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Re: IPSEC: on behavior of acquire

To: hadi@xxxxxxxxxx
Subject: Re: IPSEC: on behavior of acquire
From: Aidas Kasparas <a.kasparas@xxxxxx>
Date: Mon, 04 Apr 2005 01:02:01 +0300
Cc: ipsec-tools-devel@xxxxxxxxxxxxxxxxxxxxx, netdev <netdev@xxxxxxxxxxx>, nakam@xxxxxxxxxxxxxx
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jamal wrote:
On Sun, 2005-04-03 at 04:28, Aidas Kasparas wrote:

jamal wrote:


Exactly what i was trying to emulate - lost messages.

Your emulation was not correct. More correct would have been to start KE daemon, let it fully initialize (open pfkey socket, inform kernel that it is interested in acquire messages), then stop it (via debugger or kill -STOP) and only then send pings or other traffic and see what will happen. This is because there are different paths in xfrm+pfkey for cases 1) when there is no KE daemon and 2) when daemon is, but for some reason it does not establish a SA and therefore reaction to traffic is different.



I dont think that would work.
To summarize what happens in the kernel: everything leads to km_query()
as you have indicated in your text. If the kernel finds someone/thing has either a pfkey or netlink socket
open it sends a acquire to them. In the code you are probably looking at
(before i created the patch) - the first user/daemon the kernel sees
(either pfkey or netlink based) that has a socket open
will receive an acquire and the kernel will give up after that.

As an example, if the first pfkey user was just doing "setkey -x" and
the second was infact pluto, then pluto will never see the acquire. This is what got me looking at it to begin with. Look at the
earlier postings on the subject.

While I agree that code before your patch would not allow to cooperate tools using different ways to manage SAD/SPD (pfkey vs netlink), I have one setup in production where two instances of racoon runs simultaneously and both gets required pfkey-messages.

So in other words, just killing the ike server as you propose would mean
the kernel has no open sockets and will therefore never bother to send
an acquire.

I proposed to stop KE server, not to kill it.


Still all this is moot and is distracting us from the main discussion.
Lets define "lost"  simply as the case where an acquire never got to the
server (which may be sitting elsewhere on the network).

ACQUIREs _never_ _leaves_ _the box_ they are generated. It is allways kernel-to-userspace_process communication. It could be made reliable. And present situation IS sufficiently reliable.

In that case
what i did is sufficient. i.e. The methods to create this are not the
issue. The issue at stake is the behavior of the kernel in generating
the acquires.


See below.


Please refer to my earlier definition of what "lost" means. It doesnt
matter where the breakage happens really.
Think of everything to the right of "xfrm" in your diagram as a black
box (i.e that second thing could be pfkey or netlink - thats not the
issue). Think of some message that is supposed to reach the KE daemon
(make it interesting and say it is remote KE) then think of that message
never making it because something in the blackbox swallowed it.
If that packet is the first one and it needs to do so for the sake of setup for subsequent packets - then the desire to have it reach its
destination is very imprtant. There is no progress for it or subsequent
packets if it doesnt make it.

OK, let's talk about architecture xfrm <-> blackbox. In this architecture communication between these two elements (I do not speak about any comms in the blackbox) can be of two types:
1) reliable (messages always reach blackbox or error is reported);
2) unreliable (messages may fail even to reach blackbox).

With good blackboxes good ipsec system can be built using any of comm types. But:
a) (1) will be more reliable;
b) (1) will be more simple (at least xfrm side, as it will not require retransmisions);
c) (1) is implemented now (as a function call).

What I want to say is xfrm-to-blackbox interface is good as it is. The problem may only be in how good the blackbox is. And here we have to look inside blackbox and start talk about particular implementations of that blackbox. Retransmitions, if they needed, needs to be inside that blackbox.


The solution being proposed for Linux to treat that xfrm piece in the
same fashion as ARP is correct. Read the email from Alexey. Imagine if
ARP was only issued once(as does pfkey) or forever(as does netlink).


I have read email from Alexey. I think that xfrm_lookup() function implements functionality very similar to functionality which Alexey described.

And I think that direct comparison of ARP messages and pfkey messages is not fair, because pfkey acquire messages goes over reliable traffic and are used only to _initiate_ the process of SA negotiation. ARP has to receive information from other boxes which send it only as a direct responce to some packet. More, ARP is designed to be used [amogst others] on networks which loose some traffic by design.

I believe this is an issue with ipsec architecture itself - someone
needs to write an IETF draft on it.


I still do not see the topic for such draft.



Note: Sometimes theres no app. Example a packet coming into a gateway.


What do you have in mind?

If it is ISAKMP negotiation from remote peer, then it comes over UDP/500 or UDP/4500 over IP socket and not via acquire message via pfkey socket.

If it is ESP/AH packet with unknown SPI, then kernel simply drops it and do not send any acquire messages.



I was thinking more of this second scenario with incoming from clear
text domain and gateway encrypting assuming proper policy setup.

If you're talking about network behind security gateway communicating to host or network for which there is security policy configured on gateway, then acquire message will be generated on that security gateway, when that packet will be considered for forwarding. Again, that acquire messages never will leave security gateway.

I would have to go and reread the "opportunistic" encryption draft closely to make sense.


Speaking of "opportunistic" encryption. I never understood it. Ipsec-tools do not implement it. And in the year or so when I'm involved with it, I don't remember anybody even asking or mentioning about this feature. Therefore, I don't care about it -- users do not need it.


--
Aidas Kasparas
IT administrator
GM Consult Group, UAB


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