Code-Memo

DNS concepts

What DNS does

DNS maps names to data, most commonly:

• A / AAAA: hostname → IPv4/IPv6
• CNAME: alias → canonical name
• MX: mail routing
• TXT: arbitrary text (SPF/DKIM/verification)

Key terms

• Resolver: client-side component querying DNS (e.g., systemd-resolved)
• Recursive resolver: does the full lookup on your behalf
• Authoritative server: source of truth for a zone
• Zone: administrative domain (e.g., example.com)
• TTL: caching time

Common Linux places

• /etc/resolv.conf (nameserver config; may be managed)
• systemd-resolved often exposes stub 127.0.0.53

Check resolver status (systemd):

resolvectl status

Query tools

dig example.com A
dig +short example.com AAAA
nslookup example.com
host example.com

Troubleshooting checklist

• Verify connectivity to resolver (network first)
• Check which nameserver is used (resolvectl status)
• Compare dig @8.8.8.8 example.com vs system resolver
• Watch for split DNS (VPN) and search domains

Related

• Network troubleshooting