewReader(c).ReadString('\n') if err != nil { fmt.Println(err) return } if strings.TrimSpace(string(netData)) == "STOP" { fmt.Println("Exiting TCP server!") return } fmt.Print("-> ", string(netData)) t := time.Now() myTime := t.Format(time.RFC3339) + "\n" c.Write([]byte(myTime)) } }

• This file creates the main package, which declares the main() function. The function will use the imported packages to create a TCP server.
• The main() function gathers command line arguments in the arguments variable and includes error handling.
• The net.Listen() function makes the program a TCP server. This functions returns a Listener variable, which is a generic network listener for stream-oriented protocols.
• It is only after a successful call to Accept() that the TCP server can begin to interact with TCP clients.
• The current implementation of the TCP server can only serve the first TCP client that connects to it, because the Accept() call is outside of the for loop. In the Create a Concurrent TCP Server section of this guide, you will see a TCP server implementation that can serve multiple TCP clients using Goroutines.
• The TCP server uses regular File I/O functions to interact with TCP clients. This interaction takes place inside the for loop. Similarly to the TCP client, when the TCP server receives the STOP command from the TCP client, it will terminate.

You can now test your TCP client and server. You will need to execute the TCP server first so that the TCP client has somewhere it can connect to.

1. Run your TCP server. From the directory containing the tcpS.go file, run the following command:

    go run tcpS.go 1234
   

   The server will listen on port number 1234. You will not see any output as a result of this command.

2. Open a second shell session to execute the TCP client and to interact with the TCP server. Run the following command:

    go run tcpC.go 127.0.0.1:1234
   

   Note

   If the TCP server is not running on the expected TCP port, you will get the following error message from tcpC.go:

   dial tcp [::1]:1234: connect: connection refused
   

3. You will see a >> prompt waiting for you to enter some text. Type in Hello! to receive a response from the TCP server:

    Hello!
   

   You should see a similar output:

   >> Hello!
   ->: 2019-05-23T19:43:21+03:00
       

4. Send the STOP command to exit the TCP client and server:

    STOP
   

   You should see a similar output in the client:

   >> STOP
   ->: TCP client exiting...
       

   The output on the TCP server side will resemble the following:

   -> Hello!
   Exiting TCP server!
       

Note

The TCP server waits before writing back to the TCP client, whereas the client writes to the TCP server first and then waits to receive an answer. This behavior is part of the protocol definition that governs a TCP or a UDP connection. In this example, you have implemented an unofficial protocol that is based on TCP.

In this section, you will create a UDP client and server. After creating the client and server, you will run them both to test their connection with each other. A UDP client can be generic and can communicate with multiple UDP servers. On the other hand, a UDP server cannot be completely generic, because it typically implements a specific functionality. In the case of our UDP server example, it will return random numbers to UDP clients that connect to it.

The UDP client that you will create in this section will allow you to interact with any UDP server.

1. In your current working directory, create a file named udpC.go with the following content:

   File: ./udpC.go

   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

   package main import ( "bufio" "fmt" "net" "os" "strings" ) func main() { arguments := os.Args if len(arguments) == 1 { fmt.Println("Please provide a host:port string") return } CONNECT := arguments[1] s, err := net.ResolveUDPAddr("udp4", CONNECT) c, err := net.DialUDP("udp4", nil, s) if err != nil { fmt.Println(err) return } fmt.Printf("The UDP server is %s\n", c.RemoteAddr().String()) defer c.Close() for { reader := bufio.NewReader(os.Stdin) fmt.Print(">> ") text, _ := reader.ReadString('\n') data := []byte(text + "\n") _, err = c.Write(data) if strings.TrimSpace(string(data)) == "STOP" { fmt.Println("Exiting UDP client!") return } if err != nil { fmt.Println(err) return } buffer := make([]byte, 1024) n, _, err := c.ReadFromUDP(buffer) if err != nil { fmt.Println(err) return } fmt.Printf("Reply: %s\n", string(buffer[0:n])) } }

   • This file creates the main package, which declares the main() function. The function will use the imported packages to create a UDP client.
   • The main() function gathers command line arguments in the arguments variable and includes error handling.
   • Regular File I/O functions are used by the UDP client to interact with the UDP server. The client will terminate when you send the STOP command to the UDP server. This is not part of the UDP protocol, but is used in the example to provide the client with a way to exit.
   • A UDP end point address is returned by the net.ResolveUDPAddr() function. The UDP end point is of type UDPAddr and contains IP and port information.
   • The connection to the UDP server is established with the use of the net.DialUDP() function.
   • bufio.NewReader(os.Stdin) and ReadString() is used to read user input.
   • The ReadFromUDP() function reads a packet from the server connection and will return if it encounters an error.

You are now ready to create the UDP server. You will write the UDP server code to respond to any connected client with random numbers.

1. In your current working directory, create a file named udps.go with the following content:

   File: ./udpS.go

   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

   package main import ( "fmt" "math/rand" "net" "os" "strconv" "strings" "time" ) func random(min, max int) int { return rand.Intn(max-min) + min } func main() { arguments := os.Args if len(arguments) == 1 { fmt.Println("Please provide a port number!") return } PORT := ":" + arguments[1] s, err := net.ResolveUDPAddr("udp4", PORT) if err != nil { fmt.Println(err) return } connection, err := net.ListenUDP("udp4", s) if err != nil { fmt.Println(err) return } defer connection.Close() buffer := make([]byte, 1024) rand.Seed(time.Now().Unix()) for { n, addr, err := connection.ReadFromUDP(buffer) fmt.Print("-> ", string(buffer[0:n-1])) if strings.TrimSpace(string(buffer[0:n])) == "STOP" { fmt.Println("Exiting UDP server!") return } data := []byte(strconv.Itoa(random(1, 1001))) fmt.Printf("data: %s\n", string(data)) _, err = connection.WriteToUDP(data, addr) if err != nil { fmt.Println(err) return } } }

   • This file creates the main package, which declares the main() function. The function will use the imported packages to create a UDP server.
   • The main() function gathers command line arguments in the arguments variable and includes error handling.
   • The net.ListenUDP() function tells the application to listen for incoming UDP connections, which are served inside the for loop. This is the function call that makes the program a UDP server.
   • The ReadFromUDP() and WriteToUDP() functions are used to read data from a UDP connection and write data to a UDP connection, respectively. A byte slice is stored in the data variable and used to write the desired data. The buffer variable also stores a byte slice and is used to read data.
   • Since UDP is a stateless protocol, each UDP client is served and then the connection closes automatically. The UDP server program will only exit when it receives the STOP keyword from a UDP client. Otherwise, the server program will continue to wait for more UDP connections from other clients.

You can now test your UDP client and server. You will need to execute the UDP server first so that the UDP client has somewhere it can connect to.

1. Run your UDP server. From the directory containing the udpS.go file, run the following command:

    go run udpS.go 1234
   

   The server will listen on port number 1234. You will not see any output as a result of this command.

2. Open a second shell session to execute the UDP client and to interact with the UDP server. Run the following command:

    go run udpC.go 127.0.0.1:1234
   

3. You will see a >> prompt waiting for you to enter some text. Type in Hello! to receive a response from the UDP server:

    Hello!
   

   You should see a similar output:

   The UDP server is 127.0.0.1:1234
   >> Hello!
   Reply: 82
       

4. Send the STOP command to exit the UDP client and server:

   You should see a similar output on the client side:

   >> STOP
   Exiting UDP client!
       

   The output on the UDP server side will be as follows:

   -> STOP
   Exiting UDP server!
       

This section demonstrates the implementation of a concurrent TCP server. The benefit of a concurrent TCP server is that it can serve multiple clients. In Go, this is accomplished by creating a separate Goroutine to serve each TCP client.

The example TCP server keeps a running count of the number of TCP clients it has served so far. The counter increases by one each time a new TCP client connects to the TCP server. The current value of that counter is returned to each TCP client.

1. In your current working directory, create a file named concTCP.go with the following content:

   File: ./concTCP.go

   1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59

   package main import ( "bufio" "fmt" "net" "os" "strconv" "strings" ) var count = 0 func handleConnection(c net.Conn) { fmt.Print(".") for { netData, err := bufio.NewReader(c).ReadString('\n') if err != nil { fmt.Println(err) return } temp := strings.TrimSpace(string(netData)) if temp == "STOP" { break } fmt.Println(temp) counter := strconv.Itoa(count) + "\n" c.Write([]byte(string(counter))) } c.Close() } func main() { arguments := os.Args if len(arguments) == 1 { fmt.Println("Please provide a port number!") return } PORT := ":" + arguments[1] l, err := net.Listen("tcp4", PORT) if err != nil { fmt.Println(err) return } defer l.Close() for { c, err := l.Accept() if err != nil { fmt.Println(err) return } go handleConnection(c) count++ } }

   • This file creates the main package, which declares the handleConnection() and main() functions.
   • The main() function will use the imported packages to create a concurrent TCP server. It gathers command line arguments in the arguments variable and includes error handling.
   • Each TCP client is served by a separate Goroutine that executes the handleConnection() function. This means that while a TCP client is served, the TCP server is free to interact with more TCP clients. TCP clients are connected using the Accept() function.
   • Although the Accept() function can be executed multiple times, the net.Listen() function needs to be executed only once. For this reason the net.Listen() function remains outside of the for loop.
   • The for loop in the main() function is endless because TCP/IP servers usually run nonstop. However, if the handleConnection() function receives the STOP message, the Goroutine that runs it will exit and the related TCP connection will close.

In this section, you will test the concurrent TCP server using the netcat command line utility.

1. Run your concurrent TCP server. From the directory containing the concTCP.go file, run the following command:

    go run concTCP.go 1234
   

   The command creates a TCP server that listens on port number 1234. You can use any port number, however, ensure it is not already in use and that you have the required privileges. Reference the list of well-known TCP and UDP ports, if needed.

2. Use netcat to establish a connection with the TCP server. By default, netcat will establish a TCP connection with a remote host on the specified port number.

    nc 127.0.0.1 1234
   

3. After issuing the previous command, you will not see any change in your output. Type Hello! to send a packet to the TCP server:

    Hello!
   

   The TCP server will return the number of current client connections as its response. Since this is your first connection established with the TCP server, you should expect an output of 1.

   Hello!
   1
       

   If you’d like, you can open a new shell session and use netcat to establish a second connection with the TCP server by repeating Step 2. When you send the server a second Hello!, you should receive a response of 2 this time.

4. You can also connect to the TCP server using the TCP client you created in the Create the TCP Client section of the guide. Ensure you are in the directory containing the tcpC.go file and issue the following command:

    go run tcpC.go 127.0.0.1:1234
   

5. You will see a >> prompt waiting for you to enter some text. Type in Hello! to receive a response from the TCP server:

    Hello!
   

   You should see a similar output indicating 3 client connections:

   >> Hello!
   ->: 3
       

6. Send the STOP command to exit the TCP client:

   You should see a similar output on the client:

   >> STOP
   ->: TCP client exiting...
         

   The output on the TCP server side will be as follows:

   .Hello!
   .Hello!
   .Hello!
         

   Note

   From the shell session running the TCP server, type CTRL-c to interrupt program execution and then, CTRL-D to close all client connections and to stop the TCP server.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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