How do I diagnose intermittent network outages?

pente · n00b Joined: 21 Mar 2013 Posts: 29

My gentoo desktop had no network problems until it was moved to its current network, a standard home network. The network has a router with several devices connected over wifi; the desktop is the only device connected with an ethernet wire. The other devices on the network include another gentoo machine and a mac; everything is using dhcp.

Frequently (more than once a day) and unpredictably the desktop loses network access for seconds to minutes, sometimes as long as 15 minutes. The other devices do not experience this. When this happens, ping fails to reach the router, other devices on the network, and devices outside the network. Ping variously gives "Destination Hose Unreachable" error messages or not.

Example of pinging other devices on the same network. Note when the network comes back, several pings return at the same time.

DespLock · n00b Joined: 27 Jul 2020 Posts: 65

Hi pente,
to have any idea of what is going on with your network, you (and we) need some more information.

pls issue the following commands once:

Buffoon · Veteran Joined: 17 Jun 2015 Posts: 1369 Location: EU or US

I'd run ethtool to see the status of connection for starters.

Tony0945 · Posted: Thu Jan 14, 2021 2:14 am Post subject:

Bad cable? Can you use another? or is it snaking through the walls? Bad switch? Is it connected to the switch built into the router or different one? try a different slot on the switch.
Swapping cables is a pretty standard test. Did you terminate the cable? Or is it premade? Corroded connections? Cat-6?
One test I made in a similar situation was to connect to an AP. PC had no problems. Intermittent on the 60 foot Cat-6 cable. Wound up replacing the connector on the other end. Didn't like the way the wires looked. Solved the problem. Great cable. US government surplus. US government always buys top quality supplies (well except for mess hall food and the raw food was probably grade A).

pente · n00b Joined: 21 Mar 2013 Posts: 29

Thanks for the tips so far.

Tony0945 · Posted: Thu Jan 14, 2021 2:51 am Post subject:

I use netifrc so I can't help with networkmanager. What does "ifconfig -a" show?

pjp · Administrator Joined: 16 Apr 2002 Posts: 20067

Do your logs have anything relevant? Particularly dmesg for interfaces (likely beginning with eth and enp). Also dhcp logs, probably in /var/log/syslog or /var/log/messages. Especially useful if you can identify a time period when the problem occurred. Don't forget to verify that your time is correct so that the logged time is appropriate. Also, check whichever system is handing out the addresses.
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DespLock · n00b Joined: 27 Jul 2020 Posts: 65

Hi pente,
on a first look your config seems to be ok and i can't find any unusual except for

pietinger · Posted: Thu Jan 14, 2021 11:12 am Post subject:

pente,

first of all, diagnosing intermittent network outages is one of the hardest jobs.

You said, the only change was moving your desktop to another router. Therefore my first thinking is: Cable or Router.

If it is not the cable, it should be the router. And yes there exists buggy ones.

You are using dhcp. This is the first I would change. Give your desktop a static IP address. No more changes (also no updates). Only one by one.

If you have still problems, you know it wasnt dhcp ... and we have to investigate further ... else :-)

pietinger · Posted: Thu Jan 14, 2021 11:18 am Post subject:

Buffoon · Veteran Joined: 17 Jun 2015 Posts: 1369 Location: EU or US

DespLock · n00b Joined: 27 Jul 2020 Posts: 65

Tony0945 · Posted: Thu Jan 14, 2021 4:20 pm Post subject:

Buffoon, no need to be nasty. He said he found the documentation but didn't understand it. No crime in that.

Tony0945 · Posted: Thu Jan 14, 2021 4:21 pm Post subject:

Pente, what is mke and model of the new router and the old router if you still have it?

gengreen · Apprentice Joined: 23 Dec 2017 Posts: 150

I got a similar problem a while ago my is :

pente · n00b Joined: 21 Mar 2013 Posts: 29

I saw some odd things in dmesg, more on that at the end. I have no information on the old router, it was in a commercial setup, not a typical home router. New router is asus tm-ac1900.

C5ace · Posted: Fri Jan 15, 2021 9:53 am Post subject:

If you have or can borrow a second PC buy or make up a short crossover network cable connect both PCs and run ping.

Alternatively get one of those $10.00 Ethernet hubs and two normal short patch cables to connect the two PCs.

If either works, it's the cable or your router.

Now remove one of the connectors of your long cable and crimp on a new one wired for crossover and try again. If this works, your cable is OK and your router is bad.

Note, some cable modems and old commercial routers require crossover cables between the PC's and modem or router. Telstra in Australia used to provide such cable modems to their customers.
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DespLock · n00b Joined: 27 Jul 2020 Posts: 65

Hi pente,
with your last logs it is most UN-likely a hardware defect or the cable. In that case you would have errors in the RX erorr/TX error fields visible too (beside the link error in dmesg):

pente · n00b Joined: 21 Mar 2013 Posts: 29

Unfortunately the suggested ethtool commands did not yield any change in behavior. It's unclear to me if I should expect upgrading the kernel to resolve the issue.

Since I figured I am due for a kernel upgrade regardless, I just got through the kernel configuration marathon and am about to give it a try. Along the way I noticed the config_generic_phy option which sounded important, but about which I could find very little information, all of which suggested it was related to ethernet in some way. It is disabled in my current kernel. Probably a red herring, but I thought I'd ask if there is a chance this or some other kernel option might be relevant to my issue?

Some new information: the home network is apparently configured as some kind of mesh network. I am unclear about how those work, so I'm not sure of the implication of losing ping to other devices on the network. As I believe other devices on the network are connected (wirelessly) to the device the desktop has a wired connection to, I think it is unlikely the problem lies with the mesh.

Also, on further observation I now believe the connection failures are correlated with network usage (less frequent on weekends), although I'm not sure if it has more to do with the desktop's usage or others'.

pente · n00b Joined: 21 Mar 2013 Posts: 29

I believe I have applied all of the suggestions mentioned above, except the part about a crossover cable that I didn't understand how to do or what the goal was. I have upgraded the kernel and @world, and replaced the cable, in addition to the other steps I've mentioned in my previous comments.

There has been no change in the network interruptions I have been experiencing. Any suggestions on how to diagnose the problem would be appreciated, thanks.

Ralphred · Guru Joined: 31 Dec 2013 Posts: 495

gentoo_ram · Posted: Mon Mar 08, 2021 9:53 pm Post subject:

I saw similar behavior with this driver when my computer was plugged in to a cable modem once. Intermittent drops of signal. Once I got the cable modem replaced, the issue went away. Whatever your computer is plugged into may be having problems.