Hot answers tagged

10

Ip addresses are meaningless without a subnet mask. When you say the scope encompasses the whole 10.2.0.0 range and then say that range is 10.2.0.0-10.2.63.254 you're implying a subnet mask of /18 (255.255.192.0) because that's the only subnet mask that gives you that ip address range, but we wouldn't know that is the subnet mask you're using because you ...


7

OK, I have a couple thoughts: There are as many DHCP stacks as there are stars in the sky. OK, not quite, but you get the idea. Embedded networking stacks are especially known for having non-complete "standards" implementation. As such, it's highly likely that your devices will end up booting before your DHCPd is ready, will APIPA, and won't ever retry ...


5

There are three scenarios for a Windows DHCP client that I can think of off the top of my head. I can't speak to non-Windows DHCP clients but I have to assume they operate the same way. A running Windows DHCP client that has an active lease while the DHCP server is unavailable: The DHCP client will continue using it's currently leased ip address. When it ...


5

The subnet mask is mostly used to work out whether another IP address can be accessed on the local network, or if it needs to go through a router. Your workstations with the old /24 subnet mask will have been able to access everything else that was inside the old /24 network, because the wrong mask will still give the right answer for those addresses. They ...


3

DHCP uses broadcast traffic for it's discovery process. Routers do NOT forward broadcast traffic. If your VM's on the "inside"/eth1 side of your router are getting leases from your modem/router on the "outside"/eth0 side then you have a bridge somewhere between the 2 networks; either your "router" is actually a bridge (do you have an interface called br0 or ...


3

I finally found the answer! After a lot of digging into various wireshark captures I noticed that when the problem was happening that there was a lot of IPV6 broadcast and discovery traffic. After additional investigation I found that various computers on our network had Intel nic cards that had a wake on lan function. The computers would sleep and would ...


3

Scope options and reservations are not replicated automatically. See this comment from "Microsoft Windows DNS, DHCP and IPAM Team Blog". There is a Powershell Cmdlet in Server 2012R2: Invoke–DhcpServerv4FailoverReplication –ComputerName dhcpserver.contoso.com This will replicate all of the failover scopes on the DHCP server service running on the ...


2

In short: you can't In order for the DHCP server to know if a device is inside the domain, it have to communicate with a domain controler, thus it first have an IP address. This is why the settings you want are greyed out. A trick can be to name your computer with a special prefix. Then the DHCP server can detect it when the computer negociate it DHCP ...


2

While this answer is simple I'm posting it here because if anyone else attempts a DHCP failover setup as I did and they received this error, then hopefully this will help them. Sometimes it's not apparent on the web pages showing a failover configuration that the 'Configure Failover' wizard is not run from the DHCP server that will be the 'failover server'. ...


2

It's not all that difficult. When the servers are being turned off enable DHCP on the router. Disable DHCP on the router when the servers are being turned on. Any DHCP server on the network can service clients. A client will use whichever DHCP server it gets a response from first. So in your scenario, you'll need to enable DHCP on the router when you turn ...


2

Windows clients usually give the server about 60 seconds (give or take) to get it together. After that they switch to a fallback mode in which the devices check every 5 minutes. If 5 minutes is too long to wait, you could reboot the switch they're connected to. Even a warm boot if the switch has that feature would work.


2

Assuming you are using ISC DHCPd it is based on the subnet: subnet 10.10.0.0 netmask 255.255.255.0 { range 10.10.0.1 10.10.0.100; } subnet 10.20.0.0 netmask 255.255.255.0 { range 10.20.0.1 10.20.0.100; } Your eth0 interface should have an IPv4 address in the 10.10.0.0/24 subnet (I am assuming the subnets are a /24, you didn't mention) and the ...


2

Yes, a request for extension of the lease don't need the DISCOVER/OFFER part.


2

This is a simple matter of the mask - you have entered 255.255.254.0 versus 255.255.255.0 Since your mask needs to match the range - and 10.10.14.0 does - it works.


2

I know it sounds insane, but I'm curious if something like this is possible? To answer your actual question, no you can't do that you need to know the MAC address to set a reservation.


2

Could you not shorten the lease time on your CISCO device before finally closing it down ? That would ensure that the clients were more likely to re-request using the new DHCP device.


2

Use security on the switch port, such as 802.1x. Don't rely on DHCP as a security mechanism.


2

In the DHCP console: Enable the Allowed List filter: Right-click on IPv4 and go to Properties Click the Filtering tab Check the box next to Enable Allow List Make sure the box next to Enable Deny List is NOT checked This will cause the DHCP server to only issue IP addresses to clients within the Allowed filter list. Add a MAC to the Allowed list: ...


2

I found the problem: my DNS settings on the client were static. The IP was assigned via DHCP. This is not normal and what brought me to check that is that I have had the same issue of the DNS switching to a static IP before - when going between my home and work network. Again - this is not a normal situation and if I find out what causes the DNS to switch ...


2

Static IPs are normally outside the pool range as you do not want static IPs assigned to other hosts. As long as the static IPs are in an IP range local (or relayed) to the DCHP server they can be served. The leases file records which addresses from the pool have been assigned to a host and when that assignment expires. This is used to ensure that ...


2

Some DHCP server software operate differently when it comes to reservations: some require that all reserved addresses be outside the dynamic range but be within the subnet. some require that all reserved addresses be within the dynamic range, and it then knows not to use these when allocating other hosts dynamically. some allow a reserved address to be ...


1

As we make very clear when you sign up to serverfault this site is for professional sysadmins - not beginners - and this is a staggeringly basic question. You're asking for a /23 but 10.10.15.x is in the second 256-address block, not the first, that's 10.10.14.0 - so it's fixed your mistake for you. If you only want 10.10.15.0 then use a /24 not /23.


1

You are making a few different mistakes in your configuration. First of all site-local addresses have been deprecated for many years. Any documentation telling you to use addresses starting with fec0 is outdated and needs to be updated. The replacement for site-local addresses is called unique local addresses. These are constructed by using fd as the first ...


1

You are getting an offer from your DHCP server, but when you try to request the address the other DHCP server jumps in and NAKs it. It can do that because the client is broadcasting the request. It's quite likely that it's VMware Player itself trying to manage DHCP for you. Anyway, you have the IP of the problem DHCP server (192.168.117.254) to help you ...


1

The DNS servers of everything should go through AD. On the DC, I usually set the primary to localhost and the secondary to another DC. You can do it in both interfaces if you want; it doesn't make much difference. This use case of yours is normal. I'm guessing from what you wrote that you plan to add a second interface on the DHCP/AD server for your ...


1

VLANs would be the proper way to do this. It sounds like you have all of the DHCP servers and clients in one broadcast domain - no bueno. Could you explain what you're trying to accomplish by doing this? That might help determine the best solution.


1

The problem was that the DHCP-server had a subnet of 255.255.255.0 After I changed that, It worked..


1

Well I have pretty much come to a conclusion. In short, yes, it will work (if the reservation already exists). However like so many things from MS there are some caveats since there can be errors if adding a NEW reservation outside of the address pool. It seems the best method is to utilize exclusion zones on your address pool to limit what range of ...


1

For small site you can simply remove the DHCP's role from the server and leave it to the router for ever, I seen that often. You setup the DHCP on the router to give the domain dns suffix and to give only the domain dns. Server on or at off, the DHCP will never fail. The problem you will face is more for the DNS. Usually you give only the DC's IP as a ...


1

Yes, managing a DHCP server is completely optional for Cobbler. Just set manage_dhcp: 0 in your /etc/cobbler/settings file.



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