netdev - SO_REUSEADDR with UDP (again)

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Message-ID: <20100413093408.GA16595@myhost.felk.cvut.cz>
Date:	Tue, 13 Apr 2010 11:34:08 +0200
From:	Michal Svoboda <michal.svoboda@...nts.felk.cvut.cz>
To:	netdev@...r.kernel.org
Subject: SO_REUSEADDR with UDP (again)

Hello,

(redirected here from LKML)

I found SO_REUSEADDR on UDP sockets to behave somewhat nasty. If you
create a UDP socket with that flag and bind it to a port, then anyone
doing the same later will "steal" your packets, ie.

1. process A binds to port 12345 with SO_REUSEADDR, packets to that port
   go to process A
2. process B binds to port 12345 with SO_REUSEADDR, packets to that port
   now go to process B
3. A dies, fires up again, packets go back to A
4. A dies, does not fire up, packets go to B, as if they were stacked

And this works even if A and B are owned by different users, thus anyone
can "steal" packets from anyone as long as they use SO_REUSEADDR.
However, in most programs that's the default.

Furthermore, one can lock-out a particular source from being "stolen" by
using connect() to that source, ie.

1. process A binds to port 12345 with SO_REUSEADDR, gets the packets
2. B does the same, gets the packets, but also connect()s to the source
   of the packets
3. A can now restart or try to bind again, but does not get the packets
   (from that source)

(I haven't tested the case if A also issues a connect() even if it does
not receive packets.)

All of this seems confusing to me, and the fact that users can steal
packets from each other seems like a mild security risk. I've found some
discussions about this from circa 2002, but the above cases were not
mentioned. So - a problem or not?

Michal Svoboda

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