What is error code 504?

Where can I find L2TP/IPsec Error Code in packet/tcpdump?

I am trying to diagnose various issues related to VPN specifically L2TP/IPsec. I need to diagnose the issues using the Error codes in the reply packet from server. After trying to understand the packets exchanged between host and remote machine using Wireshark as well as tcpdump, I am unable to find the error code in the packet received from server. In RFC it mentions that there is an Error code and Optional Error Message, but I cannot see it in the dump. After using false shared key, the VPN fails but in wireshark or tcpdump no error code/ error message can be seen. tcpdump Command used is: tcpdump -i en1 -vvv 'host hostname && ((dst port 500 and udp) || ip proto 50)' Filter in Wireshark used is: isakmp or ah or esp Please share some help to get error codes in the L2TP/IPsec Phase 1 and Phase 2 packets exchanged between host and remote Server.
Answer:

I have tried using 'tcpdump ip proto l2tp', but this is also not helping me to capture l2tp packets.

Anon User at Quora Visit the source

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