VPN & IPSec

Why this matters

Module 05's firewalls decide what's allowed through, and TLS (TLS, SSL and HTTPS) protects one connection's payload. Neither solves a different, common need: connecting two entire private networks — your office LAN and your cloud VNet, or a remote employee's laptop and your company network — as if they were on the same private network, across the public internet, where anyone in between could otherwise read or tamper with every packet. A VPN (Virtual Private Network) is that connection: an encrypted tunnel carrying private traffic across an untrusted network. This is also directly the mechanism behind Azure's VPN Gateway and every "connect on-prem to the cloud" story in the next track — understand it here as pure networking, and the Azure version is just this with a managed control plane.

Concepts

What a VPN actually does: tunneling

A VPN's core trick is tunneling: taking an entire original packet (source, destination, payload — often using private addresses that aren't routable on the internet at all, exactly module 01's private ranges) and wrapping it inside a new outer packet whose source and destination are the two VPN endpoints' public addresses. The outer packet is what actually traverses the internet; at the far end, the VPN device strips the outer wrapping and delivers the original inner packet onto the remote private network. From either private network's point of view, the other network is just... there, reachable, the way module 05 described a directly-connected subnet — the tunnel makes physically distant networks behave as logically adjacent ones. Encryption is a separate, additional property layered on top of tunneling (you can tunnel without encrypting — GRE does this — but a VPN specifically adds encryption so the tunneled traffic is unreadable in transit).

Site-to-site vs remote-access VPNs

Site-to-site VPN — connects two entire networks (your office and your cloud VNet, or two office branches) via VPN devices/gateways at each site. Every host on either private network can reach every host on the other, transparently, with no per-device VPN client needed — the tunnel is between the networks, not between individual machines.
Remote-access VPN — connects a single device (an employee's laptop) into a private network. The device runs VPN client software, connects to a VPN gateway/concentrator, and — once connected — behaves as if it's physically on that private network, able to reach internal-only resources it couldn't reach from the open internet.

Both use the same underlying tunneling/encryption mechanisms; the difference is entirely about what's on each end of the tunnel — a whole network's gateway device, or one client machine.

IPSec: the standard framework for encrypting IP traffic

IPSec (IP Security) isn't one protocol but a suite — a standard, widely-implemented framework most site-to-site VPNs (and many remote-access ones) are built on. Two protocols within it matter most:

AH (Authentication Header) — guarantees the packet wasn't tampered with and really came from who it claims (integrity + authentication), but does not encrypt the payload — the data itself is still readable in transit. Rarely used alone in practice, because "tamper- proof but readable" isn't what most VPN use cases actually need.
ESP (Encapsulating Security Payload) — provides integrity and authentication and encryption (confidentiality) — the payload is actually unreadable to anyone intercepting it. This is what almost every real-world IPSec VPN uses; AH-only deployments are uncommon.

IPSec also runs in two modes, which decide how much of the original packet gets wrapped:

Transport mode — encrypts only the payload of the original IP packet, leaving the original IP header exposed. Used for host-to-host protection where both ends are the actual traffic's real source/destination — less common for VPNs specifically.
Tunnel mode — encrypts the entire original packet (header included) and wraps it inside a brand-new outer IP packet with the VPN gateways' own addresses. This is what site-to-site VPNs use: it's what lets private, non-routable inner addresses traverse the public internet safely inside a routable outer packet, exactly the tunneling behavior described above.

IKE: how two devices agree on encryption without a pre-shared secret sent in the clear

Before ESP can encrypt anything, both VPN endpoints need to agree on encryption keys and algorithms — and they have to do that negotiation itself securely, without ever transmitting the actual shared key in the open. IKE (Internet Key Exchange) is the protocol that does this, typically in two phases: Phase 1 establishes a secure, authenticated channel between the two endpoints themselves (using Diffie-Hellman key exchange — the same category of "agree on a shared secret over a public channel without ever transmitting the secret itself" trick TLS's handshake uses); Phase 2 uses that now-secure channel to negotiate the actual IPSec (ESP) parameters — the keys and algorithms that will encrypt the real tunneled traffic. The practical takeaway: a VPN connection has two distinct negotiation layers (IKE, then IPSec/ESP) with their own separate configuration and their own separate ways to fail — "IKE Phase 1 failed" and "Phase 2/IPSec failed" are genuinely different troubleshooting situations, not interchangeable "the VPN is down."

SSL/TLS VPNs: the browser-friendly alternative

Not every VPN uses IPSec. SSL VPNs (sometimes marketed as "TLS VPN") use the same TLS mechanism that HTTPS uses instead of IPSec's separate IKE/ESP machinery — commonly delivered as a browser-accessible portal (clientless, reaching specific internal web apps through a gateway) or a lightweight client that tunnels traffic over a standard TLS connection. The practical advantage: TLS traffic on port 443 passes through far more restrictive firewalls/proxies than IPSec's protocols and ports typically do, since it's indistinguishable at the network level from ordinary HTTPS traffic — a real, common reason organizations choose SSL VPN specifically for remote-access scenarios where users connect from unpredictable, locked-down networks (hotel wifi, a client's corporate firewall).

The classic VPN failure mode: overlapping private ranges

Module 05 already warned about this in passing; it's worth stating directly here because VPNs are where it bites hardest. Two private networks connected by a VPN — say, your office (192.168.1.0/24) and a partner's office (also, coincidentally, 192.168.1.0/24) — cannot be unambiguously routed once tunneled together: a packet destined for 192.168.1.50 could mean either network's host. This is not a VPN misconfiguration to "fix" with a setting — it requires re-addressing one of the two networks (or, in some products, NAT applied specifically at the VPN boundary to translate the overlap away) before the tunnel can work correctly. Always check both sides' address ranges before building a site-to-site VPN, not after it mysteriously misroutes.

Command reference

This module's hands-on work uses strongSwan (a widely-used open-source IPSec implementation) on WSL2 Ubuntu, connecting two network namespaces the way earlier modules built small labs.

Command	What it does	Example
`sudo apt install strongswan`	Installs the strongSwan IPSec daemon and `ipsec` CLI	`sudo apt install strongswan`
`sudo ipsec status`	Shows current IPSec connection states	`sudo ipsec status`
`sudo ipsec statusall`	Detailed status including IKE/ESP algorithm negotiation	`sudo ipsec statusall`
`sudo ipsec up <name>`	Manually brings up a configured tunnel	`sudo ipsec up site-to-site`
`sudo ipsec down <name>`	Tears down a tunnel	`sudo ipsec down site-to-site`
`sudo tcpdump -i any esp`	Captures ESP traffic to confirm packets are actually encrypted on the wire	`sudo tcpdump -i eth0 esp`
`ip xfrm state`	Shows the kernel's active IPSec security associations (keys/algorithms in use)	`ip xfrm state`

Flag/output notes:

sudo ipsec statusall output distinguishes IKE_SA (the Phase 1 channel) from CHILD_SA (the Phase 2/ESP tunnel) — if a tunnel isn't passing traffic, this is the first place to see which phase actually failed, rather than treating "the VPN is down" as one undifferentiated problem.
sudo tcpdump -i any esp filters specifically for ESP-protocol packets — seeing them on the wire (and being unable to read their contents, unlike plain IP traffic) is the direct, observable proof that tunnel-mode encryption is actually happening, not just configured.
ip xfrm state is the kernel's own view of active IPSec security associations — useful to confirm the negotiated encryption algorithm matches what you configured, and that a security association actually exists at all (an empty result means no active tunnel, regardless of what a config file says should exist).

Hands-on exercises

Requires sudo apt install strongswan and two network namespaces connected via a shared "internet" link (reuse the netns-building approach from module 05/12).

Build two "sites" and confirm they can't reach each other's private subnet without a VPN. Set up two namespaces, each with its own private subnet behind it (simulating two offices) and a shared public link between their "gateway" interfaces (simulating the internet). Confirm a host on site A's private subnet cannot ping a host on site B's private subnet yet — there's no route, and even if there were, the addresses are private and this is meant to simulate crossing the public internet. This is your before-state.
Configure a site-to-site IPSec tunnel and confirm connectivity. Install strongSwan on both gateway namespaces, configure a basic pre-shared-key tunnel in /etc/ipsec.conf (site A's private subnet as leftsubnet, site B's as rightsubnet, each gateway's public-facing address as left/right), sudo ipsec up <name> on both sides, and confirm with sudo ipsec status that the tunnel is ESTABLISHED. Re-run the ping from exercise 1 — expect it to succeed now, tunneled across the "internet" link.
Watch encryption happen on the wire. With the tunnel up, run sudo tcpdump -i <public-interface> esp on the shared link while pinging across the tunnel from exercise 2. Expected: you see ESP packets on the public-facing capture (proving the traffic is actually tunneled and encrypted at that point), while a capture taken inside either private subnet shows the original, unencrypted ICMP packets — the encryption boundary is exactly at the tunnel, not before or after it.
Break Phase 1 and diagnose it specifically. Deliberately mismatch the pre-shared key on one side's config, restart the tunnel, and run sudo ipsec statusall. Expected: the IKE_SA never reaches an established state — confirm the log output (sudo journalctl -u strongswan or /var/log/syslog) shows an authentication failure specifically at the IKE/Phase 1 stage, distinct from what a Phase 2/subnet-mismatch failure would show (exercise 5). Fix the key mismatch and confirm ESTABLISHED returns.
Break Phase 2 instead and compare the failure signature. Restore the correct key, but now deliberately misconfigure one side's rightsubnet to a different (wrong) CIDR than site B's actual private range. Expected: sudo ipsec statusall now shows the IKE_SA established (Phase 1 succeeded — the endpoints authenticated each other fine) but the CHILD_SA (Phase 2/the actual tunnel) fails or never installs correctly — a visibly different failure point than exercise 4's, proving the two phases really do fail independently and distinguishably.
Diagnose and fix: the overlapping-subnet VPN. Reconfigure site B's private subnet to be identical to site A's (e.g. both 10.10.10.0/24) and attempt to bring the tunnel up again with otherwise-correct config. Reason through (you don't need to fully resolve strongSwan's specific error) why this is fundamentally different from exercises 4 and 5's misconfigurations — this isn't a wrong key or wrong subnet typo, it's a structurally ambiguous routing situation that no IPSec setting alone can resolve. State what the actual fix has to be (from the Concepts section).

Independent challenge

No lab given. A company has a working site-to-site VPN between their office and their cloud VNet. Remote employees, working from home or hotel wifi, also need access to internal-only resources, but IT reports that IPSec-based remote-access VPN connections frequently fail to establish from hotel and coffee-shop networks specifically, while the existing site-to-site tunnel (between two fixed, known networks) has never had this problem. Explain the likely networking reason IPSec struggles specifically in these unpredictable-network scenarios, and recommend (with justification, using this module's concepts) an alternative VPN approach better suited to this specific remote-access case — while keeping the existing, working site-to-site tunnel as-is.

Stuck? One hint

Many hotel/public/corporate-guest networks restrictively firewall outbound traffic to "web-looking" ports/protocols only, and IPSec's IKE/ESP traffic (not standard TCP port 443 HTTP(S) traffic) is a common, specific casualty of that kind of restrictive filtering — it simply gets blocked by a firewall the user has no control over, long before it reaches the company's VPN gateway at all. An SSL/TLS VPN for the remote-access case specifically solves this: it rides over a standard TLS connection on port 443, indistinguishable from ordinary HTTPS traffic to any firewall in between, so it survives exactly the restrictive networks IPSec struggles on. The fixed, known site-to-site tunnel between two company-controlled networks has no such unpredictable-firewall problem in the path, which is why it's never shown this symptom — this is precisely the site-to-site-vs-remote-access, IPSec-vs-SSL-VPN distinction the Concepts section draws, applied to a real mixed deployment where both approaches coexist for different use cases.

Common mistakes & troubleshooting

Treating "the VPN is down" as one problem instead of checking IKE Phase 1 vs IPSec/Phase 2 separately. Exercises 4 and 5 show these fail independently with different signatures — statusall's IKE_SA vs CHILD_SA distinction is the first thing to check, not a symptom to guess at.
Building a site-to-site VPN without checking for overlapping private ranges first. Exercise 6's trap, and a genuinely common real-world VPN failure — this requires re-addressing one side, not a config tweak, so it's much cheaper to catch before building the tunnel than after.
Assuming AH is "IPSec" the way ESP is. AH authenticates but never encrypts — almost every real VPN you'll actually deploy or troubleshoot uses ESP (with encryption), not AH alone.
Assuming transport mode is what site-to-site VPNs use. Site-to-site tunnels need tunnel mode specifically, to wrap the entire original (often privately-addressed) packet inside a new, publicly-routable outer packet — transport mode leaves the original header exposed and doesn't provide this.
Not accounting for restrictive networks in remote-access VPN design. IPSec's non-web-standard traffic is a common casualty of hotel/guest-network firewalls — an SSL VPN riding on TLS/443 is the standard fix for this specific, real deployment problem (the independent challenge).

Checkpoint quiz

Write down your answer to each question before expanding it — checking without attempting first is the single easiest way to fool yourself into thinking you've learned this.

What does "tunneling" mean, and how is it different from (but complementary to) encryption?
What's the difference between a site-to-site VPN and a remote-access VPN?
What's the difference between AH and ESP within IPSec, and which one do almost all real deployments actually use?
Why do site-to-site VPNs use IPSec tunnel mode rather than transport mode?
What does IKE do, and why does it matter that it happens in two distinct phases when you're troubleshooting a broken tunnel?
Why can two private networks with identical (overlapping) address ranges not be connected by a VPN without first re-addressing one of them?
Why might an SSL/TLS VPN succeed on a restrictive public network where an IPSec VPN fails?

Show answers

Tunneling wraps an entire original packet inside a new outer packet addressed between two VPN endpoints, so the inner packet (which may use private, non-internet-routable addresses) can traverse a public network via the outer packet's public addressing. Encryption is a separate, additional property — you can tunnel without encrypting (e.g. GRE) — a VPN specifically combines tunneling with encryption so the tunneled contents are also unreadable in transit.
A site-to-site VPN connects two entire networks via gateway devices at each end, transparently linking every host on both sides with no per-device client needed. A remote-access VPN connects a single client device into a private network, requiring VPN client software on that device.
AH provides integrity and authentication but does not encrypt the payload (contents remain readable in transit); ESP provides integrity, authentication, and encryption. Almost all real-world IPSec deployments use ESP, since "tamper-proof but readable" alone rarely meets actual VPN requirements.
Tunnel mode encrypts and wraps the entire original packet (including its header) inside a new outer packet with the VPN gateways' own addresses — this is what allows privately-addressed inner traffic to be carried across the public internet inside a publicly-routable outer packet. Transport mode only encrypts the payload and leaves the original header exposed, which doesn't provide the address-hiding tunneling behavior site-to-site VPNs need.
IKE negotiates encryption keys/algorithms between the two VPN endpoints without ever transmitting the shared secret itself in the open — Phase 1 establishes a secure channel between the endpoints (via Diffie-Hellman), Phase 2 uses that channel to negotiate the actual IPSec/ESP tunnel parameters. It matters for troubleshooting because Phase 1 and Phase 2 fail independently with different symptoms (an authentication mismatch fails Phase 1; a subnet/policy mismatch can fail Phase 2 even with Phase 1 succeeding) — checking which phase actually failed narrows the diagnosis immediately instead of treating "VPN down" as one undifferentiated problem.
Because once tunneled together, a destination address like 192.168.1.50 would be ambiguous — it could mean either network's host — and no VPN setting can resolve that ambiguity; the routing itself is broken by definition until the overlap is removed by re-addressing one side (or applying boundary NAT specifically to translate the overlap away, where supported).
Because SSL/TLS VPN traffic rides over a standard TLS connection, typically on port 443, which is indistinguishable at the network level from ordinary HTTPS browsing traffic — restrictive networks (hotels, guest wifi, locked-down corporate firewalls) that block IPSec's IKE/ESP traffic specifically usually still allow standard HTTPS traffic through, so the SSL VPN survives filtering the IPSec VPN does not.

10 — CDN & reverse proxy: VPNs connect private networks securely across the public internet; now look at the opposite direction — serving public content to everyone, fast and close to them, and shielding your real servers while doing it.