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Busted Windows 8, 10 update blamed for breaking Brits' DHCP

BT, Plusnet subscribers told to reboot or use static IP addresses

Folks using Windows 10 and 8 on BT and Plusnet networks in the UK are being kicked offline by a mysterious software bug.

Computers running the Microsoft operating systems are losing network connectivity due to what appears to be a problem with DHCP. Specifically, it seems some Windows 10 and 8 boxes can no longer reliably obtain LAN-side IP addresses and DNS server settings from their BT and Plusnet broadband routers, preventing them from reaching the internet and other devices on their networks.

(The link between BT and Plusnet is that, while the latter bills itself as a friendly independent ISP, it's really a subsidiary of the former.)

BT and Plusnet told The Register Microsoft is investigating the blunder. Redmond also confirmed on Thursday in its support forum that it’s looking into the problem.

Until Microsoft devises a fix, if necessary, the software giant's advice is simple: reboot your PC. Plusnet has told its customers to use static LAN IP addresses and fixed DNS settings, rather than obtaining this information automatically via DHCP, to reach the web.

Subscribers have told El Reg that a simple reboot is not always effective at clearing the problem: sometimes it takes multiple restarts to pick up network settings automatically.

We've also heard that it's not just BT and Plusnet subscribers affected: Windows 10 users using other broadband providers and a range of modems – from Netgear to Draytek – cannot get DHCP to work, we're told.

The cause of the bug is so far unclear, although Pluset has blamed an unspecified “third-party update.” It is understood that at the heart of the cockup is a recent Windows Update release – and it's hard to tell which one is at fault because Microsoft is now deliberately vague about what exactly is in each of the sets of patches it pushes to people's machines.

If an operating system upgrade is the cause, it would be the second case of egg-on-face for Microsoft's Windows Update team in as many weeks.

Nineteen flavors of rack and x6 systems from Lenovo froze upon installation of Microsoft’s security bug fix MS16-140 for Windows Server 2016, 2012 R2 and 2012. Lenovo told customers not to install the update until they’d patched their UEFI firmware to cope with the upgrade.

Whatever the cause is this time, the result is broken DHCP with PCs being left with no automatically assigned IP addresses. The scale of the issue is unclear, although in our view, it’s mushrooming and it’s striking PCs apparently randomly. Acer, Dell and HP machines have all been affected, we're told.

Reg reader Richard Inskip said he has seen “hundreds” of cases cropping up online in support forums, and has had to fix 29 adrift computers on his own – 15 on a single network. Adam Chambers, another reader, attested to the seemingly random pattern, claiming to have set up two identical PCs up but with just one dropping off the network.

Chambers noted the following commands, run one after the other, seemed to resolve the problem:

netsh winsock reset catalog
netsh int ipv4 reset reset.log

These will repair and reset gummed-up parts of the Windows networking stack. ®

Thanks to reader David Gosnell for the original tip. Click here for an update.

Updated at 13.44 on December 14 to add: Microsoft has released a KB on the issue.

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