ASA NAT Troubleshooting: Stale Manual NAT Overrides Static NAT
Fix an ASA static NAT publish hijacked by a stale manual rule. Follow packet-tracer, NAT hits, syslog 106023, and Zeek S0-to-SF recovery in this lab.
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01Fundamentals
Models, addresses, and the basics that connect everything.
↗ 02Switching & VLANsBuild and troubleshoot Layer 2 networks.
↗ 03RoutingUnderstand forwarding decisions and paths.
↗ 04Security & accessExplore controls, authentication, and firewalls.
↗ 05WirelessFrom WLAN fundamentals to controller operations.
↗ 06Linux & toolsDiagnose and verify from the command line.
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Build a known capture, process it with Zeek, and follow connection UIDs through DNS and HTTP results. Check the records against the packets.
DNS over TLS vs DNS over HTTPS: Test Both with kdig
Query one signed answer over four transports, test certificate identity and DoH paths, then carry a DNSSEC-bogus answer over verified TLS.
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mTLS Client Certificate Rejected? Debug It with OpenSSL
A trusted server can still reject its client. Reproduce three client-certificate failures, compare both sides of TLS, and verify recovery.
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Keep the destination fixed while changing SNI, the expected hostname, and the chain. Verify each fault with strict OpenSSL checks.
IPv6 Packet Too Big: Why TCP Uploads Stall and How to Fix It
Small requests pass while a 64 KiB upload stalls at a 1280-byte link. Restore Packet Too Big feedback and see how a cached PMTU hides the fault.
Linux rp_filter: Diagnose Asymmetric Routing Packet Drops
A captured SYN can still fail source validation before local delivery. Compare strict and loose checks, then fix the route and keep strict mode.
Linux Firewall Allows TCP but It Fails: Route, Listener, NAT
Reproduce a missing route, stopped listener, loopback binding, and later firewall drop. Choose the next check from the packet’s last known point.