Requesting Autoconfiguration Service

To enable autoconfiguration for your existing network, email net-trouble@illinois.edu with the following information:

• The name and IP CIDR of the network
• Which type of autoconfiguration you would like to enable (see below for details)
• If the network already has a DHCP server, let us know and provide details so that we can assist you in migrating your existing DHCP configuration into IPAM.
• Optionally, any other special configuration changes you require at this time (e.g. DHCP options set at the network level)

For new networks, work with your network designer to request autoconfiguration service as part of the network creation process.

Autoconfiguration Design Guide

There is no one-size-fits-all optimal approach to autoconfiguration; which type you choose, and which DHCP objects you configure in IPAM, will vary depending on how you want to use and administer your network.

The first step is to determine which of the following client behaviors you want to support:

• dynamic address allocation: each client automatically obtains an available IP address which is arbitrary and unpredictable. It doesn’t matter which client gets which IP, or if a particular client gets a different IP next time it comes back. This behavior is the easiest to configure, and requires very little administrative maintenance once set up.
• fixed address allocation: each client automatically obtains an IP address which has been administratively reserved for that client’s exclusive use.  This behavior requires more configuration work in IPAM (new clients must be added and old ones removed), but allows individual client IPs to be stable and predictable.
• manual client configuration: each client device is manually configured with the necessary network parameters, and does not use autoconfiguration at all. This is usually only necessary for devices which do not support autoconfiguration.

It is possible to mix and match, using different behaviors for different clients on the same network, but care must be taken to avoid conflicts!

Next, select the autoconfiguration type that best supports your desired behavior(s) – these are covered in detail in the following sections – and request that type of autoconfiguration service on your network.

Finally, if you’ve chosen DHCPv4 and/or IPv6 Stateful autoconfiguration, configure the desired DHCP objects in IPAM (see Using IPAM for detailed instructions):

• DHCP Ranges (Dynamic Pools) for dynamic address allocation
• DHCP-enabled Host Records and/or DHCP Fixed Addresses for fixed address allocation

  Notes for Microsoft DHCP users

  A Microsoft DHCP “Reservation” corresponds to a “Fixed Address” in IPAM. Avoid the temptation to create a “Reservation” in IPAM; that term refers to a different concept entirely and will not accomplish what you intend.

  Also, there is no need to place Fixed Addresses inside a DHCP Range (a “scope” in Microsoft terms); in fact, it’s best practice to avoid doing this. If you don’t want to use dynamic address allocation behavior on your network, you should not create a DHCP Range at all.

IPv4 Autoconfiguration Types

For IPv4, autoconfiguration is synonymous with DHCPv4, so the standard choices are simple; DHCPv4 is either enabled or disabled for any given network.

Notes:

• Several best practice recommendations are discussed in DHCP Ranges (Dynamic Pools).
• One strategy for preserving flexibility on a mixed-behavior net is to create a DHCP Range near the end of the net (i.e. highest-numbered IPs), and allocate Fixed Addresses, DHCP-enabled Hosts, and static IPs for manual configuration (if any) starting from the beginning of the net (lowest-numbered IPs).

IPv6 Autoconfiguration Types

For IPv6, there are multiple standard implementation choices for autoconfiguration, each supporting different behavior(s):

Notes:

• Where feasible, choose Stateless over Stateful autoconfiguration for your IPv6 networks.
  • Stateless is simpler than Stateful, and significantly easier on the DHCP servers.
  • If your network includes clients without DHCPv6 support, they will still be able to obtain an IPv6 address using Stateless autoconfiguration. Such clients will not learn the IPv6 DNS resolver address or other option values, but in many cases clients can function successfully without this information (e.g. by using IPv4 for DNS resolution, if available).
  • It is easy to migrate between Stateless and Stateful in the future if your requirements change.
• Most modern clients do support DHCPv6, as shown in https://en.wikipedia.org/wiki/Comparison_of_IPv6_support_in_operating_systems.

Limitations of DHCPv6 fixed address allocation

The DHCPv4 protocol specification (RFC 2131) includes a “client hardware address” (chaddr) field which in practice contains the MAC address of the requesting client’s network interface.  This makes it easy for a DHCPv4 server to identify clients by MAC, and to assign fixed address allocations based on the client’s MAC.

The DHCPv6 protocol specification (RFC 3315) does not include a client hardware address field; instead, it defines the DHCP Unique Identifier (DUID) which is intended as an opaque, globally unique, and stable identifier for the whole client device (not just for one of its network interfaces).  Because the MAC address is not included in DHCPv6 messages, there was originally no way for a DHCPv6 server to assign fixed address allocations based on the client’s MAC address; instead, you must configure them based on the client’s DUID.

The challenge is that it can be difficult or impossible to learn the client’s DUID ahead of time.  While the MAC address is a property of the physical hardware, the DUID (for most general-purpose computers, at least) is a software property generated and stored on disk by the operating system.  Depending on your operating system, the DUID may not even exist until it is needed for the first time – and it will almost certainly change if you reinstall the operating system, dual-boot to a second operating system, etc.

More recently, RFC 6939 provides a mechanism for the router (acting as relay agent) to provide the client’s MAC address so that DHCPv6 servers can use it to assign fixed address allocations.  However, as of this writing (April 2025) this option is not yet supported by most building routers on our campus.

Recommendations:

• Where possible, sidestep this issue entirely by using dynamic address allocation instead of fixed.
• If you do require fixed address allocation, make sure you understand the challenges discussed in this section and that you have a strategy for ascertaining client DUIDs.

Other Information

DHCP Standards details important guidelines and recommendations for using the DHCP service, as well as the default configuration parameters that will automatically apply to all networks unless you choose to override them.

Email Alerts explains the automated email notifications that may be sent to network administrators by the DHCP service.

Introduction

DNS Traffic Control (DTC) enables the IPAM authoritative nameservers to dynamically adjust their query responses for a particular domain name and record type, potentially returning a different address (A or AAAA) record each time depending on:

• health checks
• the source IP address of the query (note that this source IP usually belongs to the recursive DNS resolver rather than the end user)

  RFC7871 defines an EDNS0 Client Subnet option for recursive DNS resolvers to tell the authoritative nameserver about the originating client’s IP, but many recursive DNS resolvers do not implement this option (in part due to some privacy shortcomings).

DNS Traffic Control can be used to provide active/passive failover or active/active load distribution (not true load balancing, see below) between multiple target IP addresses for the same service.  The target IP addresses must be stable and known in advance.

The query response synthesized by DNS Traffic Control is returned instead of any normal DNS resource records which may be configured in IPAM for the queried domain name and type.  However, if DTC is not able to synthesize a response (e.g. because none of the eligible target IPs appear to be healthy), then the normal DNS resource record set is returned as a fallback.

Responses for other record types, such as TXT, are completely unaffected by DNS Traffic Control.

Example

The following illustration shows a DTC Load-Balanced Domain Name (LBDN) with two DTC Pools comprising a total of three DTC Servers.

Using multiple Pools makes it possible to implement fairly sophisticated logic, but this is not always necessary; many real-world use cases require only a single Pool (which can be either active/passive or active/active depending on the load balancing method chosen).

For introductory purposes this example shows only DTC Servers with IPv4 addresses, but we can add more DTC Servers with IPv6 addresses to get equivalent behavior for AAAA record queries as well (using the same Pools and LBDN).  Note that DNS Traffic Control automatically ignores DTC A records while synthesizing an AAAA response, and vice versa.

About TTLs

The scalability of DNS depends upon caching DNS records in accordance with their TTL (time-to-live) values.  The effectiveness of DNS Traffic Control as a failover mechanism depends upon using very short TTL values, typically 60 seconds or less, to limit the cache lifetime of its synthesized query responses.  It also depends on client applications issuing a new DNS query before reconnecting to your service, instead of just retrying against the same remembered IP address.  Note as a consequence that DNS Traffic Control is generally much more effective at mitigating protracted outages than brief intermittent outages.

About CNAME Records

When you create a Stand-alone CNAME Record pointing to a domain name handled by DNS Traffic Control, the alias name automatically benefits from the DTC behavior of the target name:

www.example.illinois.edu. 3600 IN CNAME example.illinois.edu.
example.illinois.edu.     60   IN A     18.217.84.3

No DTC configuration is required for the alias name www.example.illinois.edu, because that portion of the response is always the same; only the A record for example.illinois.edu is dynamically synthesized.

Note also that you do not need to set a short TTL on the CNAME record; the TTL values in the sample response shown above appropriately reflect our expectations that the CNAME record will be stable but the A record might change suddenly.

Glossary

DTC Server: represents a single target IP address which is capable of providing your service.  Note that DTC Server objects do not necessarily correspond 1:1 to your real servers, since

• a single DTC Server might target a Server Load Balancing (SLB) VIP which distributes incoming client requests among multiple real servers.
• a real server with both an IPv4 address and an IPv6 address will require two separate DTC Server objects, one per address (assuming you wish to use DNS Traffic Control for both A and AAAA responses).

Health Monitor: a test performed independently by each authoritative nameserver to determine whether a DTC Server is healthy

DTC Pool: a grouping of one or more DTC Servers with a load balancing method for choosing among them

Load-Balanced Domain Name (LBDN): a slightly misnamed object which maps one or more domain name patterns to a grouping of one or more DTC Pools, with a load balancing method for choosing among them

DTC LBDN Record: a manifestation of the LBDN which automatically appears within associated DNS Zones based on the LBDN’s domain name patterns.  Technically LBDN Records are separate objects from the LBDN, but you don’t normally need to worry about this distinction.

DTC A (or AAAA) Record: record data associated with a DTC Server which is used to synthesize DTC query responses.  Usually auto-populated based on the DTC Server’s IP.

Load Balancing Methods

Global Availability: always selects the first available target from an ordered list.  Use this for active/passive failover.

Ratio:Fixed: randomly selects an available target each time, optionally “weighted” to select certain targets more often than others.  Use this for active/active load distribution.

All Available (Pool only): instead of returning just one address record at a time, return the address records for all healthy Servers in the Pool (omitting any unhealthy ones).  This is an alternative active/active method which behaves more like traditional “round-robin DNS” (see also below), but cannot be used in DNSSEC signed zones because the number of distinct responses it might hypothetically produce is an exponential power set.

Topology: selects a target by evaluating the source IP address of each incoming query against a custom-configured set of rules (note that this source IP usually belongs to the recursive DNS resolver rather than the end user).  Use this e.g. if you need to provide a different answer to on-campus vs off-campus clients – or at least, to clients currently using an on-campus DNS resolver vs clients currently using an off-campus DNS resolver.

Round Robin: rotates through the available targets in ordered sequence, returning the “next” one for each successive query (1, 2, 3, 1, 2, 3, …).  This alternative active/active method involves maintaining state on the nameserver, which will expire if no matching queries are received for a while (empirically around 60s) causing the sequence to restart deterministically from 1.

Configuring DNS Traffic Control

Unfortunately we are not able to offer any self-service configuration of DTC objects within IPAM at this time.  We are hopeful that future software releases from the vendor will provide enough granular permissions that we can safely open up at least partial self-service, but for now, all DTC configuration changes should be made by request to hostmgr (except fallback records, which are not themselves DTC).

The following documentation will walk you through the design process step by step; we encourage you to read through all the steps, and then submit one request with as much detail as possible.

1. Configure Non-DTC Fallback Records

First, follow the usual self-service steps to create one or more normal Stand-alone A and/or AAAA Records for each fully-qualified domain name (FQDN) which will be handled by DNS Traffic Control.  Ultimately these static records will be unused most of the time, but they will be returned as a fallback whenever DNS Traffic Control is unable to synthesize a satisfactory query response targeting a known healthy DTC Server.  Broadly speaking, this can happen for two reasons:

1. Your servers are working, but something is wrong with DNS Traffic Control’s monitoring of your servers.  Consider in this scenario what static DNS response would generally give clients the best chance of successfully using your service.

   Note that it is common for DTC to return fallback records for up to 1 second after a behind-the-scenes service restart in which other DTC configuration changes are being applied (even if those changes have nothing to do with your LBDN).

2. None of your servers are working.  Consider in this scenario what static DNS response would be most useful to have already cached (by recursive DNS servers and/or client applications) at the moment when your servers begin working again.

Be sure to set a custom TTL (e.g. 60 seconds) on your fallback records so that they will not remain cached for too long once DTC is again able to synthesize a response.

The two most common choices for a fallback resource record set are:

• all of the eligible target IPs (essentially falling back to traditional “round-robin DNS“)

  Note: in this case, if you want the target IPs to have matching PTRs pointing back to the service FQDN (and assuming the target IPs do not have other PTRs already) you may opt to create a fallback Host record rather than stand-alone A/AAAA records.

• just one of the eligible target IPs (this might be preferable in some active/passive use cases where there is a significant penalty associated with actually using the secondary server)

Once your DTC LBDN is configured, Grid Manager will display the fallback records in strikethrough font as a visual reminder that they are masked by DTC:

2. Choose or Customize a Health Monitor

Next, decide how IPAM should determine the health of your DTC Servers (i.e. target IP addresses).  You can use one of the default pre-defined Health Monitors (icmp, http, or https), or email hostmgr to request a customized Health Monitor for your service.

Try to choose a health monitor that will not be blocked by firewall policy; by default, each authoritative nameserver performs its own health monitor testing independently of the others, and not all of them have University of Illinois System IP addresses.  If necessary we can work around this with consolidated health monitor settings (configured per Pool, details further down).

Please provide the following details when requesting a customized Health Monitor:

• Interval (seconds): how long to wait between the end of one health monitor cycle (either receiving a response or timing out) and sending the next request to the same DTC Server.  Default: 5s
• Timeout (seconds): how long to wait for a response before giving up.  Default: 15s
• Retry Up Count: number of consecutive successes for a currently unhealthy server to be marked healthy.  Default: 1
• Retry Down Count: number of consecutive failures for a currently healthy server to be marked unhealthy.  Default: 1
• protocol: choose one of
  • ICMP (ping): sends an ICMP or ICMPv6 echo request and expects an ICMP/ICMPv6 echo response.  No additional parameters.
  • HTTP: sends an HTTP request and optionally examines the response
    

    Parameter	Default	Remarks
    Port	80
    HTTP Request	GET /	May be a multi-line message including HTTP headers.  Recommended examples: GET / HTTP/1.0 GET / HTTP/1.1 Host: example.illinois.edu Connection: close Note that we do not generally recommend using the HTTP/0.9 request format (e.g. “GET /” without HTTP-Version) because HTTP/0.9 responses do not include a status code; see RFC 1945 for more information.
    Response Code Check	any response code is valid	may require a specific HTTP response status code (e.g. 200) or accept any
    Response Content Check	do not check	may search first 5KB of headers, body, or both using a POSIX Extended Regular Expression, and optionally perform additional validation on the matched content

  • HTTPS: sends an HTTPS request and optionally examines the response
    

    Parameter	Default	Remarks
    Port	443
    HTTP Request	GET /	(same as HTTP above)
    Enable Certificate Validation	false	boolean
    Enable SNI (Server Name Indication)	false	boolean (the SNI Hostname to use is configured in each DTC Server)
    Response Code Check	any response code is valid	(same as HTTP above)
    Response Content Check	do not check	(same as HTTP above)

  • TCP: opens a TCP connection to the specified port (successful when the handshake completes)
    

    Parameter	Default
    Port	no default (must be customized)

  • SIP, PDP, SNMP: talk to us if you feel that one of these might be appropriate for your service
• internal display name

  There is a single global namespace for all DTC Health Monitor objects, so try to choose something reasonable that begins with your department or group.

• Owned By Domain: the name(s) of one or more domain models (from Contacts Database) that should confer “ownership” rights to this object.  Self-service management of DTC objects is not possible at this time, but any user with permissions on any of the named models will be authorized to request changes through hostmgr.

You can examine Health Monitors under Data Management > DNS > Traffic Control, Toolbar: Manage Health Monitors

3. Configure DTC Servers

Please email hostmgr to request creation or modification of DTC Server objects.  Provide the following details for each:

• the IPv4 or IPv6 address which should be returned in response to A or AAAA queries when this DTC Server is selected as the answer
• (optional) any Health Monitors for this DTC Server which must target a different IP address than the one returned in response to queries
• (optional) an alternate SNI Hostname to be used for HTTPS Health Monitors
• internal display name

  This name has no functional effect on DNS behavior; it is used only for administrative purposes within Grid Manager.  There is a single global namespace for all DTC Server objects, so try to choose something reasonable that begins with your department or group.

• Owned By Domain: the name(s) of one or more domain models (from Contacts Database) that should confer “ownership” rights to this object.  Self-service management of DTC objects is not possible at this time, but any user with permissions on any of the named models will be authorized to request changes through hostmgr.

Once configured, you can examine DTC Server objects under Data Management > DNS > Traffic Control.

4. Configure DTC Pools

Please email hostmgr to request creation or modification of DTC Pool objects.  Provide the following details for each:

• which DTC Servers should be members of this Pool

  Note that it is possible to add the same DTC Server to multiple Pools.

• which Load Balancing method should be used to select a member (choices are described above under “Load Balancing Methods”)
• which Health Monitors should be applied to all members (targeting the main IP address configured for each DTC Server object)
• (optional) Consolidated Health Monitor settings: should off-campus nameservers use health status information obtained by the on-campus nameservers, instead of performing their own independent tests?

  This workaround makes it possible to use DNS Traffic Control for internal use cases where your entire service (and perforce also the health monitor target) is only reachable from on campus.

• internal display name

  This name has no functional effect on DNS behavior; it is used only for administrative purposes within Grid Manager.  There is a single global namespace for all DTC Server objects, so try to choose something reasonable that begins with your department or group.

• Owned By Domain: the name(s) of one or more domain models (from Contacts Database) that should confer “ownership” rights to this object.  Self-service management of DTC objects is not possible at this time, but any user with permissions on any of the named models will be authorized to request changes through hostmgr.

Once configured, you can examine DTC Pool objects under Data Management > DNS > Traffic Control.

5. Configure the LBDN

Please email hostmgr to request creation or modification of a DTC LBDN object for your service.  Provide the following details:

• one or more fully-qualified domain names (FQDNs) for which this LBDN object should synthesize responses, e.g. example.illinois.edu

  Best Practice

  Wherever possible, configure the LBDN to synthesize responses for a single primary FQDN, and implement any additional FQDNs using CNAME Records:

  www.example.illinois.edu. IN CNAME example.illinois.edu.
  myothersubdomain.illinois.edu. IN CNAME example.illinois.edu.

  Assign multiple FQDNs to the same LBDN only if they cannot be implemented as aliases (e.g. because they reside at the apex of a zone).

• the TTL value to return for synthesized responses, e.g. 60 seconds
• one or more DTC Pools to use

  Note that it is possible to add the same DTC Pool to multiple LBDN objects.

• which Load Balancing method should be used to select a Pool (if using more than one)
• internal display name

  This name has no functional effect on DNS behavior; it is used only for administrative purposes within Grid Manager.  There is a single global namespace for all DTC Server objects, so try to choose something reasonable that begins with your department or group.

• Owned By Domain: the name(s) of one or more domain models (from Contacts Database) that should confer “ownership” rights to this object.  Self-service management of DTC objects is not possible at this time, but any user with permissions on any of the named models will be authorized to request changes through hostmgr.

Once configured, you can examine LBDN objects under Data Management > DNS > Traffic Control.

You can also see their DTC LBDN Records which appear within the associated DNS Zones.

Monitoring DNS Traffic Control

You can see the health status of your DTC objects in Grid Manager by navigating to Data Management > DNS > Traffic Control.

Notice the “Last Status Update” time!  When your server goes down or comes back up, the underlying health check and actual DNS query responses will update quickly, but it may take a few minutes for the new status to be displayed in Grid Manager.

Select an object and click “Show Visualization” from the Toolbar panel on the right to see at a glance how this object relates to other DTC objects.

Comparisons with other technologies

The following sections discuss similarities, differences, and trade-offs between DNS Traffic Control and several other technologies which can be used for similar purposes.

DTC vs Traditional Round-Robin DNS

Publishing several static (non-DTC) A or AAAA records for the same domain name, a technique known as “round-robin DNS“, is a simple and time-honored way to achieve basic active/active load distribution and (sometimes) fault tolerance among multiple target IPs.

Most recursive DNS resolvers will randomly permute the order of the record set for each response, and most clients will initially try whichever IP appears first in the list they receive.  If that first IP doesn’t work, many clients are smart enough to automatically try another one, although this may involve a noticeable delay (outcomes vary widely depending on client implementation).  Without manual intervention, the unhealthy IP will continue to appear in round-robin DNS responses and thus continue to impact the end-user experience for some fraction of new clients until service is restored.

DNS Traffic Control attempts to improve upon this user experience by no longer returning an address record for the unhealthy IP, thereby (eventually, once the old response has expired from all caches) directing all new connections to a healthy IP.

Of course DTC also offers some additional options which can’t be implemented using traditional round-robin DNS, including active/passive and topology-sensitive behaviors.  But if simple active/active is what you want, it’s worth considering whether traditional round-robin DNS might be good enough (perhaps with a moderately low TTL to facilitate manual changes in response to a protracted outage) before deciding to implement DTC.

Note that despite having a similar name, the Round Robin DTC load balancing method has nothing to do with traditional round-robin DNS.

DTC vs network-layer Server Load Balancing (SLB)

In typical internet applications (e.g. web browsing), the client first uses DNS to resolve a target domain name into a destination IP address, and then initiates a connection to a server listening on that IP address.

DNS Traffic Control alters the first part of this process by returning a different DNS record so that the client will connect to a different IP address.  Network-layer Server Load Balancing (SLB) alters the second part of the process by distributing client requests made to the same virtual server IP address (VIP) among different real servers behind the scenes.

SLB offers two comparative advantages over DTC:

• SLB offers better fault tolerance for individual real server failures, because failover is transparent to the client; existing clients need only reconnect to the same VIP, and new clients are not impacted at all.  By contrast, failover in DNS Traffic Control requires client cooperation and is subject to the potential pitfalls described in “TTL Caveat” above.
• Because SLB handles individual client connections, it can perform true load balancing (i.e. spreading the workload evenly among multiple active real servers).  The best DNS Traffic Control can do is to provide different answers to different recursive DNS resolvers and trust to the law of large numbers to distribute the overall workload, which may not be effective e.g. if many clients are all using the same recursive DNS resolver, or if a few clients generate disproportionately large numbers of connections.

The primary limitation of SLB is that it is only available in certain locations and places additional constraints on the networks your real servers may belong to; see Server Load Balancing (SLB) for details.

The comparative advantage of DNS Traffic Control is that it can provide failover and load distribution between multiple geographically-distributed sites where sharing the same virtual IP address is not possible, thereby offering some protection (subject to “TTL Caveat” above) against a total failure of one of the sites.

DTC and SLB can also be used in tandem, with DTC directing clients to one of several SLB VIPs (which may in turn be located at different sites).  Note in this scenario that DTC will need a way to monitor the health of each SLB VIP.

DTC vs Global Server Load Balancing (GSLB)

GSLB is a term commonly used by many other vendors to describe the same functionality which we (and our vendor) call DNS Traffic Control.  We prefer the term DNS Traffic Control because it is more descriptive of the actual behavior and less easy to confuse with SLB.

DTC vs Amazon Route 53 DNS Failover

Amazon Route 53 DNS Failover provides functionality very similar to DNS Traffic Control, but for DNS domains or subdomains which have been delegated to Route 53 (see AWS Authoritative DNS Guide for Illinois) instead of domains which are served by IPAM.

The main advantage of Route 53 DNS Failover is that it can take advantage of what AWS already knows about your target resources, e.g. automatically evaluating the health of alias records for Elastic Load Balancers.

DTC vs Virtual Alias records

See Virtual Alias records vs DNS Traffic Control.

Introduction

This document describes the standard as set forth by Technology Services pertaining to the governance and rules of the Domain Name System (DNS) Service at the University of Illinois, Urbana-Champaign campus.

EDUCAUSE and the American Registry for Internet Numbers (ARIN) have delegated second level domains and IP address blocks (respectively) to Technology Services, acting on the authority of the University of Illinois at Urbana-Champaign, with the agreement that DNS service will be properly maintained and configured.

Technology Services provides DNS service to the Urbana-Champaign campus and controls allocation, management and delegation of these zones.

Terminology

American Registry for Internet Numbers (ARIN): An organization that allocates IP address space to the University of Illinois at Urbana-Champaign.
Delegation: handing over authority for a portion of the DNS namespace (thereby establishing a new zone)
Domain: a logical subtree of the hierarchical DNS namespace, headed by a domain name such as illinois.edu or example.com
Domain Name: a string of labels separated by dots which identifies an entity on the Internet. Examples: illinois.edu, example.com, www.techservices.illinois.edu.
Domain Name System (DNS): A method of translating a readable name to an Internet Protocol (IP) address.
EDUCAUSE: Grants .edu domains and offers resources for the advancement of higher education to the education community.
FQDN (Fully Qualified Domain Name): The full entry in DNS for a machine, e.g. host.unit.illinois.edu.
Hostmgr (pronounced “host manager”): role within Technology Services who handles campus DNS change requests
Hostname: A domain name given to a network-attached device.
IP Address: A number to identify a device on a network.
IP Address space/block: A group of IP Addresses.
Subdomain: a logical subset (subtree) of another domain, named by prepending one or more additional labels. Examples: unit.illinois.edu and foo.unit.illinois.edu are both subdomains of illinois.edu.  Note that a subdomain does not necessarily require a separate zone.
Subnet: A subset of an IP address space.
Time to live (TTL): The amount of time the authoritative nameserver caches a record when queried by a caching server.
Third-level name: a domain name consisting of three labels.  Example: unit.illinois.edu.
Zone: a portion of DNS namespace delegated to and configured in a particular set of authoritative name servers.  All subdomains of the apex name are part of the zone unless delegated again to form a new zone.

Roles, obligations, and resources

Units DNS requests and maintenance are processed via the unit’s IT professional for network support (aka network admin).
The IT professional for network support is designated by the unit head in agreement with Technology Services.

To identify your network admin, please contact the Technology Services Help Desk.

The primary contact for DNS for Technology Services is hostmgr at illinois.edu.

General usage of DNS

The use of any domain name or IP address space that is managed by or associated with the University of Illinois at Urbana-Champaign must conform to the policy on Appropriate Use of Computers and Network Systems, which includes a prohibition on use of network resources for commercial or profit-making purposes or other purposes that interfere with the mission of the University.

Inter-unit DNS entries

Units wishing to create pointers from a domain they control to another unit (for example, cs.illinois.edu hostnames pointing into Beckman Institute subnets) should do so in cooperation with Technology Services. Technology Services will set up a discussion so that both groups are aware of the request and can coordinate the use of the new DNS entries.

External domain entries

Units should create and maintain their entries in cooperation with hostmanager, either via delegation or using the self-service management interface. Please see the section on non-.edu domains.

Reverse DNS (in-addr.arpa)

All hosts must have PTR records. This rule is enforced to help the campus be in compliance with industry best practice. As numerous services use PTR records, it can cause considerable problems to users if an IP does not have a matching PTR record.

Time-To-Live (TTL) recommendations

The Time-To-Live (TTL) DNS parameter is used to control how long DNS resolvers cache a DNS record. Setting a TTL value too low reduces the efficiency of caching and increases DNS server traffic/load. Setting a TTL too high means that DNS changes may not be recognized in a timely manner.

The campus standard for TTL records is 1 hour (3600 seconds). Contact hostmgr for changes to a domain’s TTL. TTL changes to individual records can be done by IT Professionals with appropriate access to the record in the IPAM appliance web interface.

Assignment of domains and subdomains

Acceptable Name Guidelines

Units requesting a new third-level name (e.g. xyz.illinois.edu) must meet the following guidelines from the Office of Strategic Communications and Marketing for acceptable domain names. (Pre-existing domains are ‘grandfathered’ under prior guidelines.)

• The domain name can only contain the characters (a-z), -, and (0-9), which complies with the preferred name syntax guidelines in RFC 1035 (Section 2.3.1)
• In general, avoid acronyms and initialisms unless they are universally recognized. For example, creativeservices.illinois.edu not cs.illinois.edu (which could be confused with Computer Science.)
• Supportive of brand – the domain name should not be contrary to the Illinois brand. The UIUC initialism should not be used in newly created domain names.
• Appropriateness – Language which could be considered derogatory, offensive, or misleading should be avoided.
• Respectful of scope and role – Third-level campus domain names should reflect high-level organizations in the university (colleges, departments, inter-departmental organizations,) or campus-wide services (www, campusrec, careercenter). Since third-level names apply at the campus level, the requesting unit must have the authority to represent the organization or service indicated or implied by the name. For example, research.illinois.edu could only be requested by a unit on campus that speaks for all research at the campus level.

Name requests which violate these guidelines must be justified and go through an approval process.

Non-.edu domains

Domains outside the .edu namespace (.org, .net, etc) both incur cost and require the approval of the Office of Strategic Communications and Marketing. The following requirements apply in addition to the normal considerations for domain creations:

• Must be registered through Technology Services.
• Department/Unit pays registration, renewal, and/or transfer fees equal to the amount that Technology Services pays the registrar, plus $10 per year to Technology Services.
• Changes to the domain must be kept to a minimum. There may be an additional fee for excessive changes to the domain. Typical traffic would be an initial set up and a few changes per year.

Approval / escalation process

Technology Services works with the Office of Strategic Communications and Marketing to obtain approval for new domain requests.

Third-level subdomains of illinois.edu

Every third-level subdomain of illinois.edu MUST have a corresponding Domain model in Contacts Database, which must be kept up to date by the responsible department/unit.

Third-level subdomains can be implemented in one of two ways.

Separate zone

A third-level subdomain may be created as a separate DNS zone, which will be made available for self-service management via the IPAM web interface (and may contain an arbitrary number of fourth-level records).

This option has one significant drawback (resulting from an inherent technical limitation of DNS): if mysubdomain.illinois.edu is a separate DNS zone, then there cannot be a CNAME record at mysubdomain.illinois.edu.  In practice, this typically rules out web hosting solutions that do not offer static IP addresses.

Records in the illinois.edu zone

Alternatively, a third-level subdomain of illinois.edu may be registered as a CNAME record in the second-level illinois.edu zone, and accompanied by a small number of fourth-level subdomains (each of which may be either an additional CNAME record or a separate zone).  For example:

• mysubdomain.illinois.edu  (CNAME record in the illinois.edu zone)
• www.mysubdomain.illinois.edu  (CNAME record in the illinois.edu zone)
• etc.mysubdomain.illinois.edu  (separate zone with self-service management)
• name3.mysubdomain.illinois.edu  (either CNAME record or separate zone)
• name4.mysubdomain.illinois.edu  (either CNAME record or separate zone)

Changes to CNAME records in the illinois.edu zone must be made by request to Hostmgr, but fourth-level zones will be made available for self-service management via the IPAM web interface (and may contain an arbitrary number of fifth-level records).

Consider placing records that don’t have specific name requirements (including e.g. dev/test records) underneath a single fourth-level zone.

This option provides somewhat more flexibility for web hosting solutions (since the target of the CNAME record is another domain name, which does not need to resolve to a static IP address).  It is also more lightweight, and therefore generally preferable for new subdomains which do not require a large number of fourth-level records.

Requirements for Third Level Domain Requests

The domain name must follow the Acceptable Name Guidelines and all policies outlined in this standard. The person or group making the request must have a university affiliation (registered organization, staff, faculty, student) and the request should be routed through the unit’s Network Admin.

Requested third-level domains must not already exist and must not be reserved for future use. Contact Hostmanager to check domain availability.

There is no charge for approved third-level subdomains of illinois.edu.

The domain request must include:

• the desired domain name
• a description of the name and purpose
• contact information (considerations for hosted vs. delegated)
• organization codes

See Requesting DNS Domains to start the domain registration request.

After approval, Technology Services will process the request and setup access to the domain through the IPAM web interface, or delegate it to the unit’s servers if requested (see below.) Domain requests will be acknowledged within one business day and processed within 5 business days.

Hosts and domains beyond the third level

Domain names beyond the third level are administered by the unit responsible for the third-level subdomain, subject to the following criteria:

• The names follow the acceptable name guidelines
• The length of any fully qualified domain name does not exceed 255 characters.

Note that separate DNS zones beyond the third level are usually not necessary unless the third level is registered as a CNAME or the fourth level requires delegation.  For example, you can use IPAM to create records named foo.bar.mysubdomain.illinois.edu inside a third-level zone mysubdomain.illinois.edu even though there is no fourth-level zone bar.mysubdomain.illinois.edu.  From the perspective of a DNS client, the end result is the same; the only practical difference is how things look when you log in to manage them in IPAM (i.e. whether you navigate into a separate Zone to look at all the “bar” records, or whether they are displayed in the third-level zone alongside everything else).

Implementation of changes / requests

• Changes submitted via the IPAM appliance web interface will take effect immediately.

• Requests emailed directly to Hostmanager will be hand processed as time allows. Upon completion, Hostmanager will send a completion email to the requestor. We ask that you give your request a full 24 hours for processing and activation.

• Requests that have a sensitive time-line, either during regular business hours or outside of them, should be emailed to host manager and clearly communicated in the body of the email the desired time of activation. We ask that you coordinate all time sensitive requests a full 24 hours in advance.

• Requests to modify TTLs
  The campus standard TTL is set at the domain level and inherited by all records of that domain. The campus standard is 1 hour. This duration can be lowered to help propagate changes to high profile machines (i.e Mail servers, Web servers, etc..) to external nameservers. TTL changes to individual records can be done by IT Professionals with appropriate access to the record in the IPAM appliance web interface; if a TTL change is needed for a record you do not have access to, please contact hostmgr.

Delegation of domains to unit-managed DNS servers

Units may request Technology Services to delegate management of the domain to unit-managed DNS servers. Delegated domains must follow other standards and practices in this document, but are served from unit-managed servers.

Definition of delegation

“Administrative responsibility over any zone may be divided, thereby creating additional zones. Authority is said to be delegated for a portion of the old space, usually in form of sub-domains, to another nameserver and administrative entity.” — Wikipedia entry for DNS

Delegation is appropriate when the unit:

• wishes to use its own tools and servers for managing its DNS records,
• has business requirements that are not met by Technology Services DNS tools,
• has technical integration requirements (such as Active Directory dynamic DNS)

What can be delegated?

• unit third-level domains (e.g. unit.illinois.edu)
• non-university domains (e.g. .org domains registered to the unit)
• reverse IP lookup zones (PTR zones, e.g. 100.168.192.IN-ADDR.ARPA).
  • The DNS architecture requires having 256 address blocks for reverse lookup delegation. Units with less than a 256 address subnet cannot have reverse DNS delegated.

Delegation considerations

In a delegated DNS environment, the unit’s DNS servers are the primary responders to queries to delegated domains.

Because your DNS servers are the authoritative “holders” of your domain information, they should be well-managed and highly available.

Technology Services must have contact information on file for the DNS server administrators.

DNS outages often appear as full network outages, which harm the functionality and image of the university and your unit. The Internet takes DNS seriously and so do we.

DNS servers need to be in the Fully Open firewall group to provide DNS responses to off-campus requests.

Technology Services needs to know if you are using a Windows Active Directory domain controller as the DNS server.

Recommended values for DNS delegations

Technology Services recommends that nameservers for delegated zones be configured with the following values. Note that Technology Services does not prohibit DNS administrators from making other choices, so long as the configuration still complies with the official requirements of delegation (as set forth in the delegation agreement).

Recommended values

SOA values

• Refresh: 1 hour. Exception: If departmental name servers cannot send NOTIFYs to Technology Services name servers, please contact hostmgr to determine an appropriate value.
• Retry: 15 mins
• Expiry: 2 to 4 weeks
• Minimum: 15 mins

TIP: Don’t forget that when modifying a high-profile record (e.g. for a public-facing webserver) it is a good idea to reduce the individual record TTL in advance of making the change (e.g. to a value of 1-5 minutes), and to restore it afterward to the default setting.

Firewall groups for true Delegations

Departmental name servers for true delegations MUST be placed in the Fully Open firewall group to allow udp/53 and tcp/53 from any host (including off campus).

Firewall groups for primary/secondary pseudo-delegations

Any departmental name servers which advertise themselves in an NS RRset SHOULD allow udp/53 and tcp/53 from any host (including off campus).

“Hidden primary” departmental name servers which only participate in primary/secondary pseudo-delegations and are not listed in any NS records MAY allow udp/53 and tcp/53 traffic from off campus but are not required to do so.

Recursion

All departmental name servers must either disable recursion and use the campus resolver (IPv4:130.126.2.131) or restrict recursion to only known hosts.

Delegation request process

Send an email to hostmgr with the required information.

• domains and zones to be delegated
• DNS server names and IP addresses
• Contact information for the zones
• Reason for delegation

Domain lifecycle

Changes to domains

• Domain removal
  If you no longer are in need of a domain associated to you, email Hostmanager. Once the requested date of removal is met, the domain will no longer resolve and (for non-.edu domains) billing will cease.

• Transfers within the University
  If you no longer are in need of a domain and another department or unit would like to be associated with that domain, please contact Hostmanager. Hostmanager will coordinate between both departments to reach an agreement.

DNS technical considerations

Campus resolver IP address

The DNS resolver address for campus is 130.126.2.131.

Welcome! This space (go.illinois.edu/ipam) contains detailed technical documentation for IT Professionals regarding the IP Address Management (IPAM) service.

Information for everyday computer users is available at Networking, DNS Basics.

Use the sidebar on the right to navigate.

Quick links:

How do I log in?
Getting Started with IPAM

What’s New

See IPAM News Blog for the latest news regarding the IPAM service.  Use the “subscribe” link at the bottom if you would like to receive notifications of new posts.

Contact Us

General questions or support requests regarding the IPAM service should be directed to Illinois IPAM Hostmgr at hostmgr@illinois.edu (pronounced “host manager”).

Documentation Overview

DNS Standards provides general information and guidelines about the use of DNS on campus.

Requesting DNS Domains explains how to obtain a new domain.

DHCP Standards provides general information and guidelines about the use of DHCP on campus.

Requesting DHCP for Networks explains how to enable autoconfiguration for your network, and provides a conceptual overview of the different behaviors.

Using IPAM explains how to manage DNS and DHCP configuration for your existing networks and authoritative zones using IPAM Grid Manager.

Other Tools contains related information which may be helpful to IPAM users.

Known Issues describes known problems or limitations of IPAM which may be relevant to IT Professional customers.

Introduction

A traditional CNAME record (see also Stand-alone CNAME Records) defines a static, explicit alias in the DNS, with the oft-lamented technical limitation that a CNAME record cannot be placed at the apex of a zone (e.g. example.com).

Virtual Alias records are a non-standard but increasingly common DNS feature specifically designed to work around that limitation while still accomplishing a similar goal.

When we configure a virtual A Alias record in IPAM, our authoritative nameservers periodically query other nameservers to find the current A record(s) of the desired target name (e.g. abc123.cloudfront.net), and synthesize corresponding A record(s) for the alias name (e.g. example.com).  The result is a dynamic, transparent alias:

1. IPAM authoritative nameserver receives a query matching the virtual alias:  example.com. IN A?

2. IPAM authoritative nameserver sends a query for the target name: abc123.cloudfront.net. IN A?

3. IPAM authoritative nameserver receives this response for the target name:

   abc123.cloudfront.net. 60 IN A 198.51.100.17
   abc123.cloudfront.net. 60 IN A 198.51.100.18

4. IPAM authoritative nameserver returns this synthesized response for the virtual alias name:

   example.com. 60 IN A 198.51.100.17
   example.com. 60 IN A 198.51.100.18

The intermediate response is cached according to its TTL (time-to-live) value, so any further identical queries received by the same IPAM authoritative nameserver within the next 60s will skip steps 2 and 3, and return the same synthesized records with a lower remaining TTL.  Each individual IPAM authoritative nameserver maintains its own independent cache.

When the external nameserver’s response for the target name changes, IPAM’s response for the virtual alias name will also change (once the old response expires from cache).

If no A records are found for the target name, then IPAM will return zero A records for the virtual alias name.

This virtual A Alias record is only used to answer A record queries, but we could additionally and independently configure a virtual AAAA Alias record to answer AAAA record queries.

Key differences vs CNAME records

• A CNAME record affects query behavior for all record types, but each virtual Alias record applies to only one record type (e.g. only A records, or only AAAA records).  This enables virtual Alias records to coexist with records of other types, which is why they can be placed at a zone apex while a CNAME record cannot.

• Virtual Alias records are transparent (invisible) to clients, ultimately yielding the same query response as an equivalent set of static records:

  example.com.  IN A  198.51.100.17
  example.com.  IN A  198.51.100.18

  whereas CNAME records are explicitly returned and must be interpreted by the client:

  www.example.com.        IN CNAME  abc123.cloudfront.net.
  abc123.cloudfront.net.  IN A      198.51.100.17
  abc123.cloudfront.net.  IN A      198.51.100.18

  In this case only the CNAME record comes from our IPAM authoritative nameserver; the client’s recursive resolver combines that with A records from some other authoritative nameserver to form the sample response shown.

Limitations

Virtual Alias records cannot coexist in IPAM with other records of the same type (i.e. you cannot define both a virtual A Alias and a stand-alone A record for the same name).  There is no “fallback” functionality.

Virtual Alias records can be difficult to troubleshoot, since the external behavior of their target names might change unpredictably. 

Virtual Alias records cannot be used in DNSSEC signed zones.

At this time, we are offering virtual Alias records only for the zone apex of a second-level non-.edu domain (e.g. example.com), not for .edu subdomains.

(Note however that third-level .edu subdomains can be registered as a third-level CNAME record in the illinois.edu zone instead.)

Configuring virtual Alias records

Unfortunately we are not able to offer self-service configuration of virtual Alias records within IPAM at this time.  Changes should be made by request to hostmgr; be sure to include

• fully qualified domain name (FQDN) for the virtual alias
• which record type(s) should be aliased
• the target FQDN

Virtual Alias records vs DNS Traffic Control

Virtual Alias records and DNS Traffic Control both involve synthesized responses, but they are aimed at different use cases.

Consider using DNS Traffic Control if your service has a stable set of eligible target IP addresses, but some of them might be unhealthy from time to time.

Consider using a Virtual Alias record if your service has a stable target DNS name, but the set of IP addresses that name might resolve to is unpredictable (and it’s not possible to use a CNAME record).

This page contains information about configuring DHCP Ranges (Dynamic Pools) and DHCP options.

Introduction

A DHCP Range provides a pool of IP addresses that can be assigned interchangeably to any eligible DHCP client.

To configure an individual IP address for use exclusively by a single DHCP client, see DHCP Fixed Addresses or Adding DHCP to a Host Record.

The instructions below assume that you are working in DHCP View (see Getting Started with IPAM). Some DHCP configuration tasks can also be performed from IPAM View, but the exact sequence of steps may differ.

Creating A New IPv4 DHCP Range

To create a new DHCP Range:

1. Open the Network in which you want to add a new range (see Getting Started with IPAM).
2. Click the dropdown arrow next to the Add (+) icon above the table in the main workspace, then choose “Range”.
3. Click “Next” at the bottom of the dialog window.

4. Enter the desired Start (first) and End (last) IP addresses for the range.

   Make sure the range will be large enough to comply with Range Size Guidelines.

5. Optionally enter an internal display name and/or a comment.

6. Click “Next”.

7. Important: for “Served by”, choose “IPv4 DHCP Failover Association” and then click the “Select Association” button to automatically select “DHCP-A” which is the only failover association available.
   

   Do NOT select “Grid Member”.  Although a DHCP Range served by a single Grid Member may appear at first glance to function correctly, this configuration is unsupported because it does not properly utilize the redundancy provided by DHCP Failover.

8. Click “Save & Close” at the bottom of the dialog window.

Editing a Range

To edit an existing Range:

1. Navigate to the Range you want to edit (e.g. by opening its Network in DHCP View, see Getting Started with IPAM).
2. Select the checkbox for the Range object and click the Edit (notepad) icon above the table.  This opens the Edit dialog box.
3. Be sure to enable Advanced Mode for the dialog.
4. Make any desired changes (see subsections below).
5. Click “Save & Close”.

Setting DHCP Options

See DHCP Standards for the option values that will automatically apply unless you choose to override them.

To configure additional DHCP options:

1. Choose the “IPv4 DHCP Options” subscreen in the Edit dialog.
2. Scroll down to the “Custom DHCP Options” section at the bottom.
3. Add your new option in the last row:
   
   1. First field:
      • Choose a custom vendor option space if you wish to set a vendor-specific option which will be automatically encapsulated inside option 43 (see RFC 2132 Section 8.4) when replying to clients with the appropriate vendor class identifier (option 60).
      • Otherwise, choose “DHCP” to set a standard DHCP option.

   2. Second field: choose which option to set.  Option names are not standardized and may vary by implementation, so check the numeric option code (displayed in parentheses) to be sure you have the right one.

      If you need to set a custom DHCP option or vendor-specific option that is not yet defined in Grid Manager (i.e. does not appear in the drop-down lists), please contact hostmgr@illinois.edu.
   3. Third field: type the desired value.  The format of this value depends on the type defined for the option; see DHCP Option Data Types in the vendor documentation.
4. To add another option, click the + button.

To configure BOOTP/PXE settings:

1. Choose the “IPv4 BOOTP/PXE” subscreen in the Edit dialog (requires Advanced Mode).
   Click the Override button next to the setting you wish to override, and enter a new value.

   Hint: most PXE clients require values for Next Server and Boot File (which respectively populate the “siaddr” and “file” protocol fields defined by RFC 2131); please note that these settings are not the same thing as options 66 and 67. Clients utilizing the Boot Server (“sname” protocol field) setting are much less common in our experience.

If you need to enter a value that contains backslash characters, they should be doubled, e.g. foo\\bar instead of foo\bar.

Exclusion Ranges

To exclude certain addresses within a DHCP Range from being dynamically assigned to clients:

1. Choose the “Exclusion Ranges” subscreen in the Edit dialog (requires Advanced Mode).
2. To add a new exclusion range:
   1. Click the Add (+) icon above the table to create a new row.
   2. Click the empty “Start Address” field in the new table row that appears, and type the first address to be excluded.
   3. Click the “End Address” field and type the last address in the range to be excluded. To exclude only a single IP address, enter the same value for Start Address and End Address.
3. To remove an exclusion range:
   1. Select the checkbox to the left of the exclusion range to be deleted.
   2. Click the Delete (trash can) icon above the table.

Email Alerts

By default, the DHCP servers will automatically alert you by email whenever the current utilization of one of your DHCP Ranges exceeds 95% (and again when it drops back below 85%), with a maximum of one email alert per Range per day.  The recipient list for these emails is automatically populated (at the Network level) based on the network contacts listed in Contacts Database.

From: <no-reply@dhcp-a1.techservices.illinois.edu>
Subject: DHCP high threshold crossed:

Message: DHCP high threshold crossed:
  Member: 192.17.2.10
  Network: 172.21.195.0/28/default
  Range: 172.21.195.7/172.21.195.14///default
  High Trigger Mark: 95%
  High Reset Mark: 85%
  Current Usage: 100%
  Active Leases: 5
  Available Leases: 0
  Total Addresses: 5

Reporting: DHCP
Node: 192.17.2.10
Time: Mon Mar  7 15:01:49 2018


Network Name: 0210-citesdhcp-net

Utilization graphs: 
https://ipam-tools.techservices.illinois.edu/dhcpmon/view.fcgi?subnet=0210-citesdhcp-net

Help: https://netwiki.techservices.illinois.edu/public/home/ipamdocs/using-ipam/dhcp-ranges-dynamic-pools/#Email_Alerts

You can customize the behavior of Email Alerts for this Range by choosing the “IPv4 DHCP Thresholds” subscreen in the Edit dialog (requires Advanced Mode).

• To adjust the threshold values (or disable notifications altogether) for this Range, click the Override button for the “DHCP Thresholds” area.

  • Be sure to check both “Enable DHCP Thresholds” and “Enable Email Warnings” if you do still want to receive notifications.  Do NOT check “Enable SNMP Warnings”.

    screenshot

    

  • Note that actual utilization must be strictly greater than the High Trigger Mark (or strictly less than the Low Trigger Mark) in order to trigger an alert.

    Hint: do NOT set High Trigger Mark to 100. If you only want notifications when the range is completely full, set High Trigger Mark to 99.

• To manually specify the list of email addresses which will receive threshold alert notifications for this Range (overriding the default recipient list for the Network), click the Override button for the “Email Addresses” area and populate the table as desired.
• To return either area to its default behavior, click the corresponding “Inherit” button.

Restricting Access to a Range

By default, IP addresses in a DHCP Range can be dynamically allocated to any client machine that is connected to the appropriate network. To restrict the use of a DHCP Range to specific clients:

1. Create a new MAC Address Filter and populate it with the desired MAC addresses (see MAC Address Filters), OR identify a suitable existing MAC Address Filter.

   You can use the same MAC Address Filter to restrict access to multiple DHCP Ranges.
2. Choose the “Filters” subscreen in the Edit dialog for the Range (requires Advanced Mode).
3. Click the Add (+) icon for the Class Filter List table, and select your MAC Address Filter.
   • By default the filter will be added with Action “Grant Lease”, meaning that clients matching this MAC Address Filter will be permitted (and clients not matching any MAC Address Filter listed in the Class Filter List will be denied).
   • If instead you want to deny leases to MACs matching this filter, click on “Grant Lease” and it will become a drop-down, then choose “Deny Lease” instead.

To return to the default behavior of permitting all clients, simply remove all Filters from the Class Filter List.

Advanced note: some use cases may involve adding an IPv4 Option Filter to the Class Filter List instead.  If a Class Filter List contains more than one type of filter (e.g. one or more MAC Address Filters AND one or more IPv4 Option Filters), then clients will be checked against the criteria for each filter type separately, and must satisfy them all in order to receive a lease.

Using Multiple DHCP Ranges

In most cases, you will want to create only one DHCP Range per network. If your network has a discontinuous collection of IP addresses which should be assigned interchangeably, it is better to create one large DHCP Range with Exclusions then to create several small DHCP Ranges which are otherwise identical, because Email Alerts apply individually to each Range.

It may make sense to create multiple DHCP Ranges if each one is intended to be used by a different (non-overlapping) set of clients.  If this is your goal, make sure that you associate at least one Filter with each Range.

Hint: if you configure one DHCP Range with action “Grant Lease” for a particular Filter, and a second DHCP Range with action “Deny Lease” for the same Filter, then any clients matching that Filter will be placed in the first range and all other clients will be placed in the second range.

Placing Fixed Addresses inside a DHCP Range