PING: network gurus

cartman · 12-03-2014, 07:33 PM

I've got a strange one that has me stumped.

So on my home network, I have a couple of Windows machines (one 8, the other 7) and a couple of Ubuntu boxes. I just picked up a Raspberry Pi to mess around with, and I got it configured this evening.

I added a dedicated lease on my DHCP server for the new Pi, and confirmed it got the assigned address. Now here is where the fun comes in. From either of the Ubuntu boxes, I can SSH into the Pi. I cannot from either of the Windows boxes. They are all on the same subnet. I get a reply from the IP of the Windows box of "Destination Host Unreachable" if I ping the address when I ping from one of the Windows machines. And, sure enough, the arp address of the Pi isn't in the Windows arp cache. If I ping the Windows box from the Pi, I can then SSH into the Pi, at least until the arp cache times out, then it loses the arp address again. If I set a static ARP entry on the Windows box, then it is able to connect to the Pi with no issues.

I've done some Googling, and have seen similar incidents, but the fix always seems to be add a static ARP entry. But that just doesn't sit well with me. There has to be a reason for the ARP issue between Windows and the Pi, but damned if I can find it.

Lonnie · 12-03-2014, 09:10 PM

Strange one. I would do a pcap capture with wireshark then compare the arp broadcast coming from the Ubuntu box vs the Windows box. Obviously the Pi is not liking something with the way the Windows one is asking for its address.

Also you can tcpdump on the Pi and see if arp is reaching it at all.

weegeebored · 12-03-2014, 11:30 PM

Typically the "unreachable" message means that that Windows thinks that the destination IP address is on a different subnet and that the default gateway is misconfigured or missing. What network ID and mask are you using?

SteveMax58 · 12-04-2014, 07:45 AM

Is there any difference in how these devices connect to your network? Like some devices (windows) are on wifi while the others are hardwired?

SteveMax58 · 12-04-2014, 07:49 AM

I guess where I'm going with that question is that while adding the static arp entry to the windows machine might make it remember, the connecting router should be informing the windows device that the address is reachable (well, assuming the windows device isnt being stupid...quite possible).

Using private IP space, right?

Toddzilla · 12-04-2014, 07:49 AM

Quote:

Originally Posted by cartman

I've got a strange one that has me stumped.

So on my home network, I have a couple of Windows machines.

cartman · 12-04-2014, 08:59 AM

Network config:

all on the same subnet 192.168.38.0, mask 255.255.255.0

The two linux boxes and one of the windows boxes are physically connected, the windows laptop is wireless. Pi is also connecting via WiFi.

On the windows laptop, I have VMWare Workstation. There is a CentOS VM on there that uses a bridged network connection off of the host. The CentOS VM can hit the Pi no problem, but the Windows host can't until I ping it from the Pi. So it has to be something with the ARP request sent from Windows, or the ARP ack sent from the Pi.

SteveMax58 · 12-04-2014, 10:28 AM

So if you access the Pi device from an ubuntu device...then try 1 of the windows devices to access Pi....it still doesnt work?

It sounds like an issue of arp cache clearing timers. Your windows device clears too frequently to remember (without the static entry). Your Pi device may not be performing gratuitous arp regularly (which you may want to enable if this device needs to be accessed by the others). And your router may be clearing its own arp cache too frequent for the level of use occurring with the pi device. The ubuntu devices are likely holding their arp longer (perhaps forever until overwritten is the only explanation that comes to mind).

cartman · 12-04-2014, 10:37 AM

Quote:

Originally Posted by SteveMax58

So if you access the Pi device from an ubuntu device...then try 1 of the windows devices to access Pi....it still doesnt work?

It sounds like an issue of arp cache clearing timers. Your windows device clears too frequently to remember (without the static entry). Your Pi device may not be performing gratuitous arp regularly (which you may want to enable if this device needs to be accessed by the others). And your router may be clearing its own arp cache too frequent for the level of use occurring with the pi device. The ubuntu devices are likely holding their arp longer (perhaps forever until overwritten is the only explanation that comes to mind).

When I ping the Pi from the Windows device, the Pi's MAC never shows up in the Windows ARP cache.

SteveMax58 · 12-04-2014, 10:40 AM

Quote:

Originally Posted by cartman

When I ping the Pi from the Windows device, the Pi's MAC never shows up in the Windows ARP cache.

Have you tried it right after the Ubuntu ping? Like within a few seconds?

cartman · 12-04-2014, 10:44 AM

Yeah, when I ping the Windows box from the Pi, the Pi is in the Windows ARP cache. But if the Pi gets aged out, and isn't in the ARP cache, when I ping it from a Windows box I get 'Host Unreachable', and the Pi doesn't get re-added to the ARP cache.

SteveMax58 · 12-04-2014, 11:37 AM

I understand windows is not retaining arp entries of the Pi device(for very long). But when it has no entry for an IP, it should ask the network its connected to where to find it by sending a broadcast arp request for the IP. If a device on the network has the ip it should respond with a broadcast back informing the rest of the devices that "thats me!"

So thats why its curious why another device's (presumed) arp request (Ubuntu for instance) response (from Pi) isnt being stored by windows.

The fact that you have a VM on the windows device that does retain it suggests to me that your problem is at the windows management layer rather than the NIC itself(perhaps thats obvious already to you).

I would say a pcap of the windows request as well as a pcap when another device (like the ubuntu device) would be good to sift thru. Or if thats too much trouble, maybe google a utility for windows to manage arp better.

SteveMax58 · 12-04-2014, 12:37 PM

One other thought....do you have a gw address configured? Just wondering if the arp request isnt getting thru the windows layer because of that (likely windows fault in any event but could possibly explain the inconsistency from device to device).

weegeebored · 12-04-2014, 05:54 PM

Wi-fi through a wireless router? Any other router? Physical connections via a switch? Could there be a firewall (Windows) blocking (incoming) broadcasts?

12-03-2014, 09:10 PM	#2
Lonnie High School Varsity Join Date: Dec 2004 Location: Midlothian, TX	Strange one. I would do a pcap capture with wireshark then compare the arp broadcast coming from the Ubuntu box vs the Windows box. Obviously the Pi is not liking something with the way the Windows one is asking for its address. Also you can tcpdump on the Pi and see if arp is reaching it at all. __________________ Lonnie Last edited by Lonnie : 12-03-2014 at 09:11 PM.

12-04-2014, 08:59 AM	#7
cartman Death Herald Join Date: Nov 2000 Location: Le stelle la notte sono grandi e luminose nel cuore profondo del Texas	Network config: all on the same subnet 192.168.38.0, mask 255.255.255.0 The two linux boxes and one of the windows boxes are physically connected, the windows laptop is wireless. Pi is also connecting via WiFi. On the windows laptop, I have VMWare Workstation. There is a CentOS VM on there that uses a bridged network connection off of the host. The CentOS VM can hit the Pi no problem, but the Windows host can't until I ping it from the Pi. So it has to be something with the ARP request sent from Windows, or the ARP ack sent from the Pi. __________________ Thinkin' of a master plan 'Cuz ain't nuthin' but sweat inside my hand So I dig into my pocket, all my money is spent So I dig deeper but still comin' up with lint Last edited by cartman : 12-04-2014 at 09:00 AM.

12-04-2014, 10:44 AM	#11
cartman Death Herald Join Date: Nov 2000 Location: Le stelle la notte sono grandi e luminose nel cuore profondo del Texas	Yeah, when I ping the Windows box from the Pi, the Pi is in the Windows ARP cache. But if the Pi gets aged out, and isn't in the ARP cache, when I ping it from a Windows box I get 'Host Unreachable', and the Pi doesn't get re-added to the ARP cache. __________________ Thinkin' of a master plan 'Cuz ain't nuthin' but sweat inside my hand So I dig into my pocket, all my money is spent So I dig deeper but still comin' up with lint

12-03-2014, 11:30 PM	#3
weegeebored High School Varsity Join Date: Sep 2006	Typically the "unreachable" message means that that Windows thinks that the destination IP address is on a different subnet and that the default gateway is misconfigured or missing. What network ID and mask are you using?

12-04-2014, 07:45 AM	#4
SteveMax58 College Starter Join Date: Dec 2006	Is there any difference in how these devices connect to your network? Like some devices (windows) are on wifi while the others are hardwired?

12-04-2014, 07:49 AM	#5
SteveMax58 College Starter Join Date: Dec 2006	I guess where I'm going with that question is that while adding the static arp entry to the windows machine might make it remember, the connecting router should be informing the windows device that the address is reachable (well, assuming the windows device isnt being stupid...quite possible). Using private IP space, right?

12-04-2014, 10:28 AM	#8
SteveMax58 College Starter Join Date: Dec 2006	So if you access the Pi device from an ubuntu device...then try 1 of the windows devices to access Pi....it still doesnt work? It sounds like an issue of arp cache clearing timers. Your windows device clears too frequently to remember (without the static entry). Your Pi device may not be performing gratuitous arp regularly (which you may want to enable if this device needs to be accessed by the others). And your router may be clearing its own arp cache too frequent for the level of use occurring with the pi device. The ubuntu devices are likely holding their arp longer (perhaps forever until overwritten is the only explanation that comes to mind).

12-04-2014, 11:37 AM	#12
SteveMax58 College Starter Join Date: Dec 2006	I understand windows is not retaining arp entries of the Pi device(for very long). But when it has no entry for an IP, it should ask the network its connected to where to find it by sending a broadcast arp request for the IP. If a device on the network has the ip it should respond with a broadcast back informing the rest of the devices that "thats me!" So thats why its curious why another device's (presumed) arp request (Ubuntu for instance) response (from Pi) isnt being stored by windows. The fact that you have a VM on the windows device that does retain it suggests to me that your problem is at the windows management layer rather than the NIC itself(perhaps thats obvious already to you). I would say a pcap of the windows request as well as a pcap when another device (like the ubuntu device) would be good to sift thru. Or if thats too much trouble, maybe google a utility for windows to manage arp better.

12-04-2014, 12:37 PM	#13
SteveMax58 College Starter Join Date: Dec 2006	One other thought....do you have a gw address configured? Just wondering if the arp request isnt getting thru the windows layer because of that (likely windows fault in any event but could possibly explain the inconsistency from device to device).

12-04-2014, 05:54 PM	#14
weegeebored High School Varsity Join Date: Sep 2006	Wi-fi through a wireless router? Any other router? Physical connections via a switch? Could there be a firewall (Windows) blocking (incoming) broadcasts?

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