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Building IPsec S2S tunnels between an Azure workload (Linux VM or VNet) and an on-premise datacenter behind a Mikrotik router. Covers Strongswan on the Azure VM, IPTables for SNAT variants, Azure VPN Gateway with LAN and SNAT scenarios, plus the ports and protocol numbers (ESP 50, AH 51, UDP 500, UDP 4500) that must clear every hop.