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ds18s20
Posts: 81
Joined: Fri Dec 23, 2011 5:44 am
Location: San Francisco

Sharing strange dhcpcd behaviour - any remedies?

Tue May 10, 2016 2:38 pm

Hi everyone,

Recently I observed a very strange behavior of dhcpcd which I have't seen before: basically my ISP dropped the connectivity to most of the public Internet for a short time. During that time my Pi was only affected by means of not being able to reach it's DNS resolvers (those are public IPs which belong to the ISP). There wasn't any loss of physical connectivity so to speak, meaning no interface was down etc as the internal LAN was intact.

So after connectivity was restored the Pi could not resolve any FQDNs. I figured it must have flagged the DNS IPs as timed out after many unsuccessful attempts to resolve during the outage. I let it sit. Well 12 hours later the Pi was still unable to resolve so I had to restart dhcpcd and immediately DNS started working.

I'm running the following Pi and dhcpcd (which came with the OS, no additional software installed):

Code: Select all

 cat /proc/version
Linux version 3.18.11+ ([email protected]) (gcc version 4.8.3 20140303 (prerelease) (crosstool-NG linaro-1.13.1+bzr2650 - Linaro GCC 2014.03) ) #781 PREEMPT Tue Apr 21 18:02:18 BST 2015

dhcpcd 6.7.1
Copyright (c) 2006-2015 Roy Marples


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MarkHaysHarris777
Posts: 1820
Joined: Mon Mar 23, 2015 7:39 am
Location: Rochester, MN
Contact: Website

Re: Sharing strange dhcpcd behaviour - any remedies?

Tue May 10, 2016 3:18 pm

Depends why your ISP was down. They may have swapped out servers, or added some, in any event you're probably noticing an arp cache anomaly. (address resolution protocol) This has nothing to do with DNS. Its the resolution of IP addresses to mach addresses. All machines on the net communicate (actually) via mach addresses. The arp cache is a table of IP <---> mach addresses that is used to match up the appropriate mach addy to the desired IP addy... if the mach addresses change parts of the WAN and|or local net will stop working until the arp cache is flushed and repopulated (rebooting a router does this, or restarting some services like dhcpd). In rare cases arp cache issues will force the reboot of your entire network; it depends which arp cache is honked.
marcus
:ugeek:

Dutch_Master
Posts: 362
Joined: Sat Jul 27, 2013 11:36 am

Re: Sharing strange dhcpcd behaviour - any remedies?

Tue May 10, 2016 3:28 pm

Mind, mach in the above is actually the MAC address: https://en.wikipedia.org/wiki/MAC_address :P

:mrgreen:

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ds18s20
Posts: 81
Joined: Fri Dec 23, 2011 5:44 am
Location: San Francisco

Re: Sharing strange dhcpcd behaviour - any remedies?

Wed Nov 02, 2016 6:07 pm

If it was relevant I would say sure but none has to do with the issue, I'm addressing a defect with the daemon flagging DNS addresses as bad and not checking periodically to unflag them so to speak

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