Course Intermediate

Linux Network Administration and Troubleshooting

By the end of this course

Reads and changes a server's network configuration
Narrows a connectivity problem layer by layer and counts the candidate causes each test eliminates
Configures firewall rules and remote access without locking itself out

Start course

Network Configuration

Interfaces and addresses, the routing table, name resolution configuration, and the inventory of listening ports; how many candidate causes each reading eliminates.

01 Interfaces and Addresses The interface list gives three distinct answers against eight candidate causes; two answers alone eliminate seven candidates each, the third eliminates only two and leaves six remaining.
02 The Routing Table The routing table gives three answers against eight candidate causes and leaves five in the worst case; a wrong gateway and a missing route still sit in the same group after two echo tests.
03 Name Resolution The name resolution test, when it gives the expected answer, eliminates only one of eight candidates; when it fails, it eliminates seven, and this asymmetry explains why the course's weakest test is still done.
04 Ports and Listening Services The local listening list eliminates only one of eight candidate causes; a single connection attempt made from outside eliminates seven at once, and across eight faults the total is 14 against 42.

Troubleshooting

The step cost of the layered diagnosis method, correctly reading reachability tests, packet capture's elimination power, and the trace left by intermittent faults.

01 The Layered Diagnosis Method Measuring diagnosis order over eight candidate causes and eight tests: the layered order spends 33 steps, the reverse order 38, and a greedy order that consults the oracle at every step spends 10; the greedy order is not a usable procedure because it requires knowing the answer, so it is an unreachable lower bound.
02 Reachability Tests Reading echo and path-test tools correctly: no response from a remote address fits four separate candidate causes, three reachability tests plus a connection attempt together still cannot split two candidate pairs, and what splits them is reading the configuration.
03 Packet Capture Measuring capture's elimination power: an unfiltered capture produces 14400 lines and eliminates 6 candidates without finishing the diagnosis; a one-line connection attempt eliminates 7 and finishes it. Elimination per line comes out to 0.0004 against 7.0000.
04 Common Fault Patterns The signatures of name-resolution, firewall, and path faults, and evidence vanishing on its own: an intermittent fault lasts 84 seconds in a 600-second window, a recording starting at second 300 catches 42/84, one starting at second 500 catches 13/84, and a dropped packet's trace stays at 0 lines while recording is off.

Access and Filtering

Key-based remote access, tunneling and the jump server, the packet-filtering framework, the default-deny policy, and counting the surface address translation opens.

01 Secure Shell Key-based authentication and server hardening are established; the nine candidate causes of a session failing to open are eliminated with seven tests, and a forty-six-line verbose client output eliminates five candidates against a three-line server log's seven.
02 Tunneling and the Jump Server Splitting the path in two brings the candidate-cause set up to ten; a single end-to-end attempt eliminates eight and leaves three indistinguishable groups between two hops, a test done from the hop eliminates nine, and the tunnel entry point's bind address raises indirect access from zero to seventy-two.
03 The Packet-Filtering Framework The seven hops a packet passes through inside the machine, the table-and-chain distinction, and rule evaluation order are established; a thirty-four-line rule list eliminates zero, a counter reading eliminates zero on a loaded machine and six on a quiet one, and open tracing eliminates eight.
04 Firewall Rules The default-deny policy's five steps are applied in a hundred twenty orders; forty produce a full lockout, twenty a partial one, and because the final rule set is the same across all hundred twenty orders, a dry run prevents zero of them; on writing errors a dry run eliminates five candidates and an end-to-end attempt eliminates five.
05 Address Translation The surface that port forwarding opens outward is counted point by point: four rules open thirteen points and reach eight pairs today, the translated log line eliminates zero of twelve candidate sources, and at the six-hundredth second only eleven of forty translation entries remain resolvable.