Domain name system (DNS)

hierarchy: 13 root servers
UDP 53
TCP 53: between authoritative servers
- data synchronization
- packet size > 512 bytes
server types
- authoritative: responsible for DNS zone
- non-authoritative: serve requests using cache
modes
- recursive
  - server itself polls upstream servers, then returns ultimate reply to the client
  - client interacts with one server
  - high load on server
- iterative
  - server redirects to other servers
  - root servers operation mode
TLD: top level domain
RIR:
- authoritative NS for /8 for reverse lookup
- serves info according to RIR DB

; use DNS
(config)# ip domain-lookup

; DNS server address
(config)# ip name-server <IP>

NetBIOS name system (NBNS)

255 bytes max
NS: authoritative server
MX: mail exchange
A: IPv4 address
AAAA: IPv6 address
CNAME: alternative name
PTR: reverse DNS (.in-addr.arpa or .ip6.arpa)
SRV service record
- weighted load balancing
- AD + DNS modify weight so that host selects closest controller
- format: _service._proto.name TTL class SRV priority weight port target
HTTPS
- server public key for TLS ECH
- ALPN
- IP addresses
NSEC
- list of DNS record types that exist for the name

serve addresses in varying order: clients use first address from list → load-balancing
GeoIP-based
weights in SRV record