(solved) Off-topic...stomped by network mis-behaviour

[tillkrueger]

I have just lost 4hrs of my life and a lot of hair over the weirdest thing...my iMac (and only my iMac) is losing contact to the internet seconds after I plug in the ethernet cable...it will briefly connect and start refreshing web-pages, then fall back into some mysterious mode where all web-services will fail to connect...Dropbox, the Adobe CC client, AnyDesk, web browsers, etc. will all go blank...my network settings for ethernet (and WiFi) are set to DHCP (I also tried to set them manually, same thing), and do fetch an IP and DNS, but within a few seconds of unplugging and re-plugging ethernet, they will all go blank again...for about 3-5 second I can load pages fine, but then something goes awry.

I have rebooted the router multiple times, and have done the same with my iMac. My MacBook Pro, which sits right next to my iMac works just fine...iPad Pro and iPhone, both connected via WiFi, also work as expected. But something with my iMac is going awry and I am at the end of my wits after wasting the entire afternoon trying to get it to behave.

Curious, also, is that when I unplug ethernet and enable WiFi, then the WiFi connection will work for a few seconds before it, too, will fail to stay connected to the internet. So I think I can rule out a faulty ethernet cable (purchased new last week).

What could be causing such behaviour?

[Frank1940]

Defective iMac?  or a firewall problem?   or bad memory?  

[tillkrueger]

I never touch the MacOS Firewall (always OFF), and there is no firewall in any of the routers here that wouldn't also affect the other machines, which all work fine.

So yeah, a hardware issue in the iMac would explain it, but I somehow doubt it. (just a feeling)

I had just upgraded to 10.14.1 prior, so I am thinking that a software issue is more likely...but I also doubt that that particular upgrade did it, knowing Apple's usually pretty stringent beta cycles.

My last backup is 8 days old but I have added about 100GB of data to the boot-drive since (not entirely sure what added that much), so I am somewhat reluctant to take that big a step back, but I may have to in order to find out whether this really is a hardware issue or not.

I'll make a backup of what I have first, then do a restore to the last backup after that to see where that leaves me.

[Frank1940]

I would also do a Google first and see if anyone else has reported any issues with the update. 

[tillkrueger]

good tip...but a first search came up empty...it's early, though, so I'll check back in the morning...by then the current backup will be done and I can try rolling back to the last one for confirmation, bc I know everything was working fine 8 days ago. If it doesn't anymore, then my iMac's hardware has gone faulty...here's to hoping that it didn't.

enough head scratching for the day...g'nite to ya, lad!

[tillkrueger]

Although I did just find this article, which proves that Apple isn't beyond breaking a network port through an update...it was back in 2016, and it's unlikely that this is the issue, but it does happen, apparently...time will tell.

[JonathanM]

If you are still having issues, try mapping out the IP's for all devices behind your router. It almost sounds like you have some other device with the same IP as your problem child is grabbing. Normally that's not supposed to happen, but obviously it can.

 

Quick way to check if that is indeed the issue is to find the local IP of the iMac, power it off completely and make sure it's not connected to the network in any way, then ping that local IP from another machine. If you get a response instead of a time out, there's your problem.

[tillkrueger]

that's a good theory, jonathanm, but I had already restored to the previous backup overnight and am booted into it now, without any such issues...also, I did have those same issues even when using manually assigned (and unique) IP numbers...I have a system for assigning IP numbers that I use for all of my machines and devices, so that double IP issues won't happen.

I will do the upgrade to 10.14.1 again and see whether it will still be ok after that...will report back.

[tillkrueger]

Wow, who'd h'thunk it? the 10.14.1 upgrade raked havoc on my system again...after the update, network behaviour was totally messed up again...not sure what is up with that, but my hardware (it's an iMac 27 late 2012) does not like Apple's latest software. I'll be interested to see what comes up on Google over the course of the next while. I'll be staying with 10.14 for a while then. Restoring, yet again, as I type this.

[tillkrueger]

While everything works exactly as designed now, there is one very annoying problem I still have.

My IT guy in Berlin set up a VPN profile for me, so that I can securely log into my VLAN to administer my unRAID without leaving tons of ports open on the exposed public IP that he assigned to it (which has only ports 80 and 443 exposed)...the issue is that when I connect to this VPN profile from my iMac, the VPN assigned IP is not accessible...when I do the same from my rMBP, all works as designed.

When I try to ping that IP from my iMac, I get this:

till$ ping 10.0.195.2
PING 10.0.195.2 (10.0.195.2): 56 data bytes
ping: sendto: Cannot allocate memory

What could cause a "Cannot allocate memory" error? I have connected through this VPN profile successfully, in the past, but now it's not working anymore when everything else network-related is working perfectly well.

[tillkrueger]

just found some info about this particular error, having to do with routing tables that are set up...gonna inquire deeper and report back.

[Jcloud]

Have you tried setting up a new network location and test? https://support.apple.com/en-us/HT202480

 

I know this is one of Apple's quick-fix tries for networking issues on software level, and it's not that intrusive of a fix.

[tillkrueger]

I have not tried that yet, Jcloud, but shall do so right now.

I have isolated the problem, though, to the fact that when I use *only* WiFi, I can successfully connect through the VPN, but when plugging in ethernet again, it stops working...there seems to be something going on with the IP assignments between ethernet and WiFi...totally erratic behavior, where I could get it work, briefly, 15 minutes ago, then it stopped working again.

I'll try that test and report back, thanks.

[tillkrueger]

nope, it doesn't make any sense to me...I'll be throwing in the towel for the time being.

all things being equal, my rMBP is logging in fine and connecting to my unRAID system, and my iMac is not...both connect via WiFi DHCP, both have ethernet unplugged (and why should I have to unplug ethernet anyway?!), but only one machine works as designed.

wasted too many hours hours since yesterday and must get some real work done...so frustrating!

[tillkrueger]

I'm gonna mark this issue as "resolved", even though there are still a few odd behaviours pertaining to networking. But I am getting most of what I need to get done accomplished, and what doesn't work properly on my iMac, seems to work just fine on my rMBP, so I'll just chalk this off as "good enough" for the moment. Got more important things to figure out.

thanks y'all!