[erlang-questions] Socket connects to itself

Tony Rogvall <>
Tue Oct 13 16:57:46 CEST 2009


On 13 okt 2009, at 16.38, Per Hedeland wrote:

> Tony Rogvall <> wrote:
>>
>> On 13 okt 2009, at 12.37, Per Hedeland wrote:
>>
>>> Tony Rogvall <> wrote:
>>>>
>>>> In C you could of course check the bound port before connecting.
>>>
>>> Actually no, you couldn't, since the bind happens implicitly as part
>>> of
>>> the connect() call. What you *could* do in C but not in Erlang would
>>> be
>>> to explicitly bind the socket before the connect, but of course that
>>> carries a whole can of worms that you really don't want to open for
>>> this
>>> case.
>>
>> Hmm. I normally bind my ports before connecting ;-)
>
> You do such weird things...:-)

Talk about it ;-)
I did some experiments (yes I am a bit bored right now).
I tried some of the low levels calls for gen_tcp:connect to see what  
is going on:

On my mac:
{ok,S} = inet:open(-1,{0,0,0,0},0,[{active,true}],tcp,inet,inet_tcp).
{ok,{_,Port}} = inet:sockname(S).
prim_inet:connect(S, {127,0,0,1}, Port, infinity).
Result is {error,einval}


Same thing on my linux (ubuntu 9.04) / vmware (on my mac :-)

{ok,S} = inet:open(-1,{0,0,0,0},0,[{active,true}],tcp,inet,inet_tcp).
{ok,{_,Port}} = inet:sockname(S).
prim_inet:connect(S, {127,0,0,1}, Port, infinity).
Result is {ok,#Port<..>}

I like this one ;-) No one is listening .... how about you?

/Tony



>
>> Sometime you
>> need to control the source address and source port.
>
> Yes, in some rare cases you do, but in 99.9% of them the kernel knows
> better than you, and you'll just risk messing things up that way -
> e.g. binding to an address that isn't reachable from the destination.
> Not to mention trying to bind to a "free" port different from the
> destination port, to avoid the problem here.
>
>> So that is also
>> what Erlang does ;-) The connect defaults to bind to ifaddr = any and
>> port = 0
>
> Ah, it was a while since I last had the pleasure of digging through  
> that
> code, my memory is fading...
>
>> This bind, I guess, is the operating system default so the feature/
>> problem reported
>> may occur anyway.
>
> Indeed, such a bind should never be problematic, but is also just a
> waste of cycles.
>
>> The check would probably be a bit hard to implement anyway. If  
>> connect
>> is connecting to a local address and the socket is bound to any
>> (0.0.0.0) address,
>> the way to determine if this connect is an "invalid" connect, one  
>> must
>> scan
>> the interface addresses and probably the routing tables (and more) to
>> see
>> if this case will apply.?
>
> Well, you could take a simpler approach and just consider it invalid  
> if
> the port was the same as the destination port. I'm not sure that there
> is a fool-proof way (for all implementations) to make sure that you
> don't get the same port again if you re-try, though...
>
>> Is this the "can" you where talking about ;-)
>
> No, rather the problems that you can run into if you try to bind to a
> *specific* address/port (unless you really need to), per above.
>
> --Per
>
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