This page contains information about creating and managing stand-alone DNS records.
Contents
Introduction
We use the term “stand-alone” to refer to any standard DNS record type (A, AAAA, CNAME, MX, SRV, TXT, etc) which does not have special proprietary behavior in Grid Manager. There are many different types of stand-alone records, but the steps for managing them are very similar; we will discuss a few of the most common ones.
This page does not contain many screenshots, but some of the screenshots from the Host Records page may also be helpful here.
Stand-alone A Records
An address (A) record maps a fully-qualified domain name to a single IPv4 address:
www.example.com. IN A 198.51.100.17
Publishing multiple A records for the same name is legal and results in “round-robin DNS” behavior.
Best practice: use stand-alone A records only when you need an A record with no matching PTR (see below for further discussion). If you want a matching pair of A and PTR records, create a Host Record instead.
To create a stand-alone A record (with no matching PTR):
- Open the DNS Zone in which you want to create the new record (see Getting Started with IPAM).
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Click the dropdown arrow next to the Add (+) icon above the table in the main workspace, then choose “Record” and finally “A Record”.
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If necessary, click the “Select Zone” button and choose the zone which will contain the desired fully-qualified domain name (e.g. to create “myrecord.sandbox.illinois.edu”, you would select the zone “sandbox.illinois.edu”).
- Type the leading portion of the Name (e.g. “
myrecord“) into the text box to the left of the selected zone name, so that both pieces together form the desired fully-qualified domain name.- You may leave this text box empty to create a record with the same name as the zone itself (e.g. “sandbox.illinois.edu”).
- You may type e.g. “
foo.bar” in the text box to create a record named “foo.bar.sandbox.illinois.edu” even if there is no zone “bar.sandbox.illinois.edu”. - You may type “
*” or e.g. “*.bar” in the text box to create a record with a wildcard domain name (see RFC 4592).
- Enter the target IP Address to which your A record should resolve.
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Important: UNCHECK “Create associated PTR record”
If you do want a matching PTR, you should Cancel this operation and create a Host Record instead. Leaving this box checked creates a stand-alone A record and a separate stand-alone PTR record, which is undesirable because they can easily get out of sync in the future. - Click “Save & Close”.
AAAA records are exactly like A records, but they target IPv6 addresses instead of IPv4 addresses.
www.example.com. IN AAAA 2001:db8::17
When should I use stand-alone A records?
Use stand-alone A (and AAAA) records when you do not want a matching PTR record.
Common reasons for this include:
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the IP address already has a PTR record pointing to a different fully-qualified domain name
- the IP address belongs to a network whose reverse-mapping DNS is not managed in IPAM
Otherwise, it is preferable to use Host Records (which automatically manifest as matching pairs of A/AAAA and PTR records).
Note that it is best practice to avoid creating multiple PTR records for the same IP address. While not technically an error, this may cause problems for software which expects reverse lookups to return a single name (an expectation subtly encouraged by language such as “primary” and “the host name” in https://tools.ietf.org/html/rfc1035#section-3.5).
When you want several fully-qualified domain names (FQDNs) to resolve to the same IP address, the recommended best practice is:
- Create a Host Record for the FQDN that you consider to be primary.
- Where possible, implement each additional FQDN as a Host Alias or stand-alone CNAME record pointing to the primary FQDN.
- Create a stand-alone A record with no PTR for each additional FQDN which cannot be implemented as a CNAME record.
Example: Host Record for server17.mysubdomain.illinois.edu, stand-alone CNAME records (pointing to server17.mysubdomain.illinois.edu) for www. and mysubdomain.illinois.eduwww.example.com, and stand-alone A records (pointing to the same IP as the Host Record) for mysubdomain.illinois.eduexample.com (which cannot be implemented as CNAME records since each resides at the apex of a zone).
MX Records
A mail exchanger (MX) record indicates the fully-qualified domain name of a mail server which can accept incoming email messages for a domain:
illinois.edu. IN MX 10 incoming-relays.illinois.edu.
Note that successful use of this record also entails resolving A (and/or AAAA) records for the mail server name.
To create an MX record:
- Open the DNS Zone in which you want to create the new record (see Getting Started with IPAM).
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Click the dropdown arrow next to the Add (+) icon above the table in the main workspace, then choose “Record” and finally “MX Record”.
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If necessary, click the “Select Zone” button and choose the zone which will contain the desired fully-qualified domain name (e.g. to create “myrecord.sandbox.illinois.edu”, you would select the zone “sandbox.illinois.edu”).
- Type the leading portion of the Mail Destination (e.g. “
myrecord“) into the text box to the left of the selected zone name, so that both pieces together form the desired fully-qualified domain name.- You may leave this text box empty to create a record with the same name as the zone itself (e.g. “sandbox.illinois.edu”).
- You may type e.g. “
foo.bar” in the text box to create a record named “foo.bar.sandbox.illinois.edu” even if there is no zone “bar.sandbox.illinois.edu”.
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In the “Mail Exchanger” field, enter the target fully-qualified domain name of the mail server to which the MX record should point.
Per RFC 2181, the target of an MX record MUST NOT be an (explicit) alias (i.e. a Host Alias or CNAME record). You are responsible for following this rule; it is not enforced automatically. -
In the Preference field, enter a priority value for this record. (10 is selected by default)
- Click “Save & Close”.
Stand-alone CNAME Records
A CNAME record defines a static, explicit alias in the DNS which affects query behavior for all record types:
www.illinois.edu. IN CNAME illinois.edu.
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Query:
www.illinois.edu. IN A?
Answer:www.illinois.edu. IN CNAME illinois.edu. illinois.edu. IN A 192.17.172.3
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Query:
www.illinois.edu. IN MX?
Answer:www.illinois.edu. IN CNAME illinois.edu. illinois.edu. IN MX 10 incoming-relays.illinois.edu.
Common points of confusion:
- This CNAME record helps your browser find the IP address of a web server for www.illinois.edu. It does not tell your browser to redirect HTTP requests for http://www.illinois.edu/ to a different URL (only the web server itself can do that).
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A CNAME record cannot coexist with other records (e.g. no other records are permitted at www.illinois.edu)
RFC 1034: “If a CNAME RR is present at a node, no other data should be present; this ensures that the data for a canonical name and its aliases cannot be different.”
A few special record types for DNSSEC are exempted by later RFCs.
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A CNAME record cannot be placed at the apex of a zone (e.g. illinois.edu).
This follows from the previous point, because the apex of a zone is required to have NS and SOA records.
- CNAME stands for “canonical name”, but that term (correctly applied) refers to the target name, not the alias name. Best practice: use the terms “alias” and “target” to avoid confusion.
To create a stand-alone CNAME record:
- Open the DNS Zone in which you want to create the new record (see Getting Started with IPAM).
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Click the dropdown arrow next to the Add (+) icon above the table in the main workspace, then choose “Record” and finally “CNAME Record”.
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If necessary, click the “Select Zone” button and choose the zone which will contain the desired fully-qualified domain name of the alias (e.g. to create “myalias.sandbox.illinois.edu”, you would select the zone “sandbox.illinois.edu”).
- Type the leading portion of the Alias name (e.g. “
myalias“) into the text box to the left of the selected zone name, so that both pieces together form the desired fully-qualified domain name.- You may type e.g. “
foo.bar” in the text box to create a record named “foo.bar.sandbox.illinois.edu” even if there is no zone “bar.sandbox.illinois.edu”. - You may type “
*” or e.g. “*.bar” in the text box to create a record with a wildcard domain name (see RFC 4592).
- You may type e.g. “
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In the “Canonical Name” field, enter the target fully-qualified domain name to which the alias should point.
The target of a CNAME record should not be another (explicit) alias. So-called “CNAME chains” are not technically an error, but create inefficient behavior and are discouraged as a bad practice (see RFC 1034 sections 3.6.2 and 5.2.2).
- Click “Save & Close”.
When should I use stand-alone CNAME records?
A Host Alias is functionally equivalent to a stand-alone CNAME record pointing to the Host’s primary FQDN, but carries trade-offs with respect to ease of future maintenance. Which option is preferable depends on the situation.
Advantages of using a Host Alias:
- A Host Alias will automatically be kept up to date if you change the Host’s primary Name, whereas a stand-alone CNAME record will be left “dangling” if the target Host record is renamed or deleted.
Advantages of using a stand-alone CNAME record:
- Modifying an existing stand-alone CNAME record to point to a different target is a simple one-step operation. The corresponding process for a Host Alias requires editing the old Host (to remove the Host Alias) and then editing the new Host (to add the Host Alias).
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If a Host Alias resides in a different zone (from the primary Name of the Host) which is not managed by the same set of people, the disparity in permissions may impede self-service changes to the record (possibly requiring an escalation to hostmgr). A stand-alone CNAME record presents no such problem; it is governed by the permissions on the zone containing the alias name, while any target record(s) are governed by the permissions of the zone containing the target fully-qualified domain name.
Best practice: always use a stand-alone CNAME record in the case where the desired alias name and the canonical (target) name reside in different zones which may not be managed by the same set of people.
Stand-alone PTR Records
A PTR record is a pointer to another fully-qualified domain name. PTR records (unlike CNAME records) are simple data; they do not alter DNS behavior, may coexist with other records, and have no inherent special meaning. Their significance is understood by convention from where they are placed in the namespace (e.g. “17.100.51.198.in-addr.arpa” is understood to represent the IPv4 address 198.51.100.17).
PTR records are most commonly used for reverse-mapping DNS (i.e. mapping from an IP address to a fully-qualified domain name). In general, you should never create a stand-alone PTR record in IPAM for this purpose; instead, create a Host Record which will automatically manifest as matching pairs of A (or AAAA) and PTR records.
If you do need to manage stand-alone PTR records for reverse-mapping DNS, just Open the Network in IPAM View (see Getting Started with IPAM); it is not necessary to navigate the arpa zones.
Stand-alone PTR records in forward-mapping DNS zones (used infrequently for other purposes such as DNS-SD) are not a special case, and can be managed just like the other types of stand-alone records described on this page.
Editing Stand-alone DNS Records
- Navigate to the record you want to edit (see Getting Started with IPAM).
- Select the checkbox for the record and click the Edit (notepad) icon above the table. This opens the Edit dialog box.
- Make any desired changes.
- Click “Save & Close”.
Deleting Stand-alone DNS Records
- Navigate to the record you want to delete (see Getting Started with IPAM).
- Select the checkbox for the record (making sure no other checkboxes are selected), and click the Delete (trash can) icon above the table.
- If you’re sure, click “Yes” when the confirmation dialog appears.