Network

Interfaces and addresses

Command	Shows
`ip -br a`	Interfaces with state and addresses, one per line.
`ip r`	Routing table, including the default gateway.
`ip route get 1.1.1.1`	Which interface and gateway a packet to that address uses.
`ip neigh`	ARP/neighbor cache (replaces `arp`).
`hostname -I`	All local IP addresses.
`ethtool eth0`	Link speed, duplex and whether a cable is detected.

Connectivity and DNS

When something is unreachable, test from the bottom up; the first step that fails is where to look.

Step	Test	Checks
1	`ip -br a`	The link is up and has an address.
2	`ping <gateway>`	The local network works.
3	`ping 1.1.1.1`	Routing to the internet works.
4	`dig example.com`	Name resolution works.
5	`nc -zv host 443`	The service itself answers.

Command	Effect
`ping -c 4 host`	Sends four ICMP echo requests.
`tracepath host`	Shows the route and where packets get lost (`mtr` live).
`dig example.com`	DNS lookup; `+short` for just the answer, `@1.1.1.1` for another server.
`dig -x 192.0.2.1`	Reverse lookup.
`resolvectl status`	DNS servers in use with systemd-resolved.
`nc -zv host 443`	Tests whether a TCP port is open.
`curl -I https://example.com`	Fetches only the HTTP response headers.
`wget https://example.com/file`	Downloads a file.

Ports and connections

Command	Shows
`ss -tulpn`	Listening TCP and UDP ports with the owning process.
`ss -tupn`	Established connections.

ss options: -t TCP, -u UDP, -l listening, -p process (needs root for other users’ processes), -n numeric ports instead of service names.

Scanning with nmap

Command	Effect
`nmap -sn 192.168.178.0/24`	Finds hosts in a network without port scanning.
`nmap -sS --top-ports 100 192.168.178.0/24`	Scans the 100 most common ports (needs root).
`nmap -sV 192.168.178.151`	Detects service versions on open ports.
`nmap -Pn 192.168.178.151`	Scans even if the host doesn’t answer ping.

NetworkManager

Command	Effect
`nmcli device`	Devices and their connections.
`nmcli connection show <name>`	All settings of a connection, e.g. DHCP details.
`nmcli device wifi list`	Visible wireless networks.
`nmcli device wifi connect <ssid> --ask`	Connects to a wireless network.
`nmcli connection add type bridge ifname br0 con-name br0`	Creates a bridge.
`nmcli connection modify br0 bridge.stp no ipv4.method manual ipv4.addresses 10.0.0.1/24`	Configures it.
`nmcli connection up br0`	Activates it.

Changing the MAC address

ip link set dev eth0 down
ip link set dev eth0 address aa:bb:cc:dd:ee:ff
ip link set dev eth0 up

The change is lost on reboot. With NetworkManager, set 802-3-ethernet.cloned-mac-address (or wifi.cloned-mac-address) on the connection instead; the value random picks a new address on every connect.