Samba4 Port: iptables Remapping

Q: Background problem to solve: to port Samba4 (an OSS replacement
   for active directory) from Heimdal-krb to MIT-krb. 

   The Samba4 people have built their AD-surrogate server
   (called Samba), configured to use the Heimdal KDC via a library
   interface, so that the KDC runs in the same process as all of
   the other AD services (SMB, netlogon, cifs, samr, etc).  The
   Samba4 people repeatedly insist that they won't accept an MIT
   integration that doesn't work in exactly the same way.
   The MIT-krb people respond that spec'ing such a libkdc interface,
   getting approval from the krbdev community, and building the
   libkdc interface will take "a long time".
   Part of the technical disagreement is two conflicting realities:  
   unix deployments of mit-krb take it for granted that you don't
   run non-kdc services on the same box that hosts a kdc;  while
   samba4 people know that windows clients inflexibly expect the
   kdc, cifs, netlogon ntlm, samr, etc all have to be served from
   the same ip-address.

   So, I'm looking for a better solution.
   Specifically, I believe the Samba4 people are more devoted to 
   uniformity than to their libkdc approach;  they might accept an
   alternate solution, as long as both MIT-krb & Heimdal kdc will 
   work identically with that one solution.
   So, here's what I'm hoping can be made to work:  
   * I want to keep the Samba4 kdc (Heimdal or MIT's) in a 
     separate process on the Samba-server host;
   * When the Samba server catches a client request for AD-style 
     krb-service, I want the Samba server to pretend that it is 
     tunnelling the client request to the kdc, via an IPC connection.
   * A curlicue is that in unix krb deployments, the kdc ignores
     the client's ip-address for authentication purposes, but for
     AD, the kdc usually enforces access-control on the client's
     IP-address.  this means that the tunnel needs to present the
     krb-protocol packets to the kdc, with the client's source-
     address intact.
   Is there some way to make this pseudo-tunnelled approach work,
   without reimplementing the TCP/IP stack?
   I'm told that tun/tap kernel modules might help, but I dunno.
   One asset in my favor:  the kdc knows how to deliver krb service
   via UDP; though AD clients use only TCP-mediated krb-service. 
 
A: How about:
   * iptables redirection:  iptables can take a port and redirect
     it.  You can also tunnel between machines with ipsec.
   * xinetd can also do redirection and tunneling
   * sshd or stunnel can do it (stunnel might be better than sshd).
Q: So you're saying, the samba4 server catches (readrecv()) the
   client's kdc-request, and the samba4 server can then tunnel
   the client's request to the kdc, inside some other IPC-
   connection protocol, so that the client & the kdc don't know
   about the tunnel?  is this easy to set up?
A: I was thinking iptables could probably do the proxy work and
   send the packet to another machine.
   if that doesn't work, then xinetd might be possible
Q: The Samba4 people really-really want everything on one host;
   ease-of-administration is one of their selling-points.
A: The only issue with doing it in iptables is that you have to
   make sure not to delete tha NAT rules when locking down the
   system
Q: But having a host-to-host tunnel solution available for
   krb-admins who want it, would be good from MIT's POV, and
   might be acceptable to the samba4 people, as non-default
   behavior.
A: Would the traffic be something you wouldn't want 3rd parties
   to see?  in other words, clear text or encrypted?
Q: Doesn't matter;  the krb protocol is GRAS (generally recognized
   as safe).
A: xinetd wouldl add some overhead.
Q: I believe kdc performance isn't a critical issue in Samba
   deployments;  AD users typically deply 1 AD-server per subnet
   or per building, so it's rare for an AD server to see heavy
   kdc-traffic.
Q: In unix-based krb deployments, you usually see 1 master kdc
   & a couple slaves handling a large organization (thousands
   of user-accounts).  So in unix deployments, performance-hits 
   would be bad.
A: Sounds like kernel-level redirection might be best
Q: kernel-level redirect is the iptable-remapping option?
A: Yes;  you would want to use the routing capabilities of the 
   linux kernel.
A: So the samba-server-host would be a router for the kdc port.
Q: Is this feature universal across linux distros?
A: Yes;  linux makes for a very good router.
A: Cisco uses it for low-end models
Q: Do I have hope of seeing this feature on Solaris or on *bsd?
A: I think most Unix boxes can act as routers
A: What you are wanting to do is to route just one port and 
   leave others alone.
Q: I think so;  I believe the Samba service catches different
   services on different ports, as usual.
A: If that's the case, it should be easy
Q: So you're suggesting that the Samba server can route kdc
   traffic to the kdc, on the same host or on a different host,
   in such a way that the client doesn't have to know the kdc's
   real IP-address / port?
A: Yes
Q: And in such a way that the kdc does see the client's real
   IP-address, just as if the kdc readrecv()'ed the client's
   original kdc-request-packet?
A: http://www.netfilter.org/documentation/HOWTO/netfilter-extensions-HOWTO-4.html#ss4.5
A: ^^^ might be useful
A: Now the question is did it make it into iptables mainline by now