Linux Dns Server

Linux DNS Server

Distro support

BIND and unbound run on both families; dig, rndc, named-checkconf, and named-checkzone are identical. The differences are package names, config file locations, and SELinux. Body uses Debian/Ubuntu; the RHEL family (Fedora, RHEL, CentOS Stream, Rocky, Alma, Oracle) equivalents are in the matrix.

RHEL-family gotcha: SELinux governs BIND. Zone files must carry the named_zone_t context (restorecon -Rv /var/named) and writes to /var/named need the right labels; serving from a non-standard path requires semanage fcontext. There is no SELinux step on Debian/Ubuntu. (Deep SELinux coverage lands in linux-server-hardening / Phase 2.)

In sk-* scripts use the common.sh primitives (pkg_install, svc_name) instead of hardcoding the family. See linux-bash-scripting and docs/multi-distro/plan.md.

Use when

• Managing authoritative DNS zones, records, serials, or server reloads.
• Validating zone files or setting up reverse DNS on Ubuntu/Debian.
• Debugging BIND9 or unbound behavior on a server that owns DNS service.

Do not use when

• The task is client-side DNS resolution from an application host; use linux-network-admin.
• The task is mail authentication policy only; use linux-mail-server.

Required inputs

Workflow

1. Identify the zone and the authoritative server role before editing.
2. Make the smallest correct record or config change and validate syntax first.
3. Reload the service only after passing the relevant checks.
4. Query the authoritative server directly to verify the expected answer.
5. Stop if zone ownership, serial policy, validator output, transfer/recursion scope, or rollback is unresolved.
6. Recover a failed reload by restoring the saved zone/config, validating it, reloading the prior version, and querying each authority again.

Quality standards

• Validate zone syntax before reload every time.
• Keep serial management explicit and predictable.
• Verify from the authoritative source, not only from cached resolvers.

Anti-patterns

• Editing a zone without advancing its serial. Fix: apply the site's serial policy exactly once per published change.
• Reloading named before named-checkconf and named-checkzone. Fix: block reload on either validator failure.
• Querying only a caching resolver. Fix: query each authoritative server directly and then check public delegation.
• Allowing unrestricted recursion or zone transfer. Fix: restrict by role, network, TSIG, and explicit ACL.
• Bypassing an SELinux denial with permissive mode. Fix: restore correct zone paths/contexts and keep enforcement.

Outputs

References

• references/bind9-reference.md
• references/zone-file-syntax.md

Evidence Produced

Capability contract

Read-only inspection is the default for diagnosis. Editing zones/configuration, reloading DNS, changing delegation, opening firewall access, or enabling transfer/recursion requires explicit authority. Do not expose TSIG private material.

Degraded mode

Without access to the authoritative host, validate supplied text where possible and mark live reload, delegation, propagation, and SELinux checks not assessed. Do not treat a local syntax pass as published DNS success.

Decision rules

Worked example

To add api.example.org A 192.0.2.40, confirm the authoritative zone and serial policy, edit the managed source, validate configuration and zone, reload, query each authoritative server with recursion disabled, and record the returned address, TTL, and new SOA serial.

This skill is self-contained. Every command below is a standard BIND tool (named, rndc, named-checkconf, named-checkzone, dig) present on both Debian/Ubuntu and the RHEL family; only package names and paths differ (see Distro support). The sk-* scripts in the Optional fast path section are convenience wrappers — never required.

This skill owns authoritative DNS serving — running BIND9 or unbound as a server that answers queries for zones you control.

It does not own:

• Client-side DNS lookups from a server — linux-network-admin.
• MX records and email auth — linux-mail-server.
• DNS records at a registrar — those belong to whoever manages the domain at its registrar; this skill handles self-hosted authoritative DNS.

Informed by Linux Network Administrator's Guide (BIND chapters) and the Canonical Ubuntu Server Guide (BIND9 packaging).

When to use

• Hosting authoritative zones for your own domains.
• Setting up a recursive resolver for an internal network.
• Validating zone files before reload.
• Debugging zone transfer failures or serial mismatches.
• Adding, removing, or editing DNS records in a managed zone.
• Setting up reverse DNS (in-addr.arpa / ip6.arpa).

When NOT to use

• Debugging outbound DNS from a server — linux-network-admin's diagnostics.
• Cloudflare / Route53 / Google Cloud DNS — those are managed DNS APIs.

Standing rules

1. Always named-checkconf before reloading. A bad config takes down every zone.
2. Always named-checkzone for every modified zone. A bad zone file takes down that zone.
3. Bump the serial on every change. Format: YYYYMMDDNN. Slaves won't pull the change if the serial hasn't moved.
4. Reverse zones must match forward zones. Every A record you own should have a PTR in the in-addr.arpa zone you control (or upstream).
5. Zone files live in /etc/bind/zones/ by convention. Permissions: bind:bind, mode 0644.
6. rndc reload <zone> is safer than systemctl reload bind9. It reloads one zone at a time.
7. Restrict recursion. Only answer recursive queries from trusted networks. Open recursive resolvers are used in amplification attacks.

Quick reference — manual commands

Install and baseline

sudo apt install bind9 bind9-utils dnsutils

# Files land in:
#   /etc/bind/named.conf           — main config, don't edit directly
#   /etc/bind/named.conf.options   — global options
#   /etc/bind/named.conf.local     — your zones
#   /var/lib/bind/                 — dynamic state (slaves, journals)
#   /var/log/                      — if you configure file logging

sudo systemctl status bind9 --no-pager
sudo journalctl -u bind9 -n 50 --no-pager

Validate and reload

# Validate main config
sudo named-checkconf

# Validate a specific zone file
sudo named-checkzone example.com /etc/bind/zones/example.com.zone

# Reload via rndc (preferred — doesn't flush cache)
sudo rndc reload                              # reload everything
sudo rndc reload example.com                  # reload one zone

# Dump the current cache (debugging)
sudo rndc dumpdb -cache
sudo less /var/cache/bind/named_dump.db

# Flush cache for a zone (after manual changes at upstream)
sudo rndc flush

Query your own server

# Ask your BIND directly (bypass systemd-resolved)
dig @127.0.0.1 example.com SOA
dig @127.0.0.1 example.com NS
dig @127.0.0.1 example.com MX
dig @127.0.0.1 example.com A +all        # show all sections

# Check from outside (replace with your public IP)
dig @<public-ip> example.com SOA

# AXFR zone transfer test (from an allowed secondary)
dig @<master> example.com AXFR

Adding a record

# 1. Edit the zone file
sudo nano /etc/bind/zones/example.com.zone

# 2. Bump the serial (YYYYMMDDNN format)
#    From 2026041001 to 2026041002 etc.

# 3. Validate
sudo named-checkzone example.com /etc/bind/zones/example.com.zone

# 4. Reload that zone only
sudo rndc reload example.com

# 5. Verify
dig @127.0.0.1 new-record.example.com A +short

Full BIND9 reference (named.conf structure, options, logging, views, TSIG, rndc control channel, systemd integration, common errors) — see references/bind9-reference.md.

Full zone file syntax reference (SOA, NS, A, AAAA, CNAME, MX, TXT, SRV, PTR, CAA, reverse zones, slave config, 4–6 complete examples) — see references/zone-file-syntax.md.

Typical workflows

Workflow: Adding a record to a managed zone

sudo nano /etc/bind/zones/example.com.zone
# Increment SOA serial
# Add the new record (A, CNAME, MX, TXT, etc.)

sudo named-checkzone example.com /etc/bind/zones/example.com.zone
sudo rndc reload example.com

dig @127.0.0.1 new.example.com A +short

Workflow: Serial mismatch between master and slave

# Compare serials
dig @master.example.com  example.com SOA +short | awk '{print $3}'
dig @slave.example.com   example.com SOA +short | awk '{print $3}'

# If they differ after a master reload:
sudo journalctl -u bind9 -n 100 --no-pager | grep -i -E "transfer|notify"
# Look for: "transfer of 'example.com/IN' from ...: failed"
# Common cause: slave firewall blocks 53/tcp (AXFR is TCP)

# Force the slave to retry
sudo rndc retransfer example.com              # on the slave

# Check the journal log on the slave
sudo ls /var/lib/bind/

Workflow: Validating a zone before import

# Against a file you received but haven't installed
named-checkzone example.com /tmp/imported-zone.zone

# Dump the parsed zone to verify what BIND sees
named-compilezone -o - example.com /tmp/imported-zone.zone | less

Workflow: Setting up a reverse zone for your subnet

# For 192.0.2.0/24, the zone is 2.0.192.in-addr.arpa
sudo nano /etc/bind/named.conf.local
# Add a zone stanza pointing at /etc/bind/zones/db.192.0.2

# Create the zone file with SOA + NS + PTR records
# (see references/zone-file-syntax.md for the template)

sudo named-checkconf
sudo named-checkzone 2.0.192.in-addr.arpa /etc/bind/zones/db.192.0.2
sudo rndc reload

dig @127.0.0.1 -x 192.0.2.10 +short

Troubleshooting / gotchas

• Zone loaded but queries return SERVFAIL. Check named-checkconf for missing includes. Check that the zone stanza in named.conf.local points at a file BIND can read (bind:bind, mode 0644).
• Serial didn't increment, slaves aren't pulling. BIND compares serial numbers literally — a new file with the same serial is treated as identical. Always bump.
• rndc reload works but queries show old data. BIND caches recursive answers. sudo rndc flush clears the recursive cache; zone data is authoritative and updates immediately.
• AXFR failing with "REFUSED". Master's allow-transfer doesn't list the slave's IP, or TSIG key mismatch.
• Running both systemd-resolved and bind9. Port 53 conflict. Disable resolved's stub listener: DNSStubListener=no in /etc/systemd/resolved.conf, then systemctl restart systemd-resolved.
• Logs going to syslog with nothing in /var/log/named.log. BIND defaults to syslog (journalctl -u bind9). File logging requires explicit logging { ... } stanzas — see the reference.

References

• references/bind9-reference.md — full BIND9 reference: named.conf structure, options, logging, views, TSIG, rndc, systemd.
• references/zone-file-syntax.md — zone file syntax with all record types, reverse zones, slave config, complete worked examples.
• Book: Linux Network Administrator's Guide — BIND chapters.
• Book: Ubuntu Server Guide — BIND9 package layout.
• Man pages: named(8), named.conf(5), named-checkconf(8), named-checkzone(8), rndc(8).

Optional fast path (when sk-* scripts are installed)

Running sudo install-skills-bin linux-dns-server installs:

These are optional wrappers around named-checkconf, named-checkzone, and rndc.

Scripts

This skill installs the following scripts to /usr/local/bin/. To install:

sudo install-skills-bin linux-dns-server

Currentness reference

Read current DNS operations and source gate before making version-sensitive BIND, DNSSEC, packaging, directive, or command claims. Verify current support with ISC and protocol behaviour with the relevant IETF RFC; the book is conceptual background only.