UDP and TCP packet drops

[sheharyaar]

There is a strange behavior with the firewall dropping TCP and UDP packets. After hosting a server using netcat on cloud (both EC2 and DigitalOcean),

I tested TCP using netcat and telnet. Both connected successfully. For UDP testing, I used netcat only. The servers were created on even reserved category ports which are not in use.

The strange behaviour I noticed was that, both TCP and UDP connections were dropped on both netcat and telnet exactly after 6 data transfers ( the 7th couldn't
be processed by the server and the conenction was reset ).

More information from important tools like wireshark or tcpdump would be helpful.
Information can be gathered about this behaviour by changing the size of the messages and forcing fragmentation. Tinkering with MTU size can also provide some insight.

[sheharyaar]

Another weird things happens. When connected with -v option : nc -v <IP> <port> , netcat displays XXXXX ( 5 Xs) and then accepts only a single data transfer and then exits. So still a total of 6 data transfers.

[sheharyaar]

If network allows 6 data packets, maybe we can use IKEv2 as it requires 4-6 packet transfers to create SA.

[Yureien]

We can try checking port wise. Maybe some ports have been allowed, like 3389 is for Windows UDP?