[erlang-questions] Socket option FREEBIND

Andreas Schultz <>
Mon Oct 23 09:44:32 CEST 2017


----- On Oct 20, 2017, at 8:22 PM, Michael L Martin <> wrote: 

> Posting again to all...

> iex(4)> {:ok, s} = :gen_tcp.connect('localhost', 80, [{:raw, 0, 15, <<1>>},
> {:ip, {10, 138, 69, 63}}])
Very small nitpick (does not impact the function), setsockopt boolean and integer options usually use a 32bit integer. Debugging tools like strace are much easier to read then. 
So I would recommend to write the options as {:raw, 0, 15, <<1:32/native>>} 

> {:ok, #Port<0.1310>}
> iex(5)> :inet.getopts(s, [{:raw, 0, 15, 4}])

> {:ok, [{:raw, 0, 15, <<1, 0, 0, 0>>}]}

> iex(6)> :gen_tcp.close(s) :ok

> iex(7)> {:ok, s} = :gen_tcp.connect('localhost', 80, [{:raw, 0, 15, <<1>>},
> {:ip, {10, 138, 69, 64}}])
10.138.69.64 does not exist on you system, so attempting to use it will fail (as you just discovered). 

> ** (exit) :badarg

> (kernel) gen_tcp.erl:149: :gen_tcp.connect/4

> iex(7)>

> I see badarg when trying to connect using a bogus IP address. But, the option is
> being set now.

> Using IP_TRANSPARENT instead of IP_FREEBIND:

> iex(8)> {:ok, s} = :gen_tcp.connect('localhost', 80, [{:raw, 0, 19, <<1>>},
> {:ip, {10, 138, 69, 64}}])
IP_TRANSPARENT is very different from IP_FREEBIND. You need iptables rules to make it work (and a lot more about what it is supposed to to before playing with it). 

> At this point, :gen_tcp.connect/3 blocks. The ip address does not exist, so now
> I add it:

> sudo ip addr add 10.138.69.64 dev eno1

> ip addr list dev eno12: eno1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc
> pfifo_fast state UP group default qlen 1000
> link/ether 8c:89:a5:c6:e1:7f brd ff:ff:ff:ff:ff:ff
> inet 10.138.69.63/16 brd 10.138.255.255 scope global eno1
> valid_lft forever preferred_lft forever
> inet 10.138.69.64/32 scope global eno1
> valid_lft forever preferred_lft forever
> inet6 fe80::8e89:a5ff:fec6:e17f/64 scope link
> valid_lft forever preferred_lft forever

> I would think that :gen_tcp.connect would then connect, but it's still blocked,
> and eventually times out.

> After the timeout:

> iex(8)> {:ok, s} = :gen_tcp.connect('localhost', 80, [{:raw, 0, 19, <<1>>},
> {:ip, {10, 138, 69, 64}}])
> {:ok, #Port<0.1300>}
> iex(9)> :inet.getopts(s, [{:raw, 0, 19, 4}]) {:ok, [{:raw, 0, 19, <<1, 0, 0,
> 0>>}]}
> iex(10)>
As I have said before, using FREEBIND for connect makes no sense. 
What you probably want it to bind a listening socket to an IP address that will later be added by systemd. For that include the raw option in the listening call. 

Regards 
Andreas 

>> On 2017-10-20 11:10 AM, Michael L Martin wrote:

>>> 6, 15, <<1>>}, {:ip, {10, 138, 69, 64}}])

>>> ** (MatchError) no match of right hand side value: {:error, :eaddrnotavail}

> On 2017-10-20 12:33 PM, Richard Jones wrote:

>> I'm able to make outbound connections bound to an arbitrary IP from my
>> range by specifying a socket option like: {raw, 0, 15, <<1>>}

>> note that that protocol part of {raw, ???, _, _} is 0, not 6, since
>> it's an IP level socket option (not tcp-level).

>> btw there's also a system-wide option so you don't need to specify
>> ip_freebind per socket:

>> echo 1 > /proc/sys/net/ipv6/ip_nonlocal_bind

>> RJ

>> On 20 October 2017 at 17:17, Michael L Martin [ mailto: |
>> <> ] wrote:

>>> 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 [
>>> mailto: | <> ] wrote:

>>> Raw socket options looks like it should be the answer, but I'm not getting
>>> good results:

>>> iex( [ mailto: |  ] )28> {:ok, s} =
>>> :gen_tcp.connect('localhost', 80,
>>> [{:raw, 6, 15, <<1>>}, {:ip, {10, 138, 69, 63}}])

>>> {:ok, #Port<0.12110>}

>>> iex( [ mailto: |  ] )29> :inet.getopts(s,
>>> [{:raw, 6, 15, 32}])

>>> {:ok, []}    <== option not set?

>>> iex( [ mailto: |  ] )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( [ mailto: |  ] )35> {:ok, s} =
>>> :gen_tcp.connect('localhost', 80,
>>> [{:raw, 6, 19, <<1>>}, {:ip, {10, 138, 69, 63}}])

>>> {:ok, #Port<0.12113>}

>>> iex( [ mailto: |  ] )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 |
>>> http://erlang.org/doc/man/inet.html#setopts-2 ] - {raw, Protocol,
>>> OptionNum, ValueBin}

>>> On 16 October 2017 at 15:09, Michael L Martin [ mailto: |
>>> <> ] 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,

>>>> _______________________________________________
>>>> erlang-questions mailing list [ mailto: |
>>>>  ] [
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>>>> http://erlang.org/mailman/listinfo/erlang-questions ]

>>> --
>>> Guilherme

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