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Yu Watanabe
Asked: 2018-11-25 19:51:28 +0800 CST

Why is Ubuntu keep sending TCP SYN packets to port 53 of dns server?

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I would like to get help with my Ubuntu desktop 18.04.01 .

I just installed Ubuntu 18.04.01 running top of kvm (fedora server 28) and having hard time setting up dns access.

While diagnosing why DNS queries from Ubuntu desktop is not responded from my router, I found below packets was sent from Ubuntu.

12:29:53.596586 IP 192.168.11.100.45415 > 192.168.11.1.domain: 31141+ AAAA? ntp.ubuntu.com. (32)
12:29:53.597703 IP 192.168.11.100.37220 > 192.168.11.1.domain: Flags [S], seq 574811215, win 29200, options [mss 1460,sackOK,TS val 3560337742 ecr 0,nop,wscale 7], length 0
12:29:53.597902 IP 192.168.11.100.37222 > 192.168.11.1.domain: Flags [S], seq 3169117583, win 29200, options [mss 1460,sackOK,TS val 3560337742 ecr 0,nop,wscale 7], length 0
12:29:53.598020 IP 192.168.11.100.37224 > 192.168.11.1.domain: Flags [S], seq 4965594, win 29200, options [mss 1460,sackOK,TS val 3560337742 ecr 0,nop,wscale 7], length 0

What is this SYN request from Ubuntu desktop ?

Best Regards, Yu Watanabe

ubuntu

1 Answers

  1. Best Answer

    Thank you for the answer. Joeqwerty and Tero Kikanen.

    So the cause of the TCP packets was that Ubuntu (Guest OS) was unable to communicate with DNS.

    However, root cauase of this problem was the bridge interface was not sending out dns packets from guest OS to the dns server. I use kvm on fedora sever 28 and Ubuntu runs on top of it.

    In my case I disabled the netfilter for the bridge interface.

    https://wiki.libvirt.org/page/Networking#Creating_network_initscripts

    The final step is to disable netfilter on the bridge:

 # cat >> /etc/sysctl.conf <<EOF
 net.bridge.bridge-nf-call-ip6tables = 0
 net.bridge.bridge-nf-call-iptables = 0
 net.bridge.bridge-nf-call-arptables = 0
 EOF
 # sysctl -p /etc/sysctl.conf

    Now TCP packets are not sent out.

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