Linux networking

Addresses, routes and names 4 topics

• Entry check: are you ready for networking?
  A forty-five minute check of the command-line skills this course builds on, with a pointer to the Linux from zero topic to revisit for each one.
• Interfaces, addresses and subnets
  What an interface and an address are, how the prefix splits an address into network and host, the size of the common subnets, private ranges, and how to add an address…
• Routes and the gateway
  How a computer decides where to send a packet: the connected and default routes, longest-prefix matching, ip route get, static routes, the symptoms of missing and wrong…
• Names: the hosts file and DNS
  How a name becomes an address: the hosts file and name servers in order, the common record types, dig versus getent, NXDOMAIN, split DNS, and the usual reasons two…

Is it alive? 4 topics

• Is it alive? ping and traceroute
  What ping and its output mean (loss, round-trip time, Destination Host Unreachable versus silence, name failures), why no reply is a clue and not a verdict, traceroute…
• Ports and sockets: ss and nc
  Ports as doors, listening sockets with ss, exposed versus local-only addresses, testing a door with nc, and the difference between succeeded, refused and timed out.
• HTTP with curl, and TLS
  Requests and responses, status codes by class, curl options for headers, redirects and codes, https and certificates (self-signed, expired), and why -k is only for…
• A layered troubleshooting method
  Link, address, gateway, outside, names, port and application as a ladder; testing from the bottom up and stopping at the first failure; two faults at once; one change at…

Services across the network 4 topics

• ssh: logging in, keys and copying
  Remote shells and commands with ssh, the host key check and knownhosts, key pairs with ssh-keygen and ssh-copy-id, ~/.ssh/config aliases, scp, and the changed-host-key…
• DNS answers, servers and split DNS
  Reading a full dig answer (status, TTL, record, server), asking one server with dig @server, split DNS, nslookup, getent and resolvectl, and caching and TTL.
• DHCP and time
  DHCP as discover, offer, request, acknowledge, the content of a lease and its renew and rebind times, why DHCP fails, static addresses outside the pool, and keeping…
• Watching the wire: tcpdump and nmap
  tcpdump lines and TCP flags (the handshake, refusal versus silence), the usual options, nmap and the states open, closed and filtered, comparing scans over time, and…

Firewalls, NAT and VPN 4 topics

• Host firewalls with iptables
  A firewall as an ordered list of rules with a default policy, rules with matches and targets, append, insert and delete, DROP versus REJECT, why the order matters, the…
• ufw: a firewall in plain words
  ufw as readable rules on the same firewall, default deny, the safe order (defaults, ssh, services, enable, check), restricting by source, limit and logging, deleting by…
• The gateway: forwarding, NAT and port forwarding
  Forwarding and ipforward, private and public addresses, masquerade (source NAT) and its table, destination NAT as port forwarding, the usual failures, and making changes…
• A VPN with WireGuard
  A VPN as a tunnel with its own interface, key pairs as identity, the client and server files (Interface, Peer, Endpoint, AllowedIPs, keepalive), wg-quick and wg show…

Infrastructure behind the network 4 topics

• The network map: components and the path of one request
  The parts of the office network (switch, gateway, name server, DHCP and time, firewall, VPN), a card for each with role, resource profile, failure modes, sizing…
• Capacity: bandwidth, latency and the cost of waiting
  Bandwidth versus latency, two cards (the access link and the connection table), utilisation, transfer time and the bandwidth-delay product, measuring delay, loss, link…
• Failure, redundancy and recovery
  A failure table (failure, symptom, first check, safe action), what a second copy does to availability, why redundancy is not a backup, keeping configuration copies off…
• Operating it: monitoring, documentation and change
  Checks, scripts and cron, alerts that say what is wrong, how long and what first, waiting for several readings, heartbeats for the monitor, the diagram and address…

Interview ready 2 topics

• Interview ready: addresses, routes and names
  The questions people really ask about addresses and masks, routing and the default gateway, the order of a name lookup, ping and traceroute, with model answers, a…
• Interview ready: services, firewalls and a mock round
  The questions people really ask about firewalls, ssh, NAT and forwarding, VPNs, time, availability and monitoring, with model answers, strong and weak answers, four…

The network troubleshooting notebook

A notebook of five faults from the simulated office (a wrong gateway, a mistyped name server, a stale hosts line, a closed port and a firewall in the wrong order), each with the ladder of checks, the first failure and the fix.

The access and firewall pack

A written ssh key setup, a default-deny firewall for the web server, a database restricted to the office, a port forward, and a WireGuard client with proof of a handshake.

The network map and handover

A card for each part of the network, a capacity sheet with arithmetic, a failure table with a drill, a change log, and a one-page handover.

Why take this course

• Trace a fault from the cable up
• Know what is in force, not just what you typed
• Protect and publish services without locking yourself out

You will build

• A troubleshooting notebook
• An access and firewall pack
• A network map and handover

What you can do afterwards

• Run a small network with confidence
• Explain your choices and their risks
• Answer networking interview questions with specifics