I have a local network (doesn't really matter if it's VPN or real local network - I've tried both).One computer running Linux Mint opens a socket withmint$ nc -l 4242And the second one running OpenSUSE can connect and send messages to the socket:suse$ nc 10.8.0.10 4242But if I try to open a socket on Suse and connect from Mint - the connection won't establish. I don't have ufw firewall installed on Suse at all.I tried to send TCP packets from Mint to a Windows PC and it worked well, so I guess, that the problem is in Suse machine.I have also tried picking higher port numbers (55555 for example) just in case but with no luck.iptables -L -v on Suse:Chain INPUT (policy DROP 0 packets, 0 bytes)pkts bytes target     prot opt in     out     source               destination          272 23240 ACCEPT     all  --  lo     any     anywhere             anywhere              28  5183 ACCEPT     all  --  any    any     anywhere             anywhere             ctstate ESTABLISHED   0     0 ACCEPT     icmp --  any    any     anywhere             anywhere             ctstate RELATED  15  4984 input_ext  all  --  any    any     anywhere             anywhere               0     0 LOG        all  --  any    any     anywhere             anywhere             limit: avg 3/min burst 5 LOG level warning tcp-options ip-options prefix SFW2-IN-ILL-TARGET    0     0 DROP       all  --  any    any     anywhere             anywhere            Chain FORWARD (policy DROP 0 packets, 0 bytes)pkts bytes target     prot opt in     out     source               destination            0     0 LOG        all  --  any    any     anywhere             anywhere             limit: avg 3/min burst 5 LOG level warning tcp-options ip-options prefix SFW2-FWD-ILL-ROUTING Chain OUTPUT (policy ACCEPT 47 packets, 7142 bytes)pkts bytes target     prot opt in     out     source               destination          272 23240 ACCEPT     all  --  any    lo      anywhere             anywhere            Chain forward_ext (0 references)pkts bytes target     prot opt in     out     source               destination         Chain input_ext (1 references)pkts bytes target     prot opt in     out     source               destination            2  1956 DROP       all  --  any    any     anywhere             anywhere             PKTTYPE = broadcast   0     0 ACCEPT     icmp --  any    any     anywhere             anywhere             icmp source-quench   0     0 ACCEPT     icmp --  any    any     anywhere             anywhere             icmp echo-request  13  3028 DROP       all  --  any    any     anywhere             anywhere             PKTTYPE = multicast   0     0 DROP       all  --  any    any     anywhere             anywhere             PKTTYPE = broadcast   0     0 LOG        tcp  --  any    any     anywhere             anywhere             limit: avg 3/min burst 5 tcp flags:FIN,SYN,RST,ACK/SYN LOG level warning tcp-options ip-options prefix SFW2-INext-DROP-DEFLT    0     0 LOG        icmp --  any    any     anywhere             anywhere             limit: avg 3/min burst 5 LOG level warning tcp-options ip-options prefix SFW2-INext-DROP-DEFLT    0     0 LOG        udp  --  any    any     anywhere             anywhere             limit: avg 3/min burst 5 ctstate NEW LOG level warning tcp-options ip-options prefix SFW2-INext-DROP-DEFLT    0     0 DROP       all  --  any    any     anywhere             anywhere            Chain reject_func (0 references)pkts bytes target     prot opt in     out     source               destination            0     0 REJECT     tcp  --  any    any     anywhere             anywhere             reject-with tcp-reset   0     0 REJECT     udp  --  any    any     anywhere             anywhere             reject-with icmp-port-unreachable   0     0 REJECT     all  --  any    any     anywhere             anywhere             reject-with icmp-proto-unreachableWhat can cause this issue?
TCP: One PC can connect to other's listening port but not vice versa
networking;tcp;socket;netcat
use this command:sudo iptables -I INPUT -p tcp --dport 4242 -j ACCEPTthe last line of your suse INPUT chain is:   0     0 DROP       all  --  any    any     anywhere             anywhere            that means DROP all INPUT packet, with this commandsudo iptables -I INPUT -p tcp --dport 4242 -j ACCEPTwe Insert new rule for accept input packet top and before run DROP ruleand this rule not work for new connection :ACCEPT     all  --  any    any     anywhere             anywhere             ctstate ESTABLISHEDbecause that stats is ESTABLISHED to this mean:RELATED - The connection is new, but is related to another connection  already permitted. ESTABLISHED - The connection is already  established.