Low level socket programming in Elixir

Questions / Help
1

I am trying to get low level socket programming in elixir

{:ok, socket} = :socket.open(:inet, :raw, :icmp)
:socket.bind(socket, %{:family => :inet, :port => 0})
:socket.ioctl(socket, :gifconf)

Gives me

{:ok, [%{addr: %{addr: {127, 0, 0, 1}, family: :inet, port: 0}, name: 'lo'}]}

But ip link gives me

#ip link
#1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
#    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
#2: veth0a@if3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default #qlen 1000
#    link/ether 5e:c8:38:a3:e6:50 brd ff:ff:ff:ff:ff:ff link-netns netns_br0

How do you get these two interfaces?

For testing, you can create execute this script, A bunch of helper functions to create Linux bridges, network namespaces, and interconnect everything using veth pairs. · GitHub

2

With which user to you run this?
When you are on Linux you normally need to be root to access raw ICMP sockets.

1 Like
3

I created a new linux namespace with the help of attached gist.

sudo nsenter --net=/var/run/netns/netns_veth0
4

If I understand nsenter correctly, it just spawns a new shell inside the network namespace, but you probably still need root priviliges or the CAP_NET_RAW capability to access raw sockets (but I never worked with these namespaces).

5

Not really, you can run ICMP socket as a regular user if your user ID is within sysctl net.ipv4.ping_group_range. However that will require you to use datagram socket instead of raw socket:

{:ok, socket} = :socket.open(:inet, :dgram, :icmp)

It also has slightly reduced API, as for example on Linux you cannot specify id and seq fields for ICMP packet, as these will be always set for you.

There is library that I have created in the past that provide slightly higher API for ICMP sockets (still quite limited):

It can be used in form of (to match the top message):

{:ok, socket} = :gen_icmp.open(raw: true)

:gen_icmp.echoreq(socket, {8, 8, 8, 8}, <<1,2,3,4>>)

receive do
  {:icmp, _addr, {:echorep, %{data: <<1,2,3,4>>}}} ->
    IO.puts("received pong")
end

And about the question of “how to get list of interfaces in the system”. Instead of playing with socket directly (especially as you are doing network namespacing, so obviously your view on interfaces will be limited) you should use:

:inet.getifaddrs()

That will return data like that:

{:ok,
 [
   {'lo0',
    [
      flags: [:up, :loopback, :running, :multicast],
      addr: {127, 0, 0, 1},
      netmask: {255, 0, 0, 0},
      addr: {0, 0, 0, 0, 0, 0, 0, 1},
      netmask: {65535, 65535, 65535, 65535, 65535, 65535, 65535, 65535},
      addr: {65152, 0, 0, 0, 0, 0, 0, 1},
      netmask: {65535, 65535, 65535, 65535, 0, 0, 0, 0}
    ]},
   {'gif0', [flags: [:pointtopoint, :multicast]]},
   {'stf0', [flags: []]},
   ...]}

That describe all interfaces available in your system and do not require any special privileges like being able to create ICMP raw socket.

2 Likes
6

To add some context, I was trying to replicate Networking Lab: Ethernet Broadcast Domains in elixir, the code in there uses python

7

But in this case you should not use ICMP protocol, but “just raw socket”, aka:

{:ok, socket} = :socket.open(:inet, :raw)

So this is pretty much different from your original question where you ask for the fetching list of the interfaces from raw ICMP socket.

:socket.ioctl(socket, :gifconf)

Will will return you only the configuration for the interface the socket is listening right now, not all that are available in the system.

Also I see the problem with your code - you define address map like %{:family => :inet, :port => 0} which is incorrect (weird that socket do not scream about it, or maybe it does, but you do not check the value returned by :socket.bind/2). It should be %{:family => :inet, :port => 0, :addr => address} as :addr field is mandatory according to the specs.

1 Like
8

I’m learning a ton about low level socket work from @whatyouhide’s protohacker YouTube videos.

Definitely worth a watch

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