Ruadh2 Author

Level 1

16 points

Network Issues

I have an odd network behaviour that started a couple of days after I started with my new iMac 27" running Catalina.

I network it using Ethernet. When I boot up I get network settings as follows:

Which looks fine, but I have no connectivity to the router or anything. So, checking on thethe Network Utility, I get:

So, simultaneously it has been allocated an IP address and it hasn't.

I can fix this, but in an inefficient and annoying way.

I physically remove the ethernet cable from the back of the iMac for 20s or so and then add it back.

Then I get the following sequence:

For about 35s nothing happens, then the following IP address comes up, which shows I'm not connected.

Then at a about 1m 25s without doing anything other than waiting I get this:

Which looks exactly the same as the settings at boot, but when I check in Network Utility:

So, now I really have an IP address and all works fine.

Any ideas how I can get it to do this first time without this wasteful manual handling?

iMac, 10.12

Posted on Aug 13, 2020 12:00 AM

Top-ranking reply

Ezequiell

Level 1

28 points

Posted on Aug 17, 2020 8:47 PM

Have you tried removing all the interfaces you have added?

I see you have USB modem 2, and Ethernet 2, etc...

Remove all the interfaces you aren't using, including the Ethernet one, and leave WiFi alone. Then re-add the Ethernet interface again (Should be named just Ethernet and see if the behavior remains the same.

I'm guessing you have some faulty configuration that is preventing you from truly getting an IP address.

Also, you can type "ifconfig en0" in a Terminal and see what it says. I trust that cli command over what any GUI app will tell you.

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24 replies

BDAqua

Level 10

250,253 points

Aug 17, 2020 10:46 AM in response to Ruadh2

It certainly is taking a bit to get a DHCP addy.

BDAqua

Level 10

250,253 points

Sep 11, 2020 4:50 PM in response to Ruadh2

Thanks for the report! :)

Ruadh2 Author

Level 1

16 points

Sep 12, 2020 11:27 PM in response to BDAqua

No worries. It's always important to check in with the final answer, as it's often left field of efforts to solve the problem!

Aug 17, 2020 8:33 PM in response to Ruadh2

See what answers, once the network connects...

ping -n4 192.168.0.255

(or use Netowkr Utility and ping that address a few times.)

You should see responses from locally-connected devices; from those configured to respond to pings.)

What’s on the other end of the Cat 6 cable? The firewall/gateway/modem? Or a network switch?

Ethernet 2 looks a little odd.

Any add-on security or anti-malware apps, add-on VPN clients, add-on cleaner apps, or add-on firewall apps installed here?

Aug 20, 2020 4:23 AM in response to Ruadh2

Remove Little Snitch.

Your network is segmented, and intermittent.

The image shows two hosts, .163 and .202, and .163 is slower to respond for whatever reason.

Your text indicates a different network, with .10, .105, and .130, and seemingly with packet loss.

Little Snitch can alter connectivity and has occasionally caused issues, so it’s a candidate cause.

And I can infer that there might be 9 Km between two sections of your network? If that’s the case, you will want two separate subnets, two DHCP servers, and routing.

Your job is to figure out why your network is segmented. Whatever network link or cable or port is physically between the .163 and .202 section, and the .10 and .105, and .130 section, is a good suspect, particularly if there’s some sort of hardware separation there. Bad cable, bad port, bad Wi-Fi, bad point-to-point link, etc. If there are multiple Wi-Fi routers here, subnet addressing will be required, or switching the Wi-Fi from routing mode to Access Point (bridged) mode, if the Wi-Fi boxes support that.

And Little Snitch is always on the removal list, whenever troubleshooting network issues. Same as VPN client, anti-malware, and cleaner add-on apps.

Ruadh2 Author

Level 1

16 points

Aug 24, 2020 6:08 PM in response to MrHoffman

Thanks again.

MrHoffman wrote:

Remove Little Snitch.

Your network is segmented, and intermittent.

The image shows two hosts, .163 and .202, and .163 is slower to respond for whatever reason.

Your text indicates a different network, with .10, .105, and .130, and seemingly with packet loss.

Little Snitch can alter connectivity and has occasionally caused issues, so it’s a candidate cause.

And I can infer that there might be 9 Km between two sections of your network? If that’s the case, you will want two separate subnets, two DHCP servers, and routing.

Your job is to figure out why your network is segmented. Whatever network link or cable or port is physically between the .163 and .202 section, and the .10 and .105, and .130 section, is a good suspect, particularly if there’s some sort of hardware separation there. Bad cable, bad port, bad Wi-Fi, bad point-to-point link, etc. If there are multiple Wi-Fi routers here, subnet addressing will be required, or switching the Wi-Fi from routing mode to Access Point (bridged) mode, if the Wi-Fi boxes support that.

Little Snitch turned off - no difference.

The wired network is no wore than two metres from router to any extremity.

I'm investigating the segmented network. I'll have to get some spare cables and do some testing on the weekend.

There are at three Wifi routers in the room, but one of them is turned off. I'm setting up a new modem-router as part of moving to the National Broadband Network (our VDSL2 broadband provider), and I've been testing this bug, which is why I have two going at the same time.

Ruadh2 Author

Level 1

16 points

Sep 8, 2020 12:02 AM in response to MrHoffman

MrHoffman wrote:

....

The network is segmented. The ping responses show that. The question then becomes why, and what is on which segment.

I'm sorry, I think I've just confused myself with the multiple modem-routers that I was running for testing. There were actually two different networks, using the same brand, but different unit, of modem-router. So, the IP address was identical, but they weren't on the same network.

So, my ISP has sent me a third modem-router (I actually have four, including a mobile broadband modem). This one is a different brand and has a different IP address, so I'll get less confused. Okay, all setup and going and rebooted and the same error:

So, exactly the same behaviour. The new 27" iMac on Catalina boots up as if fully connected to the Ethernet network, but Network Utility can't find the IP address that Network preferences said that it found. Remove the Ethernet cable from the iMac and reinsert, and after a minute and a half or so, all is connected okay. I then connect a MacBookPro to exactly the same node, and it immediately connects.

So, just to recap, I can replicate this error on the 27" iMac, irrespective of what cable I use, or whether its DHCP or static, or what modem I use. However, I can't replicate it on a MacBookPro running Sierra on exactly the same cable and node. The MBP just boots straight onto the network on Ethernet. Unless there are any more suggestions, I think this is something for Apple and I'll tell you how I get on.

Ruadh2 Author

Level 1

16 points

Sep 10, 2020 2:01 PM in response to MrHoffman

Normally directly by Ethernet. I don't use wireless unless I'm using the laptop away from it's office desk stand (eg. using it in the basement workshop, which has no wired connection).

I'm sure that there aren't two routers as, though I have four router-modems (! Technicolor from previous ISP, duplicate Technicolor from previous ISP, mobile broadband Netgear M1 for backup and Netcomm for current ISP) only one has the x.20.x IP address. So, I can be sure that I only had that one operating at the time of the above tests. I do have a Netgear Prosafe GS105 switch in the network though, as explained above.

I actually have a couple of switches in the network and I use lots of wired and wireless devices elsewhere (I have a second office which is entirely on wireless), but I had disconnected everything that I could to isolate the iMac and one other test device (the MBP) for the ping tests.

Aug 24, 2020 6:16 PM in response to Ruadh2

Remove Little Snitch.

The network is segmented. The ping responses show that. The question then becomes why, and what is on which segment.

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Network Issues