This page contains instructions for viewing and clearing DHCP Leases.

Introduction

A DHCP Lease represents a DHCP server’s knowledge that a particular IP address has been allocated to a particular client for a specified period of time.

The primary purpose of Lease objects is to track the state of IP addresses which are dynamically allocated from a DHCP Range. “Active” indicates a dynamically assigned lease which is still valid (i.e. has not yet expired), while “Free” or “Backup” indicates an address which is currently available for allocation.  “Abandoned” indicates an address which cannot be leased by DHCP because it is (or was) already in use on the network; see Abandoned Leases for more detail.

Grid Manager may also display “Static” Lease objects for IP addresses that have been assigned to a client via DHCP Fixed Address (or DHCP-enabled Host); note that these behave differently from regular Leases because fixed addresses are timeless.

Displaying Leases

There are three different ways to display Leases in Grid Manager. In all cases, the Lease objects on the servers are updated in real time; use the Refresh icon at the bottom of the window to see the latest data.

Viewing Leases in a single DHCP Range

You can view all free and assigned Leases in a particular DHCP Range by opening the Range object:

1. In DHCP View, Open the DHCP Network to which the DHCP Range belongs (see Getting Started with IPAM).

2. Open the Range by clicking on its address range displayed in the IP Address column.

   You can add the MAC Address column to this table.

Any “Static” Leases for IP addresses within the DHCP Range will also be included.

To see the details of an assigned Lease, select it and choose “Lease Details” from the toolbar panel on the right.

Viewing Leases in a Network

You can see the status of each IP address on a Network, including whether it is currently assigned as a Lease, by opening the Network in IPAM View (see Getting Started with IPAM).

The Lease State and MAC Address are visible in columns of the table (if you have selected List display).

To see the full details of an assigned Lease:

1. Open the IP address (in List display), or select it on the graphical map (in IP Map display).
2. Select the Lease from the Related Objects table.
3. Choose “Lease Details” from the toolbar panel on the right.

Viewing all Leases in all of your Networks

You can view all Lease objects on all of your Networks at once (including “Static” Leases) by navigating to Data Management > DHCP > Leases > Current Leases using the rows of tabs at the top of the screen.

This is by far the slowest method of examining Leases, because Grid Manager has to check every Lease object in its database against your individual user permissions in order to determine which ones should be displayed.  Performance can be improved by using a Filter to narrow the scope of your interest; for example,

• IP Address belongs to “192.168.0.0/26” (pictured in screenshot above) for a CIDR match, or
• IP Address begins with “192.168.0.” for a plain string match.

To see the details of an assigned Lease, select it and click the Lease Details (paper) icon above the table.

Clearing Leases

The DHCP protocol specification does not empower the server to revoke a client’s lease once it has been issued.

However, you can force the DHCP servers to forget that a lease has been assigned (thereby making it available for assignment to other clients) by selecting it and clicking “Clear Lease” from the toolbar (in IPAM view, click the drop-down arrow next to “Clear” and then choose “Clear Lease”).

Note that “Static” Leases cannot be cleared this way, but will be automatically removed by Grid Manager if you modify your DHCP configuration in a way that would permit the IP in question to be allocated to a different client.

Abandoned Leases

An Abandoned lease marks an IP address which is configured for DHCP dynamic allocation but has been abandoned by the DHCP server as unfit for use, due to one of the following occurrences:

• After receiving a DHCPDISCOVER, the server selected this IP address to be offered, but received a response to its ping check.
• After sending a DHCPACK for this IP, the server received a DHCPDECLINE message from the client indicating that the IP address is already in use (typically because the client issued an ARP request for the IP address and received a response, as described in RFC 2131 section 4.4.1).

Abandoned leases do not expire but remain Abandoned indefinitely, at least until the supply of Free/Backup leases becomes exhausted.  When a DHCPDISCOVER is received and there is no Free/Backup lease available to fulfill it, DHCP will automatically attempt to reclaim one Abandoned address and offer it to the new client (after performing the usual ping check).  However, if the reclaimed address is still in use, it will be marked Abandoned again and no DHCPOFFER will be sent, meaning the client will fail to obtain a lease (even though there might be other Abandoned addresses which aren’t actually still in use – in that case, future DHCPDISCOVER attempts by the same or other clients will eventually fare better).

What to do about Abandoned leases:

• Try to avoid them in the first place by making sure that clients on your network are correctly configured to use DHCP and, if your network includes any manually-configured devices, making sure that DHCP is not configured to hand out those IP addresses.

• If you do accumulate a significant number of Abandoned leases, identify any addresses which are not actually still in use on the network, and manually Clear them from Grid Manager.

  Hint: to see only the Abandoned addresses within a particular network, open the Network in IPAM view (as in Viewing Leases in a Network above), and apply Filter: Lease State equals Abandoned

This page contains instructions for creating and managing MAC address filters for DHCP.

Introduction

A MAC Address Filter is a Grid Manager object which keeps track of a set of MAC addresses.

MAC Address Filters are typically used to restrict access to IPv4 DHCP Ranges. Once you have configured your MAC Address Filter the way you want it using the instructions below, see Restricting Access to a Range to apply it to a DHCP Range.

Creating a new MAC Address Filter

If you need a new MAC Address Filter, please email hostmgr to request one. Provide the following details:

• a desired name for the new MAC Address Filter object.

  There is a single global namespace for all Filters, so try to choose something reasonable that incorporates the department or group for which this filter is relevant. If in doubt, explain what you plan to use it for, and we will help you choose a suitable name.

• the name(s) of one or more network models (from Contacts Database) that should confer “ownership” rights to this MAC Address Filter. Any user with permissions on any of the named network models will automatically be given permission to manage this MAC Address Filter as well (updated nightly).

  These network model names will be stored in Grid Manager as values of an Extensible Attribute called “Owned By Network”. Modifying or removing this attribute may result in loss of permissions on the MAC Address Filter.

Managing MAC addresses in a MAC Address Filter

To open a MAC Address Filter:

1. Navigate to Data Management > DHCP > Filters using the rows of tabs at the top of the screen.
2. Open the desired MAC Address Filter by clicking on its name in the table.

This will display a table containing all MAC addresses currently present in the MAC Address Filter.

If your table contains many rows, see Using the Table Controls in Getting Started with IPAM.

Adding a MAC address

After opening a MAC Address Filter as described above,

1. Click the Add (+) icon above the table.
2. Enter the desired MAC address.
3. Optionally, you may set an expiration time at which this MAC address will be automatically removed from the MAC Address Filter.
4. Click “Next” at the bottom of the dialog window.

5. Optionally, you may enter a username in the “Register as User” field to associate with this MAC address (for your own tracking purposes).  If you don’t want to do this, just leave it blank.

6. Click “Save & Close”.

Removing a MAC address

After opening a MAC Address Filter as described above,

1. Select the checkbox for the MAC address you wish to remove (making sure no other checkboxes are selected).
2. Click the Delete (trash can) icon above the table.
3. If you’re sure, click “Yes” when the confirmation dialog appears.

go.illinois.edu/ipamtraining (this page)

1. Open https://dev.ipam.illinois.edu
2. Double-check that the login screen says “DEVELOPMENT GRID”!
3. Click “SSO Login” or see go.illinois.edu/ipamlogin for detailed login instructions
   (If you don’t have any Contacts Database permissions, contact hostmgr to request demo access to dev.ipam)
4. Follow along with the live training instructions or the self-guided tutorial in Getting Started with IPAM

Dummy Objects

Dummy objects (in development grid only) that everybody has permissions on, for training purposes:

• sandbox.illinois.edu
• 192.168.0.0/26 (sandbox-net)
• 2001:db8::/64 (sandbox-net)
• sandbox-macfilter

Sample dig commands

• dig @dev.ipam.illinois.edu hostname.sandbox.illinois.edu IN A
  (variations: “IN AAAA”, “IN CNAME”, “IN any”, etc)
• dig @dev.ipam.illinois.edu -x 192.168.0.5

IPAM Service Documentation

The service documentation at go.illinois.edu/ipam contains step by step instructions for common tasks.

Lease Time Guidelines

The default DHCP lease time is 1 day, which is well suited for nets with low turnover (i.e. the population of client machines is fairly stable and doesn’t change much).

Network administrators may choose to configure a different lease time on individual IPv4 DHCP Ranges (Dynamic Pools), subject to the following guidelines which are designed to keep overall load on the DHCP servers from growing unsustainably:

Because IPv6 subnets are so spacious, there is generally no reason to reduce the DHCPv6 lease time parameters; IPv6 nets with higher host turnover which require Stateful autoconfiguration can simply be configured with larger DHCP Ranges in order to avoid running out of leases.

Range Size Guidelines

IPv4 DHCP Ranges (Dynamic Pools) should be sized so that the number of unassigned IP addresses in the Range (after all desired clients have obtained their leases, and not counting any addresses within Exclusion Ranges) will be at least 1-5% of the total, with a minimum of at least 2 unassigned IP addresses (even for very small Ranges) in order to properly utilize the redundancy provided by DHCP Failover.

Note: Email Alert thresholds should also be adjusted accordingly (to higher than 95%) if the expected number of unassigned IP addresses is less than 5% of the total.

Service Defaults

The following DHCP settings serve as defaults; they are configured at the top level of the Grid hierarchy and automatically inherited by all Networks. An individual Network, DHCP Range, DHCP Fixed Address, or DHCP-enabled Host can override one of these defaults by specifying its own value for the setting.

Client options

For DHCPv4:

For DHCPv6:

Service settings

For DHCPv4:

• Lease Time: 1 day (see Lease Time Guidelines above)
• Authoritative: true

• Ping Check: 1 request, with 1 second timeout

  This setting cannot be overridden for individual Networks or Ranges.

• Enable DDNS Updates: false

  ddns-updates false prevents the DHCP server from attempting to do dynamic DNS updates on behalf of clients that choose to send a Client FQDN value in their DHCPREQUEST. This setting is not one of the options transmitted to clients, and will not affect their ability to perform their own dynamic DNS updates in Active Directory (or any other DDNS server).

For DHCPv6:

• Valid Lifetime: 2 days

• Preferred Lifetime: 1 day

• Enable DDNS Updates: false

This page contains information about other DNS and DHCP tools available to campus IT professionals.

Production IPAM

Production IPAM Grid Manager is located at https://ipam.illinois.edu/

See Using IPAM for instructions.

Development IPAM

The development IPAM system located at https://dev.ipam.illinois.edu/ is a good place to safely experiment with new CSV imports, API scripts, or interface features you haven’t used before. Please note that we enable IT Pros to use dev.ipam on an “as is” basis, with the important caveats that it is not always available, often contains data that is inaccurate or extremely out of date, and may be completely overwritten at any time without warning.

dev.ipam is not reachable from off-campus, so you may need to use the VPN.

Test authoritative DNS queries sent to dns-dev1.techservices.illinois.edu (from on campus) will be answered using the data configured in dev.ipam:

dig +norecurse @dns-dev1.techservices.illinois.edu foo.sandbox.illinois.edu a

dhcpmon

dhcpmon provides graphs for each subnet showing the utilization of dynamically-allocated DHCP addresses over the past day, week, month, and year.

Please note the following limitations:

• actual usage data may be up to 15 minutes older than the graph timestamp
• abandoned lease counts are updated only once per day
• addresses from multiple Ranges are aggregated together
• unused addresses within Reserved Ranges are misleadingly counted as available even though in fact they can never be assigned to a DHCP client (see also Known Issues)

In general, dhcpmon works well for the common case of nets containing a single DHCP Range (with optional Exclusion Ranges).  If your net contains multiple DHCP Ranges and/or any Reserved Ranges, be very careful when drawing conclusions from dhcpmon data.

Domain Request Form

The Domain Request Form is located at go.illinois.edu/domainrequest

See Requesting DNS Domains for more information.

Contacts Database

Network administrators can now keep their identities up to date using Contacts Database.

dig

dig is the preferred client tool for troubleshooting DNS issues because of its flexibility and detailed output. This section is intended as a quick-start guide to help you get it running on your workstation.

See also https://help.dyn.com/how-to-use-binds-dig-tool/

Windows:

1. Browse to https://downloads.isc.org/isc/bind9/cur/9.16/ and download the latest stable release of BIND 9 as a Windows zip file (e.g. “BIND9.16.7.x64.zip“).
2. Extract the zipfile’s contents to a temporary directory on your workstation.
3. Run “BINDInstall.exe” and choose the “Tools only” option (since you don’t want to install the BIND server on your workstation).

4. Open a Command Prompt and run dig from the directory where you installed it, e.g.

   cd "C:\Program Files\ISC BIND 9\bin"
   dig

   Note that previous versions installed dig under C:\WINDOWS\system32\dns\bin or C:\WINDOWS\SysWOW64\dns\bin

5. Optionally, you may add the above bin directory to your PATH for convenience (the details of this step are outside the scope of this document, but try searching the web for “set windows path”).

Mac OS X:

• Modern versions of OS X come with dig pre-installed. Just open a Terminal window and run dig.

Linux:

• dig is readily available as a package for most linux distributions. Look for a package called “bind-utils”, “dnsutils”, or “dnstools”.

To test that your installation of dig is working, run a simple query:

dig @8.8.8.8 www.illinois.edu a

This should return the IP address of our web server (in the ANSWER section).

Plenty of other documentation has already been written about the details of working with dig; just search the web for “dig tutorial”.
See also the dig manual page.

Host Manager

If you have questions about campus DNS services, email hostmgr@illinois.edu.