[erlang-questions] HTTP requests and the meaning of eaddrinuse

Per Hedeland <>
Sun Feb 1 21:27:42 CET 2009


Johnny Billquist <> wrote:
>
>Per Hedeland wrote:
>> 
>> Yes, that's what you get (on connect()) on all Unices where I have seen
>> it, and it does make some amount of sense I think - if there are no
>> local ports left, any attempt to form a connection would have to use a
>> port that is already "in use".
>
>Well, I think that it's a flaw in the design of the interface. If you 
>use port 0 (ask the system to give you a port) you should never get 
>eaddrinuse. That is sort of guaranteed by the call (the semantic is to 
>give you a free port).

Well, we're talking about connect() here - you're not asking for a free
port, you're asking for a connection - just that the stack has to give
you a port to establish the connection (unless you have bound the
socket, which is "unusual" on connect()).

> I wonder if all systems do this, or if different systems have 
>different "solutions". If noone can tell for other systems than Linux I 
>guess I'll have to go in and examine the source myself soon. :-)

Looking at the FreeBSD kernel code, it seems it will actually return
EADDRNOTAVAIL or possibly EAGAIN if it fails to find a "free" port - but
will return EADDRINUSE if the chosen port ends up conflicting with a
connection in TIME_WAIT. The latter is probably the most common failure
case.

>You have the sockopt SO_REUSEADDR which circumvents this check. Is it 
>possible to set that from Erlang?

Certainly (read the man page), but I'm not sure how or if it applies to
connect() (and can't be bothered to figure out that part of the kernel
code:-). It obviously wouldn't make sense to not try to find a "free"
port at all if it is set (but it may affect the exact meaning of
"free"), and it certainly won't allow a connection that conflicts with
an existing one.

>> Forgot one that I actually used recently on FreeBSD - it randomizes the
>> selection of local port to make spoofing harder, and every now and then
>> a test case that did a bunch of localhost connections would get the
>> EADDRINUSE. Turning off the randomization (sysctl
>> net.inet.ip.portrange.randomized 1 -> 0) made it cycle nicely through
>> the available range => problem solved. A quick check on a Linux 2.6.20
>> system doesn't show any such randomization nor reveal a way to turn it
>> on or off, but it may well be there somewhere...
>
>Whoa? It don't actually check that the random port is actually free 
>before giving it to you???

The check is the same whether randomizing or not, but the probability of
hitting one of your own old connections in TIME_WAIT is higher when
randomizing is used.

But anyway, I think we're getting rather off-topic here, and the
original answer of "you're running out of ports" to the original
question is probably as good as it gets - I'll shut up now.

--Per



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