I have a subnet 192.168.100.x where all IPs a served DHCP over WiFi. I have
- XPS laptop 192.168.100.121 (Ubuntu 22.04)
- Inspiron laptop 192.168.100.107 (Ubuntu 22.04)
- Windows laptop 192.168.100.113 (Windows 10)
- Many other devices on the same subnet
I can ping all devices & laptops from all laptops with the single exception of the XPS laptop to the Inspiron laptop and vice-versa which fails with 'Destination Host Unreachable' both ways. To be explicit:
FROM the XPS - 192.168.100.121
PING 192.168.100.107 (192.168.100.107) 56(84) bytes of data. From
192.168.100.121 icmp_seq=1 Destination Host Unreachable
...
PING 192.168.100.113 (192.168.100.113) 56(84) bytes of data. 64 bytes
from 192.168.100.113: icmp_seq=1 ttl=128 time=4.26 ms
FROM the Inspiron - 192.168.100.107
PING 192.168.100.121 (192.168.100.121) 56(84) bytes of data. From
192.168.100.107 icmp_seq=1 Destination Host Unreachable
...
PING 192.168.100.113 (192.168.100.113) 56(84) bytes of data. 64 bytes
from 192.168.100.113: icmp_seq=1 ttl=128 time=5.02 ms
FROM the Windows laptop - 192.168.100.113
Pinging 192.168.100.121 with 32 bytes of data: Reply from
192.168.100.121: bytes=32 time=79ms TTL=64
...
Pinging 192.168.100.107 with 32 bytes of data: Reply from
192.168.100.107: bytes=32 time=112ms TTL=64
If I do an arp -a on the XPS
dlinkrouter (192.168.100.1) at 60:63:4c:7b:f6:79 [ether] on wlp59s0 ?
(192.168.100.187) at 00:11:32:9c:7a:72 [ether] on wlp59s0
Inspiron (192.168.100.107) at <incomplete> on wlp59s0 Windows
(192.168.100.113) at d8:5d:e2:0b:22:6d [ether] on wlp59s0
If I do an arp -a on the Inspiron
XPS (192.168.100.121) at <incomplete> on wlp1s0
Windows (192.168.100.113) at d8:5d:e2:0b:22:6d [ether] on wlp1s0
dlinkrouter (192.168.100.1) at 60:63:4c:7b:f6:79 [ether] on wlp1s0
If I do an arp -a on the Windows laptop
Interface: 192.168.100.113 --- 0x14
Internet Address Physical Address Type
192.168.100.1 60-63-4c-7b-f6-79 dynamic
192.168.100.107 60-57-18-28-48-8f dynamic
192.168.100.121 9c-b6-d0-b7-0a-5d dynamic
192.168.100.164 10-62-e5-bc-d6-ad dynamic
192.168.100.187 00-11-32-9c-7a-72 dynamic
192.168.100.255 ff-ff-ff-ff-ff-ff static
224.0.0.22 01-00-5e-00-00-16 static
224.0.0.251 01-00-5e-00-00-fb static
224.0.0.252 01-00-5e-00-00-fc static
239.255.255.250 01-00-5e-7f-ff-fa static
255.255.255.255 ff-ff-ff-ff-ff-ff static
I am no network engineer. Much googling suggests that I am not getting the ARP response with the MAC address in but I have no idea why it is just between the two Ubuntu systems. Lots of posts blame routers firewalls or 802.11 implementations but I think, because every other communication through the router works fine, I have ruled that out. I even set up an ssh server on the Inspiron which results in the same 'Destination Host Unreachable' from the XPS but the Windows Laptop can connect no issue.
Any ideas on where to go next?
=========================== UPDATE 1 ============================
I have, unsatisfactorily, fixed the issue by manually adding the IP:HWaddr pairs to the ARP caches thus:
On the XPS - 192.168.100.121
sudo arp -s 192.168.100.107 60:57:18:28:48:8f
On the Inspiron - 192.168.100.107
sudo arp -s 192.168.100.121 9c:b6:d0:b7:0a:5d
So I can ping both ways now. However, I do not understand why the ARP request broadcast was not being responded too in the specific instance of requests from these laptops too each other but are in all other circumstances. I will probably make these changes permanent i.e. fix the IPs and the ARP cache entry and dig some more when I have time. My line of inquiry at the moment is understanding whether my router is failing to proxy the ARP request or indeed if it needs too (both laptops are on the same WiFi network BSSID - not sure if this acts like a 'physical' network).
=========================== UPDATE 2 ============================
I made a startup service to do the static ARP mapping. Not ideal but I put the details on GitHub in case it helps others.
=========================== UPDATE 3 ============================
It's the router - the internet did know what it was talking about. After much testing & many seemingly inconsistent results I narrowed it down to the router not proxying ARP requests between the 3 WiFi networks it supports 2.4GHz/5GHz A/5GHz B. My router is a Dlink DIR-3040 with 'smart-connect' which 'intelligently' puts devices on the best of the bands. I have turned off that feature and put the devices I want to connect on the same band and so far so good.
