Linux Networking & Cybersecurity Essentials

System Identity & Hostname

Viewing and Changing Hostname

hostname

sudo hostnamectl set-hostname new-hostname

• hostname: Displays the system's current hostname.

• hostnamectl: Modern tool for persistent change (affects /etc/hostname).

Network Interfaces (ip command)

ip addr show

ip link show

Commands ip link show and ip addr show are used to inspect network interfaces on a Linux system.

• ip link show displays low-level details about each network interface, such as operational status and hardware (MAC) addresses.

• ip addr show includes all of that and adds IP address assignments (IPv4 and IPv6), making it more comprehensive.

Use of ifconfig command is currently not recommended

Interfaces Overview

Interface	Type	Likely Purpose
lo	Loopback	Localhost interface (127.0.0.1)
enp0s31f6	Ethernet NIC	Wired network card
wlp2s0	Wireless NIC	Wi-Fi interface
virbr0	Virtual Bridge	Virtual bridge (e.g., created by KVM/QEMU)

Field Descriptions (ip link show)

2: enp0s31f6: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN mode DEFAULT group default qlen 1000
    link/ether e4:b9:7a:6c:63:f7 brd ff:ff:ff:ff:ff:ff

• 2: Interface index.

• enp0s31f6: Interface name.

• Flags in <>:

  • NO-CARRIER: No physical connection (e.g., cable unplugged).

  • BROADCAST, MULTICAST: Supports broadcast/multicast traffic.

  • UP: Interface is administratively up (enabled).

• mtu 1500: Maximum Transmission Unit in bytes.

• qdisc fq_codel: Queuing discipline (used for traffic shaping).

• state DOWN: Operational status (DOWN = inactive).

• link/ether e4:b9:7a:6c:63:f7: MAC address.

• brd ff:ff:ff:ff:ff:ff: Broadcast address.

IP Address Information (ip addr show)

This command provides all the interface-level details shown by ip link show, and additionally displays assigned IP addresses, both IPv4 and IPv6, along with information about their validity periods.

lo (Loopback)

inet 127.0.0.1/8 scope host lo
inet6 ::1/128 scope host noprefixroute

• IPv4: 127.0.0.1 — the standard local loopback address.

• IPv6: ::1 — loopback address for IPv6.

• valid_lft forever: These addresses never expire.

• Used for internal system communication, always active.

enp0s31f6 (Ethernet)

link/ether e4:b9:7a:6c:63:f7

• MAC: e4:b9:7a:6c:63:f7

• No IP address is assigned.

• Interface is marked DOWN with NO-CARRIER, indicating the cable is likely unplugged or the connection is inactive.

wlp2s0 (Wireless)

link/ether 98:3b:8f:05:90:60 brd ff:ff:ff:ff:ff:ff

inet 192.168.168.87/24 brd 192.168.168.255 scope global dynamic noprefixroute wlp2s0

inet6 2401:4900:640e:743e:3dbf:d692:999c:3c28/64 scope global temporary dynamic 

inet6 2401:4900:640e:743e:980:4d6e:252b:d7d0/64 scope global dynamic mngtmpaddr noprefixroute 

inet6 fe80::91c3:4fa6:62f2:b273/64 scope link noprefixroute

• MAC: 98:3b:8f:05:90:60

• IPv4: 192.168.168.87 — dynamically assigned (likely via DHCP).

• IPv6:

  • Global addresses: dynamically assigned and temporary (privacy extensions).

  • Link-local address: fe80::... used for local network operations.

• Interface is UP and LOWER_UP, indicating a working wireless connection.

virbr0 (Virtual Bridge)

inet 192.168.122.1/24 brd 192.168.122.255 scope global virbr0
link/ether 52:54:00:02:0d:74

• MAC: 52:54:00:02:0d:74

• IPv4: 192.168.122.1 — typically assigned by virtualization tools (e.g., libvirt).

• No IPv6 address.

• Interface is up but has NO-CARRIER, which is expected unless a VM is currently using the bridge.