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Topic: Weird Network Problem in the Booth
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Leo Enticknap
Film God
Posts: 7474
From: Loma Linda, CA
Registered: Jul 2000
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posted 07-15-2019 08:41 PM
Unlikely, I'd guess, if communication was previously OK, nothing was changed or messed with, and now, for no apparent reason, communication is no longer OK. That is why I thought of a DHCP server: it will allocate addresses without any human interaction, other than connecting a device that then asks for one. If the management LAN is also the house LAN for Internet access (not recommended per DCI, but is the reality in a lot of theaters), all it takes is for an usher to connect their iPhone to the house wifi, and, if the DHCP range is not set outside the static range, this can happen.
For this reason I would always separate the DHCP range from the static range: for example, configure a router's DHCP server to give out addresses in the 172.16.1.200 to 253 range, and use 2 through 199 for static addresses (with 1 for the gateway/router). But if you don't do this, and allow static and DHCP-allocated addresses to co-exist on the same subnet, you are at risk of address conflicts happening.
You could try scanning the subnet using an IP scanning program (Advanced IP Scanner is my favorite, and works in Ubuntu under WINE; Angry IP Scanner is available in a native Debian version for Ubuntu) but there are many to choose from), and if it reveals a device with the same address as your Christie that is not your Christie (if it has no hostname, check the MAC address), then you know that you have an IP address conflict on your hands.
The only other thing I can think of is that major updates to Ubuntu can sometimes mess with the IP address settings; so it might be worth double checking that the IP settings in the Ubuntu PC are correct, especially if you have a static address, subnet mask and gateway configured. In particular, this happened the last time I updated between LTS versions (I think it was 16.04 to 18.04, but can't remember for sure). For this and several other reasons, I prefer not to do major updates to Ubuntu, period, but to nuke the partition and do a clean install of the new version.
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Marcel Birgelen
Film God
Posts: 3357
From: Maastricht, Limburg, Netherlands
Registered: Feb 2012
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posted 07-15-2019 11:48 PM
I'd recheck the subnet mask on both the Ubuntu box and the projectors. If you're absolutely sure they're correct, then I'd say resume with the following step.
You didn't indicate it, but I guess both Ubuntu box and the projectors are in the same broadcast domain/IP range, so the Ubuntu box should use ARP to communicate to the projector. Check if it resolves to the right MAC address:
First, on the Ubuntu box do a quick ping:
ping <IP address of one of the projectors>
When nothing was magically healed, this should give time-outs, you can kill the command with Control-C. This step is important for the next one, because without any reason to communicate with the host, the ARP table will not be populated with its MAC address.
Continue with:
arp -an | grep <IP address of one of the projectors>
It should return a MAC address, if it doesn't, there is a Layer 2 or lower communication problem, most likely switch related. If it does return a MAC address, check if it is the MAC address of the projector and not something else. If it is something else, then there is an IP conflict. To get a clue about what kind of device it is that is interfering, you could check the vendor of the MAC address online. This often gives an indication in the domain to be searched. Alternatively, if you have a managed switch, you could log-in onto it and possibly check via that route, on which port a certain MAC address is located. The syntax differs per switch brand, unfortunately.
In order to debug your Layer 2 problem, one thing you can still do is doing a tcpdump. Make sure tcpdump is installed on your Ubuntu machine, otherwise install it via:
sudo apt-get install tcpdump
Then, open a second terminal or secondary SSH session. In the first, do something like:
ping <ip address of a projector>
in the second, something like:
sudo tcpdump host <ip address of a projector>
You can kill both commands with Control-C, but the tcpdump window should give you an indication of the traffic flow and if anything is answering on your ARP requests and if something is giving you an ICMP ECHO reply on your pings.
If there is clearly no response, then I'd proceed to look at the logs of the switch, if there are any and maybe I'd schedule a reboot of the thing. I've seen weird stuff happening with switches before...
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