Question

One way traffic only after IPSec tunnel established successfully

Asked by: macassist

I'm trying to get some general idea about why traffic would only flow one way through an IPSec tunnel.

Both end devices (a Billion BiPAC 7402NX and a Cybertec 2000 series 3G modem/router) report a successfully established IPSec tunnel, but when I ping any device on the Cybertec end's subnet (or even the Cybertec's LAN address), I can see the traffic going out through the Billion, but no reply coming back.

I'm guessing there's some sort of routing problem at the Cybertec end and that it's sending traffic for the other end's LAN subnet over the WAN interface rather than through the tunnel (despite being told the tunnel is responsible for traffic to the the Billion's LAN subnet) - could there be any other causes?

I'll post config screens and tunnel establishment messages if necessary, but for the moment was trying to grasp generally what might be going wrong.

TIA

Sean

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Asked On
2009-08-02 at 04:16:53ID24619697
Topics

IPSec Security Protocol

,

Virtual Private Networking (VPN)

Participating Experts
1
Points
500
Comments
7

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Answers

 

by: QlemoPosted on 2009-08-02 at 05:18:26ID: 24998606

As long as both devices are the default gateways on their respective networks, only firewalls blocking unknown traffic could be of interest.

 

by: macassistPosted on 2009-08-03 at 00:59:52ID: 25002481

It turns out the 3G wireless modems wanted the Remote ID to be set to the Billion's LAN address, and the Local ID set to their LAN address (with the appropriate changes at the Billion end to "cross over" the IDs), and I've now been able to get the traffic going both ways.

This may be where my ignorance comes in, but I wouldn't have expected the Local and Remote IDs to affect routing when I had said in the tunnel configuration that the relevant subnets are at each end. Am I mistaken (i.e. are the Remote and Local IDs often used in this way) or is that odd?

TIA

Sean

 

by: QlemoPosted on 2009-08-03 at 03:30:09ID: 25003115

Are you talking about Phase 1 IDs or Phase 2 IDs (sometimes called Proxy ID)? I suppose latter, because if Phase 1 IDs, no connection should have been established.

Using Proxy ID for routing restriction is common on low-end devices. Some business devices use the Proxy ID for further restriction (what I consider as use- and senseless), i.e. the existing firewall rules are narrowed down to the networks negotiated. That way you can provide a more restrictive Proxy ID, but not a more generous one.

I see I have been right with my assumption of firewalls being in the way. I suppose the IDs are not creating the routing entries, but the firewall rules.

 

by: macassistPosted on 2009-08-03 at 06:08:47ID: 25003955

Well, the 3G modems have the Remote and Local IDs I was talking about in their Phase 1 config screens. From your comments, I'm guessing this behaviour isn't what one might expect of a well-behaved IPSec end-point - am I right?

As I mentioned before, if those IDs matched the other endpoint's (reversed, to maintain Remote vs Local identity), both ends reported successful establishment of the IPSec connection, but I couldn't get any meaningful traffic flow (Billion only showed some Tx traffic, no Rx). Using the local IP address as the IDs seems to have solved the problem, but I'd not seen that in my (limited) experience with IPSec.

Thanks for your help so far - any and all further info/insight gratefully received.

Sean

 

by: QlemoPosted on 2009-08-03 at 06:44:41ID: 25004200

Yes, you are right. If something in P1 does not fit, the tunnel is not established (normally). But each device handles this different, ignoring standards (or interpreting them "very broad").

 

by: macassistPosted on 2009-08-09 at 20:30:52ID: 31610624

Thanks for all your comments - I've given your comment Accepted Solution status because it was helpful for me to understand where things were and weren't behaving as per "normal" IPSec behaviour.

Cheers

Sean

 

by: QlemoPosted on 2009-08-09 at 22:40:12ID: 25057373

That's the important part:

  1. get it to work 
  2. understand why
     
:-)

20120131-EE-VQP-002

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