DNS issue within the local network

135 pts.
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CCNA
Certifications
Cisco certifications
Default gateway
DNS configuration
DNS server
IP address
LAN
Networking certifications
Routers
Switch configuration
TCP/IP
Dear members of this site, I am presently doing CCNA and took some exams of MCSE 2003 track. I have a very important question to ask, it is actually regarding a DNS issue within the local network. If there are 2 Computers (ComputerA and Computer B) both are connected to a switch making a local network and switch is connected to a Routers LAN interface. The WAN interface of the Router is connected to ISP (Maybe ISP Modem). we have to enter the IP address of the LAN interface of the Router in the default gateway of the TCP/IP properties of the computers which are in local network inorder to make sure that they are able to access the network outside the Router(External Network). But can it be possible that the DNS settings and default gateway of the Computers within the local network have the same IP addresses? I have seen some senarios in which the computers within a local network have the same IP address in there default gateway and DNS settings. And the computers were able to browse website without any problem. I wanted to know that how can it be possible? Can we make a Router also function as a DNS Server and assign it the same IP address? I hope my question is clear Regards,
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Yes, that is possible. Sometimes the router (default gateway) does DNS proxy lookup on behalf of the LAN clients. The router is not a full DNS server but does cache some DNS information to make lookups faster for the LAN.

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You do not necessarily have to configure the DNS server on the router as it might be using a dynamic address assignment (DHCP) from the provider, then the DNS server information will be provided as part of the address assignment. If the address is statically assigned, then the provider may give you a DNS server address on their network to place in the configuration.

If the DNS server is down, name resolution will not work. That is why a minimum of two DNS servers are listed in client configurations. This provides redundancy if one goes down.

See the router manufacturer for specifics on how the DNS configuration is done.

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  • Fahad
    Thank you for your reply, so it means that the Routers LAN interface is actually acting as a DNS Proxy . And we have to configure it in the Router ourself ,right? And the DNS Server could be either in the Local Network or it could be a DNS Server of ISP. But what if this DNS Server goes down, will the DNS Proxy in the Router still be able to resolve the web address into IP addresses? How do we link this DNS Proxy with the actual DNS Server whether it is in the local network or in the ISP network? I hope my question is clear, Regards,
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  • ISMS
    The router will cache the dns addresses from the ISP. In case the DNS server of ISP goes down, then the router will also not be able to pass on the DNS requests.
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  • petkoa
    Well, Fahad, if you don't think about the "router" from the question just as a highly-specialized device which is able of just, ehr, routing, there are at least two scenarios which will explain the situation with DNS/gateway sitting on the same IP without the "complications" of proxy-DNS or caching-DNS. (1) If you have no dedicated specialized router, you can use an usual multi-purpose OS, installed on an usual multi-purpose PC to do the routing. In this case nothing can prevent you from installing the DNServer on the same hardware - you end up with the gateway and DNS with the same IP. (2) if you have a specialized router but it is quite smart, you can do some address/port translation on it - e.g., with a rule like this: "all UDP packets coming to port 53 on 10.0.0.1 (gateway IP) should be redirected to the same port on 10.0.1.23 (the real IP of the DNS)", you can set DNS/gateway on a single IP on LAN host, and rearrange the packaet flow to correct server transparently to users. The question what will happen if actual DNS goes down has nothing to do with routing - in fact both DNS and routing systems provide for redundancy, and if one have enough resources to use it, he/she'll never have any problems :o))) Good luck with your exams, Petko
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  • Fahad
    Dear ISMS I agree with you ,the Router will not be able to pass on the domain name to IP resolution request from the Router but the DNS cache inside the Router will definitely resolve the web address(domain name) to IP address if it contains the entry. But if the Router DNS cache is clear and the ISP DNS Server is down, then you cant do anything. What do others think? Regards,
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  • Fahad
    Petkoa, Thank you for your detailed explanation, I wanted to confirm that can we assign multiple IP addresses on a single port on a Router just as you have mentioned, where you said that we can redirect the UDP traffic entering the default gateway of the Router to the DNS Server . My other question is that is it the UDP or TCP traffic that is actually handled by the DNS Server since as far as I remember, the DNS runs under both UDP and TCP. Regards,
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  • petkoa
    Hi Fahad, DNS in fact serves bothg TCP and UDP port 53, however for different purposes: on UDP-53 it answers DNS-queries and on TCP-53 it answers zone synchronization requests from slave DNServers - you see the difference in size and importance of stateless UDP and staedull TCP transfers... BR, Petko
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