Sometimes can't access device on local network

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Sometimes can't access device on local network

Colin Law
I have an odd situation that occurs occasionally.  I have a conventional wired local network (192.168.1.nnn) with in addition some devices connected via wifi. The main router (192.168.1.1) is also the wifi hub.

Consider three of these machines. A, B and C.  A and B are wired and C is wifi, all with fixed IP addresses.

Normally all is well but occasionally I am not able to access C from A. When I ping it I get
$ ping 192.168.1.85
PING 192.168.1.85 (192.168.1.85) 56(84) bytes of data.
From 192.168.1.92 icmp_seq=1 Destination Host Unreachable

92 is the device I am running on (A).

However I am able to access B from A and also able to access C from B, which is particularly odd. So, even though I cannot ssh into C from A, I can ssh into B from A and from there ssh into C.

Can anyone suggest how I might go about debugging this?
I don't want to reboot things at the moment to see what fixes it in case anyone suggests something I should look at, so I don't want to stop it failing yet. It does not happen very often.

I don't have any other network problems that I am aware of.

Colin

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Re: Sometimes can't access device on local network

Colin Law
On 31 December 2017 at 12:13, Colin Law <[hidden email]> wrote:
I have an odd situation that occurs occasionally.  I have a conventional wired local network (192.168.1.nnn) with in addition some devices connected via wifi. The main router (192.168.1.1) is also the wifi hub.

Consider three of these machines. A, B and C.  A and B are wired and C is wifi, all with fixed IP addresses.

Normally all is well but occasionally I am not able to access C from A. When I ping it I get
$ ping 192.168.1.85
PING 192.168.1.85 (192.168.1.85) 56(84) bytes of data.
From 192.168.1.92 icmp_seq=1 Destination Host Unreachable

92 is the device I am running on (A).

However I am able to access B from A and also able to access C from B, which is particularly odd. So, even though I cannot ssh into C from A, I can ssh into B from A and from there ssh into C.

The plot thickens! I see now that C is not responding to arp requests. Any machines that already have C in their arp table continue to be able to communicate but any that do not cannot. Now I need to work out why it is not responding.

Colin


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Re: Sometimes can't access device on local network

Colin Law
On 31 December 2017 at 15:02, Colin Law <[hidden email]> wrote:
On 31 December 2017 at 12:13, Colin Law <[hidden email]> wrote:
I have an odd situation that occurs occasionally.  I have a conventional wired local network (192.168.1.nnn) with in addition some devices connected via wifi. The main router (192.168.1.1) is also the wifi hub.

Consider three of these machines. A, B and C.  A and B are wired and C is wifi, all with fixed IP addresses.

Normally all is well but occasionally I am not able to access C from A. When I ping it I get
$ ping 192.168.1.85
PING 192.168.1.85 (192.168.1.85) 56(84) bytes of data.
From 192.168.1.92 icmp_seq=1 Destination Host Unreachable

92 is the device I am running on (A).

However I am able to access B from A and also able to access C from B, which is particularly odd. So, even though I cannot ssh into C from A, I can ssh into B from A and from there ssh into C.

The plot thickens! I see now that C is not responding to arp requests. Any machines that already have C in their arp table continue to be able to communicate but any that do not cannot. Now I need to work out why it is not responding.

It was not so much that C was not responding to ARP requests, but that the router (a BT Home Hub running LEDE) was not forwarding them across the wifi for some reason. Re-booting the router fixed it.  Presumably it will happen again in a couple of weeks.  I can't find anything similar in a short google search. I will post a question on the LEDE forum.

Cheers and have a good new year

Colin


Colin



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Re: Sometimes can't access device on local network

Liam Proven
On 31 December 2017 at 16:47, Colin Law <[hidden email]> wrote:
>
> It was not so much that C was not responding to ARP requests, but that the
> router (a BT Home Hub running LEDE) was not forwarding them across the wifi
> for some reason. Re-booting the router fixed it.  Presumably it will happen
> again in a couple of weeks.  I can't find anything similar in a short google
> search. I will post a question on the LEDE forum.

I was about to point the finger of suspicion at the wifi router. Any
odd behaviour, reboot that, is my general maxim.

I mean, I forget myself sometimes, but it's _always_ worth a try.

I am not a fan of HomeHubs. I replaced them at a couple of client
sites, some years back, with more standard/customisable devices (e.g.
DrayTek).

Normally I also issue what is become a tiresome catchphrase of mine --
"update the firmware!" -- but HomeHubs do that sort of thing
automagically. (In theory.)

> Cheers and have a good new year

"Šťastný nový rok" from Prague!



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Re: Sometimes can't access device on local network

Colin Law
On 2 January 2018 at 13:02, Liam Proven <[hidden email]> wrote:
On 31 December 2017 at 16:47, Colin Law <[hidden email]> wrote:
>
> It was not so much that C was not responding to ARP requests, but that the
> router (a BT Home Hub running LEDE) was not forwarding them across the wifi
> for some reason. Re-booting the router fixed it.  Presumably it will happen
> again in a couple of weeks.  I can't find anything similar in a short google
> search. I will post a question on the LEDE forum.

I was about to point the finger of suspicion at the wifi router. Any
odd behaviour, reboot that, is my general maxim.

I mean, I forget myself sometimes, but it's _always_ worth a try.

I am not a fan of HomeHubs. I replaced them at a couple of client
sites, some years back, with more standard/customisable devices (e.g.
DrayTek).

Normally I also issue what is become a tiresome catchphrase of mine --
"update the firmware!" -- but HomeHubs do that sort of thing
automagically. (In theory.)

I am running LEDE on the hub, not the standard firmware, so I was not expecting this sort of issue.

Colin
 

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Re: Sometimes can't access device on local network

Liam Proven
On 2 January 2018 at 14:33, Colin Law <[hidden email]> wrote:
>
> I am running LEDE on the hub, not the standard firmware, so I was not
> expecting this sort of issue.

Oh, I apologise, I missed that. I wasn't expecting it because I didn't
know it was possible to run 3rd party firmware on HomeHubs at all.
Looking at some installation pages, it looks very difficult --
requiring soldering in serial ports and so on -- and nothing I would
try myself.

Still, sounds more like some temporary glitch than anything likely to
be readily troubleshot.

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Re: Sometimes can't access device on local network

Colin Law
On 2 January 2018 at 13:52, Liam Proven <[hidden email]> wrote:
On 2 January 2018 at 14:33, Colin Law <[hidden email]> wrote:
>
> I am running LEDE on the hub, not the standard firmware, so I was not
> expecting this sort of issue.

Oh, I apologise, I missed that. I wasn't expecting it because I didn't
know it was possible to run 3rd party firmware on HomeHubs at all.
Looking at some installation pages, it looks very difficult --
requiring soldering in serial ports and so on -- and nothing I would
try myself.

It is not difficult if you have a bit of experience with a fine tip soldering iron, you just have to solder in four wires (including ground) to connect to for initial programming.   The advantage is that they are incredibly cheap (mine cost £12.50 from ebay). For anyone interested you want a Home Hub 5A and instructions are

Colin


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