Guest can reach host, but can't reach outside network

When a guest interface is of type='network' (i.e. using a libvirt-managed "virtual network"), sometimes the guest will be able to reach the host (both via its IP address on the virtual network, e.g. for the "default" virtual network, and via the host's publicly visible IP address, but it won't be able to reach any IP address outside of the host (on the physical network). There can be a few possible reasons for this:

1) The iptables rules setup on the host has been disturbed.

This can happen if, for example, a firewall management application decides to clear all iptables rules and reload its own rules. Since it doesn't know about the rules added by libvirt, they will be lost. Alternately, sometimes another application inserts a new rule before libvirt's rules which takes precedence over the rules added by libvirt.

To see if this is the source of the problem, simply restart libvirtd, e.g.:

service libvirtd restart

This does not disrupt running guests in any way, and libvirtd will reload all of its iptables rules when it restarts.

2) IP forwarding has been disabled on the host.

To see if this is the problem, run this command:

sysctl net.ipv4.ip_forward

If the result is:

net.ipv4.ip_forward = 1

then IP forwarding is not your problem. If the result is:

net.ipv4.ip_forward = 0

then it definitely is at least one of your problems. libvirtd normally enables ip_forwarding whenever it starts a network that has <forward mode='route|nat'>, but it does not 1) permanently set it in /etc/sysctl.conf, or 2) re-set it if libvirtd is reloaded.

This is normally not a problem, but can become a problem if some other application or service sets IP forwarding back to 0. For example, when NetworkManager is started/restarted, it will run "sysctl -p", which reloads all of the sysctl settings in /etc/sysctl.conf. The default value of net.ipv4.ip_forward in that file is "0", meaning that every time NetworkManager is reloaded, it will disable IP forwarding.

Temporary Solution

# sysctl -w net.ipv4.ip_forward=1

Permanent Solution

Change the setting in /etc/sysctl.conf to

net.ipv4.ip_forward = 1

and run

sysctl -p

3) The libvirt network has <forward mode='route'/> and the outside network doesn't have a route to reach libvirt's virtual network

If your libvirt network is using <forward mode='route'/>, you are responsible for informing the rest of your network that the gateway to this network is the external/public IP address of your virtualization host. Instructions on how to do this are dependent on the setup of your network, and beyond the scope of this troubleshooting guide.