Network

Network diagnostics

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

ping — check whether packets reach a host

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

ip — addresses, interfaces and routes

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

The veth pair and network namespaces

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

Persistent networking with netplan

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

ss — sockets and ports

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

sysctl — kernel parameters at runtime

sysctl reads and changes kernel parameters; -w lasts until reboot, a file in /etc/sysctl.d/ makes it permanent.

lsof — open files and ports of processes

lsof shows which process holds a file, a port, or a deleted file that is still open.

nginx — web server and reverse proxy

nginx serves sites from disk and forwards requests to applications; it picks a site by Host, nginx -t checks the config.

rsyslog — system logs

Writes text logs to /var/log and forwards them over the network; it does not rotate them itself, that's logrotate's job.

Firewall — nftables / ufw

The Linux firewall: filtering traffic with simple ufw and low-level nftables, checking the rules.

Unix sockets — processes talking through a file

A unix socket is a file through which processes on one machine exchange data; connecting needs write permission on it.

The classic iptables firewall

The classic Linux firewall — chains, rules, NAT, and why nftables replaced it.

ssh, scp — connecting and copying over SSH

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

sshd — the SSH server

The OpenSSH server; edit the config with a second session open — a mistake in PermitRootLogin locks you out immediately.

SSH keys and ssh-agent

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

VLANs and interface bonding

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

NFS network filesystem

Network directories over NFS — exporting on the server, mounting on the client, permissions.

ssh-keyscan — collecting host keys

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

DNS and name resolution

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

PostgreSQL — access, pg_hba.conf, dump and restore

PostgreSQL is a relational DBMS: roles and databases, pg_hba.conf access rules, pg_dump dumps and pg_restore restores.

nmap: which ports are visible from outside

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

rsync — file synchronization and copying

Copies only the differences between trees; --delete mirrors the source into the destination, removing anything extra.

HTTP — requests, responses, status codes and headers

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

curl — HTTP requests from the console

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

wget — downloading files over the network

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

tcpdump — capturing and analyzing network traffic

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

ethtool — network card settings

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

Docker — containers: running, ports, volumes, networks

Docker runs programs in isolated containers started from images and manages their ports, volumes and networks.

nc — checking ports and transferring data

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

docker compose — a container stack in one file

docker compose describes a container stack with network, volumes and variables in compose.yaml and starts it at once.

whois — domain and IP data

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

TLS and certificates — openssl, the chain, verification

A TLS certificate binds a server key to a site name; openssl reads its fields, checks the chain and issues a new one.

conntrack — connection tracking

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

chrony: time synchronization over NTP

An NTP client and server: keeps the system clock in step with time servers and can serve time to your machines.

Prometheus — metrics collection

Collecting metrics with a pull model — targets, PromQL, exporters, alerts.

WireGuard — VPN on keys

A fast VPN on a key pair instead of passwords; the private key in wg0.conf needs permissions, like an SSH key.

Samba — file sharing over SMB

An SMB server for Windows-compatible shares; the SMB password is separate from the system one, easy to forget to set.

KVM and libvirt

Virtual machines on Linux — managed with virsh, networks, storage, console.

Postfix — a mail server

Sending and receiving mail — config, the queue, logs, typical delivery debugging.

BIND9 — DNS server

Running your own DNS — zones, records, config checks, resolving debug.

DHCP server

Handing out addresses on a network — pool, reservations, leases, diagnostics.

HAProxy — load balancer

Load balancing and proxying TCP/HTTP — backends, health checks, statistics.

keepalived — high availability with VRRP

High availability through a virtual IP — failover between nodes using VRRP.