I am having an issue with my smartphone device and packets monitoring: I am trying to find a reason why I cannot establish a VPN connection via PPTP protocol.I have the following successful scenario:Device set using IPv4Connecting to VPN OKNow a failing scenario:Device set using IPv6Connecting to VPN fails (after 30s timeout)I checked tcpdump result and found out the following: during exchanges for connection establishment, both IPv6 and IPv4 scenario are the same until my device is sending a PPP LCP Configuration-Request to server.Then a Ack comes back from server for IPv4 scenario, and nothing comes back for IPv6 scenario. On server side they affirm that in both scenario the Ack is send back.QuestionsIf tcpdump does not show this packet, does that mean it has not been received ?If this packet have been received but dropped, how can I find out (tools, methodology) ?I am not a Linux nor a network expert so I might miss some trivial hints.Thanks,Linux version 3.10.40
check tcp packets reception below tcpdump level
networking;tcp;ethernet;ipv6;tcpdump
If tcpdump does not show this packet, does that mean it has not been received ?Yes, that's what it means. Packet capture directly sees what's being received on the wire. If there were a firewall/filter rule dropping the packet, it would still show up in the packet capture and then it would be dropped at a later time.Most likely something else somewhere along the network path is filtering the ConfAck. You could inquite whether the ConfAck in the IPv6 case is much larger than the ConfAck in the IPv4 case. I don't see why it would be, but if it were much bigger, enough to require fragmentation somewhere along the way due to an MTU issue, I guess that could cause the problem?