DHCP and DNS: network configuration and name resolution

DHCP distributes network configuration; DNS resolves names into resource records. Troubleshoot address assignment and name resolution separately, using the client's actual settings, packet captures and server evidence.

DHCP provides more than an IP address

The typical IPv4 DORA exchange starts with Discover, a client broadcast seeking a server. Offer proposes an address, for example 192.168.1.50 with a lease. Request identifies the offer the client wants, and Acknowledge confirms it. DHCP also supplies the mask, default gateway, DNS servers, search domain and other options. A valid address with incorrect options can still leave a client unable to reach its destination.

DHCP commonly uses UDP ports 67 and 68. Broadcasts do not automatically cross routers. A relay forwards requests from a segment to a central server and includes information about the originating network. Scope, relay and VLAN must agree. Reservations associate an address with a client identifier but do not replace documentation of critical devices.

DNS is a distributed, cached database

A stub resolver normally asks a recursive resolver, which uses its cache or follows the hierarchy through root, top-level-domain and authoritative name servers. DNS commonly uses port 53. TTL determines how long an answer may be cached.

A successful lookup does not prove that the service is reachable. Conversely, an IP address may work while name resolution fails. Keep these questions separate during diagnosis.

A records associate names with IPv4 addresses; for example bissolux.com → 192.0.2.10.
AAAA records associate names with IPv6 addresses, for example 2001:db8::10.
CNAME records point an alias to a canonical name, such as www.bissolux.com → bissolux.com.
MX records identify mail destinations; TXT records can carry verification and email policy information.

Diagnose the real path

For DHCP, inspect interface state, VLAN, client address, lease, relay and scope. An IPv4 address in 169.254.0.0/16 often indicates that no lease was obtained. A packet capture reveals which DORA step is missing rather than merely showing that another attempt failed.

For DNS, compare the configured resolver, query type, response code, authoritative data and cache. NXDOMAIN says the name does not exist according to the answer. SERVFAIL more often indicates processing, delegation or validation failure. Split DNS may intentionally return different internal and external answers.

Practical checklist

01
Record address, mask, gateway, DNS servers and lease duration.
02
Compare VLAN, relay address and DHCP scope.
03
Query the internal resolver and authoritative server explicitly.
04
Inspect response code, record type, TTL and delegation.
05
Verify changes with packet captures or logs.

Frequently asked questions

Why can I ping an IP address but not a hostname?

IP connectivity exists, but name resolution does not. Check the resolver configuration, DNS record, search suffix, cache and DNS server reachability.

Why is a relay needed when a VLAN uses a central DHCP server?

The client's initial broadcast stays in its local segment. A router does not ordinarily forward it; a relay connects the segment to the central DHCP service.

DHCP and DNS: network configuration and name resolution

Knowledge you can put to work.