Day 1 - Network Configuration and Diagnostics Lab: Bring Up an IP by Hand and Open the Route

📌 Key Takeaways

• Network configuration always has two layers: temporary (ip, ifconfig) and persistent (ifcfg-*, nmcli con). If the task says "must survive a reboot," it always means persistent configuration.
• ip command syntax: ip addr add address/prefix dev interface, ip route add default via gateway. default is the default route.
• Even after you change a profile with nmcli con modify, you must reactivate it with nmcli con up for it to take effect. Saving and applying are two separate steps.
• The two entries in ifcfg-* that are easy to leave blank are BOOTPROTO (static/dhcp) and ONBOOT=yes. If ONBOOT is no, the interface does not come up after a reboot.
• Diagnose layer by layer: ip a → ping gateway → ping 8.8.8.8 → dig. The point where it breaks is the cause.
• If ping 8.8.8.8 works but domain names do not, it is a DNS problem. If the gateway responds but the outside world does not, the default route is missing.

Two Layers of Network Configuration: Temporary vs Persistent

• First, burn this into your head: Linux network configuration is always split into two layers.

• An address added with ip addr add disappears at reboot.
• To make it stick, write it into a config file (/etc/sysconfig/network-scripts/ifcfg-*) or an nmcli connection profile.
• When you see a condition like "must persist after reboot" on the exam, always think persistent configuration (a file or nmcli con).

One-line summary: ip/ifconfig are temporary, ifcfg-*/nmcli con are persistent. "Persist after reboot" = persistent configuration.

Working with Addresses, Links, and Routes Using the ip Command

• ifconfig/route are deprecated, and the modern standard is the ip command (iproute2 package).
• The syntax consists of an object and an action.

# Working with addresses
ip addr show                       # Show addresses of all interfaces (= ip a)
ip addr show eth0                  # Only a specific interface
ip addr add 192.168.10.5/24 dev eth0    # Assign an address
ip addr del 192.168.10.5/24 dev eth0    # Remove an address
 
# Link = the interface itself
ip link show                       # List interfaces and their state
ip link set eth0 up                # Bring the interface up
ip link set eth0 down              # Bring it down
 
# Routing
ip route show                      # Routing table (= ip r)
ip route add default via 192.168.10.1        # Set the default gateway
ip route add 10.0.0.0/24 via 192.168.10.254  # Static route to a specific network
ip route del 10.0.0.0/24                      # Delete a route

• The ip command specifies the netmask together with the address as a CIDR prefix, like /24.
• It is easy to confuse this with the old ifconfig syntax such as 192.168.10.5 netmask 255.255.255.0.

Memorize: ip addr add address/prefix dev interface, ip route add default via gateway. default means 0.0.0.0/0 (the default route).

ifconfig and route (legacy, still on the exam)

ifconfig                                   # Show active interfaces
ifconfig eth0 192.168.10.5 netmask 255.255.255.0   # Address + netmask
ifconfig eth0 up                           # Bring up
route -n                                   # Routing table (no name resolution)
route add default gw 192.168.10.1          # Default gateway
route add -net 10.0.0.0 netmask 255.255.255.0 gw 192.168.10.254  # Static route

• ifconfig writes the netmask in dotted decimal, like netmask 255.255.255.0.
• route is written in the form add default gw or add -net ... netmask ... gw.
• You need to tell the syntax of the two tools apart precisely.

nmcli: The Command-Line Gateway to NetworkManager

• Since RHEL 7 the standard manager is NetworkManager, and its CLI is nmcli.
• The decisive difference from the ip command is that configuration made with nmcli is persistent.

# Check status
nmcli                              # Overall summary
nmcli device status                # Per-device status (= nmcli d)
nmcli connection show              # List connection profiles (= nmcli c)
 
# Create a static IP profile
nmcli connection add type ethernet con-name myeth ifname eth0 \
  ipv4.method manual \
  ipv4.addresses 192.168.10.5/24 \
  ipv4.gateway 192.168.10.1 \
  ipv4.dns 8.8.8.8
 
# Modify an existing profile
nmcli connection modify myeth ipv4.addresses 192.168.10.6/24
nmcli connection modify myeth +ipv4.dns 1.1.1.1   # Add a DNS server (+)
 
# Apply (you must reactivate for it to take effect)
nmcli connection up myeth
nmcli connection down myeth

• Key properties: ipv4.method (manual = static / auto = DHCP), ipv4.addresses, ipv4.gateway, ipv4.dns.
• After modifying, you must bring it up again with nmcli connection up (or reload) for it to take effect.

⚠️ Trap: Changing a profile with nmcli con modify does not apply it immediately. You must reactivate it with nmcli con up <profile>. It is the same "saving and applying are separate" pattern as firewalld's --reload.

Writing ifcfg-* Files by Hand (Persistent Config on RHEL/CentOS)

• The file that nmcli ultimately generates, or that you write by hand, is /etc/sysconfig/network-scripts/ifcfg-<interface>.
• It can be asked as a fill-in-the-blank in the lab section, so you need to memorize the directives exactly.

# /etc/sysconfig/network-scripts/ifcfg-eth0  (static IP example)
TYPE=Ethernet
BOOTPROTO=static          # static (or none) = static, dhcp = automatic
NAME=eth0
DEVICE=eth0
ONBOOT=yes                # Activate automatically at boot (the key one!)
IPADDR=192.168.10.5
NETMASK=255.255.255.0     # or PREFIX=24
GATEWAY=192.168.10.1
DNS1=8.8.8.8
DNS2=1.1.1.1

• To get an address via DHCP, set BOOTPROTO=dhcp and leave IPADDR/NETMASK/GATEWAY empty.
• After editing, apply with nmcli con reload → nmcli con up eth0, or systemctl restart NetworkManager.

Fill-in-the-blank example: BOOTPROTO=____ (static/dhcp), ONBOOT=____ (yes). With ONBOOT=no the interface does not come up after a reboot, a classic exam trap.

Persistent DNS Configuration: /etc/resolv.conf

• The nameservers used for name resolution are defined in /etc/resolv.conf.
• However, if NetworkManager manages the interface, DNS1/DNS2 in ifcfg populate this file.

# /etc/resolv.conf
nameserver 8.8.8.8
nameserver 1.1.1.1
search example.com        # Automatic domain completion

Diagnostic Tools: ping, ss, dig, traceroute

• Once you have configured something, diagnostics follow to confirm "does it really work?"
• The lab section expects you to think in terms of diagnosing layer by layer.

# 1) L3 reachability: ping
ping -c 4 192.168.10.1             # Send 4 packets then stop (-c count)
ping -c 4 8.8.8.8                  # Check external reachability
ping -c 4 www.google.com          # Check name resolution and reachability together
 
# 2) Ports and sockets: ss (replaces netstat)
ss -tuln                           # TCP/UDP listening ports (numeric)
ss -tnp                            # Established TCP + process
# t=TCP, u=UDP, l=listen, n=numeric (no name resolution), p=process
 
# 3) Name resolution: dig / nslookup
dig www.example.com                # Detailed DNS lookup
dig www.example.com +short         # Result only
dig @8.8.8.8 example.com MX         # Query a specific server for MX records
nslookup www.example.com           # Interactive/simple lookup
 
# 4) Path tracing
traceroute 8.8.8.8                 # Hop-by-hop path trace

• Combine the ss options -t (TCP) -u (UDP) -l (listen) -n (numeric) -p (process).
• ss -tuln is the standard command for seeing "what ports are open right now."

Diagnostic ladder: ① Is there an IP? (ip a) → ② L2/L3 via ping gateway → ③ External reachability via ping 8.8.8.8 → ④ DNS via ping domain/dig. Where it breaks is the cause.

⚠️ Confusing: If ping 8.8.8.8 works but ping google.com does not, it is a DNS (resolv.conf) problem. IP reachability is fine and only name resolution fails.

Diagnostic Scenario: "The Internet Doesn't Work"

• Here is the lab-style thought process in one place.

Example problem: "ping gateway succeeds + ping 8.8.8.8 fails" → default route is missing. ip route add default via <gw>.

Hostname and Name Resolution Order

• The network lab section also covers setting the hostname and local name resolution.

hostnamectl                          # Show current hostname and OS info
hostnamectl set-hostname web01.example.com   # Change the persistent hostname
hostname                             # Print only the current hostname

• Name resolution follows the order of the hosts: line in /etc/nsswitch.conf.

# /etc/nsswitch.conf
hosts:  files dns         # files (/etc/hosts) first, then dns

• If files comes first, mappings written in /etc/hosts take priority over DNS.
• Write mappings for local testing directly in /etc/hosts.

# /etc/hosts
127.0.0.1   localhost
192.168.10.50   web01.example.com web01

Resolution priority: By default /etc/hosts (files) > DNS (/etc/resolv.conf). If /etc/hosts has a wrong mapping, you end up at the wrong IP even when DNS is healthy, so check it too when diagnosing.

Renewing a DHCP client: To get the lease again, run dhclient -r (release) and then dhclient (request again), or nmcli con down/up. In a NetworkManager environment, nmcli is more consistent.

Try It Yourself

• Run the following yourself and get used to the output (a test machine or virtual machine is recommended).

# Get the current addresses, routes, and sockets at a glance
ip a
ip r
ss -tuln
 
# Add a temporary address, check it, then delete it
ip addr add 10.10.10.10/24 dev eth0
ip addr show eth0
ip addr del 10.10.10.10/24 dev eth0
 
# Diagnose reachability and name resolution
ping -c 3 127.0.0.1          # Loopback (your own TCP/IP stack)
dig +short openai.com

Typing it yourself until your hands remember is the only secret to passing.

📖 Glossary

• Temporary / persistent configuration : Configuration that stays only in memory and disappears at reboot (ip) / configuration saved in a file that persists (ifcfg-*, nmcli con).
• iproute2 : The modern networking toolset that replaces ifconfig and route. Its main command is ip.
• CIDR prefix : Notation that writes the netmask as a bit count, like /24. The ip command uses this form.
• Default gateway (default) : The exit for all packets whose destination is unknown. Set with ip route add default via <IP>.
• NetworkManager / nmcli : The standard network management daemon since RHEL 7 / its command-line tool. Configuration it creates is persistent.
• ipv4.method : The nmcli property that sets the address assignment method. manual is static, auto is DHCP.
• BOOTPROTO : The directive in ifcfg-* that sets the address assignment method. static (or none) and dhcp.
• ONBOOT : The directive that decides whether the interface is brought up automatically at boot. If it is not yes, the interface does not come up after a reboot.
• ss : The command for viewing open sockets and ports (replaces netstat). -t TCP -u UDP -l listening -n numeric -p process.
• dig : A tool that queries a DNS server directly to check name resolution. +short shows only the result.
• nsswitch.conf : The file that sets the lookup order, whether names are looked up in /etc/hosts (files) first or DNS first.
• hostnamectl : The systemd command for viewing and changing the system hostname. set-hostname is applied persistently.