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Currently working with a small business which is using an ISP supplied domestic ADSL modem/router to connect their internal setup to the internet and this is also being used for DHCP.

They're having repeated problems where 1 or more clients lose internet connectivity. During these times all clients/servers can connect internally without issue, all clients and servers can ping the modem/router but can't connect out over any protocol (web, email or ping).

Unfortunately this domestic modem/router doesn't have much of an interface but during these episodes I can ping to a public IP from the router web console.

The customer had been restarting everything every time to resolve but the issue is actually resolved by performing a DHCP renew on the affected device.

In summary during the fault time all 192.168.1.0/24 hosts can connect with each other and to the gateway (192.168.1.1). That gateway can connect to the internet but the hosts can't.

I suspect that this is related to DHCP lease expiry but wouldn't that stop internal network traffic too?

How can I get my customer back online reliably?

(I'd love to recommend that they invest in some proper business grade tech but they don't have the budget so I am left to try and resolve with what they have.)

yknivag
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    This is a brilliant and clear example of consumer-grade hardware being insufficient for the needs of a business. The domestic modem/router cannot handle as many clients as a business-grade one would. Your customer is getting what they pay for. – Jenny D Jul 08 '16 at 08:57
  • Depends how "small" the small business is... If the OP can note how many clients are simultaneously connecting (PCs, network printers, wireless devices (including mobile phones), servers etc.) then it gives us an idea how many clients are being serviced. – Kinnectus Jul 08 '16 at 09:05
  • @BigChris It's not merely a matter of the number of clients; it's the number of concurrent NAT mappings - which can be a lot, for instance, if the users are keeping multiple tabs open in their web browsers. Each NAT mapping is held in a state table; when that table gets full, it can't handle any more, and you get this exact problem. – Jenny D Jul 08 '16 at 09:26
  • Learnt something new today :) – Kinnectus Jul 08 '16 at 09:34
  • By small I mean very small. Less than a dozen hosts split over wired and wireless. If it were the NAT table then how would a DHCP renew resolve it? Wouldn't the router need to be restarted to flush the NAT table (there isn't a function in the console to do that). – yknivag Jul 08 '16 at 10:05
  • @jenny-d what more specific information would you like me to include exactly? The problem is clearly described, the desired end state is obvious and if I knew how to reproduce this intermittent issue on demand it would not be intermittent and neither would I need to ask what was causing it as I would know it's cause. – yknivag Jul 08 '16 at 10:48
  • @yknivag My vote was cast to close with the reason "Questions should demonstrate reasonable business information technology management practices. ". When different people vote to close for different reasons, only one of the reasons will be shown. Using consumer-grade hardware for is clearly off-topic as you can see on http://serverfault.com/help/on-topic – Jenny D Jul 08 '16 at 10:51
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    @yknivag The question about why a DHCP renew will resolve the issue should be put to the manufacturer's support. My guess is that the devices does flush the relevant NAT entries at that point. However, the reason why the workaround works is not really important; the answer to your customer's question is that if they're not willing to pay for proper equipment they're going to have to keep doing `dhcp renew` manually whenever stuff breaks. These are the only two options - pay for the good stuff, or suffer with the bad stuff. – Jenny D Jul 08 '16 at 10:56
  • @jenny-d it's quite absurd really. The question is off topic here because this business (like thousands of others) is struggling along with consumer grade equipment and yet it is off topic at superuser because the consumer equipment is being used by a business. They both seem pretty petty reasons to me to close a topic. The reason I asked the follow-up question was because this has been working for quite a time and there has been no change in their work pattern so I'm struggling to see really how this is a load related issue when there is no increase in load. – yknivag Jul 08 '16 at 21:57
  • An upgrade in the software on the workstation can affect network usage, even if the users do not alter their own patterns. However, the best suggestion I can give you is one I've already make - contact the support for the modem and/or the ISP. If you feel that your question should not have been closed, you may post to [meta] and request to have it reopened. I should warn that I believe this is unlikely to be successful since a moderator was part of the closing decision here. – Jenny D Jul 09 '16 at 09:05

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Looks like the router loses its routing / NAT configuration when a DHCP lease expires.

Since this is a software problem in the router, you need to find if there are upgrades for the software for the router. If there aren't any upgrades available, they need to buy some corresponding device.

Tero Kilkanen
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