ASA - ICMP works on a L2L tunnel but TCP fails.

Question

All,
I have just started to work with the ASA's and I have a couple of problems with two 5510 8.4(1) ASA's supporting a L2L tunnel.
Problem-1:
Below  is the topology and currently the only config on these ASA's is what is  required to get the LAN2LAN tunnel setup and nothing more. ASA01 and ASA02 are the tunnel termination devices.
LAN A->Routing device->ASA-01 ----->Internet<------------ASA-02<-Routing device<-LAN2
Below is what is working
- Tunnel is established between the ASA's.
- I can ping from LAN A to LAN B and viceversa.
Below is not what is working
- I cannot RDP from a device in LAN A to LAN B and vice versa.
What we found in troubleshooting when we initiate a RDP session from a server in LAN-A to Server in LAN-B.
- The packet capture on  ASA - A shows that the SYN leaves the ingress(LAN interface).
-  The packet capture on ASA - B shows that the SYN is leaving the LAN interface.
-  Dont see a SYN-ACK on ASA-B. First we thought there might be a  different reason(detailed below as problem-2) but we dont see the  syn-ack on ASA-A either.
- Doing a asp-drop capture on ASA-B we saw that the SYN,ACK from server in LAN-B is being dropped with the following message
Drop-reason: (tcp-not-syn) First TCP packet not SYN
Any ideas on why ASA-B doesnt treat this is as a established tcp session?
Problem -2
On the packet capture wizard in ASDM if I do a  capture on the LAN interface of the ASA02 I can only see packets  leaving the ASA towards the LAN but I do not see anything coming back  into the interface from the LAN interface. This works the same whether I  do a ICMP or a TCP session(RDP).
For example - Ping from a server on LAN A to LAN B
- On ASA01
The packet capture wizard shows both icmp-echo from LAN-A and icmp-reply from LAN-B
- On ASA02
The packet capture wizard shows icmp-echo from LAN-A both not the icmp-reply from LAN-B.
I am not sure what the reason for both the problems above and the reasons might just be that my skill level with ASA's are just not there yet. Any guidance will be great appreciated.
Thanks,
Vishnu

Answer 1

Hello Vishnu,
Any ideas on why ASA-B doesnt treat this is as a established tcp session?
This is happening because the ASA is not seeing the entire 3 way hanshake, Are you sure all the packets are going across the ASA??? I would recommend you to do captures on both inside interfaces just for RDP traffic and attach them to this post so I can correlate to determine if indeed the ASA is receving what it needs.
On the packet capture wizard in ASDM if I do a  capture on the LAN interface of the ASA02 I can only see packets  leaving the ASA towards the LAN but I do not see anything coming back  into the interface from the LAN interface. This works the same whether I  do a ICMP or a TCP session(RDP).
That's exactly the reason of why this problem is happening, Good job correlating the facts,
Resolution of the issues:
I would say the problem is on the Routing device between ASA-2 and the LAN-2...
Make sure the Routing device knows that in order to reach the LAN-1 it needs to send the traffic back to the ASA-2 as somehow this traffic is not making it on the right interface,
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Julio Carvajal Segura