Domain 3 of 5 · Chapter 7 of 8

Network Services

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Included in this chapter:

  • DHCP: the DORA lease and how it crosses subnets
  • IPv6 SLAAC vs DHCPv6
  • DNS: records, resolvers, and resolution
  • NTP, PTP, and NTS: keeping time
  • Exam-pattern recognition

The core network services: what each one provides and its key protocol fact

ServiceDHCPDNSNTP
JobHand out IP config (address, mask, gateway, DNS)Resolve names to/from IP addressesSynchronize device clocks
Defining mechanismDORA lease exchange from a scope/poolHierarchical records queried via resolversStratum hierarchy from a reference clock
Stateful by default?Yes, the server tracks each leaseCaching only; resolvers cache by TTLNo state; clients poll a server
IPv6 angleDHCPv6 (stateful) vs SLAAC (stateless)AAAA records for IPv6 hostsSame protocol; PTP for sub-microsecond
Crosses subnets viaDHCP relay / IP helper (unicast)Resolvers forward queries upstreamHierarchy of servers, no relay needed

Decision tree

What does the host need?AddressingName resolutionAccurate timeDHCP relayIPv4 across subnetsSLAACserverless IPv6Stateful DHCPv6track IPv6 leasescross-subnet IPv4IPv6, no serverIPv6, auditedDNStune TTL for cutoverNTPmillisecond syncPTPsub-microsecondNTSauthenticated timegeneral synctrading / ICSmust be trusted
Cheat sheet

Cheat sheet

  • DHCP leases IPv4 config through the DORA exchange
  • A DHCP scope is the pool plus the options handed out with it
  • A reservation gives a device a fixed address while staying DHCP-managed
  • DHCP lease time controls renew at 50% and rebind at 87.5%
  • A DHCP relay carries the broadcast across subnets to a central server
  • SLAAC autoconfigures IPv6 addresses with no DHCP server
  • EUI-64 builds the IPv6 interface ID from the 48-bit MAC
  • Stateful DHCPv6 tracks IPv6 leases; SLAAC does not
  • The RA's M and O flags decide how an IPv6 host configures
  • DNS maps names to addresses with typed resource records
  • PTR records and reverse lookup map an address back to a name
  • Authoritative servers own zones; caching resolvers just remember answers
  • TTL trades change-propagation speed against query load
  • Recursive query asks for the answer; iterative returns a referral
  • NTP synchronizes clocks because logs, certs, and Kerberos depend on time
  • NTP stratum counts hops from the reference clock
  • PTP gives sub-microsecond time by stamping packets in hardware
  • NTS authenticates NTP so an attacker cannot forge time
  • Split-horizon DNS returns different answers by query source
  • An SRV record advertises a service's host and port
  • IXFR transfers only the changed zone records
  • A secondary compares SOA serial numbers to decide on a transfer
  • DHCP Option 82 carries the client's circuit and port identity
  • DHCP Option 66 hands clients their TFTP server name

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References

  1. RFC 2131 — Dynamic Host Configuration Protocol Whitepaper
  2. RFC 4862 — IPv6 Stateless Address Autoconfiguration Whitepaper
  3. RFC 1035 — Domain Names: Implementation and Specification Whitepaper
  4. RFC 5905 — Network Time Protocol Version 4: Protocol and Algorithms Specification Whitepaper