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NO_PROPOSAL_CHOSEN strongswan ipsec tunnel

3 min read 23-10-2024
the ifix
NO_PROPOSAL_CHOSEN strongswan ipsec tunnel

When setting up IPsec tunnels using StrongSwan, one common issue that users may encounter is the NO_PROPOSAL_CHOSEN error. This error indicates a failure in the negotiation of cryptographic parameters between two endpoints of a VPN tunnel. In this article, we will explore what this error means, the potential causes, and how to resolve it effectively.

What is the NO_PROPOSAL_CHOSEN Error?

The NO_PROPOSAL_CHOSEN error occurs during the Internet Key Exchange (IKE) phase when the two ends of the IPsec tunnel cannot agree on a set of cryptographic parameters (such as encryption algorithms, key lengths, etc.). This failure can prevent the successful establishment of the VPN connection.

Example of Code / Configuration That Could Lead to the Error

For instance, consider the following configuration snippet for StrongSwan:

config setup
  charonstart=yes
  uniqueids=no

conn %default
  keyexchange=ikev2
  ike=aes256-sha256-modp1024!
  esp=aes256-sha256!
  dpdaction=clear
  dpddelay=300s
  dpdtimeout=1h

conn myvpn
  left=192.0.2.1
  leftid=@myvpnserver
  leftcert=myvpnserverCert.pem
  right=203.0.113.1
  rightid=@client
  rightsubnet=10.0.0.0/24
  auto=add

In this configuration, if the remote endpoint (right) does not support the specified algorithms (AES256, SHA256) or the specific Diffie-Hellman group (modp1024), the result would be a NO_PROPOSAL_CHOSEN error during the negotiation process.

Analyzing the Causes

Several factors can lead to the NO_PROPOSAL_CHOSEN error:

  1. Mismatch in Encryption Parameters: The most common cause is that the two ends of the tunnel have mismatched configurations regarding the algorithms used for encryption, integrity, and key exchange.

  2. Unsupported Algorithms: If one of the endpoints is configured to use algorithms that are not supported or have been disabled in the StrongSwan configuration or the peer's configuration, this can also lead to this error.

  3. Configuration Syntax Errors: Even small syntax errors in the configuration files can lead to negotiations not working as expected.

  4. Outdated Versions: Sometimes, an outdated version of StrongSwan may lead to compatibility issues with newer cryptographic standards or configurations.

Resolving the Issue

Here are steps to troubleshoot and resolve the NO_PROPOSAL_CHOSEN error:

  1. Check Logs: The first step is to check the StrongSwan logs, typically found in /var/log/syslog or /var/log/charon.log, depending on the system configuration. The logs can provide detailed information about what cryptographic parameters were proposed and which ones were rejected.

  2. Compare Configurations: Ensure that both ends of the tunnel have compatible configurations. For instance, verify that the ike and esp settings match on both sides.

  3. Testing with Default Parameters: Temporarily simplify both configurations to use default or widely compatible parameters to identify if the error persists.

  4. Consult Documentation: Always refer to the official StrongSwan documentation for detailed guidance on configuration options and supported algorithms.

  5. Update StrongSwan: If the issue remains unresolved, consider updating StrongSwan to the latest version as it may include fixes for known bugs or compatibility improvements.

Conclusion

The NO_PROPOSAL_CHOSEN error in StrongSwan can be frustrating, but understanding its causes and how to troubleshoot it can streamline the process of setting up a secure IPsec tunnel. By aligning the cryptographic parameters and ensuring compatible configurations on both ends, you can effectively resolve this issue and establish a stable VPN connection.

Additional Resources

With this information, you should be better equipped to diagnose and fix the NO_PROPOSAL_CHOSEN error in StrongSwan. Happy tunneling!

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