Re: Trying to understand router settings for DHCP & DNS server (noob question) WNR3500L

OneMist8k · ‎2016-03-10

We have an external DHCP and DNS server. The Netgear router has DHCP turned off. The clients are all Windows machines.

Using the IPCONFIG /ALL on the command line of a windows client on the network, everything is working as (I think) it should.

Default Gateway...: 192.168.0.1

DHCP Server.......: 192.168.0.5

DNS Servers.......: 192.168.0.5

All good. Now I'm trying to understand why!

Under menu Basic->Network, the "IP Address" shows 192.168.0.1, which is where the Gateway IP address is coming from. My question is: How does the router know the IP address of the DHCP & DNS?

Under menu option Basic->DDNS, the "Dynamic DNS" drop-down menu is selected to "Use External IP address Checker (every 10 minutes)". No IP address is listed. "Dynamic DNS 1" and "Dynamic DNS 2" are both set to "None".

Under menu option Advanced->DHCP DNS, the "Use Internal DNS" is checked, "Prevent DNS-rebind attacks" is checked. That is all. No IP addresses listed anywhere.

Finally, I thought maybe it was getting it from menu "Advanced->Routing" where the Static Routing Table is set up as:

Destination...: 192.168.0.5

Gateway........ 192.168.1.1

Subnet Mask.... 255.255.255.0

Metric......... 1

Interface...... LAN

Description.... SBS2003

Since the IP address of the destination is the DHCP / DNS server (192.168.0.5), one would think that is how the router is getting the DNS/DHCP IP address, right? But wait... the IP address of the Gateway is incorrect. It says 192.168.1.1 but I know the Gateway should be (and is verified by the NSLOOKUP) 192.168.0.1. The IP address in Basic->Network->LAN is in conflict with the setting here in Advanced->Routing->Gateway.

So what gives? Is the value of the "Gateway" setting here ignored? Is the whole section ignored, including "Destination"? If so, then how is the DHCP server discovered?

Just tryin' to understand...

Thanks in advance.

TheEther · ‎2016-03-10

@OneMist8k wrote:
We have an external DHCP and DNS server. The Netgear router has DHCP turned off. The clients are all Windows machines.

Using the IPCONFIG /ALL on the command line of a windows client on the network, everything is working as (I think) it should.

Default Gateway...: 192.168.0.1
DHCP Server.......: 192.168.0.5
DNS Servers.......: 192.168.0.5

All good. Now I'm trying to understand why!

Under menu Basic->Network, the "IP Address" shows 192.168.0.1, which is where the Gateway IP address is coming from. My question is: How does the router know the IP address of the DHCP & DNS?

The router doesn't need to know. When you disable the DHCP server on the router, this relinquishes the router from the responsibility of assigning IP addresses to clients as well as providing the default gateway and DNS server addresses to those same clients. The local DHCP server on your network assumes those responsibilities.

Under menu option Basic->DDNS, the "Dynamic DNS" drop-down menu is selected to "Use External IP address Checker (every 10 minutes)". No IP address is listed. "Dynamic DNS 1" and "Dynamic DNS 2" are both set to "None".

Can you provide a screenshot? I don't see this mentioned anywhere in the documentation. You can either upload a screenshot by clicking on the little icon that looks like a mountain with a moon in the sky or upload it to an image hosting site, like imgur.com, and including a link.

Under menu option Advanced->DHCP DNS, the "Use Internal DNS" is checked, "Prevent DNS-rebind attacks" is checked. That is all. No IP addresses listed anywhere.

Can you include a screenshot of this, too?

Finally, I thought maybe it was getting it from menu "Advanced->Routing" where the Static Routing Table is set up as:

Destination...: 192.168.0.5
Gateway........ 192.168.1.1
Subnet Mask.... 255.255.255.0
Metric......... 1
Interface...... LAN
Description.... SBS2003

Since the IP address of the destination is the DHCP / DNS server (192.168.0.5), one would think that is how the router is getting the DNS/DHCP IP address, right? But wait... the IP address of the Gateway is incorrect. It says 192.168.1.1 but I know the Gateway should be (and is verified by the NSLOOKUP) 192.168.0.1. The IP address in Basic->Network->LAN is in conflict with the setting here in Advanced->Routing->Gateway.

So what gives? Is the value of the "Gateway" setting here ignored? Is the whole section ignored, including "Destination"?

This static route is entirely unusable because the gateway address, 192.168.1.1, is not even in the subnet of your router's LAN.

If so, then how is the DHCP server discovered?

As I mentioned above, the router doesn't need to know the address of the DHCP server. It just routes (and NATs, too). Clients will find the DHCP server without the assistance of the router by broadcasting DHCP requests. The DHCP server will see and respond to these requests.

View solution in original post

TheEther · ‎2016-03-10

@OneMist8k wrote:
We have an external DHCP and DNS server. The Netgear router has DHCP turned off. The clients are all Windows machines.

Using the IPCONFIG /ALL on the command line of a windows client on the network, everything is working as (I think) it should.

Default Gateway...: 192.168.0.1
DHCP Server.......: 192.168.0.5
DNS Servers.......: 192.168.0.5

All good. Now I'm trying to understand why!

Under menu Basic->Network, the "IP Address" shows 192.168.0.1, which is where the Gateway IP address is coming from. My question is: How does the router know the IP address of the DHCP & DNS?

The router doesn't need to know. When you disable the DHCP server on the router, this relinquishes the router from the responsibility of assigning IP addresses to clients as well as providing the default gateway and DNS server addresses to those same clients. The local DHCP server on your network assumes those responsibilities.

Under menu option Basic->DDNS, the "Dynamic DNS" drop-down menu is selected to "Use External IP address Checker (every 10 minutes)". No IP address is listed. "Dynamic DNS 1" and "Dynamic DNS 2" are both set to "None".

Can you provide a screenshot? I don't see this mentioned anywhere in the documentation. You can either upload a screenshot by clicking on the little icon that looks like a mountain with a moon in the sky or upload it to an image hosting site, like imgur.com, and including a link.

Under menu option Advanced->DHCP DNS, the "Use Internal DNS" is checked, "Prevent DNS-rebind attacks" is checked. That is all. No IP addresses listed anywhere.

Can you include a screenshot of this, too?

Finally, I thought maybe it was getting it from menu "Advanced->Routing" where the Static Routing Table is set up as:

Destination...: 192.168.0.5
Gateway........ 192.168.1.1
Subnet Mask.... 255.255.255.0
Metric......... 1
Interface...... LAN
Description.... SBS2003

Since the IP address of the destination is the DHCP / DNS server (192.168.0.5), one would think that is how the router is getting the DNS/DHCP IP address, right? But wait... the IP address of the Gateway is incorrect. It says 192.168.1.1 but I know the Gateway should be (and is verified by the NSLOOKUP) 192.168.0.1. The IP address in Basic->Network->LAN is in conflict with the setting here in Advanced->Routing->Gateway.

So what gives? Is the value of the "Gateway" setting here ignored? Is the whole section ignored, including "Destination"?

This static route is entirely unusable because the gateway address, 192.168.1.1, is not even in the subnet of your router's LAN.

If so, then how is the DHCP server discovered?

As I mentioned above, the router doesn't need to know the address of the DHCP server. It just routes (and NATs, too). Clients will find the DHCP server without the assistance of the router by broadcasting DHCP requests. The DHCP server will see and respond to these requests.

OneMist8k · ‎2016-03-11

Ah, that explains that part. Thank you.

So now I'm concerned about the DNS server settings. From the command line of a windows client on the network the NSLOOKUP website command returns the IP address of the Windows server. So my question is the same regarding DNS: How does the network find the DNS server, if as you say, the static route is unusable? The first DNS "hop" is supposed to be the local DNS server, but I don't see it on the routing table in the router configuration.

Attached are the screenshots you requested. I've attached a few extra.

Part of the IPCONFIG command from a windows client:

Basic -> DDNS

Advanced -> DHCP/DNS

Top half of Advanced -> Routing

Bottom half of Advanced -> Routing (static routing table entry is in both shots).

NSLOOKUP yahoo.com (from Windows client on the network)

Again, thank you so much for helping me understand the config. You've been extremely helpful.

TheEther · ‎2016-03-11

@OneMist8k wrote:
Ah, that explains that part. Thank you.

So now I'm concerned about the DNS server settings. From the command line of a windows client on the network the NSLOOKUP website command returns the IP address of the Windows server. So my question is the same regarding DNS: How does the network find the DNS server, if as you say, the static route is unusable? The first DNS "hop" is supposed to be the local DNS server, but I don't see it on the routing table in the router configuration.

The address of the DNS server is typically provided by the DHCP server. It's important to understand that DHCP doesn't exist solely to assign IP addresses to hosts. Hosts can use DHCP to learn all kinds of information about the network they are attached to. See the Wikipedia article for a list of the many options available in DHCP (link). Most of them are seldom used, but basic information like host IP address, gateway address, DNS server address(es) and, sometimes, WINS server address(es) are pretty common.

It looks like you are running Shibby Tomato. Correct? If so, it has a very powerful DNS and DHCP server called DNSmasq. It should be able to do everything that a Windows-based DNS server can do and then some. It's also fantastically complicated.

OneMist8k · ‎2016-03-11

"Fantastically complicated". Now you're scaring me!

We bought the modem from our VOIP vendor, Nextiva. Shibby Tomato came pre-loaded. It replaced a Verizon G1100 modem that had a SIP ALG feature that many VOIP vendors want turned off. The Verizon router couldn't disable it, so the whole router had to be replaced.

I would love to have DHCP and DNS handled by the router. The contractor who set up our network years ago prefers it on the Windows server. In discussions last year on moving DNS and DHCP to the router, he insists an outbound server is still the way to go. Routers start breathing hard if they do too much, he says, and failures increase. I'm not a NetAdmin, so it's hard to talk the talk.

We've migrated all applications off the server except these (DHCP/DNS) and the domain controller. It isn't even a file server anymore, that has all been moved off to a Netgear ReadyNAS device. I'm thinking of getting a simple Linux distro to run Samba for domain control, but as you can tell I'm a bit out of my depth.

Our network isn't too complicated. We've got about 14 in this office. When things go bad I'm the first guy that gets called, ergo why I need to understand the plumbing a little better. (I'm a programmer, which is why they call me!)

Thank you for helping out here. This has been really helpful.