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I understand what and how to set "Connection-specific DNS suffix", and I mostly understand the purpose. My question is just about why only certain network connection/adapter has it by default (even when UI shows it empty). For example, for my case the wired/LAN connection has it like (IPCONFIG /ALL):

Ethernet adapter MAIN:    
       Connection-specific DNS Suffix  . : abc.corp.com
       Description . . . . . . . . . . . : Intel(R) Ethernet Connection

The UI (Advanced TCP/IP -> DNS Tab) shows it empty. If I set it to something, IPCONFIG will show that, but if I keep it empty, 'abc.corp.com' will be shown by command.

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When I connect to wireless, the "Connection-specific DNS suffix" remain empty by the output of IPCONFIG. (Actually, it doesn't matter if connected to wired/wireless).

So, the question is just:

  • Why wired connection (or any) has it by default. Is DHCP or DC sending it? Is it part of some TCP/IP protocol? Or is it coming via some group-policy (failed to find)?

I did /RENEW, /FLUSHDNS, /RELEASE the IPCONFIG, but it doesn't effect.

  • Did you enable the DHCP client on all network adapters? If so are you connecting to the same DHCP-server via the wired and wireless connection? – Albin Oct 23 '18 at 17:21
  • I don't know about DHCP 'client'. However, yes, I see "DHCP Enabled" in IP Settings. – Ajay Oct 24 '18 at 7:00
  • Yeah, thanks, thats what I wanted to know. Do you "connect" with ethernet and wireless to the same LAN or more specifically to the same DHCP server, or is it a different one? – Albin Oct 25 '18 at 6:59
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"Connection-specific DNS Suffix" is returned by DHCP by Option 015 Domain Name. It tells what the domain name is, for example "contoso.com". The client uses the information from Option 015 together with its host-name to form the FQDN, for example PC1.contoso.com.

The computer can have a "Primary DNS Suffix" defined, which once defined does not change. If a different "Connection-specific DNS Suffix" is received while connecting, it takes precedence, because this ensures that on the new network or VPN the computer will have a proper name on the network and can find and be found by other computers. After disconnecting from a VPN, the "Primary DNS Suffix" will again be the one used.

The existence or not of the "Connection-specific DNS Suffix" depends solely on the answer returned by the DHCP server. If none is returned, then it will stay empty and the "Primary DNS Suffix" will still be used by default.

A DHCP Option 015 name can come from the router or from the ISP, and its presence does not depend on the network type, either wired or wireless, but on whether DHCP returned an Option 015 name or not. If none is received, it will stay empty.

In the post there is the assumption that a wireless network cannot have a "Connection-specific DNS Suffix". I have searched and found several other posts in which were published the results of ipconfig /all and in which it was present on a wireless adapter : link1, link2, link3, and link4.

I think that in the context of the poster, the DHCP server was probably the wireless router, which did not return an Option 015 name. This is determined by the wireless router's firmware rather than by other factors. The router can maybe be programmed differently, but this is not the subject here.

Reference : RFC 2132, DHCP Options and BOOTP Vendor Extensions, section 3.17. Domain Name.

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The DHCP server is responsible for passing on the "Connection-specific DNS Suffix" (not the domain controller). So it's also neither part of the TCP/IP protocol nor is it coming via group-policy.

So it's independent from the network adapter therefore it does not make a difference if you use a ethernet (wired) or WLAN (wireless) connection. If you connect to the same DHCP Server with the wired and the wireless connection you should get the same suffix. All assuming there is only one DHCP server in you're LAN/WLAN which should be the case.

If this happens when you connect to the same LAN with both adapters, there might be a rough DHCP server. For example: if it's a home network and you use a (second) router in order to make a WLAN connection make sure only one of the routers has DHCP enabled. In a cooperate network most likely s.o. didn't properly isolate his test network from the main network, here's a real life example of that.

So refreshing the IP-configuration (/RENEW, /FLUSHDNS, /RELEASE) won't make any difference, because it just start a new request to the DHCP server and if it didn't pass on a Suffix the last time it won't do it the next time either (unless it's a different DHCP server). You will have to make sure to use the same DHCP for both connections/adapters.

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