I'm running Ubuntu Server 18.04.1 on a Dell 7240 laptop. I have both the ethernet and wifi connected to my network (/24). The IP addresses are reserved by the DHCP server (my Asus router) and dished out to the server fine. When I originally built this machine I suffered the same problem I'm about to descibe but I subsequently rebuilt and changed some things and the problem went away until last night when it appeared again.

The server gets the IP's from the dhcp server and can ping out and connect to the external internet, get updates etc so its routing through the gateway fine. The server can ping the gateway but the gateway can't ping the server. Moreover nothing on the rest of the LAN can ping the server, nor can the server ping anything on the LAN. Its like Ubuntu Server has suddenly turned on its firewall and is denying any and all traffic except traffic going out to the internet (I checked btw and it hasn't). Or that all ability to switch has suddenly collapsed. Its so baffling!

For extra context - this problem was happening originally and like I said I rebuilt the machine. When I rebuilt I also installed NetworkManager cli because I am more familiar with it from the desktop Ubuntu and there are more troubleshooting guides using it. Documentation and help for netplan is thin on the ground at the moment. So I changed the yaml in netplan to use NetworkManager and had a solid networking experience for a few weeks. I thought maybe NetworkManager had solved this weird issue until last night.

Thanks in advance for any help anyone can provide.

up vote 0 down vote accepted

I've encountered this issue in the past with a raspberry pi powered web kiosk that was set to statically obtain an IP address that was reserved for another device, and the symptom was exactly as you described; could ping gateway, couldn't be pinged and couldn't ping anything on the network, but still had internet access. Since your server has two interfaces, try turning one off. My suspicion is that there could be a MAC address mismatch on the part of your router that is causing a problem with address reservation.

edit: Upon some research it does indeed seem that your specific laptop has MAC address issues when it comes to its wifi chip.

  • Thanks for this and for the link about the mac addresses. I turned the wifi off using the hardware switch and retried troubleshooting but the problem persisted. However I've figured out the problem. I had made a bridge network in Docker which was incorrectly configured. Whats strange is I made this network a day or so ago and it only wreaked havoc last night. Whats also annyoing is I really did have the same problem when I originally built this system the first long before I installed Docker but its clear that was caused by something else - quite possibly the suggestion you make. – manners Nov 11 at 17:19
  • definitely keep a note of what happened, in case something similar happens in the future. – Minty Nov 11 at 18:40

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