If you would like to use the WebSocket API, it is useful if you have a server. In this article I will show you how to write one in C#. You can do it in any server-side language, but to keep things simple and more understandable, I chose Microsoft's language.
This server conforms to RFC 6455, so it will only handle connections from Chrome version 16, Firefox 11, IE 10 and over.
First steps
WebSockets communicate over a TCP (Transmission Control Protocol) connection. Luckily, C# has a TcpListener class which does as the name suggests. It is in the System.Net.Sockets namespace.
Note: It is a good idea to include the namespace with the using keyword in order to write less. It allows usage of a namespace's classes without typing the full namespace every time.
localaddr specifies the IP of the listener, and port specifies the port.
Note: To create an IPAddress object from a string, use the Parse static method of IPAddress.
Methods:
Start()
System.Net.Sockets.TcpClient AcceptTcpClient() Waits for a Tcp connection, accepts it and returns it as a TcpClient object.
Here's a barebones server implementation:
cs
usingSystem.Net.Sockets;usingSystem.Net;usingSystem;classServer{publicstaticvoidMain(){TcpListener server =newTcpListener(IPAddress.Parse("127.0.0.1"),80);
server.Start();
Console.WriteLine("Server has started on 127.0.0.1:80.{0}Waiting for a connection…", Environment.NewLine);TcpClient client = server.AcceptTcpClient();
Console.WriteLine("A client connected.");}}
TcpClient
Methods:
System.Net.Sockets.NetworkStream GetStream() Gets the stream which is the communication channel. Both sides of the channel have reading and writing capability.
Properties:
int Available This Property indicates how many bytes of data have been sent. The value is zero until NetworkStream.DataAvailable is true.
NetworkStream
Methods:
Writes bytes from buffer, offset and size determine length of message.
cs
Write(byte[] buffer,int offset,int size)
Reads bytes to buffer. offset and size determine the length of the message.
cs
Read(byte[] buffer,int offset,int size)
Let us extend our example.
cs
TcpClient client = server.AcceptTcpClient();
Console.WriteLine("A client connected.");NetworkStream stream = client.GetStream();//enter to an infinite cycle to be able to handle every change in streamwhile(true){while(!stream.DataAvailable);byte[] bytes =newbyte[client.Available];
stream.Read(bytes,0, bytes.Length);}
Handshaking
When a client connects to a server, it sends a GET request to upgrade the connection to a WebSocket from a simple HTTP request. This is known as handshaking.
This sample code can detect a GET from the client. Note that this will block until the first 3 bytes of a message are available. Alternative solutions should be investigated for production environments.
cs
usingSystem.Text;usingSystem.Text.RegularExpressions;while(client.Available <3){// wait for enough bytes to be available}byte[] bytes =newbyte[client.Available];
stream.Read(bytes,0, bytes.Length);//translate bytes of request to stringString data = Encoding.UTF8.GetString(bytes);if(Regex.IsMatch(data,"^GET")){}else{}
The response is easy to build, but might be a little difficult to understand. The full explanation of the Server handshake can be found in RFC 6455, section 4.2.2. For our purposes, we'll just build a simple response.
You must:
Obtain the value of the "Sec-WebSocket-Key" request header without any leading or trailing whitespace
Concatenate it with "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" (a special GUID specified by RFC 6455)
Compute SHA-1 and Base64 hash of the new value
Write the hash back as the value of "Sec-WebSocket-Accept" response header in an HTTP response
After a successful handshake, the client will send encoded messages to the server.
If we send "MDN", we get these bytes:
129 131 61 84 35 6 112 16 109
Let's take a look at what these bytes mean.
The first byte, which currently has a value of 129, is a bitfield that breaks down as such:
FIN (Bit 0)
RSV1 (Bit 1)
RSV2 (Bit 2)
RSV3 (Bit 3)
Opcode (Bit 4:7)
1
0
0
0
0x1=0001
FIN bit: This bit indicates whether the full message has been sent from the client. Messages may be sent in frames, but for now we will keep things simple.
RSV1, RSV2, RSV3: These bits must be 0 unless an extension is negotiated which supplies a nonzero value to them.
Opcode: These bits describe the type of message received. Opcode 0x1 means this is a text message. Full list of Opcodes
The second byte, which currently has a value of 131, is another bitfield that breaks down as such:
MASK (Bit 0)
Payload Length (Bit 1:7)
1
0x83=0000011
MASK bit: Defines whether the "Payload data" is masked. If set to 1, a masking key is present in Masking-Key, and this is used to unmask the "Payload data". All messages from the client to the server have this bit set.
Payload Length: If this value is between 0 and 125, then it is the length of message. If it is 126, the following 2 bytes (16-bit unsigned integer) are the length. If it is 127, the following 8 bytes (64-bit unsigned integer) are the length.
Note: Because the first bit is always 1 for client-to-server messages, you can subtract 128 from this byte to get rid of the MASK bit.
Note that the MASK bit is set in our message. This means that the next four bytes (61, 84, 35, and 6) are the mask bytes used to decode the message. These bytes change with every message.
The remaining bytes are the encoded message payload.
Decoding algorithm
D_i = E_i XOR M_(i mod 4)
where D is the decoded message array, E is the encoded message array, M is the mask byte array, and i is the index of the message byte to decode.
Example in C#:
cs
byte[] decoded =newbyte[3];byte[] encoded =newbyte[3]{112,16,109};byte[] mask =newbyte[4]{61,84,35,6};for(int i =0; i < encoded.Length; i++){
decoded[i]=(byte)(encoded[i]^ mask[i %4]);}
Put together
wsserver.cs
cs
//// csc wsserver.cs// wsserver.exeusingSystem;usingSystem.Net;usingSystem.Net.Sockets;usingSystem.Text;usingSystem.Text.RegularExpressions;classServer{publicstaticvoidMain(){string ip ="127.0.0.1";int port =80;var server =newTcpListener(IPAddress.Parse(ip), port);
server.Start();
Console.WriteLine("Server has started on {0}:{1}, Waiting for a connection…", ip, port);TcpClient client = server.AcceptTcpClient();
Console.WriteLine("A client connected.");NetworkStream stream = client.GetStream();// enter to an infinite cycle to be able to handle every change in streamwhile(true){while(!stream.DataAvailable);while(client.Available <3);// match against "get"byte[] bytes =newbyte[client.Available];
stream.Read(bytes,0, bytes.Length);string s = Encoding.UTF8.GetString(bytes);if(Regex.IsMatch(s,"^GET", RegexOptions.IgnoreCase)){
Console.WriteLine("=====Handshaking from client=====\n{0}", s);// 1. Obtain the value of the "Sec-WebSocket-Key" request header without any leading or trailing whitespace// 2. Concatenate it with "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" (a special GUID specified by RFC 6455)// 3. Compute SHA-1 and Base64 hash of the new value// 4. Write the hash back as the value of "Sec-WebSocket-Accept" response header in an HTTP responsestring swk = Regex.Match(s,"Sec-WebSocket-Key: (.*)").Groups[1].Value.Trim();string swka = swk +"258EAFA5-E914-47DA-95CA-C5AB0DC85B11";byte[] swkaSha1 = System.Security.Cryptography.SHA1.Create().ComputeHash(Encoding.UTF8.GetBytes(swka));string swkaSha1Base64 = Convert.ToBase64String(swkaSha1);// HTTP/1.1 defines the sequence CR LF as the end-of-line markerbyte[] response = Encoding.UTF8.GetBytes("HTTP/1.1 101 Switching Protocols\r\n"+"Connection: Upgrade\r\n"+"Upgrade: websocket\r\n"+"Sec-WebSocket-Accept: "+ swkaSha1Base64 +"\r\n\r\n");
stream.Write(response,0, response.Length);}else{bool fin =(bytes[0]&0b10000000)!=0,
mask =(bytes[1]&0b10000000)!=0;// must be true, "All messages from the client to the server have this bit set"int opcode = bytes[0]&0b00001111,// expecting 1 - text message
offset =2;ulong msglen = bytes[1]&0b01111111;if(msglen ==126){// bytes are reversed because websocket will print them in Big-Endian, whereas// BitConverter will want them arranged in little-endian on windows
msglen = BitConverter.ToUInt16(newbyte[]{ bytes[3], bytes[2]},0);
offset =4;}elseif(msglen ==127){// To test the below code, we need to manually buffer larger messages — since the NIC's autobuffering// may be too latency-friendly for this code to run (that is, we may have only some of the bytes in this// websocket frame available through client.Available).
msglen = BitConverter.ToUInt64(newbyte[]{ bytes[9], bytes[8], bytes[7], bytes[6], bytes[5], bytes[4], bytes[3], bytes[2]},0);
offset =10;}if(msglen ==0){
Console.WriteLine("msglen == 0");}elseif(mask){byte[] decoded =newbyte[msglen];byte[] masks =newbyte[4]{ bytes[offset], bytes[offset +1], bytes[offset +2], bytes[offset +3]};
offset +=4;for(ulong i =0; i < msglen;++i)
decoded[i]=(byte)(bytes[offset + i]^ masks[i %4]);string text = Encoding.UTF8.GetString(decoded);
Console.WriteLine("{0}", text);}else
Console.WriteLine("mask bit not set");
Console.WriteLine();}}}}