Way (non network-based) to share host (ubuntu) file system with guest (centos 7) using libvirt

Here is a better way to set up port forwarding, using a hook script (source).

In /etc/libvirt/hooks/qemu:

#!/bin/sh

GUEST_NAME=
HOST_PORT=
GUEST_IPADDR=
GUEST_PORT=

if [ "$1" = "$GUEST_NAME" ]; then
  if [ "$2" = start ]; then
    iptables -t nat -A PREROUTING -p tcp --dport "$HOST_PORT" \
         -j DNAT --to "$GUEST_IPADDR:$GUEST_PORT"
    iptables -I FORWARD -d "$GUEST_IPADDR/32" -p tcp -m state \
         --state NEW -m tcp --dport "$GUEST_PORT" -j ACCEPT
  elif [ "$2" = stopped ]; then
    iptables -t nat -D PREROUTING -p tcp --dport "$HOST_PORT" \
         -j DNAT --to "$GUEST_IPADDR:$GUEST_PORT"
    iptables -D FORWARD -d "$GUEST_IPADDR/32" -p tcp -m state \
         --state NEW -m tcp --dport "$GUEST_PORT" -j ACCEPT
  fi
fi

You should set the four variables at the top to fit your libvirt setup.

You will need to restart libvirt-bin, which on ubuntu is done with:

sudo sh -c 'service libvirt-bin stop; service libvirt-bin start'

then you will need to restart the guest. On Ubuntu, you will need to adjust /etc/apparmor.d/usr.sbin.libvirtd to allow the hook script to execute:

Next to

/usr/sbin/* PUx,

append

/etc/libvirt/hooks/* PUx,

Then reload apparmor:

sudo service apparmor reload

There's probably a way to autoconfigure $GUEST_IPADDR using virsh / dumpxml / iface-dumpxml, but I haven't found it. Alternatively, the IP can be set statically in the network xml: documentation.

As far as I can tell, network filters can only be used for restricting what happens on the virtual network, and they aren't useful for port forwarding.

The answer to this issue was that because I'd done a non-graphic install of Centos 6 minimal on the guests, the network interfaces had not been automatically set up to work. My lack of knowledge of virtualisation and Centos meant that I assumed that this would be taken care of for me, but I was incorrect in this assumption.

Fix: I needed to go into ifcfg-eth0 on the guests and turn on ONBOOT=yes and BOOTPROTO=dhcp, then restart the network adapter via ifdown eth0 then ifup eth0. After doing this, the guests received IP addresses and everything worked as expected.