DHCP Failover Issue – Standby Server Responding When It Should Not
Hi everyone, I'm encountering an issue with my DHCP failover setup in Hot Standby mode, and I need insights into why the standby server is providing DHCP leases when it shouldn’t. Setup Overview: I manage a network with over 100 sites worldwide, each having a local DHCP server. Each site has a dedicated DHCP server running on the server VLAN. Clients reside on different VLANs, and IP helpers (DHCP relay) are configured on a Checkpoint firewall at each site. The IP helper forwards DHCP requests to: The local DHCP server (primary) in the site's server VLAN. The standby DHCP server (failover), located at an on-premises data center (DC). DHCP servers are configured in Hot Standby mode using Microsoft DHCP Failover. Issue: Despite the Hot Standby configuration, I noticed that my Cisco Meraki dashboard frequently reports a new DHCP server detected, referring to the standby DHCP server, even though the primary DHCP server at the local site is available. Cisco Meraki triggers this alert when it detects DHCPACK packets from the standby DHCP server traversing the local networks. However, in Hot Standby mode, the failover server should only issue leases if the primary server is unreachable. Example: Site-1's primary DHCP server (DHCP-1) has a failover partnership with Failover-1 at the DC. Site-1's connectivity to the DC is stable, yet Cisco Meraki occasionally detects DHCPACK packets from Failover-1, triggering alerts. Troubleshooting Done So Far: Verified that failover mode is correctly set to Hot Standby (not Load Balance). Confirmed that the primary DHCP server is healthy and responding. Checked DHCP logs on both servers but found no clear failover events. Performed packet captures of DHCP traffic, but the results were inconclusive. Investigated whether Checkpoint firewall’s IP helper can prioritize the primary DHCP server, but it appears not to support this functionality. Created a PowerShell script to check for failover-related event logs (Event IDs: 20254 and 20255). This provided better visibility but did not correlate with the Meraki alerts. Questions: Are there any known scenarios where a standby DHCP server in Hot Standby mode might mistakenly issue leases, even when the primary is active? Is there any detailed information on the failover “heartbeat” mechanism between primary and standby servers? I found that it uses TCP port 647, but I couldn’t locate official documentation on the interval and failure conditions. Could failover state synchronization delays cause this behavior? Are there specific logs or PowerShell commands I should check to confirm why the standby server is responding? Is there a way to prevent the standby server from responding unless the primary is truly unreachable (e.g., registry settings, advanced configuration)? Any guidance or troubleshooting steps would be greatly appreciated! Thanks in advance.Edit subnet mask or scope in dhcp server running in windows server - Solved
it's not possible to directly change the subnet mask of an existing DHCP scope in a running Windows DHCP server. Here are the steps: 1. Export the Existing Scope Configuration: Open a command prompt with administrative privileges. Type the following command to export the scope configuration to a text file: netsh dhcp server \\<DHCP_Server_Name> scope <Scope_IP_Address> dump > C:\dhcp.txt 2. Modify the Configuration File: Open the dhcp.txt file in a text editor. Locate the line that specifies the subnet mask (e.g., SubnetMask 255.255.255.0). Change the subnet mask to the desired value. Save the changes to the file. 3. Delete the Old Scope: In the DHCP management console, right-click the scope you want to modify and select "Delete." 4. Import the New Scope: In the command prompt, type the following command to import the modified configuration: netsh exec c:\dhcp.txt 5. Verify the Changes: In the DHCP management console, check if the scope has been re-created with the new subnet mask. Right-click the scope and select "Properties" to confirm the subnet mask change. (Major Point - Ensure that your existing network address and subnet network address remain the same after making changes. If they are not the same, you need to modify the entire network address in the text file. For example, if the original subnet is 255.255.255.0 and the network address is 10.1.10.0, and you change it to 255.255.252.0, then the network address should also be updated to 10.1.8.0. Therefore, you must replace all instances of 10.1.10.0 with 10.1.8.0 in the entire text file (using Ctrl+H for the replacement). Thats it....
Can DHCP Server detect client windows version?
I have a DHCP server and several clients, some running Windows 10 and some Windows 7. I want the DHCP server to be able to identify the Windows version of the clients using the vendor class and assign the settings I've specified accordingly. Also, I don't want to configure anything on the clients; I want the DHCP server to be able to detect the Windows version by itself. Is there a way to do this?
No active Leases in Windows DHCP gui , but when check over Command Shell there are active Leases
Hello , We want to Monitor active Leases in DHCP with CheckMK. It works so far . We create an own Scope . In this Scope there are two IP Adresses . When an Server becomes one of this both IP Adresses the we become an active Lease . This Lease brings the Monitoring Software to an Alarm . We setup an Leasetime for this Lease under the Scope Settings to 5 minutes. The Target should be that the Monitoring Software brings an Alarm when one of this two Leases or both are aktive. But we want to become back an green ( good ) Monitoring Message , when the Server who becomes the IP Address is shut down and the Lease Time from 5 Minutes are gone . Means then there are no active Leases at all . But now our Problem . We shut down the Server and after 5 Minutes aktive leases under DHCP Server was empty . BUT When i put in this command : “netsh dhcp server show mibinfo” then i become back this info : C:>netsh dhcp server show mibinfo MIBCounts: Discovers = 5167. Offers = 5167. Delayed Offers = 0. Requests = 246250. Acks = 246422. Naks = 7. Declines = 0. Releases = 63. ServerStartTime = Freitag, 5. Juli 2024 23:00:21 Scopes = 24. Scopes with Delay configured= 0. Subnet = x.x.x.x. No. of Addresses in use = 1. No. of free Addresses = 1. No. of pending offers = 0. Means the System think that there is already an active IP but the GUI dont show it . How is it possible to bring back the No. of Addresses in use back to zero ? Thanks Michael
Ignoring client id in windows dhcp server
Hello. How to make Windows DHCP Server use only MAC addresses? Now virtual machines for Linux (Ubuntu) are sent instead of the MAC address Clientid, which sometimes comes to conflicts when issuing addresses, because Clientid may not be unique when cloning VM. I know that you can configure the Ubuntu so that it only gives Mac, but then you have to configure it on every VM or server.
Forcing dhcpserver to use mac address and ignore the client identfier sent by the client
It is strange that Windows Server 2022 uses -ClientId option for the Add-DhcpServerv4Reservation command, but the GUI "New Reservation" dialog uses "Mac address". I want the DHCP Server to ignore the client identifier sent by the client, and to match the reservation on the Mac address. This does not work by just entering the Mac address in the GUI as 7ab764d65e02 or using Add-DhcpServerv4Reservation ... -ClientId "7a-b7-64-d6-5e-02" Is there a way to enter a value here to force the DHCP Server to match on the Mac address? Is it possible to prefix the specified mac address? I've read about 01 as the htype value.DHCPv6 and specifying IP exlcustions for a default /64 scope
Hello, I am currently in the process of setting up dhcpv6 on my AD server and I noticed that when you specify the dhcpv6 scope you are restricted to only setting a /64 prefix. (This is way more IP's then I need for the dhcpv6 scope) I want to only hand out a /113 specific range. I noticed the only way to do this is to exclude a hole lot of IP's in the /64. How do I come up with the exclusion list so that I am only handing out that specific /113 prefix that I have? Or would it just be better to use unique /64s for each vlan? Example: vlan1 - 2001:1900:3000:4::0/64 vlan2 - 2001:1900:3000:5::0/64 vlan3 - 2001:1900:3000:6::0/64 I look forward to hearing peoples thoughts Thanks in advanceMake Print Spooler and DHCP services Redundant?
Looking to get our print Spooler and DHCP redundant/Failover. The Spooler currently Spools to about 25 Printers DHCP and the Spooler are installed on a Primary AD DC. We have About 120 users. Servers are Windows Datacenter 2022. The AD DC is one of 2 at this Location. Looking at Windows Server Failover Clustering, it looks like I can install WSFC on a AD DCs and then Add the Print Spooler to the cluster. (Yes, loosing the original \\host\share) My 2 Questions are: 1) WSFC on DCs is Supported, though Wanted real World experience. Is this fine? I dont want to have to spin up 2 more VMs to just maintain printing. Is this a good way to do redundant Print Spooling? 2) DHCP has its own built in Failover as you can tie it to another DHCP server and make them function as one. Is the built in failover Better than adding DHCP as a WSFC Resource? Thank you, Scott<-
Move single DHCP scope from one DHCP server to another
Hello I am trying to get a single DHCP scope moved from one DHCP server to another. I am moving DHCP servers with multiple scopes to new servers and the commands I am running now, will export ALL scopes on the server to and .xml file then imported on the new server and this works fine but I have another server with multiple scopes but want to selectively export specific scope(s) only. The commands I am running now are.... To Export. Export-DhcpServer -ComputerName "DHCPSERVERNAME" -Leases -File "C:\temp\DHCP-EXPORT.xml" To Import. Import-DhcpServer -Leases –File "C:\Users\admin\Desktop\DHCP-EXPORT.xml" -BackupPath "C:\Users\admin\Desktop\Backup" –VerboseDNS duplicate record issue
Currently we are seeing duplicate DNS records for multiple DNS zones. This is specific to our VPN IP scopes, as other scopes do not appear to have this problem. In an effort to correct this issue, as it appears to be occurring from DHCP not being able to update/delete DNS records due to the client being the owner of the record, the below steps have been implemented. This is a smaller environment with approx 1200 endpoints, so the slightly more aggressive DNS intervals is not a concern. DHCP lease time adjusted to 8 days from previously 1 day DNS scavenging adjusted to "No Refresh + Refresh" = DHCP lease - 1 day 3 days (no-refresh) + 4 days (refresh) and 1 day scavenging https://docs.microsoft.com/en-us/archive/blogs/askpfe/how-dns-scavenging-and-the-dhcp-lease-duration-relate I also implemented Dynamic DNS Updates per the below MVP blog, but oddly the owner of all DNS records changed from SYSTEM as the owner to being self owned, rather than being owned by the DHCP server. https://blogs.msmvps.com/acefekay/2009/08/20/dhcp-dynamic-dns-updates-scavenging-static-entries-amp-timestamps-and-the-dnsproxyupdate-group/ The DNS duplicate issue is still occurring, which I'm assuming is due to the DHCP server not owning the DNS records and deleting them when their lease expires or updating when the IP is reassigned. Searched around quite a bit on this one and I'm stumped at this point. Anyone have an thoughts/suggestions to get DNS records to be properly owned by the DHCP server?
