当我从Chrome WebSocket发送127个以上的字符时，我的Golang服务器看不到超过126个字符

Question

I'm having a blast reinventing the wheel and playing with bits to implement a simple server. It's almost functional, but I'm not sure if this issue is my client or my server. Here is the function where I pass the resulting byte array from net.Conn Read

func readWsFrame(p []byte) {
    // process first byte
    b := p[0]
    fmt.Printf("first byte: %b
", b)
    fin := b & 128 // hopefully 128, for fin
    op := b & 15   // hopefully 1, for text
    fmt.Printf("fin: %d
op: %d
", fin, op)
    // process second byte
    b = p[1]
    fmt.Printf("second byte: %b
", b)
    masked := b & 128 // whether or not the payload is masked
    length := b & 127 // payload length
    fmt.Printf("masked: %d
length: %d
", masked, length)
    // process bytes 3-7 (masking key)
    key := p[2:6]
    // payload
    d := p[6:]
    if length == 126 {
        key = p[4:8]
        d = p[8:]
        fmt.Println("med key")
    } else if length == 127 {
        key = p[10:14]
        d = p[14:]
        fmt.Println("big key")
    } else {
        fmt.Println("lil key")
    }
    fmt.Printf("masking key: %b
", key)
    fmt.Printf("masked data: %b
", d)
    var decoded []byte
    for index := 0; index < int(length); index++ {
        decoded = append(decoded, d[index]^key[index%4])
    }
    fmt.Printf("unmasked data: %b
", decoded)
    payload := string(decoded)
    fmt.Println("payload: ", payload)
}

The client code is me having the dev console open right off this web page and running

var ws = new WebSocket("ws://localhost:16163");
ws.send("a".repeat(125))
ws.send("a".repeat(126))
ws.send("a".repeat(127))
ws.send("a".repeat(400))

My server is doing what I expect until I reach 127 characters. At that point, and every amount over 126, my 2nd byte is 11111110 and the length is 126. I can see the unmasked/encoded/magic message doesn't go beyond 126 a's.

I'm sure my go code is sub-par and there might be something obvious here, but I'm looking at the bits themselves, and I can see a 0 where I am expecting a 1, please help me, thank you!

I saw a similar question about writing messages larger than 126 bytes and how I'll need extra bytes for payload size, but in this case my client is the chrome web browser.

--edit:

I realize that I will never see more than 126 characters based on the loop I have there, but I should still see a 1 in the final bit in the second byte for these larger messages, right?

--edit:

Came across this how to work out payload size from html5 websocket

I guess I misunderstood everything else I was searching for. Can someone confirm this? If the length is <126, the length is byte & 127. If the length is 126, the length is the value of the next 2 bytes. If the length is 127, the length is the next 4 bytes.

I thought initially that the length would be 127 if it payload length was 127+ hah, oops. So when the length is 126 or 127, the 2nd byte is not part of the actual length? I'll probably confirm all of this with testing, but I thank you all for resolving this issue before the weekend so I can finish this side project.

Answer 1

The code should update the length property after realizing it's 127 or 126 by reading the length data in the bytes that follow the initial length indicator.

I would consider those 7 "length" bits slightly differently. They don't really indicate length as much as they indicate encoding.

If the length is encoded using 64bits (8 bytes), than the length indicator == 127.

If the length is encoded in 2 bytes, than the indicator == 126.

Otherwise, the length is encoded in the 7 bits of the indicator itself.

For example, the length 60 can be encoded in all three ways (albeit, some use more space).

There's a C Websocket implementation here if you want to read an example decoding of the length.

Good Luck!