krb5 UDP daemons and IP pktinfo

Simo Sorce simo at
Tue Jan 5 17:22:10 EST 2016

On Tue, 2016-01-05 at 16:58 -0500, Greg Hudson wrote:
> MIT krb5 contains two UDP daemons, krb5kdc and kadmind (for password
> changes).  For UDP daemons, it is sometimes important to send replies
> from the same address as the requests were sent to, or the replies may
> be discarded by firewalls or NAT devices.  The BSD sockets interface
> does not provide this functionality if you bind the server socket to the
> wildcard address.  For IPv6, there is a standardized and widely
> implemented extension (IPV6_PKTINFO) which provides the needed
> functionality.  For IPv4, the situation is more fragmented (see below).
> As a result, we have some complicated code in lib/apputils/net-server.c
> which does the following for each of IPv4 and IPv6:
> * If pktinfo suport is present, we bind to the wildcard address and set
>   the socket to use pktinfo.
> * Otherwise, we loop over the local interfaces and bind to each one.  We
>   also bind to the routing socket.  When a message is received on the
>   routing socket indicating that local interfaces have changed, we close
>   all sockets (including active TCP connections) and rebind listener
>   sockets from scratch.
> In 2010 I asked for help doing a platform survey, and found that while
> Linux and Solaris 11 implement an IPv4 variant of pktinfo (IP_PKTINFO),
> other platforms do not (NetBSD added it in 2013).  Until yesterday, my
> operating assumption was that we were stuck with the complex code for
> the forseeable future.  Yesterday, I discovered that some operating
> systems implement the same functionality in a different way
> (IP_RECVDSTADDR).  If we handle both variants, then we can safely bind
> to the IPv4 wildcard address on Linux, Solaris, all of the BSDs, and OS
> X.
> So, I think it might be reasonable to simplify the net-server.c code at
> the expense of some degradation on the rare platforms which don't
> implement either variant of IPv4 pktinfo.  My favored form of
> degradation is that we always bind to the wildcard address, and if the
> platform doesn't have IPv4 pktinfo support, we just let sendto() pick
> the source address of the reply.  For those platforms, our UDP daemons
> will continue to work on single-homed machines, but might not behave
> properly on multi-homed machine depending on the routing configuration.
> Do other people agree that this is reasonable?

Sounds perfectly reasonable to me.


Simo Sorce * Red Hat, Inc * New York

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