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I use Ubuntu server as part of my system and now I have seen something interesting. I have a router that sends me ethernet 802.11 packets.

I generate this packet in the router and then send it under UDP to my Ubuntu Server. This operation works fine on any other Ubuntu Desktop but not on my server.

The UFW is inactive and my iptable look like this:

Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination 

And here is the UDP memory usage:

cat /proc/sys/net/ipv4/udp_mem

763227  1017637 1526454

In wireshark I don't see the packet and I really don't know what to do. Anyone have an idea?

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    If you don't see the packet(s) with wireshark or tcpdump, then I don't think they actually ever got to the server in the first place. I do not understand the significance of cat /proc/sys/net/ipv4/udp_mem with respect to your issue. May 28, 2015 at 18:18
  • if I only change the if to the other host in the network automatically I can see that in the wireshark, nothing change except destination ip
    – Alex Brill
    May 28, 2015 at 18:23
  • Oh. I thought you were running wireshark on your server, but I guess not. On which computer are you running wireshark? Can you use tcpdump on the server itself to determine if the packets actually arrive there? May 28, 2015 at 18:28
  • I running wireshark on the server and on any other host. when i provide traffic to the server i donesn't see packets, if i do the same to other host I see all packets
    – Alex Brill
    May 28, 2015 at 18:35
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    O.K. so if wireshark is running on the server and it doesn't list the packets, then the packets did not arrive at the server in the first place. Your problem is not with your server dropping the packet. May 28, 2015 at 18:40

1 Answer 1

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If you are running Wireshark on the server which you are sending the UDP packet to, and not seeing the packet, then the packet never arrived at the server in the first place, as Doug Smythies said in comments.

Since this appears to be the case, you will need to do some investigation. It is possible the Router is not creating the packet and sending it correctly. It is also possible your firewall rules at the router are blocking the packets. It is also possible the packets are just not being created the way you think they are. You should start by trying to direct actual UDP packets at the server, such as DNS queries directed at the server instead of an actual DNS server.

Unfortunately, though, without further information on the infrastructure and setup on BOTH ends of the network (your router/LAN, and your server, specifically what kind of server, if there's a firewall between the server and the internet, or whether it's an EC2 or such), we can't say more about what could be the problem.

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