[nsp] IPSEC tunnel mode

Joel Snyder Joel.Snyder at Opus1.COM
Tue Aug 26 09:03:57 EDT 2003

 >I want packets from hosts on Ethernet1 to be securely forwarded to R5
 >and then further to the Internet, without R2, R3 and R4 knowing about
 >the network on Ethernet1.
 >Do I have to build a GRE Tunnel between R1 and R5 to run IPSEC over
 >GRE, or could I just do with IPSEC tunnel mode only?

"It depends."

It depends on what you mean by not having "r2, r3, and r4 knowing about 
the network."  It also depends on what you mean by "packets" and what 
kind of routing strategy you use.

IPsec tunnel mode is the simplest answer; you can think of it as a whole 
different routing engine which somehow magically sends IP unicast 
packets across from one router to the other and which does not otherwise 
interfere with or communicate with your other IP routing engine.  Thus, 
if you have properly filtered out knowledge of R1's ethernet from your 
routing tables, then R2, R3, R4 won't know diddly about it.

If when you say "packets" you mean more than IP unicast packets, then 
you will definitely need something which instantiates an IP interface. 
By pumping GRE out over IPsec, you can ship any kind of packets you want 
over the tunnel since IPsec sees it as a GRE tunnel, not as the data in 
the tunnel.

GRE is also important if you want the tunnel's existence to be part of 
your routing infrastructure (again, not sure what you mean by "knowing" 
about the Ethernet).

If I were doing this, my definition of "packets" would be "IP unicast," 
my definition of "knowing" is "can see packets" and thus I would simply 
do an IPsec tunnel from R1 to R5 and leave it at that.  If you want to 
stick ACLs on the incoming Internet-pointing interface prohibiting 
packets from going from R2 directly to R1's Ethernet, that might also be 
  useful, but perhaps traffic from R2 to R1's Ethernet is not 
prohibited, even if unencrypted.

Now, it gets a LOT more complicated if you want R2, R3, and R4 to send 
their traffic out to R5 for encryption before it can come back to R1. 
In that case, you'd definitely want to think about using GRE because you 
will want to make the only route to R1's ethernet via R5, and GRE is one 
easy way to do that.  You can always static it up, of course.

I would tend to avoid GRE just because of the additional overhead and 


Joel M Snyder, 1404 East Lind Road, Tucson, AZ, 85719
Phone: +1 520 324 0494 (voice)  +1 520 324 0495 (FAX)
jms at Opus1.COM    http://www.opus1.com/jms    Opus One

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