If you are upgrading your machine from Debian Wheezy to Jessie, then you will find out that there is no IP forwarding anymore. In my case I was upgrading my Proxmox 3.1 to 4.2 and masqerading with iptables. So having 1 public address for multiple virtual machines.
You have to check your /etc/network/interfaces and add this line:
post-up echo 1 > /proc/sys/net/ipv4/ip_forward
This enables ip forwarding.
Should look like this:
auto lo
iface lo inet loopback
auto eth0
#real IP adress
iface eth0 inet static
address 192.168.10.2
netmask 255.255.255.0
gateway 192.168.10.1
auto vmbr0
#private sub network
iface vmbr0 inet static
address 10.10.10.1
netmask 255.255.255.0
bridge_ports none
bridge_stp off
bridge_fd 0
post-up echo 1 > /proc/sys/net/ipv4/ip_forward
post-up iptables -t nat -A POSTROUTING -s '10.10.10.0/24' -o eth0 -j MASQUERADE
post-down iptables -t nat -D POSTROUTING -s '10.10.10.0/24' -o eth0 -j MASQUERADE