Network fault challenges: fix broken networks
In every lesson a broken network is waiting; hints are few, evidence is plenty. You find the fault, fix it and write a short report.
Level 201 · 18 lessons · 15 practice labs
Lessons
Addresses and routes
- One IP on two machines
- The gateway is mistyped
- Overlapping subnets
- It goes out but never comes back
- Short task: put the four faults of round one into one report
Services
- A stale DNS record and TTL
- The address pool is exhausted
- Permission denied on an NFS share
- The “host key has changed” warning
- Failed logins caused by clock skew
- Short task: rank the service faults by impact and fix them
Performance and hidden problems
- Large files will not cross the VPN
- A slow link: speed and duplex mismatch
- The network suddenly froze: a loop
- Some machines get the wrong address
- A stateful firewall and an asymmetric path
- Short task: write a short report with root-cause analysis
Wrap-up
- Capstone: solve a full day of fault tickets and hand over
Practice labs
- The new server was left in the wrong VLAN: Find that the switch port of a new server nobody can reach is assigned to the wrong VLAN, move it to the right one and show a client on its subnet reaching it.
- The printer was replaced and nobody can print: Find the neighbour entry pinned to the old device’s MAC address on the print server, remove it and prove with a capture that frames to the new printer carry the right MAC address.
- After the reboot the floors cannot see each other: On Ekin Co-op’s router, find the forwarding setting lost at reboot and the route that was never written to the persistent configuration, make both permanent and show they survive a restart of the network service.
- Everything is slow: traffic leaves over the backup link: Show that of two default routes the slow backup link wins with the lower metric, fix the metrics and prove with traceroute that traffic leaves over the main link.
- One subnet has been missing since the maintenance: Find in the router’s table the blackhole route left over from maintenance that swallows traffic to just one subnet, remove it and show the server on that subnet is reachable again.
- The new floor reaches internal servers but not the internet: For Orchid Hotel’s new floor, prove with packet counters that the address-translation rule covers only the old subnet and that the forward chain lacks the new one, fix both and show access to the outside test server.
- It gets an address but cannot resolve any name: Find the DHCP option that hands clients a decommissioned DNS server, fix it and show a client that renews its lease resolving internal names.
- Devices in the new VLAN get no address: Show with a capture that discover packets from a new VLAN, whose DHCP server sits on another subnet, stop at the router, add the relay agent and the missing scope and prove a client gets an address.
- Every other attempt goes to the old server: Find why a name at Tulip Pharmacy sometimes resolves to the old address by comparing the serial numbers on the two DNS servers, repair the broken zone transfer and show both servers giving the same answer.
- The admin network moved and SSH shut: Find the server firewall rule that allows SSH only from the old admin subnet, update it for the new subnet and show access from other subnets is still closed.
- The connection is reset at once: service or firewall?: Prove whether the reset comes from the server or from the firewall in between by comparing captures at two points and their TTL values, fix the rule responsible and show the connection being established.
- The portal opens on some computers and hangs on others: Find why Harborview Hospital’s internal portal hangs on dual-stack clients (an AAAA record pointing at a server that does not listen on IPv6), fix the record or the service and show by measurement that both kinds of client get in without delay.
- Calls keep breaking up: which hop is losing packets?: Use mtr to find the hop where loss begins along the path, tell it apart from hops that merely rate-limit ICMP and write the result with evidence to the answer file.
- Fast link, crawling backup: a forgotten rate limit: Find the traffic-shaping rule left on the backup server’s interface by a test, remove it and show the throughput before and after with iperf3.
- New connections fail around midday: Prove from the kernel log that the connection-tracking table on Kuzey Logistics’ gateway fills up, find the internal device filling it, stop or limit the source and show new connections being established under load.
About this course
In every lesson a broken network is waiting; hints are few, evidence is plenty. You find the fault, fix it and write a short report.
What you will learn
- Addresses and routes
- Services
- Performance and hidden problems
- Wrap-up
Prerequisites
Finishing the level 101 courses of the same path, or knowing those topics, is enough.
Lab environment
Lessons run in your own Kali Linux terminal in the browser, connected to a private lab network. Nothing to install.
Frequently asked questions
- Is this course right for my level?
- The course is at level 201; it assumes you know the topics of the lower-level courses.
- Is there anything to install?
- No. Every lesson runs in your own lab environment that opens in the browser.
- How do lessons work?
- Each lesson has 8 to 10 steps and a short quiz. Early steps show the command to type, later steps give less help; if two attempts fail, the solution opens.
- Can I try it for free?
- Yes. The first three lessons of every course are free.