Strange possible DNS problem with BT Home Hub

Discussion in 'Broadband' started by Mark Carver, Aug 4, 2014.

  1. Mark Carver

    Mark Carver Guest

    Strange problem this weekend.

    A year ago, I set up extra wifi points for a friend who has a large
    sprawling home.

    She has a BT ADSL connection, with a Home Hub 3. That provides
    a wireless signal in the main part of the house.

    Last year, I extended the system. I used a pair of Netgear Powerline
    adaptors, to provide an ethernet connection to the far end of the house,
    fed from one of the LAN ports on the BT HH. At the far end
    the ethernet output from the other adaptor, feeds a simple basic 4 port
    ethernet hub. One feed from that feeding a TP Link TP-WR702N mini dongle
    (configured as a WiFi access point), the other feed, feeding another TP
    Link dongle, on the end of a 50 metre CAT 5 (in a separate building)

    All has been working perfectly for a year. All three WiFi devices have
    their own SSIDs, and I set the TP-Link dongles to Ch 1 and 13. I can't
    remember whether I set the BT HH box to Ch 6, but there's no coverage
    overlap anyway.

    On Saturday, she phoned me to say there was no internet connection
    available from either TP Link devices, but the HH WiFi was still fine.
    The TP Links were giving a signal, just an 'empty' one. She tried
    several devices, all the same. She'd rebooted everything etc.

    I talked her through switching off WiFi on her laptop, and connecting it
    via cable LAN to the four port hub. That worked fine, internet
    connection available. So that confirmed the powerline chain, and the
    little hub were OK. However still no luck with either TP Link device. At
    one point attempting to connect via one TP link box, resulted in a BT,
    'There seems to be a problem connecting to the internet' web page,
    (presumably generated by the HH itself ?).

    Yesterday, I set up a spare TP Link dongle I have to duplicate her
    arrangement, so I could talk her through the checking DHCP and DNS
    settings. She logged on to one of the TP link dongles, and all was in
    order, it had been assigned a dynamic IP address by the HH, and it
    itself had DHCP server disabled, so the IP address handout is managed by
    the HH. It was showing the primary DNS server as the BT HH
    ( (Exactly how I'd configured it a year ago).

    Then, we discovered, all was OK, internet connection available on both
    devices. Very odd. It certainly wasn't 'finger trouble' on her part,
    and the fault was originally detected by her daughter. The whole family
    had been trying to make it work, by checking leads, and rebooting all of
    Saturday afternoon before bothering me, and it was still the same
    yesterday morning, before I rang back.

    I can only think there was some sort of DNS problem, with the HH, or
    within BT's network ? I don't thing the BT HH allows WAN side access
    to remotely examine its parameters ? I might get her a 'proper' ADSL
    router, so I can remote access in (she lives 100 miles away) but how
    does BT Internet authentication work ?
    Is it locked down like Sky's ?
    Mark Carver, Aug 4, 2014
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  2. Is it possible to set static IP addresses for the access points
    (dongles) themselves, while keeping DHCP for devices fed through them?

    If network access was OK everywhere else before the access points, and
    they were producing wireless signals but with no valid network
    connection, then that suggests the problem is with the access points,
    and I reckon a local IP address conflict would account for it, so it's
    something to try.

    Who knows how it happened (if that is what happened) but just to avoid
    this possibility, I use static IP addresses on all my network devices
    that are in fixed locations, and only use DHCP for portable devices
    such as laptops and phones. Many routers are configured by default
    with DHCP starting at a high number such as 64 or 100, so you can use
    anything below that for fixed addressing, so you may not even need to
    reconfigure the Home Hub.

    Roderick Stewart, Aug 4, 2014
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  3. Mark Carver

    Mark Carver Guest

    Yes, it is. In fact I've done that on another 'remote' install of mine.
    Yes, that's possible, though the reboots that were attempted *should*
    have flushed anything such as that out ?
    Indeed, well worth a punt I think. I'm visiting in a couple of months,
    so I'll do it then. If it fails again before then, I'll talk my friend
    through the process remotely (she's getting very good at logging onto
    http set up menus now !)
    Mark Carver, Aug 4, 2014
  4. Mark Carver

    Graham J Guest

    The obvious check in these circumstances is to investigate the LAN
    separately from the WAN.

    Does the WiFi client get an IP address? (Command line: ipconfig /all)

    If so it suggests the LAN is OK - and the DHCP server in the HH is
    accessible. You can confirm this by PINGing the router.

    It's worth setting the WiFi access points with a static IP address, then
    these can be interrogated from the wireless client even if the HH DHCP
    service isn't accessible - you would have to configure the client with a
    static IP temporarily.

    Then connect a PC by wire to the HH and confirm it can be PINGed from
    other devices on the LAN - in different locations (by wire and
    wireless). You may have to prepare the PC with the necessary firewall

    This should identify any connectivity issues within the LAN.

    All the clients however connected should see the same default gateway
    and DNS server - the HH itself. Is it possible that some of the
    wireless clients are configured with manual DNS settings?

    Some HH devices have "Kiddie-proof" settings which do not allow some
    clients access to the internet. I've never investigated these; I've
    simply disabled it completely, but I've not had to be concerned about

    A "proper" router with VPN support is the way to go - you can monitor
    all the wireless devices (and other things: printers, NAS disks,
    webcams, etc.) that way. There are a few Powerline devices with
    IP-addressable management pages but most require a utility running on a
    PC - but with a VPN and the help of your user together with VNC or
    similar you can have access to the PC to see what is going on. Clearly
    this PC should be connected to the router.

    Some BT ADSL accounts require a password - if so the documentation with
    the service should show it.

    I expect the ADSL service has a dynamic IP, if so you will need to
    configure the router with a dynamic DNS account; but these are never
    entirely reliable. It would be better to get an ADSL service with a
    static IP (from Plusnet, Zen, or A&A).

    It is possible that the device connected by WiFi directly to the HH had
    cached its DNS entries whereas the devices using the other access points
    had not; this might explain the difference in ability to connect to the
    internet if the HH DNS was not actually working properly. A good test
    for this is to ping things on the internet by IP address rather than URL.

    I think BT has several DNS servers, some of which are unavailable from
    time to time. The router in its default configuration will only learn
    the DNS servers at the time it establishes its ADSL connection. I don't
    think it is told if the DNS servers become unavailable; but of course
    rebooting the router should resolve that.

    How certain are you that she actually rebooted the HH ??

    If you supply a proper router with VPN capability and change her to a
    professional ISP you should be able to monitor the complete system
    remotely, including rebooting everything. I support several clients
    that way, and it is worthwhile even for apparently quite knowledgeable
    and competent clients to make sure that you yourself have tested
    everything and seen the actual results.

    Good luck.
    Graham J, Aug 4, 2014
  5. Well... yes, though that depends on how you define "should". I think
    most people would assume it to mean "it would be nice if it did", but
    that's not always what happens in real life. I have occasionally seen
    some odd effects with networks that don't connect properly unless you
    start things in a particular order.

    Given that some of your network is going through the mains, there's
    always the possibility that a disturbance there could have kicked
    something into a non-standard state, or even that your friend could
    have unplugged one of the powerline adaptors to use the Hoover without
    thinking about it, or even remembering it.

    Some supposedly "automatic" things work better than others, so my
    inclination is generally only to use automatics where they give a
    genuine advantage. I don't think there's a particular advantage in
    automatic addressing in the fixed parts of a network that you have to
    make the effort to set up anyway, so you might as well set up the
    addresses too. Good luck with it anyway.

    Roderick Stewart, Aug 4, 2014
  6. Mark Carver

    Mark Carver Guest

    Thank you, and thank you for your thoughts !!

    All points noted. I'm fairly certain the HH was rebooted, but of course
    I can't be 100% sure. To be honest, I'm surprised this has been the only
    problem in a year, over the winter she was suffering no end of power
    cuts, and brown outs, and I was amazed this chain of devices managed
    to sort themselves out, and power back up all working every time.

    Most of the devices in use are mobile phones, and tablets. The nearest
    thing to a PC in the house is a single lap top.

    I'd already been talking to her, about upgrading to proper ISP, I was
    slightly dismayed a couple of years ago to discover she'd gone for BT,
    but previously it was TalkTalk !!

    Connection does indeed have a dynamic IP address. I do use DynDNS
    on Mother-in-Law's and my parent's router (come on Plusnet, supply a
    static option on your 'starter' account please ?) and it does the job,
    just can take a few mins to refresh fully after reboots etc, which is a
    Mark Carver, Aug 4, 2014
  7. Sometimes the order things are rebooted matters.
    Also EVERYTHING must be rebooted (powered off and on again) including,
    for example, network switches and homeplugs.
    Brian Gregory, Aug 4, 2014
  8. Mark Carver

    Graham J Guest


    The power-on order should be router first, then powerline devices, then
    APs and other things with IP addresses.

    My recommendation for the things which need static IP addresses is to
    configure the router to bind to the MAC address, and choose IP addresses
    outside the DHCP scope of the router. On some routers this might not be
    possible, they can only bind addresses within the DHCP scope. And on
    other routers (HH and the like) you may have no control over the IP
    address but the router may remember it so it becomes consistent. If the
    router has no binding mechanism then manually configure the device,
    again using an address outside the DHCP scope.

    Then, manually configure all the devices to have the IP address, default
    gateway, and DNS settings that have been assigned by the DHCP server.

    That way the devices are guaranteed to have your chosen IP paramaters,
    even if the router is not working.

    I have found a case where this fails. The TP-LINK TL-wpa281 (which came
    in a kit with the TL-PA211 powerline adapter) claims an IP address for
    itself, but it does not use DHCP - because the router does not show it
    in the list of IP addresses it has issued. So the router has no control
    over the address, it can't bind it to the MAC address. My guess is that
    the TL-wpa281 finds out the IP address of the DHCP server, then
    arbitrarily chooses its own IP address within that subnet. It isn't
    consistent across power off/on. It may check whether its choice is
    currently unused, but this does not prevent it from choosing an address
    which is manually configured in another device as yet not switched on.

    No doubt I could investigate further with Wireshark or the like ...
    Graham J, Aug 5, 2014
  9. You can get yourself into terrible knots trying to set up MAC
    filtering unless you know exactly what you're doing and take great
    care not to make mistakes. Get one digit wrong in the MAC address of
    the computer you're using to set it up, and as soon as you click the
    button to switch the filtering on, you'll be completely disconnected.
    The only way out then is to perform a hardware reset of the router,
    restoring everything, including wireless settings, to factory
    defaults, so you'll have to start all over again, which is tedious.

    I'll leave you to guess how I know this.

    The procedure of least effort is to note the IP addresses the router
    hands out automatically, and if they seem to start from a particular
    value - 64 and 100 are common choices - just set static addresses that
    are below that value on all your fixed computers and other devices.
    Make sure they're all different of course. For gateway and DNS
    settings, put the address of your router. It'll be the same as the
    others but probably ending in 1 or 254. I've always found this to be
    enough to make things work.

    Roderick Stewart, Aug 5, 2014
  10. Mark Carver

    Graham J Guest

    Most decent routers allow you to save the settings. Where I'm trying
    something new, I save the current settings, then try the new thing. If
    I get it wrong a H/W reset followed by a restore from the saved settings
    does save time.
    Graham J, Aug 5, 2014
  11. Perhaps I am. If you mean that the router keeps a table to ensure that
    each physical device always gets the same IP, I've never seen a
    setting for this, and can see two problems straight away. First, if a
    lot of devices connect to the router over a period of time (e.g.
    visitors with smartphones), then DHCP addresses will gradually become
    used up. Second, with static addresses it would have to work the other
    way round, i.e. assign the MAC address to the IP address you have
    chosen, rather than assigning an IP address as DHCP would do. And then
    what would happen if you decided to change the static address of a
    computer that had had its MAC address assigned in this way?

    If this is what you had in mind, then I can see that it's different
    from MAC filtering. Frankly, both procedures look like extra
    complication, when to solve a connection problem what you really need
    is less of it. Static IP addressing will give the software less to do,
    so there should be less to go wrong, shouldn't there?

    Roderick Stewart, Aug 6, 2014
  12. Mark Carver

    Graham J Guest

    A decent router will allow you to build a table of MAC to IP
    reservations, just for those devices that you require. Try looking at
    any of the Draytek Vigor range.
    I suspect this may happen with some HH routers that appear to remember
    MAC-IP assignments indefinitely. Not sure how it is resolved. Most
    routers forget the assignments when powered off; some (e.g. Vigor) can
    be programmed to remember specific assignments.
    No, it doesn't work that way!

    Client asks DHCP server for an IP address. DHCP server sees the MAC
    address of the client, and assigns an IP address. Most routers will
    check whether anything on the LAN is already using that IP address, and
    allocate another.

    If the router supports reservation, then the MAC address is used to look
    up from the table; if an IP entry is found then it is applied.

    MAC addresses are an essential part of the ARP (Address Resolution
    Protocol) which all networked devices use to communicate on the LAN.
    No, static IP addressing requires the user to keep notes of the
    configuration. Most people don't! Dynamic addressing solves almost all
    problems; but there are occasions when a computer expects a resource
    (e.g. a printer) to have a consistent IP address. Many printers resolve
    this by not using IP addressing for their drivers, relying on
    proprietary mechanisms.

    Filtering is a wholly different issue.
    Graham J, Aug 6, 2014
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