DNS Server Operation & Configuration

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Frederica Wilkins
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1 Introduction The internet has a tree like network of DNS servers, which are responsible for converting a URL (e.g. to an IP address. The root DNS server shares it's database with all of the outlying DNS servers. A broadband router contains a DNS server, which needs to communicate with one of these DNS servers (such as the ones provided by the ISP). DNS Server operation DNS Server configuration within a router Page 1 of 11 Date: 24/10/2014

A broadband router contains a DNS server, which needs to communicate with one of these DNS servers (such as the

2 Operation When a PC, laptop, phone, or tablet etc wants to access a website, there is a problem. The user has supplied the device with a website address (URL Uniform Resource Locator), such as:- Straight away there is a problem, the TCP/IP stack of any IPv4 networking device can only establish connections to IP addresses, not website addresses (URLs). Well what is the IP address of Google then answer I don't know. Quite a lot of the big websites use a server farm anyway, which means you would need to send the request to one of many IP addresses. This is where DNS (Domain Name Service) comes in. The device, without the user knowing, sends out a DNS query to try & find the IP address of the given URL. Another problem rears it's head what is the IP address of the DNS server? When the device connected to the Wi-Fi network, or wired connection to the router, it is allocated an IP address by the DHCP (automatic) process for itself to use, but actually it is given more than just that. A typical DHCP "offer" to any client connected to the router would contain:- IP address : x Subnet mask : Default gateway : DNS Server(s): So now the device can send the DNS query to the DNS server ( in this example). On receipt of the DNS query from your connected device, the DNS server ( in this case) will respond with a DNS Query Response. Within that response is an Answer field that contains that website's IP address. This device can now initiate a browser session to the IP address it has just been given and that web page should now be seen in the browser on the device. This might seem complex, welcome to the world of IP networking! The DNS server within the router will build up a cache storage of URL/IP address pairs, as you visit numerous websites. Those it does not know, it will refer back to the DNS servers in your router's WAN configuration (e.g. TalkTalk, or opendns etc), with what is known as a recursive query. These network DNS servers will then supply the router with the relevant IP address. The router's DNS server will update it's cache with this information & hand it out to the requesting device. However, cache storage is temporary, based on time & how many entries it can hold. So every so often the cache will flush itself, deleting all of the entries that it contained and start again. Page 2 of 11 Date: 24/10/2014

uk Straight away there is a problem, the TCP/IP stack of any IPv4 networking device can only establish connections to IP addresses, not website addresses (URLs).

3 To understand the DNS server hierarchy, have a look at:- Then basically most companies like Cisco above link into this worldwide hierarchy somewhere, as do Google, opendns, TalkTalk etc. All the router's DNS server is doing is linking into the Google, opendns, TalkTalk DNS servers etc. Page 3 of 11 Date: 24/10/2014

aspx Then basically most companies like Cisco above link into this worldwide hierarchy

Configuration Note: I am only a TalkTalk customer & have no routers other than my own one to get screenshots from. These are from a Huawei HG523a router, most other routers are similar.

4 Configuration Note: I am only a TalkTalk customer & have no routers other than my own one to get screenshots from. These are from a Huawei HG523a router, most other routers are similar. First the DSCP Server settings, this is where the DHCP protocol allocates the clients a DNS server(s) of If you want to use anything other than the allocated TalkTalk network DNS servers, do not enter them here. These should always be the same IP address as the router. Once in Advanced mode, go to:- Basic > LAN Go down to the DHCP Server section:- The rest of this document is based around the HG523a & other Huawei routers, but not the HG635. This should also be broadly similar for the DSL3780 with the Huawei look and feel. To complete a basic understanding of DNS, please read this section, even if you have an HG635, or a D-Link router. Other routers are also covered but in less detail. Page 4 of 11 Date: 24/10/2014

2.168.1.1. If you want to use anything other than the allocated TalkTalk network DNS servers, do not enter them here. These should always be the same IP address as the router.

Huawei routers not HG635 (inc some DSL3780s) Now to setup the router to communicate with the network DNS Servers on the internet. There are two ways to do this:- 1.

5 Huawei routers not HG635 (inc some DSL3780s) Now to setup the router to communicate with the network DNS Servers on the internet. There are two ways to do this:- 1. Use the dynamically allocated DNS servers from the TalkTalk network 2. Use statically assigned DNS servers, such as Google or opendns In both cases go to:- Basic > WAN Dynamically allocated TalkTalk DNS servers Leaving the DNS server entries below blank, forces the router to use the dynamically allocated DNS Servers. To see these look at your router's Internet Summary page. Note you may well have more than one WAN "interface", you want the 0_38 one highlighted with a red box:- Page 5 of 11 Date: 24/10/2014

Use statically assigned DNS servers, such as Google or opendns In both cases go to:- Basic > WAN Dynamically allocated TalkTalk DNS servers Leaving the DNS server

Statically assigned DNS servers Now to force the router's DNS server to communicate with a dedicated DNS server such as opendns or Google etc, enter the relevant IP addresses as shown below.

6 Statically assigned DNS servers Now to force the router's DNS server to communicate with a dedicated DNS server such as opendns or Google etc, enter the relevant IP addresses as shown below. opendns uses: Google uses: Irrespective of the method used to assign the network DNS server, the DHCP "offer" from the router's DHCP server to any client would still contain the local DNS server:- IP address : x Subnet mask : Default gateway : DNS Server(s): In this example, the main DNS server that the router's one would make recursive queries back to when it does not hold the relevant IP address for would be Google. Page 6 of 11 Date: 24/10/2014

4 Irrespective of the method used to assign the network DNS server, the DHCP "offer" from the router's DHCP server to any client would still contain the local DNS server:- IP address :

7 Other routers D-Link DSL routers DSL 3780 with D-Link interface style Huawei HG635 Super Router D-Link routers Go into Advanced mode & then click on Advanced along the top and DNS Setup down the left. Enable the option Use the following DNS Server address and put the required ones in there. Finally click the Add/Apply button:- Page 7 of 11 Date: 24/10/2014

8 DSL-3780s with D-Link interface style Go into Advanced mode & then click on Advanced along the top and DNS down the left. Enable the option Use the following DNS Server address and put the required ones in there. Finally click the Add/Apply button:- Page 8 of 11 Date: 24/10/2014

Enable the option Use the following DNS Server address and put the

9 HG635 Super Router The router's interface is a bit more complex than most, but logon to the router & click on Internet :- Then click on Internet Connection on the left hand side. This will show the DNS servers you are currently using, to change these, click the Edit button:- Page 9 of 11 Date: 24/10/2014

10 When the page expands, tick the Static DNS checkbox:- Page 10 of 11 Date: 24/10/2014

11 Now put the DNS servers you want to use in the latest part of the expanded page & click Save :- Page 11 of 11 Date: 24/10/2014

DNS Server Operation & Configuration

DNS Server Operation & Configuration Introduction The internet has a tree like network of DNS servers, which are responsible for converting a URL (e.g. www.google.com) to an IP address. The root DNS server shares it's database with all of