Linux bridging for KVM

Check that the kernel is set to enable IP forwarding:

sysctl -a | grep forwarding

You can enable with:

sudo sysctl net.ipv4.conf.all.forwarding=1
sudo sysctl net.ipv6.conf.all.forwarding=1

There may also be an issue with ARP proxying. Check with:

sysctl -a | grep proxy_arp

And set with the command:

sudo sysctl net.ipv4.conf.eth0.proxy_arp=1

You can put the keys and values in a file under /etc/sysctl.d to have the values reset on reboot.

Testing from another device on the router's subnet may help determine the problem.

Pinging the virtual machine may provide useful diagnostics.
Checking if you can ARP for the virtual machine will indicate if you can find the MAC address for the server. Use "arp -a" after pinging it to see if the MAC address is successfully found.
Traceroute may indicate where the problem starts.

Testing with tcpdump on the eth0 interface may also indicate where the connection is failing.

Repeated arp requests without a valid response indicates a reachability issue.
Missing echo or echo reply traffic may indicate which side has an issue.
Traceroute responses to the router or addresses behind it may provide additional information.

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