Electronic Engineering Blog

BT HomeHub 5 has issues with mDNS/Bonjour

I recently upgraded from the BT HomeHub 3 to their latest offering, the BT Home Hub 5.

However immediately on replacing the old router with the new one I started having problems with my networked printers. I have a pair of HP OfficeJet 7500A printers, attached to my home network. Once I had upgraded to the Home Hub 5, I could see the printers on the network, but I could not print to them.

To cut a long story short, I did a bit of digging about with Wireshark and Bonjour Browser and discovered the issue. The printers advertise themselves on the network using mDNS (also known as Bonjour in Apple circles). With mDNS when a computer wishes to find a provider of a certain service, such as printing, it sends out a service discovery packet on the network, and all the devices which provide that service reply with their details. This is known as ‘service discovery.’ In the case of these printers the service is known as “_ipp.tcp” (Internet Printing Protocol).

The problem that I’ve been seeing with the HH5 is that this service request and discovery process does not work for certain devices – but only when the computer issues the request via the HH5’s LAN interface (ie, the wired ethernet ports). If the computer is connected to the home hub using WiFi then the service discovery process works properly.

mDNS and Bonjour work by sending the service discovery packets to a well known ‘multicast’ address: – It looks to me like the HH5 is not passing these requests on the LAN interface correctly. Yet it does work on the WiFi interface.

It could be the size of the TXT record that is at the heart of the problem, because these printers return quite a large TXT packet, but I have not been able to verify that. Even so, it works fine over the WiFi interface, so it should work on the LAN interface.

Finally, if I go back to using my old BT Home Hub 3, then it all works fine on both the LAN and the WiFi interfaces, leading me to deduce that there is an issue with mDNS/Bonjour being used with HH5.

I’ve contacted BT and sent them packet captures and all kinds of data, but they don’t respond. It would appear as through they simply do not care about this issue.

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