Access to raw Ethernet frames from Racket

If Racket needs a packet from a socket or a port,
you might just find it's nowhere near as easy as you thought.
Up in complicated ioctls you'll be caught,
Ensuring error-freedom is particularly fraught!

Dashing to the rescue is this polished little package:
A tiny C extension, with a pinch of Racket hackage,
Relieves you of your burden and returns you to your slackage,
And competently fixes Racket's packet socket lackage.

— tonyg, with apologies to Gene Ziegler

What?

Read and write raw Ethernet frames from Racket programs.

How?

(require packet-socket)

(display (raw-interface-names))
(newline)

(define handle (raw-interface-open "eth0:1"))
(unless handle (error 'example "Couldn't open the device"))

(define packet (raw-interface-read handle)) ;; blocks

(raw-interface-write handle packet)

(raw-interface-close handle)

You will need to take care of Ethernet frame headers, footers, checksums and addressing yourself. You may find the Racket package bitsyntax useful to you in manipulating binary data structures.

You will also need to give the Racket executable CAP_NET_RAW capability on Linux in order to create packet sockets. One approach to this could be:

sudo setcap cap_net_raw+p+i+e `which racket`

Be warned that ethernet has a 64-byte minimum packet size. Less four bytes for the checksum, two for the ethertype, and twelve for the sender and target MAC addresses, that leaves 46 bytes of payload in such a minimum-sized packet. If you are expecting to receive packets with fewer than 46 bytes of payload, then make sure to handle the zero-padding that gets added to the end by the ethernet! For example, IPv4 packets have an explicit whole-packet length indicator for just such a situation.

Who?

Copyright (c) 2014 Jonathan Schuster
Copyright (c) 2014 Tony Garnock-Jones

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Bugs

• BPF exists not only on OSX, but also on many other BSDs. The #ifdefs in the support code should be widened to allow it to run there as well.