Router question

[glennd]

Just a simple question but one on which I've never been entirely clear. In my experience whenever we lose internet we also lose connectivity within the LAN. Is this because the configured DNS server is always outside of the LAN? If this is so then the answer to maintaining a functioning LAN when the internet is out is to have a DNS server within the LAN? So, I'll be looking for some device that sits between the LAN and the router that keeps the LAN running? Is this the kind of function that an Edgerouter or USG performs?

 

[mikeroq]

Typically the router has a dns server and DHCP will hand out the routers IP for dns. If the internet goes out the LAN is not affected.

What router and what are you losing access to on the lan?

 

[Sky-Knight]

Edgerouter, USG, Untangle, PFSense, OPNSense... heck a crap Linksys. ALL have a local DNS cache. Though they all have varying levels of configurability to maintain name resolution for local names.

USG and Edgerouter are basically not configurable in this space... which is stupid. Untangle is by far the easiest to use in this space. but the new *Sense platforms work too.

 

[glennd]

Typically the router has a dns server and DHCP will hand out the routers IP for dns. If the internet goes out the LAN is not affected.

What router and what are you losing access to on the lan?

that was my understanding but whenever the internet goes out the dns stops working so somehow the two are tied together. I suppose that's a function of the router. The particular router doesn't seem to matter, other than they're always isp supplied cos that's the world I work in.

Edgerouter, USG, Untangle, PFSense, OPNSense... heck a crap Linksys. ALL have a local DNS cache. Though they all have varying levels of configurability to maintain name resolution for local names.

That would seem to be better than no dns at all.

USG and Edgerouter are basically not configurable in this space... which is stupid. Untangle is by far the easiest to use in this space. but the new *Sense platforms work too.

I had a quick look at USG and found nothing about dns except a spot to type in a dns server like your standard modem/router. Still, a dns cache is a start. Remember, the goal is to keep the LAN functioning when the internet or isp modem/router goes out.

Edit: Perhaps just a better class of modem/router?

 

[britechguy]

that was my understanding but whenever the internet goes out the dns stops working so somehow the two are tied together. I suppose that's a function of the router. The particular router doesn't seem to matter, other than they're always isp supplied cos that's the world I work in.

Then you need to check to see if it's some specific ISP-provided modem-router. That's the world I work in for the most part, too, and I have never had the LAN go down when connectivity to the internet is out. And I've dealt with a lot of different D-Link, Actiontec, Motorola, and other modem-routers whether for DSL or cable. I haven't yet hit a fiber modem-router only because fiber optic service has only very recently been introduced into our area. Even smartphone hotspots have kept the LAN alive when lapses in connectivity to the internet have occurred.

 

[Sky-Knight]

@glennd I haven't seen a crap box router on the shelf that didn't have a DNS cache in it for years. BUT, your typical home junk only caches things for like 30 seconds. So if you're dependent on it for name resolution, the LAN will collapse when the Internet fails anyway.

But if you're using that junk you shouldn't be broadcast resolving names anyway... so I'm down to wondering what is down that you want to fix?

 

[mikeroq]

I've never had a lan stop functioning cause the internet was out on an isp router.

 

[Sky-Knight]

Cox is pushing these all in wonder boxes that are cable modem, wireless router combo machines. They have terribly deficient CPUs, and if the cable side goes down the CPU overloads... while the box is in that condition it gets really poor at pushing packets.

I don't allow my customers to operate like that, I demand a dedicated modem to split that load. I suspect everyone else here has similar habits, because this was just the way things worked out for a very long time and habits are slow to change. Even if you've never actually sat down and thought about it.

So if he's got an ISP provided all in wonder box, I can certainly see how it would fail to function when the ISP has issues, because I've seen here on the COX network.

 

[glennd]

Is this just your personal network or have you seen it elsewhere too? If it's just your network then what you describe as "losing the Internet" might be the whole router locking up.

I don't believe any modem/router will use a public DNS server to resolve addresses on your LAN - mostly because each node on your LAN would need a routable address, and I'd be astonished if they're not all NATted private ones.

Anyway, if you want to rule out DNS and DHCP at a stroke then you can install your own servers on any Windows machine very easily. I normally use DualServer (http://dhcp-dns-server.sourceforge.net/) for this but there are others.

I've seen it at a few different locations including me. A customer reported it yesterday, going in this morning to investigate. It could be the whole router is going out but I know there's a general trend in this town for nbn to just disappear randomly. The further problem I face is the isp supplied modem, in this case Telstra, has the voip config hidden so I can't just switch in another modem to test.

I don't believe any modem/router will use a public DNS server to resolve addresses on your LAN

Agreed, makes no sense, hence the confusion.

I think the answer to the OP is: the modem should pass on the isp's dns server to clients in the LAN but it should also handle dhcp and dns queries for the local LAN and, in theory at least, that should continue to function when the WAN is gone. If the LAN connectivity is going out, internet aside, I should be looking at the modem/router. In the case of a Telstra modem, I think I would want an alternative dhcp/dns server inside the LAN.

 

[fincoder]

@glennd I have that issue at home. I live out of town and still use ADSL1 (prefer over Skymuster Satellite), not a Telstra customer but I'm using a Telstra-issued modem (TG587 model, freebie from a customer, my wife likes the big WiFi on-off button on the front, not sure why the button's labelled ECO).

When ADSL goes down occasionally, access to my fileserver also stops. The first couple of times it happened I spent lots of time troubleshooting, swapping with another spare Telstra modem, pings etc, couldn't figure out the issue then ADSL came good and so did my LAN.

Now I know to just expect the LAN to go out when ADSL is down. I'd love to replace it but my wife wants one with a big WiFi on-off button on the front like the Telstra ones!

 

[britechguy]

I'd love to replace it but my wife wants one with a big WiFi on-off button on the front like the Telstra ones!

Serious question: Does she (or do you) actually turn WiFi on/off with any frequency?

 

[fincoder]

Does she (or do you) actually turn WiFi on/off with any frequency?

Yes she likes it turned off when not being used. House has data outlets in every room, WiFi not needed for TV, fileserver, 17in laptop when in it's usual spot. Unfortunately for her, her elderly mother needs WiFi for possible iPad facetime for a few hours a day. I actually tried a lightning-ethernet adapter for the ipad but the facetime app doesn't work over ethernet (iOS bug that I assume will never be fixed!).

 

[fincoder]

Personally it doesn't worry me and I have it on permanently in my shop, even have a UniFi AP beaming a public hotspot to cafes across the road in town.

We live out of town in the bush with amazing wildlife like wallabies, wildflowers, king parrots, black cockatoos, fairy wrens, finches and many other birds. If it isn't being used she says it shouldn't be unnecessarily disturbing the natural environment with EMF, in case it affects birds or bees or something else. That's about the extent of the reasoning...

 

[GTP]

I've never lost LAN functionality whenever the internet goes down.
I have a 16 port TP-Link managed switch on my network (I've never logged into it).
I've replaced modem/routers for clients and used the old router function for small home networks,
never had anyone say they lost the LAN when the internet drops.

 

[glennd]

Can you make the problem happen on demand on your own network by pulling out your WAN cable? If so, then half an hour with Wireshark comparing the good and bad conditions should give you your answer. Even if it doesn't then you'll still have had half an hour of playing with Wireshark, which is always fun.

If all else fails, the Scientific Method usually comes through in the end.

I had a little time this morning before I went out so I looked at my modem and noticed I've been in there playing with the DNS settings There are two spots, both optional to put whatever ip address your heart desires. I had the google dns server first and the isp dns server second. I said to myself, "That's interesting!" Then I said to myself, "I should remove those so that it uses it's default settings." Which I did and then reconnected my laptop, on the laptop the new dns server is just 10.0.0.1. The modem has 0.0.0.0 in those two spots so dhcp is clearly just adding itself to the client dns field in lieu of any other configuration.

Next I disconnected the nbn, leaving the modem alive, waited a minute and proceeded to navigate my way around the lan like nothing had happened.

So I guess that explains my own personal issue, well that one at least, there's others...

I went to the customer site I mentioned earlier and it turns out their description of the problem doesn't do justice to the actual problem (surprise, surprise!). They have deeper problems with their POS software which seems to be really flakey with losing printers and things. The actual LAN seems to be running fine. The modem could be flakey, hard to tell at this point, it was stable the whole time I was there. So I'll deal with the software issues first and then see where we stand.

 

[YeOldeStonecat]

The pure function of a router is to connect one network, to another network.
It could be branches of a large network for example. Or in the case that many people are familiar with, to connect a network (one network) to the internet (another network). When a router connects a private network to the internet, it's now called a "gateway".
All gateways are routers
Not all routers are gateways.

The function of a router does not mandate it run DHCP and DNS. When running as a gateway, they "can"...especially for smaller networks that don't have their own server.

If the internet goes out, you should still have your local area network running fine.

Most home grade routers (gateways) are configured to hand out the ISPs DNS servers. The ISPs DNS servers do not know a single thing about computers on your private network, they don't handle the name resolution for the local network If you don't have a business network with a Windows server running active directory, the only other DNS service available for your local network is possibly one that your gateway is doing. And many of them do not do that well.

So you end up relying on netbios and broadcast traffic.

 

[britechguy]

And many of them do not do that well.

My experience is that the vast majority of them do this very well. That is not to say your assertion is wrong, because there are very simply a lot more gateway modem-routers out there than I've ever laid eyes or hands on.

But I've been in this business a very long time now and have never, personally, encountered a gateway that didn't keep functioning just fine in handling the LAN traffic when its ability to connect to "the outside world" was impaired for reasons beyond its own control.

 

[YeOldeStonecat]

My experience is that the vast majority of them do this very well. That is not to say your assertion is wrong, because there are very simply a lot more gateway modem-routers out there than I've ever laid eyes or hands on.

But I've been in this business a very long time now and have never, personally, encountered a gateway that didn't keep functioning just fine in handling the LAN traffic when its ability to connect to "the outside world" was impaired for reasons beyond its own control.

I might push things more and be more demanding, leaning towards more business networks, certainly mostly with on prem servers however, some workgroups of just peer to peer. I like being able to manually set DNS host entries in a DNS server service on a firewall. Some more demanding software needs a higher level of name resolution. Most of the linux routers excel in this area. Resi grade Stinksys and Nutgears....not so much. And I can go back to the very first broadband routers, like the popular Linksys BEFSR41 or Netgear RT314. I remember dealing with some of the first couple of hundred BEFSR41 routers that rolled off the assembly line with very buggy firmware, working with Linksys support..and they got me in touch with Phillip from www.speedguide.net....he had a patch for Windows 95 related to PPPoE...and I became staff at Speedguide after that intro!

For small businesses, before broadband, we used to use multi WAN dial up routers....like WebRamp, and some 3COM Office connect products. They'd employ multiple 56k modems to connect an office network.

 

[NETWizz]

Without the Internet, the LAN side should still be operational. The difference is you would not be able to reach other subnets at least not those on the Internet.

You an verify this by pinging something by IP address within the LAN. It should work being intravlan communication does not even use the default gateway. All it really does is ARP for a MAC address of the destination device or computer and use that to send Layer-2 Frames viathe LAN media type (i.e. Ethernet). Within those frames is the Layer-3 Packet with the source and destination IP and tis payload typically consisting of TCP segments or UDP datagrams with their respective source and destination ports and the inner application specific messages they contain (i.e. HTTP, SMTP, SMB, etc.).

For routed communications, it works the same way only the computer determines the IP belongs to a non directly-connected subnet, so it forwards it to the default gateway, which is an IP within the LAN. The source and destination IP are of the computer sending and the final destination device, BUT the computer will ARP for the MAC address of the default gateway using the IP of the default gateway and address the layer-2 frame there where the Layer-2 frame contains the packet. When the router receives the frame, it unwraps that layer and examines the layer-3 IP packet against its routing table. Most likely it just has a default-route telling it to route everything via a next-hop to which it has a directly connected subnet (i.e. via your Internet IP). To get it there the router actually ARPs for the MAC address of the next hop and re-wraps the packet in a brand-new frame sourcing it from your router's external/Internet/WAN interface to the MAC address of the ISP's provider-edge router. At each hop, you have brand new Layer-2 information as a carrier on the media-type, but the packet for the most part remains unaltered at least with regard to addressing aside from the fact its priority bits are sometimes re-written, sometimes it is manipulated with NAT or PAT, and generally its TTL is decremented that said it's addressing is end-to-end where the layer-2 addressing is point-to-point.

I hope this helps a bit.

You can verify DNS by using NSLOOKUP. If you have no DNS, you would need to determine the IP of your DNS server(s) via ipconfig /all and determine if they are the router, on the Internet, on another subnet, or local. Even then, for anything outside the zone file hosted on the DNS server, the DNS server would need to use its forwarders, which would need to be reachable.

Basically, you are going to need to diagnose it, but the LAN generally should still remain operational without the Internet.

 

[Sky-Knight]

Minor point of clarification...

The difference between a router, and a gateway in @YeOldeStonecat's post is NAT. If the device is performing network address translation, it's now a gateway. The silly part is this is technically still inaccurate as a telecom gateway is a technical term for a device that bridges a protocol divide. NAT isn't a protocol... it's a service.

But he's right to point out that's a separate feature relative to being able to use a routing table to push packets into another IP range (routing).

So a typical SOHO gateway is:

1.) Router.
2.) NAT
3.) DHCP service for LAN interfaces.
4.) DNS service for LAN interfaces.
5.) Small unmanaged switch.

What changes between the residential equipment, and the commercial equipment is the degree of configurability of the first 4 things. And these days we have "gaming" equipment which usually bolts in QoS, otherwise known as traffic shaping, and perhaps some basic content controls usually aimed at limiting advertisements.

When you get into the LAN these days Windows 10, Linux, Mac, all the IoT devices... the only consistent name resolution available is via DNS. And for DNS to work you need a record. To get that record you either make it yourself, or you have some sort of dynamic updating service. Untangle like most things that use DNSMasq to handle DNS and DHCP services, will automatically make a DNS record for each hostname it collects during the DHCP assignment process as a client gets on the wire. BUT... MANY SOHO crap boxes even ones that use DNSMasq disable this feature. Now if you're using mostly Windows the unit will fall back to broadcast based name resolution and usually you're OK. Unless the Windows Firewall is off the rails... which happens frequently.

@NETWizz is 100% correct here. You need to dig into it while it's down to figure out what specifically is broken. If IP level coms work, then your issue is likely DNS. And that's "normal" for these devices that do not generally cache DNS all that long. Which is why in such environments I tend to use DHCP reservations and IP addresses in my UNC paths because Windows 10's broadcast resolution is notoriously grumpy at times.