When an IKEv2 tunnel fails to establish or drops intermittently, fast and accurate debugging is essential for network administrators, developers, and site operators. This article provides a compact yet comprehensive set of CLI commands and practical techniques to diagnose IKEv2 negotiation issues across popular platforms. The goal is to let you quickly identify where in the IKEv2 exchange things go wrong and to give actionable next steps to resolve common problems.

Understanding the IKEv2 Exchange — Quick Recap

Before diving into commands, it helps to recall the minimal IKEv2 handshake flow so you can map observed behavior to protocol steps. The two core exchanges are:

  • IKE_SA_INIT — Diffie-Hellman, nonces, and proposal/cipher selection; negotiates the first keys (IKE SA).
  • IKE_AUTH — Authentication (PSK, certificates, EAP) and creation of Child SAs for IPsec traffic.

Troubleshooting typically revolves around one of these stages failing, or packet delivery problems caused by NAT, MTU, policy mismatches, or authentication errors.

Essential Packet Capture Tips

Always start with a packet capture on both ends if possible. Packet captures reveal whether messages are being sent, whether NAT-T is applied, UDP ports in use (500/4500), and observable error notifications.

  • Capture IKEv2 traffic on Linux or *nix: tcpdump -n -s 0 -w ikev2.pcap -i eth0 udp port 500 or udp port 4500
  • Filter in Wireshark using: ikev2 or udp.port == 500 || udp.port == 4500
  • Look for IKE_SA_INIT (SA, KE, Ni, Nr) and IKE_AUTH messages and for Notify Payloads that carry errors (e.g., NO_PROPOSAL_CHOSEN, AUTHENTICATION_FAILED, INVALID_KE_PAYLOAD).

General Troubleshooting Workflow

Use the following flow to isolate issues fast:

  • Verify basic IP reachability and routing (ping, traceroute).
  • Confirm UDP port reachability (nc or hping3 can help).
  • Capture packets to see actual exchanges and identify whether negotiation reaches IKE_AUTH.
  • Increase logging on VPN daemons or devices and correlate logs with packet captures.
  • Check crypto proposals and policies (encryption, integrity, DH groups, lifetimes) on both peers for exact matches.
  • If NAT between peers is present, ensure NAT-T and UDP encapsulation handling are consistent.

Platform-Specific CLI Commands

The following sections list practical CLI commands you can run on different platforms. Use exact commands with appropriate privileges (root/admin). Commands are grouped by vendor/daemon with short notes about interpreting outputs.

strongSwan (Linux)

  • ipsec statusall — Shows connection states, established SAs, and children.
  • ipsec up / ipsec down — Manually bring up or tear down a connection.
  • journalctl -u strongswan -f — Live logs for strongSwan; shows charon debug output if configured.
  • Enable verbose logging in strongswan.conf or use charon.log settings: increase to debug for full protocol traces.
  • swanctl –list-sas — For swanctl-based setups to list Security Associations.
  • Investigate kernel XFRM state: ip xfrm state and ip xfrm policy.

Linux Kernel/OpenSwan/LibreSwan

  • ip xfrm state and ip xfrm policy — Verify IPsec SPIs and policies in the kernel.
  • Daemon logs (e.g., pluto or racoon): tail syslog or use journalctl -u ipsec for details.

Cisco IOS and IOS-XE

  • show crypto ikev2 sa — Display IKEv2 security associations and states.
  • show crypto ipsec sa — List IPsec SAs and packet counters.
  • debug crypto ikev2 protocol — Detailed logs of the IKEv2 protocol flow (use with caution in production).
  • debug crypto ikev2 packet — Shows raw packets and payload parsing for deeper inspection.
  • To stop debugging: undebug all or no debug all.

Cisco ASA

  • show vpn-sessiondb detail l2l (or show vpn-sessiondb ikev2) — Shows active VPN tunnels.
  • show crypto ikev2 sa — IKEv2 SAs on ASA.
  • debug crypto ikev2 127 — High verbosity debugging for IKEv2 on ASA (use cautiously).
  • show logging and show conn — Useful for connection state and logging buffers.

Juniper (Junos)

  • show security ike security-associations — Display IKE SAs on Junos devices.
  • show security ipsec security-associations — Display child IPsec SAs.
  • Start capturing logs: monitor start and show log kmd (IKE daemon logs).
  • Clear SAs for testing: clear security ike-security-associations or clear security ipsec-security-associations.

VyOS

  • show vpn ipsec sa — Shows active IKE and IPsec SAs.
  • Logs appear in system logs: tail -f /var/log/messages or journalctl -u ipsec for strongSwan-based setups.

Interpreting Common Error Notifications

When you see Notify payloads in captures or logs, map them to likely causes:

  • NO_PROPOSAL_CHOSEN — Mismatched crypto proposals (encryption, integrity, DH group, transform-set). Ensure both sides share identical proposals or include a compatible one.
  • AUTHENTICATION_FAILED — PSK mismatch, invalid certificate chain, or wrong identity used in authentication. Verify PSK exact bytes or certificate trust chains.
  • INVALID_KE_PAYLOAD or INVALID_KE — DH group mismatch or corrupted key exchange; check DH group (modp) parameters.
  • USE_TRANSPORT_MODE — Policy mismatches where one side expects transport mode; typically seen in host-based IPsec setups.
  • INTERNAL_ADDRESS_FAILURE — For virtual IP allocation failures (common with server-side address pools).

Key Checks and Quick Fixes

Some quick validation steps often fix the majority of negotiation issues:

  • Confirm peer IPs and routing: incorrect remote IP or NAT retranslation breaks exchanges.
  • Verify proposals: match encryption (AES/GCM/AES-CBC), integrity (SHA1/SHA256), and DH group numbers exactly.
  • Check lifetimes: mismatched IKE or CHILD_SA lifetimes can cause renegotiation problems.
  • Certificates: ensure certificate validity, correct Common Name/SubjectAltName, and proper trust anchors on both sides.
  • NAT handling: verify NAT-T is enabled if any NAT device exists between peers; check for UDP encapsulation on port 4500.
  • MTU and fragmentation: large IKE messages (cert chains) may exceed MTU; enable fragmentation or adjust MSS and MTU settings.

When to Enable High-Verbosity Debugging

High verbosity (debug level) of IKE daemons or device debugs should be used sparingly because it can be noisy and CPU intensive. Use it when:

  • Packet captures show retransmits but no authenticated exchange.
  • Notifications are ambiguous or absent in captures.
  • Intermittent failures require correlation across logs and packet traces.

Always capture logs to files and rotate/disable debug as soon as you have enough data to avoid performance impact.

Putting It All Together — Example Debug Session

A typical session might look like this:

  • Start tcpdump on both peers: tcpdump -n -s 0 -w ike.pcap -i eth0 udp port 500 or udp port 4500.
  • On strongSwan host, tail logs: journalctl -u strongswan -f. On Cisco IOS, enable protocol debug: debug crypto ikev2 protocol.
  • Attempt to bring up the connection: ipsec up myconn or perform traffic that triggers the tunnel.
  • Review captures for IKE_SA_INIT / IKE_AUTH flow. If the peer responds with NO_PROPOSAL_CHOSEN, adjust proposals to match.
  • If authentication fails, inspect PSK/cert configuration and certificate chain validation messages in logs.
  • When resolved, clear SAs and re-establish to confirm stable operation: ipsec down myconn; ipsec up myconn or clear crypto ikev2 sa on IOS.

Effective IKEv2 debugging is a combination of methodical packet capture, targeted logs, and a precise set of CLI checks on both peers. By understanding the IKEv2 handshake stages and using the commands above, you can rapidly identify whether the issue is packet delivery, a cryptographic mismatch, or authentication/certificate-related.

For more in-depth guides and platform-specific tutorials, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.