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If a device is running in DHCP mode, but has its DNS settings changed manually, which differ from the DNS settings of the router (which is acting as the DHCP server), will the routers DNS settings override the devices DNS settings, when the device makes an outgoing connection to the WAN ?

Background info :

We have an ISP that will not allow the connection from the modem unless you use their DNS servers, the ISP also dose not let you use your own modem or rotuer, although there is a work around for this using a Draytek 2862 router modem that we have implemented. At this same site we have a media server device that needs to be using Google DNS to work correctly.

The Draytek router which is also acting as the DHCP server is using the ISP DNS settings, the media server device which has DHCP enabled has a setting for DNS, so we have set the media server devices DNS to Google DNS, although the router is using the ISP's DNS.

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No.

The DNS server that is used is determined by the OS. If you tell the OS "manually" that it has to use 8.8.8.8 as nameserver, then it will do so. On a Linux system, you would, for example, overwrite /etc/resolv.conf (or tell the network manager to use a specific DNS server for this connection). I have used this set-up up to the moment where I started putting a second router between my network and the providers' router.

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I do not fully understand your question, but if I explain the route DNS takes it should answer your question regardless.

A DNS request is executed by the browser on a device.

The first thing that the OS checks, is the hosts file. If the hosts file has an entry for the DNS, then this entry is always used, regardless of what comes next.

If no entry is present, the OS will query the network card settings.

Either the ip address of one or more DNS servers were provided by the DHCP server, or it was set to manual, and configured by the user on the device itself. Both can't happen at the same time on a client.

If it was set to manual, then whatever the ip address of the DNS server is, that one will be contacted and used. This still can be the DNS server of the router, but it can also be google's DNS servers (8.8.8.8) or another one on the internet.

If DHCP provides the DNS servers, then in the DHCP server settings one or more ip addresses are listed. These ip addresses do not have to be the DCHP server's own ip address, but it can be, if the DHCP server is on a router and that router also has a DNS server.

That said, when you can have more than one DNS server, the primary DNS server is always contacted, and only if it times out, the secondary DNS server is contacted. It is possible for the DNS server it contacts to return: "Sorry, I do not have this address" in which case it does skip quickly to the secondary DNS server. Setting multiple servers is really only used for fail-over, not to get many lists of different DNS, such as one for all the domain stuff, and the fast ones on the internet for internet DNS. The reason one would use the domain servers DNS + google DNS is only in case the server's DNS server goes offline to not fully trash the internet.

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First of all, this ISP sounds very picky, second of all, the OS determines what DNS server to use, although in DD-WRT there is an option to force the DNS change to whatever the router specifies.

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    agreed the ISP is very picky (they are Sky UK). When you say the OS determines the DNS, in my case would the be the media server device (question updated to reference this)
    – sam
    Jan 27 '19 at 22:43
  • @sam Yes. The default OS settings are usually to use the router's settings, but they can be overridden. Oh also I bet the modem and router and DNS your ISP provides are all garbage and slow?
    – Menotdan
    Jan 27 '19 at 22:47
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    yes the ISPs hardware is rubbish. No QOS, VLANS, DHCP reservation, VPN server etc.. error prone and slow. I guess they strip it of all the features so they dont get people calling up when they have changed the config and broken it
    – sam
    Jan 27 '19 at 22:56
  • @sam Ya lol, I guess that the ISP is targeted more at tech newbies? It would be nice of them to at least let you use different hardware and DNS, considering most tech newbies stick with their default hardware and don't change much. Also there is a misspeeeeeeeling in the title
    – Menotdan
    Jan 27 '19 at 23:36
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I’ve replaced my Sky router with a Cisco device and can set the DNS to anything on the router and delivers to the clients via DHCP - The only way sky ‘force’ the dns server as thats what they code in their router (which you can also change by exporting the config manually finding the sky DND address’, change them and restore the config :)

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