Where should I set DNS? Router or Modem or iMac?

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Level 1

26 points

Where should I set DNS? Router or Modem or iMac?

I have the Time Cap set to "Ethernet."

I have the DSL router doing the PPPoE and everything else.

I have each of my machines set with a DNS of 8.8.8.8 in the Sys. Pref, Network pane.

Three of the machines are wired to the router, the other 3 are wireless.

What is the difference with above config in setting the DNS to 8.8.8.8 on the router OR on the modem? Assuming a local node (machine) did NOT have a DNS set would it get 8.8.8.8 whether I set it on the modem or the router?

Very curious on this.

Thanks,

Al

iMac (21.5 with 8GB RAM, Snow Leopard), Mac OS X (10.6.4)

Posted on Dec 13, 2010 8:40 PM

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Answer 1

Tesserax

Level 10

158,208 points

Dec 13, 2010 9:41 PM in response to al1776

What is the difference with above config in setting the DNS to 8.8.8.8 on the router OR on the modem?

Nothing as the DNS settings on the local machine would take precedence.

Assuming a local node (machine) did NOT have a DNS set would it get 8.8.8.8 whether I set it on the modem or the router?

If the DSL router is configured as the DHCP server, it would provide the DNS IP addresses to the network clients on the local network. If the Time Capsule is downstream of the DSL router and has the DHCP service enabled, it would create a subnet different from the DSL router's, but pass on the DNS settings from that router to the clients on its subnet. On the other hand, if you configured the TC with the DNS info, the TC would provide it to clients on its subnet.

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Answer 2

al1776 Author

Level 1

26 points

Dec 13, 2010 11:11 PM in response to Tesserax

So you are telling me that I could delete the DNS from each machine and set it in either the new modem or Time Capsule router.

I guess what I'm confused at is what device is doing what. The modem I have is an Actiontec GT701D which is actually a one-port router. I have it set to do the PPPoE and the DHCP and the DNS as that is the default set-up. The TC is set to "Ethernet" and in "bridge" mode.... whatever that means. In the configuration I have I could still set the Time Cap to do the DNS? I know I can set the modem to do it. Is there an advantage to using one over the other?

I guess it would help if I know what the difference between DHCP and PPPoE and the DNS function. I guess they are 3 discrete things and I can have either device do them?

Al

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Answer 3

Bob Timmons

Level 10

185,042 points

Dec 14, 2010 7:02 AM in response to al1776

We talked about PPPoE in your previous thread. It's a protocol for DSL devices to establish a connection to the Internet. Ideally, you want the device that is closest to the Internet connection to provide the PPPoE service. Most newer modems allow this type of setup.

Older "bridge only" or "dumb" modems that were furnished by most DSL service providers 5+ years ago required that the PPPoE service be setup on either a computer (if a user had only one computer and no router) or on a router if the user had multiple computers.

Disconnects are more likely to occur in this type of setup and the PPPoE configuration was confusing to most end users, so providers began to offer devices with the PPPoE service on board to simplify things. A good example is your Actiontec which is technically a gateway...modem and router in one enclosure. Other gateways provide 3-4 ethernet ports to simplify things even further.

So, hypothetically if you wanted your Time Capsule to provide the PPPoE service instead of your Actiontec, you could configure it to do so and then configure the Actiontec as a simple "bridge". As it stands now, you have the Time Capsule setup as a "bridge" since the Actiontec is providing the PPPoE service along with DHCP and DNS (and NAT).

Now, suppose you had a dual band Time Capsule and you wanted to use the Guest Network feature? It's not possible to do this when the TC is configured as a "bridge". If you wanted this feature, you would have to change things around to allow the TC to handle PPPoe. The TC would also be handling DHCP and other services in this type of setup.

I will let Tesserax explain a bit more about DHCP and DNS services.

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Answer 4

al1776 Author

Level 1

26 points

Dec 14, 2010 7:18 AM in response to Bob Timmons

Great explanation. Thank you.

I'm not sure what device or module goes out and gets the WAN IP (i.e. 75.45.35.25) from the ISP and where it is stored and which device uses it. Is it DHCP?

As I understand it, DHCP is the module that assigns a unique LAN IP (i.e. 192.168.0.100) to each devices on the local network and DNS is the module that goes out and obtains foreign IPs when needed. And from what I understand I can run DHCP on my gateway-router or on my Time Cap? Same for DNS? If so, is there any advantage to having DNS on the router vs. the Time Cap? (DHCP I'm not too concerned with.)

(About 10 years ago I remember running my own DNS on Slackware Linux with something called BIND, but I forgot all that stuff once I bought a Mac and didn't need to fuss with that level of detail anymore.)

Thanks again,

Al

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Answer 5

Tesserax

Level 10

158,208 points

Dec 14, 2010 8:48 AM in response to al1776

So you are telling me that I could delete the DNS from each machine and set it in either the new modem or Time Capsule router.

That is correct!

I guess what I'm confused at is what device is doing what.

Let's look at a few examples, hopefully to help understand where all these protocols/services (PPPoE, DNS, & DHCP) come into play:

Scenario 1: Modem > Computer
In this scenario, your ISP provides the DNS server IP address information to your computer via DHCP. DHCP also provides the other important IP address information: IPv4 address, subnet mask, & router IP to your computer in order to communicate on the Internet.

Since your DSL ISP requires that you are authorized to access their Internet service, they utilize PPPoE to pass your account credentials (username & password) from your computer to their authorization server. Once verified, you are granted access.

Scenario 2: Modem > Router (Actiontec) > Computer

In this scenario, your Actiontec router provides your account credential (via PPPoE) to the ISP and also acts as a DHCP server. As a DHCP server, it provides your computer with the required IP addresses and passes on the ISP's DNS IP addresses as well. You do have an option with this router to enter a different set of DNS IP addresses ... like those for Google DNS or OpenDNS if you do NOT want to use the ISP's DNS servers. In this case, the Actiontec, uses its DHCP server to provide those addresses to your computer.

Scenario 3: Modem > Upstream Router (Actiontec) > Downstream Router (TC) > Computer

Since you have the TC configured as a bridge, it is basically a "pass-through" device. That is, the Actiontec will continue to provide the IP addresses just like in Scenario 2 "through" the TC so that wired or wireless clients connected to the TC get the addresses. Also, like in Scenario 2, you can assign DNS IP addresses on the TC ... and these would be provided to clients connected to it.

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