Day 5 - Week 9 Comprehensive Review: Names, Addresses, Time, and the Web on One Page

📌 Key Takeaways

• Every network service boils down to "in which config file, with which directive, declaring what".
• Only the commands differ per service; the rhythm is the same — install → configure → syntax check → start/reapply → check status.
• Instant port recall: DNS 53 · DHCP 67/68 · NTP 123 · HTTP 80 · HTTPS 443. Daemons: bind→named, dhcp→dhcpd, chrony→chronyd, httpd, nginx.
• Instant check-command recall: named-checkconf (DNS) · httpd -t (Apache) · nginx -t (Nginx).
• Three traps: you must bump the Serial when editing a zone, 169.254 is a DHCP failure signal, stratum 0 is the reference clock and 16 is unsynchronized.

The 5 Major Services in One Table — Package, Daemon, Config, Port

Memory anchors:

• Instant port numbers: DNS 53, DHCP 67/68, NTP 123, HTTP 80, HTTPS 443.
• On the RHEL family, Apache's package and daemon are both httpd, and DNS is named (the package is bind).
• Check tools after a config change: named-checkconf (DNS), httpd -t (Apache), nginx -t (Nginx).

Every service rides the same rhythm of "install → edit config file → syntax check → start/reapply daemon → check status". Only the commands differ per service; the flow is identical. Once this rhythm is in your bones, you can handle a service you have never seen the same way.

DNS Review — Records and Zone Files

• SOA 5-timer order: Serial (serial number) → Refresh (check interval) → Retry → Expire (discard limit) → Minimum (negative cache).

⚠️ Trap review: In a zone file, if a name does not end with a dot, the zone name (origin) is appended automatically — the targets of CNAME, NS, and MX are safest with a trailing dot (FQDN). When you modify a zone you must increase the Serial for the secondary server to receive the change. You cannot place a CNAME at the top of the domain (@).

DHCP and NTP Review — Addresses and Time

• The 4 DHCP steps are DORA — Discover → Offer → Request → Ack.
• The first DISCOVER goes out as a broadcast because there is no IP yet, and beyond a router a relay is needed.

• For NTP stratum, the lower the number, the more accurate (0 = reference clock, 1 = primary server, 16 = unsynchronized).
• chrony sets sources with server/pool and the client range with allow in chrony.conf.
• Check status with chronyc sources, chronyc tracking, and timedatectl.

⚠️ Trap review: The DHCP failure signal is 169.254.x.x (APIPA). NTP stratum 0 is not unsynchronized but the reference clock itself, and 16 is unsynchronized. It is safest to keep dhcpd.conf fixed IPs outside the dynamic range.

Web Server Review — Apache vs Nginx Mapping Table

• Virtual hosts: name-based (distinguished by the Host header, many sites on 1 IP) vs IP-based (distinguished by IP, one IP per site).
• HTTP/1.1 making the Host header mandatory made name-based the standard.

Apache is stable with a process per connection (prefork) but uses a lot of memory, while Nginx is event-driven and asynchronous, strong at concurrent connections and static serving. In practice, a combination is common with Nginx in front (reverse proxy, static, TLS) and Apache or an app server behind. A reverse proxy is configured with upstream + proxy_pass, and unlike a forward proxy (acting for clients), it distributes requests in front of the servers.

Summary of Config-Check and Status Commands

# DNS
named-checkconf                         # named.conf syntax
named-checkzone example.com zonefile    # zone syntax
dig www.example.com / dig -x 1.2.3.4    # Forward/reverse lookup
 
# DHCP
systemctl status dhcpd
cat /var/lib/dhcpd/dhcpd.leases         # Lease status
 
# NTP/chrony
chronyc sources                         # Time source list
chronyc tracking                        # Sync accuracy and stratum
timedatectl                             # Time, timezone, NTP sync status
 
# Apache
httpd -t                                # Syntax check
httpd -S                                # Virtual host list
httpd -M                                # Loaded modules
 
# Nginx
nginx -t                                # Syntax check
nginx -s reload                         # Zero-downtime reapply

This week's services are the history of internet infrastructure itself. DNS (1983) solved the limits of the hosts file with a distributed system, DHCP (1993) extended BOOTP to complete IP automation, NTP (1985) synchronized the clocks of distributed systems, Apache (1995) popularized the web, and Nginx (2004) solved the C10K problem and made large-scale services possible. Today no web request is completed without passing through these five.

Integrated Scenario — The Journey of One Request

• From the moment a user types www.example.com until they receive the page, let's connect in one line how this week's services cooperate.

1. (DHCP) At boot, the PC leases an IP, gateway, and DNS server (DORA)
2. (DNS)  The local DNS asks the BIND authoritative server about www.example.com and gets the IP
3. (NTP)  Servers sync their clocks with chrony to keep certificates and logs consistent
4. (Nginx) The front Nginx receives the request, serves static files directly and proxies dynamic ones to the backend
5. (Apache) The backend Apache generates the dynamic content and responds

• These five steps must mesh smoothly for the user to see a single page.
• If any one link breaks, everything stops — DHCP failure = no IP, DNS failure = cannot resolve names, NTP drift = certificate errors, web server down = 503.
• Understanding network services as a "chain" is this week's conclusion.

📖 Glossary

• Zone file : A file gathering the records of one domain zone. It starts with a single SOA.
• Serial : The zone's serial number. You must raise this after editing for secondary servers to pick up the change.
• Trailing dot : The dot after a name in a zone file. Without it, the zone name is appended automatically.
• DORA : DHCP's 4 steps Discover → Offer → Request → Ack.
• DHCP relay : A feature that forwards requests on a client's behalf past a router that broadcasts cannot cross.
• range vs fixed-address : The IP pool to hand out dynamically / a reservation that always gives the same IP to a specific MAC.
• stratum : The distance from the reference clock. 0 is the clock itself, 1 is a primary server, 16 is unsynchronized.
• chronyc tracking : A command that shows sync quality such as the current reference server, offset, and stratum.
• DocumentRoot / root : The actual path the URL root points to. The same concept in Apache and Nginx, with different names.
• Virtual host : Configuration that serves multiple sites from one server. There are name-based and IP-based kinds.
• Reverse proxy : A relay that receives external requests in front of the server and distributes them to internal backends. upstream + proxy_pass.
• .htaccess : The distributed configuration file Apache places in each directory. Nginx does not support it.