VPN

This skill covers all VPN technologies, providing cross-platform expertise in IPsec/IKEv2, WireGuard, SSL/TLS VPN, remote access architectures, site-to-site design, and cryptographic algorithm selection. Sibling technology skills provide deep implementation details.

When to Use This Skill vs. a Technology Skill

Use this skill when the question is cross-platform or architectural:

• "Which VPN technology should I use for our remote workforce?"
• "Compare IPsec vs WireGuard for site-to-site"
• "Design a VPN architecture for 50 branch offices"
• "What crypto algorithms should I use in 2026?"
• "IPsec vs SSL VPN -- trade-offs?"

Route to a technology skill when the question is implementation-specific:

• "Configure IKEv2 tunnel between FortiGate and ASA" --> the ipsec skill
• "WireGuard AllowedIPs routing" --> the wireguard skill
• "Cisco Secure Client SAML authentication" --> the cisco-secure-client skill

How to Approach Tasks

1. Classify the request:

   • Architecture / Design -- Load references/concepts.md for protocol fundamentals
   • Technology selection -- Compare options in the platform comparison below
   • Algorithm selection -- Apply CNSA guidance below
   • Troubleshooting -- Identify the VPN technology and consult the specific technology skill
   • Migration -- Map current to target technology, identify feature gaps

2. Gather context -- Use case (site-to-site vs remote access), scale, existing infrastructure, compliance requirements, team expertise, client platform support

3. Analyze -- Consider security, performance, operational complexity, and interoperability

4. Recommend -- Provide specific guidance with trade-offs

VPN Technology Comparison

IPsec / IKEv2

Strengths:

• Industry standard with widest interoperability (every enterprise firewall supports it)
• Hardware acceleration on most platforms (FortiASIC, PAN-OS SP, Cisco ASIC)
• Strong crypto flexibility (AES-GCM, SHA-384, ECP-384, PQC transition path)
• Mature protocol suite with RFC standards (RFC 7296, 4303, 3948)
• Route-based VPN (VTI) supports dynamic routing protocols over tunnels

Considerations:

• Complex configuration with many knobs (proposals, transforms, DH groups, traffic selectors)
• Interoperability issues between vendors (proposal mismatch is the most common failure)
• Requires UDP/500 and UDP/4500 (or ESP protocol 50) -- can be blocked by restrictive firewalls

Best for: Site-to-site VPN between enterprise firewalls, hub-and-spoke with dynamic routing, DMVPN/ADVPN, compliance-driven environments requiring CNSA-grade cryptography.

WireGuard

Strengths:

• Simplest configuration of any VPN technology (~4,000 lines of kernel code)
• Highest performance (kernel-space, ChaCha20-Poly1305, near-wire-speed)
• 1-RTT handshake (<100ms connection establishment)
• Fixed cryptography (no negotiation = no downgrade attacks)
• Built into Linux kernel 5.6+; cross-platform support

Considerations:

• No built-in user authentication (key-based only; needs external layer for SSO)
• No built-in key distribution (manual or external orchestration required)
• UDP only (can be blocked; no native TCP fallback)
• No dynamic IP updates without external tooling
• No enterprise management features natively (no centralized management, no logging)

Best for: Linux-to-Linux site-to-site, developer/IT team VPNs, container networking (Calico/Flannel), mesh VPN (Tailscale/Headscale/Netmaker), simple road warrior setups.

Cisco Secure Client (AnyConnect)

Strengths:

• Full enterprise remote access platform with VPN + posture + NAC + ZTNA
• DTLS for optimal performance (avoids TCP-over-TCP)
• Deep integration with Cisco ecosystem (ISE, FMC, Umbrella, XDR)
• SAML authentication with system browser (SSO, biometric, hardware tokens)
• DAP (Dynamic Access Policies) for granular per-session access control
• Always-On VPN, split tunneling, per-app VPN (mobile), TND

Considerations:

• Requires Cisco headend (ASA or FTD)
• Client software required on endpoints
• Complex licensing model (Advantage, Premier, Secure Access tiers)
• Primarily remote access (not site-to-site)

Best for: Enterprise remote access with full endpoint compliance, organizations deep in the Cisco ecosystem, environments requiring per-session dynamic access control.

Site-to-Site vs Remote Access

Aspect	Site-to-Site	Remote Access
Endpoints	Firewall-to-firewall	Client device-to-firewall
Protocol	IPsec IKEv2, WireGuard	SSL/DTLS, IPsec IKEv2, WireGuard
Routing	Static or dynamic (BGP/OSPF over tunnel)	Full tunnel or split tunnel
Authentication	PSK or certificates (device-level)	User credentials + MFA + posture
Scale	Hundreds of tunnels per hub	Thousands of concurrent users
Topology	Point-to-point, hub-spoke, mesh	Hub-spoke (client to gateway)

Crypto Algorithm Selection (2025/2026)

Current Recommended (CNSA 1.0)

Component	Algorithm	Notes
IKE Encryption	AES-256-GCM or AES-256-CBC	GCM preferred (AEAD)
IKE Integrity	SHA-384	With GCM, use explicit PRF instead
PRF	PRF_HMAC_SHA2_384	Required with AEAD encryption
DH Group	Group 20 (ECP-384/P-384)	Group 19 (P-256) minimum
IPsec Encryption	AES-256-GCM-16	AEAD, no separate integrity needed
IPsec Integrity	SHA-256 or SHA-384 (with CBC)	Not used with GCM
PFS	Group 20 (ECP-384)	Always enable PFS on Child SAs

Post-Quantum Transition (CNSA 2.0, by 2033)

Component	Algorithm	Notes
Key Exchange	ML-KEM-1024 (FIPS 203)	Hybrid: ML-KEM-1024 + ECP-384 during transition
Digital Signatures	ML-DSA-87 (FIPS 204)	For certificate authentication
Hash/PRF	SHA-384	Still compliant

Avoid

• DH Groups 1, 2, 5, 22, 23, 24 (all weak/broken)
• MD5 or SHA-1 for integrity/PRF
• DES or 3DES encryption
• IKEv1 (disable where possible)

VPN Architecture Patterns

Point-to-Point (Site-to-Site)

Two gateways with fixed peer IPs. Route-based (VTI) preferred over policy-based (crypto map).

Hub-and-Spoke

Central hub with multiple spokes. Spoke-to-spoke traffic traverses hub unless shortcuts configured (DMVPN Phase 3, ADVPN).

Dynamic Mesh (DMVPN / ADVPN)

• DMVPN (Cisco): mGRE + IPsec + NHRP + dynamic routing. Scales to thousands of spokes.
• ADVPN (Fortinet): IKEv2 extensions for automatic spoke-to-spoke shortcuts.
• Both allow direct spoke-to-spoke tunnels on demand while hub facilitates initial contact.

Full Mesh

Every site connected to every other site. Scales as O(n^2). Only practical for <10 sites.

Common Pitfalls

1. Proposal mismatch: The most common IPsec failure. Both sides must have at least one overlapping set of encryption, integrity, DH group, and PRF algorithms.
2. PFS mismatch: One side requires PFS with specific DH group; other has PFS disabled. Fails silently or with TS_UNACCEPTABLE.
3. Traffic selector mismatch: Initiator proposes 10.1.0.0/24 but responder expects 10.0.0.0/8. Check both sides.
4. NAT-T not enabled: IPsec fails when NAT device is between peers. Enable NAT-T (UDP/4500) on all configurations.
5. Clock skew with certificates: Certificate validation fails if time differs >5 minutes. NTP is critical.
6. WireGuard key management at scale: Without orchestration (Tailscale, Netmaker, Ansible), manual key distribution becomes unmanageable beyond ~20 peers.

Technology Routing

Request Pattern	Route To
IPsec, IKEv2, site-to-site, crypto proposals, DMVPN, VTI	the ipsec skill
WireGuard, wg-quick, AllowedIPs, Tailscale, Netmaker	the wireguard skill
Cisco Secure Client, AnyConnect, DTLS, DAP, TND, posture	the cisco-secure-client skill

Reference Files

• references/concepts.md -- IPsec protocol suite, IKEv2 exchange mechanics, ESP, tunnel vs transport mode, crypto algorithms, PFS, DPD, NAT-T. Read for protocol fundamentals.