Erlang: Create a netlink socket, bind to it, send a multicast message and receive a response

[Ulf Wiger]

Have you tried https://github.com/Feuerlabs/netlink ?
(I haven't used it in ages, but the last commit was in 2019, so it
seems to still be in use.)

BR,
Ulf W

Den tors 27 aug. 2020 kl 10:13 skrev Papa Tana <papa.tana101@REDACTED>:
>
> Hi All,
>
> I'm still trying to speak netlink with the linux GTP Module using Erlang:
>
> Here is my scenario:
>
> %% load the gtp.ko module manually
> > modprobe gtp
>
> %% at userspace, create a netlink socket, and bind to it
>
> > {ok, Fd} = procket:open(0, [{family, netlink}, {protocol, 0}, {type, dgram}]).
> > Port = erlang:open_port({fd, Fd, Fd}, [binary]).
>
> %% Now, I assume that I need to send a multicast message and receive some response
> %% I assume it would be something like this:
>
> erlang:port_command(Port, <<some_binary_format>>).
> flush().
>
> %% Of course, I don't get any response because I'm sending bad message.
> I have debug some successful library written in C how he did it.
>
> Exemple of strace of libgtpnl to Linux GTP module:
>
> > strace -s 100 -f -o out -x ./gtp-link add gtp1
>
> > bind(5, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, 12) = 0
> > getsockname(5, {sa_family=AF_NETLINK, nl_pid=2966, nl_groups=00000000}, [12]) = 0
> > sendto(5, {{len=80, type=0x10 /* NLMSG_??? */, flags=NLM_F_REQUEST|NLM_F_ACK|0x600, seq=1598219164, pid=0}, "\x02\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00\x08\x00\x03\x00\x67\x74\x70\x31\x28\x00\x12\x80\x07\x00\x01\x00\x67\x74\x70\x00\x1c\x00\x02\x80\x08\x00\x01\x00\x03\x00\x00\x00\x08\x00\x02\x00\x04\x00\x00\x00\x08\x00\x03\x00\x00\x00\x02\x00"}, 80, 0, {sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, 12) = 80
> > recvmsg(5, {msg_name={sa_family=AF_NETLINK, nl_pid=0, nl_groups=00000000}, msg_namelen=12, msg_iov=[{iov_base=[{{len=36, type=NLMSG_SUCCESS, flags=0, seq=1598219164, pid=2966}, "\x00\x00\x00\x00\x50\x00\x00\x00\x10\x00\x05\x06\x9c\xe3\x42\x5f\x00\x00\x00\x00"}, {{len=0, type=0 /* NLMSG_??? */, flags=0, seq=0, pid=0}}], iov_len=4096}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 36
> > close(5)                                = 0
> > write(2, "WARNING: attaching dummy socket...
>
> As per my understanding:
>
>     - Header would be: Length=80, type=16, flags=1536, sequence number=1598219164, pid=0
>     - nl_pid = 0, nl_groups = 0
>
> Well, but I'm stuck:
>
>     1/ Do anyone know how to send such a message using Erlang?
>     2/ I have tried https://github.com/msantos/procket, my netlink socket seems ok as above, but the next step, I'm lagging
>     3/ I have take a look at https://github.com/travelping/gen_netlink as well, but first, when I tried to install tetrapak (make, make install), I got a lot of error so I cannot even install tetrapak on my machine.
>     And even if tetrapak would be installed, it will be the same as 2/, I don't find any working example even in https://learnyousomeerlang.com/ on how to successfully speak netlink using Erlang.
>
> Any hint?
> Best Regards,