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UPD2 BT Openreach FTTC Superfast Broadband Modems Break VPN Access

Tuesday, Nov 5th, 2013 (3:42 pm) - Score 4,687

The Director of broadband ISP Andrews & Arnold (AAISP), Adrian Kennard, claims to have discovered a fault with the superfast broadband FTTC (VDSL) modems that BTOpenreach distributes to new subscribers, which can affect “almost anyone running any sort of VPN” (Virtual Private Network) Internet service.

The problem does not appear to show on modems that haven’t recently had their firmware updated and those running custom (non-BT) firmware are also unaffected, which suggests that the issue might have been introduced by an automatic upgrade; a recent one.


According to Kennard (blog), the seemingly new bug results in the modem blacklisting some User Datagram Protocol (UDP) packets after a Point-to-Point Protocol (PPP) restart and this apparently causes a few problems.

Adrian Kennard explained:

If we send a number of UDP packets, using various UDP ports, then cause PPP to drop and reconnect, we then find that around 254 combinations of UDP IP/ports are now blacklisted. I.e. they no longer get sent on the line. Other packets are fine.

Sending 500 different packets, around 254 of them will not work again after the PPP restart. It is not actually the first or last 254 packets, some in the middle, but it seems to be 254 combinations. They work as much as you like before the PPP restart, and then never work after it.

We can send a batch of packets, wait 5 minutes, PPP restart, and still find that packets are now blacklisted. We have tried a wide range of ports, high and low, different src and dst ports, and so on – they are all affected.”

Sadly a lot of VPNs (e.g. OpenVPN), like so many other Internet services, make use of UDP for at least part of their communications. As a result many VPN users could be suffering from reliability problems that they might otherwise struggle to identify; especially since the device is only a VDSL Modem and you thus wouldn’t ordinarily expect it to be the cause of this particular fault.

A simple fix for the problem is to quickly disconnect the Ethernet port on the modem and reconnect, which should resolve the issue until the next PPP restart causes it to reappear. In the meantime Kennard said that BT is working on the issue and we are attempting to confirm that.

UPDATE 6th Nov 2013 (7:14am)


So far the issue has only been spotted on Openreach’s Huawei HG612 modems (i.e. not the ECI series), although it’s still unclear whether or not this affects all of the different versions of the device or just a specific one.

Meanwhile a spokesperson for Openreach confirmed to ISPreview.co.uk that it has been made aware of the issue and that they’re “looking into it“. We hope to have more information shortly.

UPDATE 12th Nov 2013

AAISP has posted an interest update on this issue (here) but so far there’s still some uncertainty about the cause. The problem also doesn’t appear to show-up on BTInfinity lines but this could be because “when the PPP re-establishes, a different IP address is allocated each time, and whatever is session tracking does not match the new connection“. On the other hand it might be an unusual issue being caused at the street cabinet but that’s only an outside theory.


So far we’re no closer to a fix and the bug testing continues.

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By Mark Jackson
Mark is a professional technology writer, IT consultant and computer engineer from Dorset (England), he also founded ISPreview in 1999 and enjoys analysing the latest telecoms and broadband developments. Find me on X (Twitter), Mastodon, Facebook and .
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