Tool cheatsheet

Linux networking Cheatsheet

Inspect interfaces, routes, ports and DNS, test connectivity, capture packets and manage firewalls with Linux networking tools.

Interfaces and routes

Show IP addresses — List interfaces and their IP addresses. Add -br for a compact view.
Show interface state — List interfaces and whether they are up.
Show the routing table — List routes, including the default gateway.
Find which route a destination uses — Show the interface and gateway used to reach an address.
Find your public IP address — Ask an external service for your public address.
Show the ARP table — List devices your machine has talked to on the local network.
Add an IP address temporarily — Assign an extra address to an interface. It is lost on reboot.
Bring an interface up or down — Enable or disable an interface.

Check that a host answers — Send four echo requests and report replies.
Trace the path to a host — Show each hop on the way to a destination.
Trace with live statistics — Combine ping and traceroute into a continuously updated report.
Test whether a port is open — Try to connect to a TCP port. -z checks without sending data.
Time each phase of a request — Show how long DNS, connecting, TLS and the first byte took.
Find the path MTU — Send a packet that must not be fragmented. 1472 bytes plus headers makes 1500. Reduce the size until it succeeds.

List listening ports — Show listening TCP and UDP sockets and the processes that own them.
Find what is using a port — Show the socket and process for a port.
Show established connections — List established TCP connections.
Summarize socket counts — Show totals by socket state.
List processes using a port — Show the process that has a port open.
Scan a host's open ports — List open ports on a host. Only scan systems you own or have permission to test.

Look up a record — Query A records. Add AAAA, MX, TXT or NS for other types.
Get just the answer — Print only the answer values.
Query a specific DNS server — Ask a particular resolver, to compare answers.
Trace DNS from the root — Follow delegation from the root servers to the authoritative answer.
Find the name for an IP address — Do a reverse DNS lookup for an address.
Show the system resolver — Show DNS servers per interface on systems using systemd-resolved.
Flush the DNS cache — Clear systemd-resolved's cache.
Override a name locally — Point a host name to an address on this machine only, in /etc/hosts.

Capture traffic on an interface — Print packets on an interface without resolving names.
Capture traffic for a port — Show only traffic to or from port 443.
Save a capture for Wireshark — Write packets to a file you can open in Wireshark.
Show the firewall rules (ufw) — Show the status and numbered rules of Ubuntu's firewall front end.
Allow a port (ufw) — Allow incoming SSH. Allow SSH before enabling the firewall.
Turn the firewall on (ufw) — Enable ufw and start it at boot.
List nftables rules — Show the active nftables ruleset.
List iptables rules — Show the filter table rules with counters and numeric addresses.

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