[erlang-questions] Socket option FREEBIND

[Michael L Martin]

Thanks for the quick response. As for what I'm  attempting to achieve, 
well, I'm not really sure.

I have a customer that is adamant that we use IP_FREEBIND, so I've been 
poking around in :gen_tcp and :inet trying to see how to do that.

Here's what happens when I try to open a listen socket:

iex(5)> {:ok, s} = :gen_tcp.listen(8000, [{:raw, 6, 15, <<1 :: 
size(32)>>}, {:ip, {10, 138, 69, 63}}])
{:ok, #Port<0.1305>}
iex(6)> :inet.getopts(s, [{:raw, 6, 15, 4}])
{:ok, []}
iex(7)> :gen_tcp.close(s)
:ok
iex(8)> {:ok, s} = :gen_tcp.listen(8000, [{:raw, 6, 15, <<1 :: 
size(32)>>}, {:ip, {10, 138, 69, 64}}])
** (MatchError) no match of right hand side value: {:error, :eaddrnotavail}

iex(8)>

So it looks like it's not working for listen sockets, either. Could very 
well be that I simply don't know what the heck I'm doing...



On 2017-10-20 12:04 PM, Andreas Schultz wrote:
> Hi,
>
> ----- On Oct 20, 2017, at 5:10 PM, Michael L Martin 
> <mmartin4242@REDACTED> wrote:
>
>     Raw socket options looks like it should be the answer, but I'm not
>     getting good results:
>
>     iex(worker@REDACTED)28> {:ok, s} =
>     :gen_tcp.connect('localhost', 80, [{:raw, 6, 15, <<1>>}, {:ip,
>     {10, 138, 69, 63}}])
>
>     {:ok, #Port<0.12110>}
>
>     iex(worker@REDACTED)29> :inet.getopts(s, [{:raw, 6, 15, 32}])
>
>     {:ok, []}    <== option not set?
>
>     iex(worker@REDACTED)29> {:ok, s} =
>     :gen_tcp.connect('localhost', 80, [{:raw, 6, 15, <<1>>}, {:ip,
>     {10, 138, 69, 64}}])
>
>     ** (MatchError) no match of right hand side value: {:error,
>     :eaddrnotavail}
>
>
> IP_FREEBIND on connect doesn't make sense, it's only really useful on 
> listen sockets.
>
> The option lets you bind to an IP address that does not yet exist on 
> the host. That is ok for listen, but when you attempt to build an 
> outgoing connection from that IP, then it has to be configured.
>
>     It appears that the IP_FREEBIND option is simply not set. If I use
>     IP_TRANSPARENT instead:
>
>     iex(worker@REDACTED)35> {:ok, s} =
>     :gen_tcp.connect('localhost', 80, [{:raw, 6, 19, <<1>>}, {:ip,
>     {10, 138, 69, 63}}])
>
>     {:ok, #Port<0.12113>}
>
>     iex(worker@REDACTED)34> :inet.getopts(s, [{:raw, 6, 19, 32}])
>
>     {:ok, [{:raw, 6, 19, <<0, 0, 0, 0>>}]}
>
>
> IP_TRANSPARENT needs special iptables rules to work correctly and is 
> for a very specific use case. I don't think it would be very useful 
> for an Erlang application to use that.
>
>     In this case, the raw option 19 (IP_TRANSPARENT) is set, but to
>     false rather than true. I believe this is because the beam file
>     needs root or CAP_NET_ADMIN.
>
> You need root and a specific iptables setup for it to work.
>
>     Any thoughts?
>
>
> What are you attempting to achieve anyway?
>
> Regards
> Andreas
>
>
>
>
>     On 2017-10-16 01:18 PM, Guilherme Andrade wrote:
>
>         I believe you can use raw socket options[1] for that, but it
>         won't be portable.
>
>         [1]: http://erlang.org/doc/man/inet.html#setopts-2 - {raw,
>         Protocol, OptionNum, ValueBin}
>
>         On 16 October 2017 at 15:09, Michael L Martin
>         <mmartin4242@REDACTED <mailto:mmartin4242@REDACTED>> wrote:
>
>             Hi all,
>
>             Is there a way to specify the FREEBIND option when opening
>             a socket? I don't see any reference to it in the
>             documentation.
>
>
>             Thanks,
>
>             _______________________________________________
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>
>
>
>
>         -- 
>         Guilherme
>
>
>
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