This is a problem that showed itself when upgrading our Proxmox 3.2 Nodes up to Proxmox 4. About halfway through the upgrade, our network adapters suddenly stopped being able to communicate with any local addresses, but could still ping outside addresses.
The cause was a minor config change that gets added in pretty stealthy. When this happens, simply add the following line to the bridge config in /etc/network/interfaces:
To make the entire config section resemble:
auto vmbr0 iface vmbr0 inet static address 192.168.3.xxx netmask 255.255.255.xxx gateway 192.168.3.xxx bridge_ports bond0 bridge_stp off bridge_fd 0 bridge_vlan_aware yes
This was pretty subtle to find, but after completing the upgrade and logging into the new Proxmox interface, it’s an option under the Network settings for that host.