My WRT54GL router running Tomato 1.28.1816 provides a DNS service to my network, and all devices have names which resolve forward and backward, including one VirtualBox virtual machine running 64 bit Ubuntu 12.x, over 64 bit Windows 7 as a host OS. The host OS runs on a PC which has a Wi-Fi connection to the router.
I can reach the VirtualBox instance by name (most commonly, to SSH into it). From time to time, however, its name disappears from DNS, becoming an unknown host. This seems to happen after the VM has been dormant for a while.
The host machine is reachable by name, but the virtual machine running in it is not similarly reachable.
The problem persists after Tomato is rebooted, even though the virtual machine appears in the list of clients as seen in the Tomato administrative UI: name, MAC and all.
From inside the virtual machine, networking works fine. Long-running connections are still connected, and machines can be reached.
What fixes the problem is to restart networking on that VM, so that it renegotiates a new lease with the router.
Also here is something strange: at all times when this issue is happening, that VM can be pinged by name from a shell on the router itself, as well as from a Linux server box which is attached to the router by copper ethernet, and has a static IP. Only wireless clients (including the VM itself) cannot resolve that VM's name. My confidence in this claim is not 100%; I must double check this the next time the issue reproduces.
Update: Dec 2013.
The issue is happening again (and as you can surmise from the date of the question and update, it is quite infrequent). I have more information. I'm logging into the router, and looking at the device list. Two things are apparent:
The Ubuntu VirtualBox instance's IP address was listed, but with no name, only its IP address (192.168.1.146).
The MAC address for this entry is wrong; it is the MAC address of the host machine; the OUI is that of ASUSTek (the manufacturer of the $15 PCI Wi-Fi card that is used, and which is the adapter to which the VirtualBox "Adapter 1" fake network interface is bridged). The entry for 18.104.22.168, the host machine, has the same MAC.
Furthermore, after I just restarted networking on the virtual machine, a new entry has appeared, bearing the correct host name. However, the bogus entry persists in the table. So there are now two entries in the device list for 22.214.171.124; one with the wrong MAC and no host name, and one with the right MAC.
It's hard to know where to lay suspicion, but I'm leaning toward an issue with VirtualBox "Bridged Adapter" mode over a wireless LAN adapter.