Introduction to strongSwan

This document gives an introduction to strongSwan for new users (or for existing users wanting to catch-up).

Prerequisites

It is assumed that the reader of this document…​

  • understands the principles of networking, from setting IP addresses and DNS servers to basic firewalling

  • grasps the basic authentication concepts based on public-key cryptography and a public-key infrastructure (PKI)

  • knows how to install binary software packages or how to compile source code following instructions

  • is adept on the console

If you don’t have such knowledge, there exist many ready-to-use appliances that provide remote IPsec access.

Securing a Network

strongSwan is a complete IPsec solution providing encryption and authentication to servers and clients. strongSwan can be used to secure communications with remote networks, so that connecting remotely is the same as connecting locally.

topology
Gateway

The gateway is usually your firewall but this can be any host within your network. Often the gateway is also able to serve a small network with DHCP and DNS. In the image above the hosts moon and sun serve as gateways for the internal hosts alice, venus and bob, respectively.

Remote Access / Roadwarrior Clients

Usually, roadwarriors are laptops and other mobile devices connecting remotely to your home network via the gateway. In the image above carol and dave represent roadwarriors who want to access either of the two networks behind the two gateways.

Remote Hosts / Host-to-Host

This can be a remote web server or a backup system. This is illustrated in the image by host winnetou and either of the gateways moon and sun. The connection between the two hosts can usually be initiated by either one of them.

Remote Sites / Site-to-Site

Hosts in two or more subnets at different locations should be able to access each other. Again referring to the image above, the two subnets 10.1.0.0/16 and 10.2.0.0/24 behind gateways moon and sun, respectively, might be connected, so that e.g. the hosts alice and bob may securely communicate with one another.

On our website you’ll find dozens of complete configuration examples covering these and similar situations.

IKE and IPsec Basics

strongSwan is basically a keying daemon that uses the Internet Key Exchange Version 2 (IKEv2) protocol to establish Security Associations (SAs) between two peers. For legacy applications IKEv1 is still supported, although we strongly discourage from using IKEv1 due to stability and some security reasons. IKE provides strong authentication of both peers and derives unique cryptographically-strong session keys. Such an IKE session is often denoted IKE_SA in our documentation. Besides authentication and key material IKE also provides the means to exchange configuration information (e.g. virtual IP addresses) and to negotiate IPsec SAs, which are often called CHILD_SAs. IPsec SAs define which network traffic is to be secured and how it has to be encrypted and authenticated.

A CHILD_SA consists of two components:

  1. The actual IPsec SAs (two of them are established, one in each direction) describing the algorithms and keys used to encrypt and authenticate the traffic.

  2. The policies (there are at least two) that define which network traffic shall use that SA.

The policies work both ways, i.e. only traffic matching an inbound policy will be allowed after decryption. Policies are derived from the traffic selectors (TS) negotiated via IKE when establishing a CHILD_SA. Unprotected traffic that the kernel receives and for which there is no matching inbound IPsec policy will be dropped. This is a security feature.

The actual IPsec traffic is not handled by strongSwan but will be relegated to the network and IPsec stack of the operating system kernel. strongSwan installs the negotiated IPsec SAs and SPs into the kernel by using a platform-dependent kernel interface.

The mentioned distinction between policies and SAs often leads to misconceptions. For instance, referring to the image above, if host moon has a site-to-site tunnel to host sun (connecting the two networks 10.1.0.0/16 and 10.2.0.0/24) and host carol has a roadwarrior connection to host sun (from which carol received a virtual IP address of 10.3.0.10). Then carol wont be able to automatically communicate with alice, even if forwarding is enabled on sun. This is because there is no IPsec policy allowing traffic between carol (10.3.0.10) and alice (10.1.0.10). An additional SA between moon and sun connecting the virtual subnet 10.3.0.0/24 with 10.1.0.0/16 would be a possible solution to this issue.

Generally IPsec processing and routing are two different topics. IPsec is often just bumped into the stack (policy based) and the original routing decision for the unprotected packet also applies to the protected packet. This is the reason that strongSwan charon IKE daemon by default installs specific routes to the remote part of the TS (in newer charon versions (>5.5.0), routes aren’t installed for transport mode CHILD_SAs). An exception to this is route-based IPsec that uses interfaces to control what packets are going to be processed by each tunnel to a unique participant. Route-based IPsec is less flexible than policy-based IPsec.

Authentication Basics

To ensure that the peer with which an IKE_SA is established is really who it claims to be, it has to be authenticated.

strongSwan provides several methods to do this:

Public Key Authentication

RSA, ECDSA or EdDSA X.509 certificates are used to verify the authenticity of the peer.

  • Certificates can be self-signed (in which case they have to be installed on all peers) or signed by a common Certificate Authority (CA). The latter simplifies deployment and configuration a lot as the gateway only needs the CA certificate to authenticate all peers that provide a valid certificate signed by that CA.

  • Certificate Revocation Lists (CRLs) or the Online Certificate Status Protocol (OCSP) may be used to verify the validity of certificates.

  • To securely store private keys smartcards may be used via the pkcs11 plugin.

  • In order to prevent man-in-the-middle attacks, the identity claimed by the peer has to be confirmed by the certificate, either by the subjectDn or a subjectAltName extension.

Pre-Shared-Key Authenticiation (PSK)

A pre-shared-key is an easy-to-deploy option but it requires strong secrets to be secure.

  • If the PSK is known to many users (which is often the case with IKEv1 XAuth with PSK) any user who knows the secret could impersonate the gateway. Therefore this method is not recommended for large scale deployments.

Extensible Authentication Protocol (EAP)

This covers several possible authentication methods, some are based on username/password-based authentication (EAP-MD5, EAP-MSCHAPv2, EAP-GTC) or on X.509 certificates (EAP-TLS). Some can even tunnel other EAP methods (EAP-TTLS, EAP-PEAP).

  • The actual authentication of users may be delegated to a RADIUS server with the eap-radius plugin.

  • EAP authentication can only be used with IKEv2 and for some methods with IKEv1 using the xauth-eap plugin.

eXtended Authentication (XAuth)

XAuth provides a flexible authentication framework within IKEv1. It is mainly used for username/password-based authentication. Also it is generally used as a second authentication method after a mutual authentication based on either X.509 certificates or PSK. With IKEv1 hybrid authentication it is however possible to authenticate the gateway with a certificate and use XAuth to authenticate the client, only.

With IKEv2 it is possible to use multiple authentication rounds (RFC 4739), for instance to first authenticate the machine with an X.509 certificate and then the user with a username/password-based authentication scheme (e.g EAP-MSCHAPv2). It is also possible to use asymmetric authentication, e.g. by authenticating the gateway with a certificate and the client with a username/password-based EAP method in the first authentication round. Please be aware that not all IKEv2 implementations support the RFC 4739 extension.

Again, our website provides dozens of configuration examples covering these and other authentication options.

Configuration Files

The recommended way of configuring strongSwan is via the powerful vici control interface and the swanctl command line tool. The swanctl.conf configuration file used by swanctl is stored together with certificates and corresponding private keys in the swanctl directory.

Global strongSwan settings as well as plugin-specific configurations are defined in strongswan.conf.

Alternatively, the legacy stroke control interface and the ipsec command line tool can be used with the deprecated ipsec.conf and ipsec.secrets configuration files.

Other Configuration Sources

The configuration may also be loaded from an SQL database or can be provided by custom plugins. Using the charon-nm daemon variant, the NetworkManager can be used to manage VPN connections.

Installation

The strongSwan installation is covered in a separate document.

Using binary packages provided by your distribution is generally recommended, as it makes maintenance easier. Unfortunately, this means that you are often not able to use the most recent version.

Invocation and Maintenance

strongSwan is usually managed with the swanctl command while the IKE charon is controlled by systemd on modern distros. With legacy installations, strongSwan is controlled by the ipsec command where ipsec start will start the starter daemon which in turn starts and configures the keying charon daemon.

IKE Connections and CHILD SAs defined in swanctl.conf can be started through three different ways:

On traffic

If start_action = trap is used, IPsec trap policies for the configured traffic (defined via local_ts/remote_ts) will be installed and traffic matching these policies will trigger acquire events that cause the daemon to establish the required IKE/IPsec SAs. This is also used for passthrough/drop IPsec policies, to let specific traffic bypass other policies/SAs or drop it completely.

On startup

CHILD_SAs configured with start_action = start will automatically be established when the daemon is started. They are not automatically restarted when they go down for some reason. You need to specify other configuration settings (dpd_action and/or close_action to restart them automatically but even then, the setup is not bullet-proof and will potentially leak packets. You are encouraged to use trap policies and read the SecurityRecommendations to take care of any problems.

Manually

A connection that uses no start_action has to be established manually with swanctl --initiate or acts passively as a responder waiting for a peer/roadwarrior to connect. Depending on the configuration, it is also possible to use swanctl --install to install policies manually for such connections as start_action = trap would do it on startup.

After an SA has been established, swanctl --terminate may be used to tear down the IKE_SA or individual CHILD_SAs.

Whenever the swanctl.conf file or credentials in the swanctl directory are changed they may be reloaded with the different swanct --load-.. commands. Already established connections are not affected by these commands (unless start_action = start is used). If a configuration update is required, the SAs or even the daemon must be restarted.

Using the different swanctl --list-.. commands will provide information about loaded or cached certificates, supported algorithms and loaded plugins.

Logging and Monitoring

If you run into problems, increasing the log level might help you understand what exactly went wrong. The different logging options are described in a separate document or the strongswan.conf man page. Recommended log settings for debugging problems may be found here.

Whenever you encounter a log message similar to received …​ error notify where the placeholder …​ is e.g. NO_PROPOSAL_CHOSEN or TS_UNACCEPTABLE, you should consult the logs of the remote peer in order to find out why that error notify was generated in the first place.

The swanctl --list-.. commands will provide information about the established and configured connections.

On Linux, the iproute2 package provides the ip xfrm state and ip xfrm policy commands to request detailed information about the IPsec SAs and policies installed in the kernel. Adding the -s option will display extensive statistical information like the number of transmitted or invalid packages. On other platforms the setkey command from the ipsec-tools package provides similar information.

tcpdump and wireshark are also often useful to debug problems.

When testing a connection with ping, make sure to select a source IP address (with the -I option) that is included in the local traffic selector (also see Site-to-Site Configurations below).

PKI

To use certificate-based authentication you’ll need to create either self-signed certificates or set up a complete public-key infrastructure (PKI), consisting of a Certificate Authority (CA), optional intermediate CAs and end-entity certificates plus Certificate Revocation Lists (CRLs) or alternative methods like OCSP to verify the validity of certificates.

One of the easiest ways to generate certificates is to use the pki tool. Since setting up a whole PKI can be quite complex we provide some simple instructions to get you started.

OpenSSL is also a widespread alternative to generate certificates, as are several GUI based CA management utilities. Microsoft’s Active Directory Certificate Services (AD CS) could also be used for large scale PKIs.

Certificate Requirements

  1. The generated end entity certificates need to authenticate the corresponding remote IKE ID for peer authentication to succeed.

  2. To authenticate against another strongSwan instance using one or several certificates (also attribute certificates might be used), the certificate must authenticate the IKE ID the host is sending.

    If Alice tries to authenticate against Bob as Alice (herself) then Alice’s certificate must contain at least one subjectAltName (SAN) field with the correct type (FQDN) and the value Alice or the subjectDistinguishedName (DN), not the commonName (CN) has to be Alice!

    In other words you can use the complete DN or any of the SAN fields (assuming the type is correct) as an IKE ID. See Notes regarding certificates for details.

  3. Additionally, the certificate has to be trusted by Bob, either by being known beforehand by Bob to being valid, or the certificate being issued by a certificate authority (CA) Bob is trusting.

  4. For authentication to succeed, the other peer has to possess the complete X.509 certificate trust chain from root certificate (the root CA) down to the end entity certificate (the host or user certificate) including all intermediate certificates (intermediate CAs). This is done either by sending any intermediate certificates to the remote host or the remote host already having them installed locally.

The requirements for certificate-based authentication with third party IKE implementations are stated in separate documents for Microsoft Windows and Apple iOS/macOS.

Notes regarding Certificates

A root CA certificate which being at the top of the X.509 trust chain, is always self-signed and can therefore be faked by anyone, is never sent to another host. Any peer must install the root CA certificate locally in a trustful way and never accept any root CA certificates received over the network.

A local certificate is only sent to another host if at least one of the following settings are true:

  1. The local host has set connections.<conn>.send_cert = always in the connection definition it is using.

  2. The remote peer requests a certificate issued by a trusted CA by sending a CERTREQ payload to the local peer that indicates one of the CAs in the path from the local host’s certificate up to its root CA certificate.

Routing

On Linux, strongSwan installs routes into routing table 220 by default and hence requires the kernel to support policy based routing.

You can make the charon daemon install the routes into any table you like or you can disable them completely. For that purpose the charon.install_routes, charon.routing_table and charon.routing_table_prio settings in strongswan.conf may be used. When a tunnel is established between two subnets, charon tries to find local IPs in the tunneled local subnets. Such an IP must be configurd with scope global to be viable for the lookup. If a valid IP is found, charon will install a route pointing to the remote subnet where the source IP is set to the found IP. This results in routes like the following:

10.1.0.0/24 via 10.2.0.1 src 10.2.0.2

In that example, the local IP would be 10.2.0.2. The remote subnet would be 10.1.0.0/24. This is done, so packets to the remote subnet are sent with the correct source IP. Thus the IPsec policies will match and traffic from the local machine to the remote subnet will be secured by IPsec.

To avoid conflicts with these routes (especially if virtual IPs addresses are used), the kernel-netlink plugin manually parses the host’s routing tables to determine a suitable source address when sending IKE packets. On hosts with a (very) high number of routes this is quite inefficient. In that case, setting charon.plugins.kernel-netlink.fwmark in strongswan.conf is recommended, as it will allow using a more efficient source address lookup.

In order to detect connectivity changes, strongSwan parses the events that the kernel sends when a route is installed or deleted and hence could cause high CPU load when e.g. running on a system that receives a lot of routes via dynamic routing. Connectivity change detection can be disabled by setting charon.process_route = no in strongswan.conf.

If IPv6 is used then make sure to bypass Neighbor Discovery Protocol (NDP) traffic if necessary. It is possible that you encounter MSS/MTU problems when tunneling traffic. Please refer to Forwarding and Split-Tunneling for details.

Remote Access Configurations

In this section we present example configurations for common remote access use cases. In these so called roadwarrior scenarios mobile clients will be able to connect to a remote network.

Because these clients most likely connect from unknown IP addresses, the gateway will use remote_addrs = %any to literally accept connections from anywhere. To simplify routing traffic back to the clients and because roadwarriors are often located behind one or more NAT devices, the use of virtual IP addresses is necessary.

The virtual IPs can either be from a distinct subnet or actually from the subnet behind the gateway by use of the farp plugin and optionally the dhcp plugin.

Whether roadwarriors will send all traffic to the gateway or use split-tunneling, i.e. only send traffic for specific destinations through the tunnel, is also something to consider. This is explained in more details in Forwarding and Split-Tunneling. That document also explains how traffic is forwarded to hosts behind the gateway.

IKEv2 Configurations

The three strongSwan gateway configurations shown for the Windows clients may be used for all IKEv2 clients:

In all three use cases the gateway is authenticated by a certificate while the clients either authenticate themselves based on certificates (1, 2) or on username/password schemes (3). The generic EAP use case (3) incorporates the EAP-TLS use case (2), so that only two configurations (1, 3) must be implemented in parallel on a strongSwan VPN gateway to leave it up to the VPN clients to select any of the three authentication methods above.

With the eap-radius plugin, user authentication may be delegated to a RADIUS server (e.g. an existing Active Directory DC).

Both the strongSwan VPN Client for Android and NetworkManager may be used with any of the strongSwan VPN gateway configurations. Alternatively the charon-cmd command line IKEv2 client provides a simple means to establish roadwarrior connections.

Our app for macOS supports IKEv2 and simple EAP authentication. With iOS 8 and macOS 10.10 Apple introduced support for IKEv2 in their clients. A GUI to configure such connections is currently not provided, so it’s necessary to write (or generate) custom configuration profiles.

Site-to-Site Configurations

We provide the following site-to-site configuration examples

The most important difference compared to the remote access case is that the initiator will not request a virtual IP address but instead use local_ts to tunnel traffic from one or more local subnets. With IKEv2 multiple subnets (in CIDR notation) can be added to local_ts/remote_ts separated by commas. If IKEv1 is used, a separate children.<child> subsection has to be added for each combination of local and remote subnet, as only the first subnet in local_ts/remote_ts will be used.

One thing that often confuses users new to IPsec is that testing a net-to-net scenario from either of the gateways often requires one to select the source address used specifically (e.g. with ping -I) because the external IP of either gateway might not be included in the tunneled subnets. If that is something you require, either add the external IPs to the list of subnets in local_ts/remote_ts or add a specific host-to-host config.

Host-to-Host Configurations

Host-to-host connections are very easy to setup. You basically have to set remote_addrs to the hostname or IP address of the peer and configure the desired authentication. Neither the local_ts nor remote_ts traffic selectors have to be set explicitly.

Again, our web site provides some practical host-to-host configuration examples.