Linux networking cheat sheet

The chapter on one page: what to remember, the commands to have at your fingertips, and flashcards for quick revision.

The chapter in eight sentences

A host is identified on the network by an address with a prefix, reaches other networks through a gateway, and asks a DNS server for names.
ip -br addr, ip route and ip neigh show addresses, routes and neighbours; ss -tulpn shows what listens, and on which address.
ping proves an address answers; it does not prove that a service works.
NetworkManager separates devices (hardware) from connection profiles (saved settings), stored in /etc/NetworkManager/system-connections/.
nmcli connection add|modify|up|down|delete builds persistent configuration; modify edits the profile and up applies it; ip addr add is lost at reboot.
hostnamectl set-hostname sets the host's own name; names of other hosts are resolved in the order of nsswitch.conf: /etc/hosts, then DNS.
getent hosts shows what programs will see; dig asks DNS only. A stale /etc/hosts line can override DNS.
Troubleshoot upward: link, address, gateway, target IP, name, port. "No route to host" with a working ping suggests a firewall, "Connection refused" means nothing listens there.

Command	Does
`ip -br addr`, `ip -br link`	Addresses, links and MACs
`ip route`	Routes and default gateway
`ip neigh`	Neighbours
`ping -c 2 HOST`	Reachability
`tracepath -n ADDR`	Path hop by hop
`ss -tulpn`	Listening ports and programs
`curl -sS -m 5 URL`	Test a service, show the error

Command	Does
`nmcli device status`	Devices
`nmcli connection show [NAME]`	Profiles / one profile
`nmcli con add type dummy ifname I con-name N ipv4.method manual ipv4.addresses A/24`	New static profile
`nmcli con modify N +ipv4.addresses A/24 ipv4.dns D ipv4.gateway G`	Edit the profile
`nmcli con up N` / `down N` / `delete N`	Apply / remove settings / delete
`nmcli -g PROP con show N`	One value

Command / file	Does
`hostnamectl`, `hostnamectl set-hostname N`	Show / set the name
`/etc/hosts`	Local names, searched first
`/etc/resolv.conf`	DNS servers and search domain (managed by NetworkManager)
`/etc/nsswitch.conf`	`hosts: files dns myhostname`
`getent hosts N`	What programs resolve
`dig +short N`, `dig -x ADDR`, `host N`	DNS only

What do address, prefix and gateway mean?: Address: this interface. Prefix: how many bits are the network (/24 = 255.255.255.0). Gateway: the router for other networks.
Show addresses, routes, listeners?: ip -br addr · ip route · ss -tulpn
List devices and connection profiles?: nmcli device status · nmcli connection show
Create a static profile on a dummy interface?: nmcli con add type dummy ifname dummy0 con-name dummy0 ipv4.method manual ipv4.addresses 10.20.30.1/24
Change a profile and apply it?: nmcli con modify NAME +ipv4.addresses A/24 · nmcli con up NAME
Where are NetworkManager profiles stored?: /etc/NetworkManager/system-connections/*.nmconnection
Show or set the host name?: hostnamectl · hostnamectl set-hostname NAME
Order of name sources?: nsswitch.conf: files (/etc/hosts), then dns (resolv.conf), then myhostname.
See what programs resolve a name to, and ask DNS directly?: getent hosts NAME · dig +short NAME
"No route to host" with a working ping?: A firewall rejects the port.
"Connection refused"?: Nothing listens on that port or address: check ss -tlnp on the server.
Troubleshooting order?: Link, address, gateway, target IP, name, port.