Overview

This guide walks through a practical, step-by-step approach to mapping L2TP-based VPN topologies using Nmap. It is written for site administrators, enterprise operators, and developers who need to discover and document the layout of L2TP or L2TP-over-IPsec deployments in a safe and repeatable manner. The focus is on actionable techniques and interpretation of results rather than high-level theory. Throughout, emphasis is placed on non-intrusive reconnaissance, correct interpretation of UDP-based services (often less straightforward than TCP), and combining Nmap output with packet captures and auxiliary tools to produce an accurate topology map.

Before you begin: scope and legal considerations

Scanning networks and devices that you do not own or are not authorized to test can be illegal and disruptive. Always obtain explicit permission from the network owner and coordinate with operations teams prior to scanning. In production environments, schedule tests to avoid impacting real users and enable logging so that security teams can correlate events.

Key L2TP and related protocols to target

L2TP (Layer 2 Tunneling Protocol) itself uses UDP port 1701. Frequently L2TP is combined with IPsec for confidentiality and integrity; in these deployments you will also see:

  • UDP 500 (ISAKMP/IKE)
  • UDP 4500 (NAT-T IKE)
  • ESP (IP protocol 50) — not a UDP/TCP port but an IP protocol number
  • Occasional interaction with RADIUS/AAA servers on ports like 1812/1813

Understanding these elements helps you infer whether L2TP is running standalone, wrapped in IPsec, or protected by vendor-specific control planes.

Step 1 — Smart reconnaissance and host selection

Start by identifying candidate nodes that might act as VPN concentrators, gateways, or edge routers. Typical methods include:

  • DNS enumeration and reverse DNS lookup of public IP ranges.
  • Passive logs from firewalls or load balancers showing connections to UDP/1701, UDP/500, and UDP/4500.
  • IP range listing for your edge networks (e.g., public WAN prefixes assigned to your organization).

Once you have an IP range, focus scans on the relevant ports. Use a conservative scanning policy for UDP, because UDP scanning can take longer and is more likely to trigger alarms.

Step 2 — Basic UDP and TCP discovery with Nmap

UDP scanning in Nmap uses the -sU option. Combine UDP and TCP scans to get a fuller picture. Example commands:

nmap -sU -p 1701,500,4500 –open -Pn -n 203.0.113.0/28

nmap -sS -p 80,443,22 –open -Pn -n 203.0.113.0/28

Notes:

  • -Pn disables host discovery—useful for hosts behind firewalls that block ICMP.
  • -n skips DNS resolution to speed up scans.
  • UDP scans are slower and may produce many “open|filtered” results; correlate with other evidence.

Interpreting UDP results

UDP ports often show as “open|filtered” since many devices simply drop packets. A definitive “open” indicates a response (for instance, an ICMP port unreachable response would show “closed”). When you see UDP 1701 responsive, that’s a strong indicator that an L2TP control plane is present or that a proxy is in front of it.

Step 3 — Service/version detection and NSE scripts

Use version detection and Nmap Scripting Engine (NSE) to collect protocol-specific details. Key command:

nmap -sU -sV –script “default or safe or +ike-” -p 500,4500,1701 -Pn 203.0.113.5

Relevant NSE scripts:

  • ike-version — queries IKEv1/IKEv2 responders to obtain vendor and version information for IPsec endpoints.
  • radius-info — probes RADIUS servers (if accessible) to identify vendor strings and supported attributes.
  • ssl- and http-* scripts — useful if management interfaces or TLS-protected endpoints exist on the VPN node.

Note: script availability and names vary by Nmap version; run nmap –script-help or list the scripts folder to confirm names.

Practical NSE usage

For IPsec-wrapped L2TP, query IKE responders to see whether they support NAT traversal and which proposals are accepted:

nmap -sU -p 500,4500 –script ike-version -Pn 203.0.113.5

The ike-version output typically returns:

  • Supported encryption/authentication transforms
  • Vendor strings identifying device type (e.g., Cisco, strongSwan, Windows RRAS)
  • Indication whether IKEv1 or IKEv2 is being used

Step 4 — Correlate L2TP control plane with IPsec transport

Distinguish three common deployment patterns and how to detect them:

  • L2TP alone: UDP 1701 open and no IPsec ports seen.
  • L2TP over IPsec (traditional): UDP 500 and/or 4500 open plus UDP 1701 reachable only after IPsec negotiation; ESP (protocol 50) seen in captures.
  • NAT environment: UDP 4500 used (NAT-T), IKE proposals show NAT detection or NAT-T attributes.

To confirm IPsec, perform a packet capture of an initiation (use a lab test, or coordinate with operations). Observe IKE packets on UDP 500/4500 and ESP traffic (IP proto 50) after SA establishment. If ESP is filtered, the device may be using UDP encapsulation.

Step 5 — Topology mapping techniques

Combine the following data to build a topology:

  • Traces from Nmap’s traceroute (–traceroute) to identify network hops between you and the VPN endpoint.
  • TTL analysis: compare TTLs of different services to understand internal routing.
  • Device fingerprints from -sV and NSE scripts to identify vendor and role (gateway vs. concentrator).
  • Correlation with infrastructure inventory (ASN, BGP prefixes, public IP assignments).

Example command for combined scan and trace:

nmap -sU -sV -p 1701,500,4500 –script ike-version –traceroute -Pn 203.0.113.0/29 -oA vpn-scan

This produces an output set (normal, XML, grepable) you can use for automated parsing and diagram generation.

Visualizing the topology

Export Nmap’s XML (-oX) and feed it to a parser (Python, xmlstarlet) to pull out nodes, ports, and script outputs. Create a graph with Graphviz or a network diagram tool. Key attributes to include:

  • IP addresses and derived hostnames
  • Open ports: 1701, 500, 4500
  • IKE vendor/version strings
  • Traceroute hop sequence
  • Observed NAT behavior (NAT-T usage)

Step 6 — Confirm flows with packet capture

Nmap provides service-level evidence but packet captures provide protocol-level confirmation. Use tcpdump or tshark on a span port or on the gateway itself:

tcpdump -nn -i eth0 udp port 500 or udp port 4500 or udp port 1701 -w l2tp-capture.pcap

Inspect the capture to verify:

  • IKE exchanges (SA initiator/responder packets)
  • L2TP control messages (L2TP control messages are PPP-like messages over UDP 1701)
  • ESP packets or UDP-encapsulated ESP following tunnel establishment

Packet captures also let you see timing, retransmissions, and potential middleboxes rewriting headers — essential for diagnosing NAT or load-balancer issues.

Step 7 — Advanced inference: multi-site and HA setups

When mapping multiple gateways or an HA pair, look for:

  • Common vendor string across multiple IPs indicating cluster nodes.
  • Shared public IP with different internal endpoints — suggests load-balancing or NAT64.
  • Common RADIUS servers or AAA endpoints referenced in logs or via accessible management interfaces.

Use timing and TTL differences to infer whether nodes are co-located or geographically distributed. If you can trigger a VPN connection from a test client, observe which public IP responds and whether the internal address changes—this reveals clustering and session affinity behavior.

Troubleshooting and common pitfalls

  • ICMP filtering complicates host reachability tests—use alternative signals (open UDP/TCP ports) to determine responsiveness.
  • UDP scans produce many false positives; always seek packet-capture confirmation for critical decisions.
  • Vendor strings may be intentionally obfuscated—treat them as one input among many rather than a definitive identifier.
  • IP protocol numbers (ESP) are not scanned by default with port-based tools; rely on packet captures or host-based logs to detect ESP traffic.

Automation and reporting

Automate repeated scans with careful rate-limiting and output to XML for machine parsing. A sample workflow:

  • Run Nmap with -oX to produce XML outputs.
  • Parse script output fields (ike-version, radius-info) to extract vendor and capabilities.
  • Feed parsed data into a templating engine to generate diagrams and CSV inventories.

This allows periodic topology audits to detect configuration drift (e.g., a VPN gateway replaced with a different vendor) and supports change management.

Summary and best practices

Mapping L2TP VPN topologies requires a combination of UDP/TCP scanning, NSE script interrogation, traceroute correlation, and packet capture verification. Use Nmap as the central discovery tool, but always corroborate findings with protocol-level captures and inventory data. Maintain a conservative, authorized scanning posture and produce clear diagrams that include ports, vendor/version, NAT/HA behavior, and traceroute-based hop details.

For more resources and tools to manage VPN infrastructure mapping and secure deployments, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.