Golang: how to check if an HTTP request is in the TCP payload bytes

Question

I am using the gopacket package and every time I have a TCP packet I want to check if the payload contains an HTTP request. Is there an easy way to do that instead of writing my own parser? There is also a function (see: func ReadRequest(b *bufio.Reader)) which returns a Request struct but I do not know what kind of input I should use. tcp.Payload is the byte[] array that seems to have the information I need to parse (see the following example):

// Get the TCP layer from this packet
if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer != nil {
    fmt.Printf("TCP ")
    // Get actual TCP data from this layer
    tcp, _ := tcpLayer.(*layers.TCP)
    srcPort = tcp.SrcPort
    dstPort = tcp.DstPort
    if tcp.SYN {
        fmt.Print("[SYN] ")
    }
    if tcp.ACK {
        fmt.Print("[ACK] ")
    }
    if tcp.FIN {
        fmt.Print("[FIN] ")
    }
    fmt.Printf("%s:%d > %s:%d ", srcIP, srcPort, dstIP, dstPort)

    fmt.Println(string(tcp.Payload))
}

After sending an HTTP request I get the following output:

PKT [001] TCP [SYN] 192.168.2.6:59095 > 192.168.3.5:80
PKT [002] TCP [SYN] [ACK] 192.168.3.5:80 > 192.168.2.6:59095
PKT [003] TCP [ACK] 192.168.2.6:59095 > 192.168.3.5:80
PKT [004] TCP [ACK] 192.168.2.6:59095 > 192.168.3.5:80 GET /certificates/test.pdf HTTP/1.1
User-Agent: Wget/1.15 (linux-gnu)
Accept: */*
Host: 192.168.3.5
Connection: Keep-Alive

Any suggestions are welcome...

Do you mean you want to check if a packet has the start of an HTTP request? A single request is likely to span multiple packets. — Mr_Pink
– Mr_Pink, Commented Aug 22, 2016 at 14:50
Yes, let's go for the simple case where the payload has only one HTTP request and without missing parts — tgogos
– tgogos, Commented Aug 22, 2016 at 14:54
Yes, if the payload contains a the complete request you can feed it to http.ReadRequst. This simpler case isn't really useful, since you're operating on individual packets of a TCP stream. It fails immediately when you consider the possibility that a request can start in the middle of the packet payload, and it's still very likely that the request will span multiple packets. HTTP is at a higher level than TCP, and you need to reassemble the stream (flow) before you can parse the HTTP correctly. — Mr_Pink
– Mr_Pink, Commented Aug 22, 2016 at 15:08

tgogos · Accepted Answer · 2017-10-19 08:24:29Z

1

With help from this question: How to parse http headers in Go the following attempt with http.ReadRequest() works...

if tcpLayer := packet.Layer(layers.LayerTypeTCP); tcpLayer != nil {
   tcp, _ := tcpLayer.(*layers.TCP)
   if len(tcp.Payload) != 0 {
      reader := bufio.NewReader(bytes.NewReader(tcp.Payload))
      httpReq, err := http.ReadRequest(reader)
      ...
  }
}

edited Oct 19, 2017 at 8:24

answered Aug 25, 2016 at 15:37

tgogos

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