{error, eagain}

[joagre]

Hi,
I experimented a bit but I must be doing something wrong.
Can someone help?

Cheers
/Jocke

jocke@xev:/src/tunctl/trunk$ getcap /usr/lib/erlang/erts-5.9.1/bin/beam.smp
/usr/lib/erlang/erts-5.9.1/bin/beam.smp = cap_net_admin+ep
jocke@xev:/src/tunctl/trunk$ getcap /usr/lib/erlang/erts-5.9.1/bin/beam
/usr/lib/erlang/erts-5.9.1/bin/beam = cap_net_admin+ep
jocke@xev:~/src/tunctl/trunk$ cat test.erl
-module(test).
-compile(export_all).

test() ->
case tuncer:create("tun0", [tun, no_pi, {active, false}]) of
{ok, Pid} ->
io:format("Device: pn", [tuncer:devname(Pid)]),
ok = tuncer:up(Pid, {10,0,0,1}),
recv(Pid);
Error ->
Error
end.

recv(Pid) ->
case tuncer:recv(Pid) of
{ok, Buf} ->
io:format("Buf: pn", [Buf]),
recv(Pid);
Error ->
io:format("Error: pn", [Error]),
timer:sleep(2000),
recv(Pid)
end.
jocke@xev:~/src/tunctl/trunk$ ./start.sh
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:4:4] [async-threads:0] [kernel-poll:false]

Eshell V5.9.1 (abort with ^G)
1> c(test).
{ok,test}
2> test:test().
Device: <<"tun0">>
Error: {error,eagain}
Error: {error,eagain}

BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
(v)ersion (k)ill (D)b-tables (d)istribution
^C
jocke@xev:~/src/tunctl/trunk$ sudo ./start.sh
[sudo] password for jocke:
Erlang R15B01 (erts-5.9.1) [source] [64-bit] [smp:4:4] [async-threads:0] [kernel-poll:false]

Eshell V5.9.1 (abort with ^G)
1> test:test().
Device: <<"tun0">>
Error: {error,eagain}
Error: {error,eagain}
Error: {error,eagain}

BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
(v)ersion (k)ill (D)b-tables (d)istribution
jocke@xev:~/src/tunctl/trunk$

[msantos]

@joagre the code looks correct! In this case, there just isn't any data coming from the device.

For testing, try setting the netmask wider:

ifconfig tun0 10.0.0.1 netmask 255.255.255.0
traceroute 10.0.0.2

You should see the traceroute logged in the erlang shell.

Similarly, if you modify your example slightly to return the PID:

-module(test).
-compile(export_all).

dev() ->
    case tuncer:create("tun0", [tun, no_pi, {active, false}]) of
        {ok, Pid} ->
            io:format("Device: ~p~n", [tuncer:devname(Pid)]),
            ok = tuncer:up(Pid, {10,0,0,1}),
            Pid;
        Error ->
            Error
    end.

Then sniff the interface:

tcpdump -s 0 -X -i tun0

And send data:

1> Dev = test:dev().
Device: <<"tun0">>
<0.35.0>
2> tuncer:send(Dev, <<"foo">>).
ok

tcpdump should print out the packet:

14:51:13.616726 truncated-ip 3
        0x0000:  666f 6f                                  foo

This is a good explanation about the behaviour of tun devices:
http://backreference.org/2010/03/26/tuntap-interface-tutorial/

Finally, it's a bit simpler to use {active,true} mode:

1> Dev = test:dev(). 
Device: <<"tun0">>
<0.35.0>
2> tuncer:setopt(Dev, {active,true}).
ok
3> flush().
Shell got {tuntap,<0.35.0>,
                  <<69,0,0,60,163,112,0,0,1,17,2,63,10,0,0,1,10,0,0,2,233,231,
                    130,154,0,40,138,20,64,65,66,67,68,69,70,71,72,73,74,75,
                    76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,
                    95>>}

Hope that helps! And let me know if you have any more questions.

[joagre]

Thanks for the reply and the informative link!

My main misunderstanding (I have many :-) was that I thought that tuncer:read/1 and tuncer:recv/1 did behave as gen_tcp:recv/1, i.e. be blocking waiting for a packet. In your opinion, how should a process using tuncer:read/1 in non-active mode be designed? Should it call tuncer:read/1 in a polling fashion with a small timeout if it receives {error, eagain}?

Thanks
/Jocke

[msantos]

Yes, currently you have to manually handle if the file descriptor is not ready. You can also pass the file descriptor into erlang:open_port/2 (this is what tuncer does for active mode). There is some discussion about the different ways of polling raw file descriptors here:

https://github.com/msantos/inert#alternatives

I'm planning on making tuncer:recv/1 into a blocking operation. However it will require adding a dependency on the library in the url.

[joagre]

Ah! I see. There is a more to this than I understood. I will use tuncer in active mode to start with and if my own project/experiment catches on I might very well reiterate to help out with tunctl. Consider it a threat. :-)

Thanks. You produce quality stuff! Invaluable.