The Tool Desk
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NSD is an authoritative-only DNS server: it publishes data for zones you operate and answers recursive resolvers, but it does not provide recursive caching for client applications. The procedure below uses Ubuntu’s package-managed NSD and an illustrative example.com zone; replace addresses and records with values for your domain.
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NSD (Name Server Daemon) is designed to serve authoritative DNS data. When a resolver asks for a record in a zone loaded by NSD, it returns the answer with the authoritative (AA) flag. It does not look up arbitrary Internet names on behalf of clients and is not a recursive cache. Run a separate recursive resolver for workstation or application lookups, or point those clients at an existing resolver.
This distinction matters operationally. A server can answer direct queries for example.com while still being unable to resolve ubuntu.com for local users. Public authority also requires delegation at the parent zone and network reachability; installing NSD alone does not change the Internet’s delegation.
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Choose the Ubuntu release and package source
Ubuntu’s package manager is the simplest installation route. NLnet Labs’ NSD 4.15.4 documentation describes distribution packages as the easy path, while warning that a distribution may lag upstream. Package versions are release-specific, so inspect the candidate on the machine you will configure instead of pinning a version copied from another release.
| Reference | Version shown | How to use the information |
|---|---|---|
| Ubuntu Noble package manpage | NSD 4.8.0-1build3 | Applies to that Ubuntu release’s package metadata; do not assume it is the version on another release. |
| Ubuntu Resolute package manpage | NSD 4.14.0-1 | Use the manpage installed on the target system for option and utility details. |
| NLnet Labs installation documentation | NSD 4.15.4 | Upstream reference; source builds are an alternative when you need a newer release or compile-time options. |
Record the release and package source before changing configuration:
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apt-cache policy nsdThe output tells you which Ubuntu archive supplies the package and which version will be installed. If a particular NSD feature is tied to a newer release, verify it against that release’s local manpage rather than relying on an upstream example.
Prerequisites for an authoritative server
- A stable IPv4 address and, if you publish IPv6, a stable IPv6 address. The addresses must remain assigned to the host.
- A hostname for the nameserver, such as
ns1.example.net, with a working local system configuration. - Control of the zone you intend to serve and permission to change its parent-zone delegation.
- Firewall and security-group rules allowing DNS on both UDP 53 and TCP 53. TCP is required for large responses and zone transfers.
- A decision about whether this machine is a primary (master) for the zone or a secondary that transfers data from another authoritative server.
- At least one independent secondary for a resilient public service. The exact number and geographic or network separation required by a registry or registrar vary by policy.
For a private, split-horizon, or laboratory zone, you can stop after local and network tests. For an Internet-facing zone, plan delegation and glue records before calling the deployment complete.
Install NSD with apt
- Refresh package metadata and install the daemon:
sudo apt update sudo apt install nsd - Confirm the installed package and utilities:
nsd -V apt-cache policy nsd command -v nsd-checkconf nsd-checkzoneThe exact version printed is the one whose defaults and manpages govern this host. Keep that value in your deployment notes.
- Inspect the package-created configuration and service definition before editing:
sudo less /etc/nsd/nsd.conf systemctl cat nsd
Building NSD from source is reasonable when the Ubuntu package is too old or you require compile-time options unavailable in the archive. A source installation has different paths, service integration, and upgrade responsibilities; do not mix its files with the distribution package without documenting the layout.
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Understand nsd.conf
The main file is /etc/nsd/nsd.conf on the Ubuntu package. Its syntax is attribute: value. Current Ubuntu references describe top-level clauses including server:, zone:, pattern:, key:, and verify:. The file specifies server behavior, zone files, and primary/secondary relationships.
Minimal server and zone declaration
Start with an explicit listener and one zone. The addresses below are documentation ranges; substitute your server’s real addresses.
server:
ip-address: 192.0.2.53
ip-address: 2001:db8:53::53
port: 53
username: nsd
zone:
name: example.com
zonefile: /etc/nsd/example.com.zoneIf the package’s existing file already defines defaults such as a zones directory, retain compatible settings and add the zone stanza in the style used there. A zone declaration needs the zone name and the path to its zone file; transfer settings are separate and should not be inferred from the declaration alone.
Keep configuration maintainable
- Put
nsd.confand every zone file under version control, but restrict the repository and working files to administrators. Transfer keys are secrets. - Use absolute zone-file paths while you are establishing the service. This makes error messages unambiguous.
- Keep one authoritative source of each zone. Do not edit generated copies on a secondary.
- Use comments for the purpose of unusual listener or transfer settings, not for storing passwords or private keys.
Create an authoritative zone file
A zone file needs an SOA record, NS records, and the address records needed to reach those nameservers and services. This example is deliberately small and is not a complete production zone:
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@ IN SOA ns1.example.com. hostmaster.example.com. (
2026101501 ; serial: YYYYMMDDnn
3600 ; refresh
900 ; retry
1209600 ; expire
300 ; negative caching TTL
)
IN NS ns1.example.com.
IN NS ns2.example.net.
ns1 IN A 192.0.2.53
ns1 IN AAAA 2001:db8:53::53
@ IN A 192.0.2.80
www IN A 192.0.2.80
@ IN MX 10 mail.example.com.
mail IN A 192.0.2.25Use fully qualified names with a trailing dot in SOA, NS, MX, and target records. The SOA serial is not decorative: increment it for every published change so secondaries know that new data is available. A date-plus-counter format such as 2026101501 is easy to audit; if you make two changes on one day, increase the counter rather than reusing the serial.
Nameserver address and glue considerations
If a nameserver’s hostname is inside the zone it serves—for example, ns1.example.com—the parent registry normally needs a glue address so resolvers can reach it without a circular lookup. The zone’s own A or AAAA record is still required. If the nameserver is in another domain, such as ns2.example.net, its address is maintained in that domain’s authoritative zone.
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Configure primary and secondary transfers
Choose one role per zone. A primary publishes the editable zone file and notifies secondaries. A secondary obtains the zone by transfer and should not be edited by hand.
| Role | Relevant settings | Purpose |
|---|---|---|
| Primary | notify, provide-xfr |
Tell named secondaries about a new SOA serial and permit their AXFR/IXFR requests. |
| Secondary | allow-notify, request-xfr |
Accept notifications from the primary and request the zone transfer. |
Primary example
For a primary at 192.0.2.53 and a secondary at 198.51.100.54, an illustrative zone stanza is:
zone:
name: example.com
zonefile: /etc/nsd/example.com.zone
notify: 198.51.100.54 transfer-key
provide-xfr: 198.51.100.54 transfer-keySecondary example
On the secondary, the same zone can be declared with the primary’s address:
zone:
name: example.com
zonefile: /var/lib/nsd/secondary/example.com.zone
allow-notify: 192.0.2.53 transfer-key
request-xfr: AXFR 192.0.2.53 transfer-keyExact transfer syntax and supported options are version-sensitive; consult the nsd.conf(5) installed with your Ubuntu package. The examples show the relationship, not a substitute for checking that manpage.
Restrict transfers and authenticate them
Never expose unrestricted transfers on an Internet-facing primary. Use the secondary’s fixed address and a TSIG key. Define the key in a key: clause and reference the same key name in notify, provide-xfr, allow-notify, and request-xfr. Generate a strong, unique secret, store it with administrator-only permissions, and install it on both servers. Do not place a real secret in public configuration examples or source repositories.
An address allow-list without a key limits where transfers can originate; TSIG also authenticates the request. Use both controls. If a transfer fails, check that the key name, secret, algorithm, source address, and system clocks match on both hosts.
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Validate before starting or reloading
Validation catches syntax and zone errors before they become an outage. Run the utilities as the account that can read the configuration and zone files:
sudo nsd-checkconf /etc/nsd/nsd.conf
sudo nsd-checkzone example.com /etc/nsd/example.com.zoneA successful check normally exits without an error message and returns status zero. A failure identifies a line, token, missing file, malformed record, or invalid owner name. Correct the first reported error, then run both checks again. If a command is not present, inspect the package’s installed manpages and paths with dpkg -L nsd; do not substitute an unrelated DNS utility and assume it validates NSD semantics.
Check file ownership and permissions as well. The daemon must be able to read its zone files, while transfer-key files should not be world-readable:
sudo stat /etc/nsd/nsd.conf /etc/nsd/example.com.zone
sudo namei -l /etc/nsd/example.com.zoneEnable the service and verify listeners
- Enable NSD at boot and start it now:
sudo systemctl enable --now nsd - Check service state and recent logs:
systemctl status nsd --no-pager sudo journalctl -u nsd -b --no-pager - Confirm that the daemon is bound to the intended addresses and both transport protocols:
sudo ss -luntp | grep ':53'You should see UDP and TCP listeners on each configured address. If only loopback is present, review the
ip-addresssettings and host firewall. If another process owns port 53, stop or reconfigure it before starting NSD.Quick wins for a faster PC:
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sudo systemctl reload nsdIf reload reports an error, inspect the journal and keep the last known-good configuration in place until the correction validates.
Query NSD directly with dig
Test the server’s address directly so a recursive cache or stale delegation cannot hide a local mistake.
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dig @192.0.2.53 example.com SOA
dig @192.0.2.53 example.com NS
dig @192.0.2.53 www.example.com A
dig @192.0.2.53 example.com MX- In the flags line, look for
aa(authoritative answer). - Confirm that the SOA serial matches the zone file you installed.
- Confirm that the NS set is the one you intend to delegate.
- Check that A, AAAA, MX, and other records return the expected values and TTLs.
Test TCP explicitly when a firewall, DNSSEC data, or a large response is involved:
dig +tcp @192.0.2.53 example.com SOA
dig +tcp @2001:db8:53::53 example.com NSRepeat the tests from another network when possible. A local query proves that NSD answered on the host; it does not prove that an external resolver can reach the address.
Publish Internet delegation and glue
- At the registrar or registry for the parent zone, create host (glue) records for in-bailiwick nameservers and enter their IPv4 and IPv6 addresses.
- Set the domain’s NS delegation to the complete authoritative set, including an independent secondary.
- Open UDP and TCP 53 at every host firewall, cloud security group, load balancer, and upstream network involved.
- Wait for parent-zone and resolver caches to expire according to their TTLs, then query the delegated nameservers directly and through several external vantage points.
- Compare the externally visible SOA serial and NS records with the primary’s zone file. A mismatch usually indicates stale delegation, an incorrect glue address, a blocked path, or a secondary that did not transfer.
Registry rules differ by top-level domain, and the NSD documentation does not prescribe a universal number of nameservers or a specific registrar workflow. Follow the parent registry’s current requirements.
NSD compared with a combined DNS server
Ubuntu’s NSD documentation contrasts NSD’s authoritative-only design with servers that can also act as a resolver and cache. The choice affects architecture rather than the zone-file fundamentals.
| Concern | NSD | Server with recursive mode |
|---|---|---|
| Primary purpose | Authoritative service for configured zones. | Can serve zones and recursively resolve/cache other names. |
| Configuration model | nsd.conf with server, zone, pattern, key, and verification clauses. |
Different syntax and separate controls for recursion and authoritative data. |
| Transfer controls | Explicit notify, provide-xfr, allow-notify, and request-xfr, preferably with TSIG and address restrictions. |
Equivalent concepts, but option names and defaults differ. |
| Recursive exposure | Not provided, reducing the risk of accidentally running an open resolver. | Must be tightly restricted with ACLs if recursion is enabled. |
| Updates | Ubuntu packages may lag the current upstream release; inspect the target release before deployment. | Package cadence and security process depend on the selected implementation. |
For an authoritative-only edge service, NSD’s narrow role can simplify isolation. If the same host must provide recursive service, design that function separately rather than assuming NSD will supply it.
Troubleshoot common failures
The service will not start
- Run
sudo nsd-checkconf /etc/nsd/nsd.confand read the first reported line. - Check for a missing zone file, incorrect path, or permissions that prevent the daemon user from reading it.
- Use
journalctl -u nsd -bto identify a port conflict or an address that is not assigned to the host.
Queries return REFUSED or no answer
- Verify that the query is sent to the NSD address, not to a recursive resolver.
- Confirm the requested zone name exactly matches the
zonestanza, including the absence or presence of a trailing dot as required by the configuration syntax. - Check that UDP and TCP 53 are permitted and that NSD is listening on the queried IPv4 or IPv6 address.
The answer is not authoritative or is stale
- Query the primary directly and inspect the
aaflag and SOA serial. - Increment the serial after every edit and reload NSD only after validation succeeds.
- On a secondary, inspect transfer logs and verify
allow-notify,request-xfr, TSIG credentials, and source addresses.
Transfers fail
- Ensure the primary’s
provide-xfrpermits the secondary’s actual source address. - Ensure the secondary requests from the primary’s reachable address and that TCP 53 is open.
- Compare key names, secrets, algorithms, and clocks on both hosts. A TSIG mismatch is an authentication failure, not a zone-content error.
Operational checklist for each change
- Edit the version-controlled zone source or
nsd.conf. - Increment the SOA serial for any published zone-data change.
- Run
nsd-checkconfandnsd-checkzone. - Reload NSD and inspect the service journal.
- Query SOA, NS, and changed records over UDP; use TCP for the same records when transport size or policy warrants it.
- For a delegated zone, verify the result from outside the server’s network and confirm that secondaries report the new serial.
Following this sequence gives Ubuntu a reproducible authoritative NSD service: the daemon serves only the zones you declare, transfers are limited to known secondaries, and both local answers and public delegation are checked rather than assumed.
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