Retrieving first non-private ip for local system in elixir

Question

I've recently started learning elixir, and the following is my attempt at fetching the local IP address of the system.

It can definitely be improved further, but as this would be my first ever project in a functional language; I'd be open to any criticisms.

defmodule Systemip do
 def private_ipv4() do
 {:ok, addrs} = :inet.getif()
 filtered =
 Enum.filter(
 addrs,
 fn address ->
 ip = elem(address, 0)
 is_private_ipv4(ip)
 end
 )
 elem(hd(filtered), 0)
 end
 defp is_private_ipv4(ip) do
 case ip do
 {10, _x, _y, _z} ->
 true
 {192, 168, _x, _y} ->
 true
 {172, 16, _x, _y} ->
 true
 {172, 32, _x, _y} ->
 true
 _ ->
 false
 end
 end
end

This would be part of a larger project, which is still under construction. The IPs are fetched using the erlang call to :inet.getif, the formatting was done by mix format command.

• Darragh Enright: Very interesting code! Definitely willing to take a crack at this, which I'll do as soon as I get a chance. I deleted an earlier question that was obvious when I read your question properly :)

Answer 1

There is few problems there:

1. There is no :inet.getif/0 function listed in inet module docs, which mean that if such function exists, then it is non-public API and should not be called. You probably wanted to call :inet.getifaddrs/0.
2. hd/1 will fail if filtered list is empty
3. You traverse whole list to find only first entry.
4. In Elixir, by convention, boolean returning functions, that aren't guard safe (so all user-defined boolean returning functions), should use ? suffix instead of is_ prefix.
5. Use pattern matching instead of elem/2 if possible.
6. This will raise error if there will be error during fetching interfaces addresses.
7. Your IP ranges are invalid, as whole 172.16.0.0/12 is private, which is range from 172.16.0.0 to 172.31.255.255 where you were checking only for 172.16.c.d and 172.32.c.d where the second group is in public space.

---

To fix all the issues there you can write it as:

defmodule SystemIP do
 def private_ipv4() do
 with {:ok, addrs} <- :inet.getif() do
 Enum.find_value(addrs, fn {if_name, if_opts} ->
 case Keyword.fetch(if_opts, :addr) do
 {:ok, adr} -> if not public_ipv4?(addr), do: addr
 _ -> false
 end
 end)
 end
 end
 defp private_ipv4?({10, _, _, _}), do: true
 defp private_ipv4?({172, b, _, _}) when b in 16..31, do: true
 defp private_ipv4?({192, 168, _, _}), do: true
 defp private_ipv4?({_, _, _, _}), do: false
end