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Advice needed for fault ticket

Hello all

I was hoping someone could provide some advice on this fault ticket we have. I will say upfront that the output I have provided is from a Juniper firewall but I have decided to post here in case this issue is vendor agnostic and more 'general'.

The situation unfolds thus: our customer has hundreds of branch sites and there is a certain application/service that is monitored remotely by a 3rd party at each site. The 3rd party has a router at each branch and this connects to our LAN. Between the 'local' 3rd party router and the 3rd party's head office is an IPSec tunnel that carries the alarms. On our LAN interface at each branch we restrict communication between these two to ESP & ISAKMP as the per the following ACL:

ip access-list extended RESTRICT-IPSEC

permit esp host LOCAL-RTR host REMOTE-RTR

permit udp host LOCAL-RTR host REMOTE-RTR eq isakmp

deny ip host LOCAL-RTR host REMOTE-RTR

And all was well in the world.

We received a fault ticket where the customer was complaining that they were receiving lots of 'rogue' traffic. They sent us some output from their app which confirmed that not only were they receiving traffic with destination port 500 (UDP), as we would expect, but other traffic with random port numbers. The IPSec traverse our DMZ/internet firewalls, the Junipers. I have attached two sets of output: one that describes the 'normal' flow we are expecting, i.e. UDP 500, the other with the random port numbers that the 3rd party is complaining as 'rogue'.

As I understand it ISAKMP is used to set the tunnel up and ESP encapsulates the payload (I think). Certainly from a rough GNS3 setup this is what appeared to be happening. Could this 'rogue' traffic actually actually be legitimate traffic, specifically the ESP-related traffic and this is actually just a big mis-understanding.

Many thanks for your help and advice.

Cisco Employee

Advice needed for fault ticket

IPsec is:

AH - protocol 51

ESP - Protocol 50

IKE - UDP/500 or UDP/4500 (with NAT-T).

With NAT-T data is also encapsulated into UDP/4500.

Looking at the trace you provided: protocol-id="50" I assume this means the IP protocol number. It would indiate this an ESP packet, however ports should not be there. ESP header does not contacin port information.

What would be interesting is to capture the "offending" traffic with wireshark and check what it truly is.


Advice needed for fault ticket

Hello Marcin

Thanks for your reply.

Unfortunately getting a packet capture in this instance is not possible. I did also think the protocol-id="50" pointed to ESP but can't figure our why the ports are there.

I'll keep this one open to see if anyone else can throw their suggestion in.