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Old 11-28-2013, 11:07 PM   #1
Registered: Aug 2011
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Distribution: ArchLinux - 3.0 kernel
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linux machines lose access to internet after random amount of time.. until ping out??

so like most infuriating network problems, this one happens only after an extended, seemingly random period of time. i have 5 static IPs on comcast business with which i have 4 linux machines connected, configured with 4 of these statics. If any one of them has had no traffic pass over their external interfaces for "some" time (usually half a day or more), these systems lose access completely from the internet. After this happens, the only way to restore connectivity is to ping (or initate any network transfer) from the machines out to the internet. When I do this on the machine, it IMMEDIATELY restores the connectivity.

Worringly, this appears like an issue with the cable modem, as, when the systems are knocked offline and I run a tcpdump on the external interface, I see absolutely NO traffic appear when I try to ping from the outside... it's like the cable modem forgets that there's a device even plugged in, and needs to be "reminded" that there's a machine there. A call to comcast support was completely worthless; they told me it was a problem with my equipment. Except that it happens with 4 different machines....

If i had to take a complete shot in the dark, i'd say that it's some sort of ARP caching that's not working properly, but i'd have no idea where to go from that. Because the packet capture shows absolutely nothing, I really don't have a lot to go by... but does anyone have any ideas? I've tried typical cable modem stuff (unplug/replug, have comcast re-push a config file down.) The modem is an SMCD3G, if that helps at all.

Last edited by psycroptic; 11-28-2013 at 11:19 PM.
Old 11-30-2013, 12:29 PM   #2
John VV
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Are these 4 office machines running the current Arch ?
if not what OS is installed

also comcast is ipv4
new Linux OS's look for the ipv6 address unless you disable that
Old 11-30-2013, 01:58 PM   #3
Registered: Aug 2011
Location: USA
Distribution: ArchLinux - 3.0 kernel
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yes, all current pacman -Syu'd Arch. and Linux actually, by default, looks for BOTH ipv6 and ipv4. In any case, it doesn't matter cause i've had "ipv6.disable=1" on the kernel line on all the machines.

i will say that, in addition to initiating traffic from the machines to restore access, I can also go into the modem config page and click "refresh" on the "connected devices" page... if that helps at all.
Old 11-30-2013, 04:00 PM   #4
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call comcast.. i suspect that this is not on your side bit comcast may kill the connection if nothing comes from your machienes.. you could write a cron script to send a single ping to a random host on the internet (google maybe?) every 10 minutes?!?
Old 11-30-2013, 04:06 PM   #5
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yes this happen to me with my IP At&T modem was going slowly but was never proved the code said that but was not consistent. They said buy a new one. but I had them replace it at their cost they wanted me to buy a different model. Wala they had boxes of my type of medems never used at the store. Many people said the same thing as they try to force us to use uverse.
Old 11-30-2013, 06:30 PM   #6
Registered: Aug 2011
Location: USA
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yeah, i was afraid of this... and yeah stuferus i'm kinda thinking a keepalive cronjob might be the only solution... but damnit i shouldn't have to do that. particularly for the ungodly amount of money i pay comcast... and they damn well shouldn't "kill" connections from IPs that, by their definition, are intended to be for servers.

even worse, they force me to use their modem, and they only offer 1 model (this POS SMC)...

(btw, to the dude who said comcast is IPv4, comcast BUSINESS, which is what i have, actually has enabled IPv6 to their clients in my area (nashville, TN))


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