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DHCP server behavior when updating the DNS servers info in the DHCP options

Member
Posts: 2
8450     0

will the DHCP client wait till the IP release in order to get the latest updated DNS server IP addresses or is there a DHCP action from DHCP server to ask all the DHCP client to update their DNS server IP address.

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Adviser
Posts: 77
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I'm not 100% sure what you mean, but the client is always responsible for any DHCP action, the DHCP server will never send/ask something on it's own. When a renew takes place, it will use the newly obtained IP addresses of the DNS servers if these where changed. So for clients with a long leasetime, say 8 days, it can take upto 4 days before it uses the new IP addresses.

 

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Member
Posts: 2
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thanks Harrys for the reply. my question is, when an administrator change a DNS server settings in a DHCP scope, will all the clients inside that scope gets the new DNS server settings immediately or it will have to have till it's lease time expired to get the update.

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Adviser
Posts: 130
8451     0

Hello There,

 

That’s not going to be immediate. As what Harry mentioned, it’ll take half the configured lease period for the changes to take effect in clients if options(DNS servers) are updated on the server right after the initial lease. Keep in mind that the update would not be in effect unless & until you restart the DHCP service following the change. Ideally a client would send a renewal request which includes the parameter request list. The server would respond back to the request with the options specified in this list. As per the example from previous reply :

 

  • Say a client gets a lease for 8 days.

 

  • You update the DHCP options on the server by day 3(List of DNS servers).

 

  • As the client contacts the server the fourth day for lease renewal, the newly configured option(DNS servers) is returned to the client. So that means, the server is not going to push anything to the client, unless requested & it doesn’t wait for the lease to ‘expire’. To make it more clear, until the client contacts the server for lease renewal, it’ll continue to use the old DNS servers that you’ve configured.

 

Hope that’ll clarify your question.

 

Best regards,

Mohammed Alman.

 

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Techie
Posts: 3
8451     0

Hi Mohammed,

I'm in the same boat (I think) - I need to change some DNS servers from internal to external IPs.  If I do this, it will propogate at half the lease time, I understand that.  In the meantime though, will it cause traffic interruption or service impact to the subnet?

 

Thanks

-Ian

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Community Manager
Community Manager
Posts: 356
8451     0

@MisterFritchy wrote:

Hi Mohammed,

I'm in the same boat (I think) - I need to change some DNS servers from internal to external IPs.  If I do this, it will propogate at half the lease time, I understand that.  In the meantime though, will it cause traffic interruption or service impact to the subnet?

 

Thanks

-Ian


Changing DHCP options requires a service restart to take effect so yes, there will be a minimal service impact as the DHCP service restarts. This is generally fairly quick and most clients won't notice anything more than a hiccup as the restart takes place.

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Member
Posts: 2
8451     0

Hi,

Apologies for continuing and old thread but I am having similar issues and Infoblox doesn't appear to be following expected behaviour.

The lease time for the scope in quesiton is 1 hour and on a wireshark trace I can see that the client is contacting the server every 30 minutes (half lease time) as expected.

The server replies with DHCP ACK including option 6 Domain Name Server parameters.

When I update the order of the DNS servers for this scope on Infoblox and restart services I would expect to see the new order in the next DHCP ACK to the clients request.

I am not seeing the new order though, anyone know why this might be?

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Expert
Posts: 292
8451     0

@memgtdg wrote:

Hi,

Apologies for continuing and old thread but I am having similar issues and Infoblox doesn't appear to be following expected behaviour.

The lease time for the scope in quesiton is 1 hour and on a wireshark trace I can see that the client is contacting the server every 30 minutes (half lease time) as expected.

The server replies with DHCP ACK including option 6 Domain Name Server parameters.

When I update the order of the DNS servers for this scope on Infoblox and restart services I would expect to see the new order in the next DHCP ACK to the clients request.

I am not seeing the new order though, anyone know why this might be?


First thing I would do is view the DHCP configuration - you can do this by selecting the DHCP member, then on the toolbar select the view configuration menu option. This will let you double check that the DNS servers are in the correct order - it might be that you have set them at the wrong level (e.g. maybe they are override at the range level but you set them at the network level).

 

Paul Roberts
PCN (UK) Ltd

All opinions expressed are my own and not representative of PCN Inc./PCN (UK) Ltd. E&OE

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Member
Posts: 2
8451     0

Thanks for your reply Paul. I've double checked and I set the DNS settings at the network level and the range is still set to inherit from the network

Re: DHCP server behavior when updating the DNS servers info in the DHCP options

Expert
Posts: 292
8451     0

Did you verify what order the DNS servers appear in the actual DHCP configuration?

 

I have seen some odd things happen with inheritance, I would try setting the DNS server option on the actual range (override the inheritance), see if that works, then try setting it back to inherit. You might need to log a ticket with support if the inheritance isn't working properly.

Paul Roberts
PCN (UK) Ltd

All opinions expressed are my own and not representative of PCN Inc./PCN (UK) Ltd. E&OE
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