follow-up: discrepancy with POSIX

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As a follow up to my question from yesterday on the netdev list what I think is a real problem. Either in the kernel or in the POSIX spec.

The POSIX spec currently says this about SOCK_DGRAM sockets:

If address is a null address for the protocol, the socket’s peer address shall be reset.

The term "null address" is not further specified but it will usually be read to allow the following scenario to work out:

fd = socket(AT_INET6, ...)

connect(fd, ...some IPv6 address...)

   struct sockaddr_in6 sin6 = { .sin6_family = AF_INET6 };
   connect(fd, &sin6, sizeof (sin6));

connect(fd, ...some new IPv6 address...)

This does not work on Linux in the moment. The socket remains connected to the old IPv6 address but the second connect() call does succeed (this does not sound OK). What does work is if the connect call to disassociate the address uses AF_UNSPEC instead of AF_INET6.

The question is: do people here think this is a problem in the POSIX spec? Binding to :: and 0.0.0.0 isn't possible, so maybe the Linux implementation should allow this?

If you think the POSIX spec is wrong (and can point to other implementations doing the same as Linux) let me know and I'll work on getting the spec changed.

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The standard way to undo connect is to use AF_UNSPEC. Code to handle that for dgram sockets is there. It's the same code for v4 and v6.

-Andi

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I quoted the standard and it does not say anything about AF_UNSPEC. So you cannot simply make such broad statements.

I also don't say that this behavior should be removed. It's certainly useful, very much so in fact.

But the spec calls for a "null address" to be used and that's in my understanding something different from using AF_UNSPEC.

I looked through Stevens TCP Illustrated Vol 2 and it seems not to mention resetting the address at all. The POSIX spec certainly got this text from .1g.

I cannot test it on other systems. If somebody has access to some certified systems (and maybe others), write a bit of code which creates a DGRAM socket, connect to one address, call connect with a "null address", then connect to another address (which likely has to use a different interface since otherwise the connect will just succeed, it seems).

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From: Ulrich Drepper <[email protected]>
Date: Wed, 19 Sep 2007 09:49:09 -0700

It just occured to me that AF_UNSPEC might be used simply because "all zeros" might be a valid real bindable address for some address family. And using AF_UNSPEC avoids that problem entirely.

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Yes, but for IPv4/6 it's not an issue. Some implementations might handle all-zeros and the spec _currently_ calls for it. In this case an alignment would be good.

I guess I'll just go ahead and file a problem report with the spec. Maybe the Unix vendors will test their implementations in provide feedback.

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Searching the web shows up this:

http://developer.apple.com/documentation/Darwin/Reference/ManPages/man2/connect.2.html

Datagram sockets may dissolve the association by connecting to an invalid address, such as a null address or an address with the address family set to AF_UNSPEC (the error EAFNOSUPPORT will be harmlessly returned).

I.e., at least Apple implements both variants.

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Ok "standard" was perhaps a poor choice of words.

AF_UNSPEC used to be introduced long ago by Alan based on some early POSIX draft iirc.

Also incidentially it's a null address:

include/linux/socket.h:#define AF_UNSPEC 0

memset(&sockaddr, 0, sizeof(sockaddr)) should give you AF_UNSPEC

-Andi

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But the spec calls for <quote>null address for the protocol</quote>.

That means the family for the null address is the same as the family of the socket.

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Spec doesn't match traditional behaviour then. IPv4 0.0.0.0 is traditionally an synonym for old style all broadcast (255.255.255.255) on UDP/RAW and it's certainly possible to connect() to that.

-Andi

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Well, determining whether that's the case is part of this exercise.

Where do you get this from? And where is this implemented? I don't doubt it but I have to convince people to change the standard and possibly introduce incompatibility.

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Sorry it's actually loopback, not broadcast as implemented in Linux. In Linux it's implemented in ip_route_output_slow(). Essentially converted to 127.0.0.1

I think it's traditional BSD behaviour but couldn't find it on a quick look in FreeBSD source (but haven't looked very intensively)

Admittedly port 0 is somewhat dodgy for UDP too, but at least in RAW context it might be valid.

-Andi

One has to set their way-back machine pretty far back to find the *BSD bits which used 0.0.0.0 as the "all nets, all subnets" (to mis-use a term) broadcast IPv4 address when sending. Perhaps as far back as the time before HP-UX 7 or SunOS4. The bit errors in my dimm memory get pretty dense that far back...

It has hung-on in various places (stacks) as an "accepted" broadcast IP in the receive path, but not the send path for quite possibly decades now.

rick jones

Well it is valid in Linux for sending. And who knows who relies on it.

-Andi

That would be BSD4.2 - BSD4.3 went to all-ones, and it *was* quite the little mess if you had both flavors of boxes on the same subnet at the same time, it would packet-storm *quite* easily.

Which is a valid address in some protocols. If I remember rightly then appletalk net 0 node 0 port 0 is valid although I'd want to look in the book to check that - ditto AF_ECONET although I doubt anyone cares too much 8)

Alan

From: Ulrich Drepper <[email protected]>
Date: Wed, 19 Sep 2007 08:21:47 -0700

The whole AF_UNSPEC thing I'm almost certain comes from BSD, which has behaved that way for centuries.

Someone needs to cull through Steven's Volume 2 to verify this, I'm too busy at the moment to do so myself.

We got it from the 1003.4g draft socket specification if I remember rightly. Its entirely plausible that got it from 4BSE.

Alan