Network

A ladder of connectivity checks — from the gateway to DNS and the destination port.

A veth pair is two virtual interfaces joined like a cable; it connects network namespaces inside one machine.

Persistent network configuration on Ubuntu — netplan YAML files, static addresses and DHCP, applying safely.

Key pairs for passwordless login; sshd rejects ~/.ssh if it is writable by the group or by others.

Logical networks over a cable and combining NICs into one interface; the bond mode must match the switch's config.

How a name becomes an IP — resolv.conf, systemd-resolved, /etc/hosts, dig.

HTTP is the web protocol: a client sends a method, path and headers; the server replies with a code, headers, body.

The ping command sends ICMP echo requests to a host and prints replies, latency and loss — the first network check.

Manage interfaces, addresses and routes (iproute2) — the replacement for ifconfig/route.

Shows who is listening on ports and which connections are open; a netstat replacement, faster with many connections.

A client for logging in and copying files over SSH; host aliases in ~/.ssh/config remove repeated connection parameters.

Pre-populate known_hosts with server keys — skip the interactive "yes" on first ssh.

A port scanner: shows which ports of a machine are open, closed or filtered, as another machine sees them.

curl sends HTTP and HTTPS requests from the console: status code, headers, the whole exchange, certificate checks.

Downloads files with resume and recursion; -r without -np and -l has no brakes and can mirror a whole site.

Captures packets to diagnose "is traffic even arriving"; without -n it resolves every IP through DNS and slows down.

Link speed and duplex, error statistics, cable type — NIC diagnostics.

Checking whether ports are reachable, simple data transfer over TCP/UDP.

Shows a domain's or IP's owner and abuse contact; for domains, some fields are hidden by the registrar's privacy shield.

The kernel's connection table — what NAT/firewall see, overflow, debugging.