Code-Memo

VPN concepts (IPSec, OpenVPN, WireGuard)

What a VPN does

Creates an encrypted tunnel over an untrusted network
Often also provides routing (full-tunnel vs split-tunnel)

Common modes

Site-to-site: connect networks (office ↔ cloud VPC)
Remote access: connect a user device to a network

IPSec (high level)

Often used for site-to-site
Components: IKE, ESP/AH, policies, SA (security association)
Common tooling: strongSwan

OpenVPN (high level)

Runs in user space over UDP/TCP
Mature ecosystem, flexible auth

WireGuard (high level)

Modern, minimal codebase, fast
Uses public keys; simpler configuration
Interfaces often named wg0

Typical troubleshooting

Routing: ip route, ip rule
DNS: split DNS and search domains
MTU: reduce MTU if things “hang” over tunnel

Useful commands

ip addr show
ip route
wg show 2>/dev/null

Routing basics