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Author Topic: Checking a device is there  (Read 358 times)

Bowdon

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Checking a device is there
« on: May 30, 2018, 11:32:16 AM »

Some background;

I was looking on the smarthub routing table page and noticed 1 device connected to the 5Ghz wireless connection. It's an LG TV. I have an LG TV that I used to connect wirelessly until I upgraded my nearby switch and now use ethernet. The ethernet shows up on the router table too.

The mac address for the wireless LG TV is different to the one connected via ethernet.

I did remove the wireless LG TV but its re-appeared this morning.

I suspect the routing table page on the smarthub as been corrupted because it doesn't show my 2 smartphones connected.

I'm trying to see if the IP address of this wireless LG TV is actually active. How would I do that?

What is the command to ping a lan ip address?

Will I be able to tell from the reply if the ip address is in use or not?
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broadstairs

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Re: Checking a device is there
« Reply #1 on: May 30, 2018, 12:11:04 PM »

Its ping 192.168.0.1 for example and if active will reply, if not it times out.

Stuart
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Bowdon

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Re: Checking a device is there
« Reply #2 on: May 30, 2018, 12:50:57 PM »

Thanks for the reply.

I pinged the router that came back good.

I then pinged the IP of this unknown wireless LG TV and it came back: destination host unreachable

Then I pinged the real LG TV connected via ethernet and that worked.

I pinged my smartphone, that doesn't even appear on the router table, and that worked.

So it looks like the wireless LG TV listing is probably a ghost listing or something.
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roseway

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Re: Checking a device is there
« Reply #3 on: May 30, 2018, 12:57:09 PM »

Possibly a neighbour who accidentally hooked up to your WiFi?
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johnson

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Re: Checking a device is there
« Reply #4 on: May 30, 2018, 01:28:18 PM »

Your TV will still have different MAC addresses on its ethernet and wireless interfaces. Are you sure the TV has 'forgotten' its wireless connection?

A sure fire way to test would be to change your wireless routers SSID, but that means selecting a new network on all wireless devices... a pain.

Are you concerned that someone in your local area is spoofing the MAC your TV used to have and has also defeated the WPA passcode? It seems unlikely.
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Bowdon

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Re: Checking a device is there
« Reply #5 on: May 30, 2018, 03:35:34 PM »

I was concerned someone had connected to my network with their LG TV. Though the router table in the smarthub doesnt seem up to date as it doesn't show my smartphones connected, even when I refresh it.

Ok I've just found where it shows the wireless mac address on my LG TV, its the same one that appears on the hub.

Interestingly I can't connect via wireless mode. I suspect the hub as confused itself and lost track of whats been connected and disconnected. Probably a full reboot of the hub would fix it.
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Weaver

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Re: Checking a device is there
« Reply #6 on: May 31, 2018, 04:56:55 PM »

I don't know what a ‘routing table’ is in this context because this is all a single lan, there is bridging here but no routing, correct? Please do forgive me if you know all this already, you probably do. A routing table is a an L3 thing (ie IPv4 and IPv6) list of IP address prefixes ie IP address ranges versus what to do with them. The kind of thing you often do see though is either or both of:
  • a dhcp allocation table - listing which IPv4 addresses have been handed out to the various devices by the DHCP server, this is a map between IP addresses and MAC addresses and vice versa
  • an arp cache listing, lists every known MAC address to IPv4 or IPv6 pairing that has been learned. This will list everything that a particular machine knows of, including devices that do not use DHCP
Sometimes things can sit in the ARP cache listing for an enormous amount of time. Devices may have a timeout after which they re-query the MAC address for a particular IP address by sending out a new probe in case an IP address has become associated with something with a different MAC address for some strange reason.

I don't know that that will help much. I rather doubt you have a security issue somehow. Maybe just a quirk of the UI of some kit somewhere.

I must say that I have failed to understand the lifetimes of some of this kind of listed info.

Sincere apologies if telling you a heap of stuff you already know.

As for checking whether a device is really there, it is worth forcibly clearing your own ARP cache out, different o/s have different commands for this, and then re-pinging the node you wish to check on. There really ought to be a tool that pings some node by MAC address, that would be very useful.

On some operating systems, the ARP cache gets binned if you bring an interface down and up again, such as unplugging a NIC or switching a NIC off then on. But I wouldn't guarantee it, would need to test this theory on your system.

[Moderator edited to fix a typo.]
« Last Edit: May 31, 2018, 06:20:19 PM by burakkucat »
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j0hn

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Re: Checking a device is there
« Reply #7 on: June 01, 2018, 01:03:15 AM »

1.  Note down the MAC address of the suspect LG device from the HomeHub DHCP table.

2.  Disconnect the Ethernet connection from your LG TV and connect via the wireless.

3.  Go back to the HomeHub and compare the MAC you noted down with the MAC of your LG TV's wireless card.

Most devices have different MAC addresses for their Wired/Wireless cards. Switching from wired to wireless will show as a completely new device to your router.
This is expected behaviour.
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Bowdon

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Re: Checking a device is there
« Reply #8 on: June 01, 2018, 11:23:36 AM »

1.  Note down the MAC address of the suspect LG device from the HomeHub DHCP table.

2.  Disconnect the Ethernet connection from your LG TV and connect via the wireless.

3.  Go back to the HomeHub and compare the MAC you noted down with the MAC of your LG TV's wireless card.

Most devices have different MAC addresses for their Wired/Wireless cards. Switching from wired to wireless will show as a completely new device to your router.
This is expected behaviour.

Yup, I disconnected the ethernet port and tried to reconnect via wireless (thinking I could only see the wireless MAC if I was connected). But it wouldn't let me connect wirelessly. Though in the menu's I had to go to the second page of the About TV page to see the wireless MAC listed and it was the same as the one I saw on the router.

For some reason I think the router seems to only be half working and not properly clearing devices when they disconnect.

I noticed this morning my phone appeared on the list, even though my phone as been connected nearly 24/7 since I got the router.

I suspect when it updated the firmware last week it messed something up. Eventually I'll probably have to turn the router off and on again to hopefully fix it.

I keep trying to use these smart hubs but they keep doing crazy things lol.

Update: yea, something is definately wrong with this hub as it won't let me manually set a lan ip address for one of the laptops. But as soon as I set it to get ip automatically it works again.

I'll wait for its usual 14 day reboot to happen and see if that changes anything, otherwise I'm going to be looking to swap it out. Hey BT I tried! :P
« Last Edit: June 01, 2018, 11:40:29 AM by Bowdon »
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