[SOLVED] Some (but not all) network hosts can't talk to each other

dlanced · 06-30-2020, 08:17 PM

I'm having trouble with my local network. All the Linux hosts get DHCP addresses and connectivity. The problem is that they're not always able to communicate (SSH, ping) with each other. I've got two WiFi routers configured on channels far apart from each other. One router (my ISP's device) is at 192.168.1.1 and my TPLink AC1200 router is configured as an access point on the same network at 192.168.1.2.

Right now I've got a Raspberry Pi that was assigned 192.168.1.16 and has internet connectivity, but can't be seen or accessed from my primary workstation - even when it's connected through the same router (that would be the TPLink, at the moment). And I can't SSH from the Pi to my workstation, But my laptop - on the same network and logged in through the same router - can SSH in and pick the Pi up on nmap.

Another issue that I think may be related is that, for some reason, I'm not always able to refer to all the hosts by "hostname.local" - although they'll generally work using the IP address (except when they're "off" the network, of course).

Any ideas?
Thanks,

ferrari · 06-30-2020, 08:28 PM

Quote:

Right now I've got a Raspberry Pi that was assigned 192.168.1.16 and has internet connectivity, but can't be seen or accessed from my primary workstation

Can you ping the Pi host via the primary workstation successfully?

Code:

ping 192.168.1.16

With respect to Avahi...

Code:

avahi-browse -art

*You may need to discover avahi-utils package first.

Check for active firewalls on the host you're trying to reach.

dlanced · 06-30-2020, 08:36 PM

Quote:

Originally Posted by ferrari

Can you ping the Pi host via the primary workstation successfully?

Code:

ping 192.168.1.16

Nothing:

Code:

$ ping 192.168.1.16
PING 192.168.1.16 (192.168.1.16) 56(84) bytes of data.
From 192.168.1.13 icmp_seq=1 Destination Host Unreachable

Quote:

With respect to Avahi...

Code:

avahi-browse -art

*You may need to discover avahi-utils package first.

I've attached the avahi output: avahi.txt
As you can see, this is a busy machine - lots of LXD bridges etc.

Quote:

Check for active firewalls on the host you're trying to reach.

[/QUOTE]
Good point. I forgot to mention that there are no firewalls on the local network.
Thanks,

ferrari · 06-30-2020, 08:58 PM

Can you confirm the ip assignments on the pi at this time?

Code:

ip address

Code:

ip route

dlanced · 06-30-2020, 09:05 PM

Quote:

Originally Posted by ferrari

Can you confirm the ip assignments on the pi at this time?

Code:

ip address

Code:

$ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN group default qlen 1000
    link/ether b8:27:eb:2e:69:db brd ff:ff:ff:ff:ff:ff
3: wlan0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether b8:27:eb:7b:3c:8e brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.16/24 brd 192.168.1.255 scope global dynamic noprefixroute wlan0
       valid_lft 85877sec preferred_lft 75077sec
    inet6 fe80::9181:da2:27d5:37e/64 scope link 
       valid_lft forever preferred_lft forever

Quote:

Code:

ip route

Code:

$ ip route
default via 192.168.1.1 dev wlan0 proto dhcp src 192.168.1.16 metric 303 
192.168.1.0/24 dev wlan0 proto dhcp scope link src 192.168.1.16 metric 303

ferrari · 06-30-2020, 09:13 PM

I know you said there was no firewall on any host....but just to be sure, run this from the Pi...

Code:

sudo iptables -S

ferrari · 06-30-2020, 09:17 PM

Regarding this comment...

Quote:

I've got two WiFi routers configured on channels far apart from each other. One router (my ISP's device) is at 192.168.1.1 and my TPLink AC1200 router is configured as an access point on the same network at 192.168.1.2.

Can you confirm that both hosts are connected to the same wifi AP?

Code:

iw dev wlan0 link

or via older iwconfig command...

Code:

iwconfig wlan0

dlanced · 06-30-2020, 09:18 PM

Quote:

Originally Posted by ferrari

I know you said there was no firewall on any host....but just to be sure, run this from the Pi...

Code:

sudo iptables -S

This is a completely fresh install on the Pi:

Code:

$ sudo iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT

The truth is that the Pi is just a current example of the problem. I've been having similar issues with other hosts on this LAN.

ferrari · 06-30-2020, 09:25 PM

If you have hosts connected via different wifi AP's this might present an issue (eg multicast traffic being blocked), although pinging should work with no issue. Some APs have features to keep wifi client hosts from being able to talk directly to each other as well.

Just in case this is applicable...
https://www.tp-link.com/us/support/faq/2089/

dlanced · 06-30-2020, 10:35 PM

Quote:

Originally Posted by ferrari

If you have hosts connected via different wifi AP's this might present an issue (eg multicast traffic being blocked), although pinging should work with no issue. Some APs have features to keep wifi client hosts from being able to talk directly to each other as well.

Just in case this is applicable...
https://www.tp-link.com/us/support/faq/2089/

That is interesting. My TPLink model doesn't seem to have that function. But in any case, in at least my current setup, all three hosts (my workstation, my laptop, and the Pi) were attached to the same router - but only two were able to talk to each other.
Thanks,

dlanced · 06-30-2020, 11:00 PM

Just a quick update: A couple of hours later, the Pi suddenly became available for SSH from my workstation. This is exactly the unpredictable behavior I've been experiencing with other hosts. Is it possible that there's something buggy about the way ARP is caching my host addresses and, for some hosts, it's taking a very long time? Full disclosure: I don't know all that much about ARP, but I'd like to hear what people who do understand it think.

tinfoil3d · 06-30-2020, 11:06 PM

dlanced, as ferrari mentioned, I also have a strong suspicion if both of your wifi AP have same keys and name then your systems might roam between them sometimes which would cause them to split. Make sure the devices are connected to the same AP.

dlanced · 06-30-2020, 11:10 PM

Thanks. I have indeed seen some flipping between APs over the past months.
But all three of the devices I've been watching through the past six hours have been on the same AP. Besides that, why should they split even if they are one different APs? They're all on the same network.

tinfoil3d · 06-30-2020, 11:33 PM

If your tp-link isn't configured as a bridge then they'd be on the same network the same way my 192.168.1.2 is on the same network as you.

ferrari · 07-01-2020, 03:08 AM

Yes, I agree... focus your attention on the second AP device. I assume it is connected via ethernet? (Refer below link)
https://www.tp-link.com/en/support/faq/417/

WDS Bridging is described here
https://www.tp-link.com/us/support/faq/440/