[Hikikomori]

Cool! I did something similar when I wanted to learn Go, but did my own parsers instead of using gopacket, I would recommend doing that yourself if you want to learn more low level stuff.

How I parsed IP for example:

  type Addr [4]uint8
  
  func (ip Addr) String() string {
   return fmt.Sprintf("%d.%d.%d.%d", ip[0], ip[1], ip[2], ip[3])
  }
  
  type Hdr struct {
   Version    uint8
   IHL        uint8
   DSCP       uint8
   ECN        uint8
   Length     uint16
   Id         uint16
   Flags      uint8
   Fragoffset uint16
   TTL        uint8
   Protocol   uint8
   Checksum   uint16
   Src        Addr
   Dst        Addr
  }
  
  func (hdr *Hdr) Parse(d []byte) error {
   hdr.Version = uint8(d[0] >> 4)
   hdr.IHL = uint8(d[0] & 0x0f)
   hdr.DSCP = uint8(d[1] >> 6)
   hdr.ECN = uint8(d[1] & 0x03)
   hdr.Length = uint16(binary.BigEndian.Uint16(d[2:4]))
   hdr.Id = uint16(binary.BigEndian.Uint16(d[4:6]))
   hdr.Flags = uint8(d[6] >> 5)
   hdr.Fragoffset = uint16(binary.BigEndian.Uint16(d[6:8])) & 0x1fff
   hdr.TTL = d[8]
   hdr.Protocol = d[9]
   hdr.Checksum = uint16(binary.BigEndian.Uint16(d[10:12]))
   hdr.Src = Addr{d[12], d[13], d[14], d[15]}
   hdr.Dst = Addr{d[16], d[17], d[18], d[19]}
  
   if hdr.IHL > 5 {
    fmt.Println("extra options detected") // TODO: support for extra options
   }
   return nil
  }

[lixiasky]

Thanks a lot for sharing this — it's super helpful!

Yeah, I’m currently using gopacket mainly to get something working fast, but I’ve been thinking about writing my own parsers from scratch to understand the protocols better.

Your Hdr example is really clean — definitely saving this as reference! I love how direct and readable it is.

I’ll definitely try going lower level when I revisit the packet layer logic. Thanks again for the nudge

[0xbadcafebee]

Seconding this. Implementing low level protocols from scratch is a great introduction to network programming (do the kids today ever do network programming, or is it all just 15 layers of libraries on top of HTTP?). Good to understand the underpinnings of the systems you work with, and how subtly complex things get down there.