strongSwan Configuration

Overview

strongSwan is an OpenSource IPsec-based VPN solution.

This document is just a short introduction of the strongSwan swanctl command which uses the modern vici Versatile IKE Configuration Interface. The deprecated ipsec command using the legacy stroke configuration interface is described here. For more detailed information consult the man pages, our new documentation site and the legacy wiki.

Quickstart

Certificates for users, hosts and gateways are issued by a fictitious strongSwan CA. In our example scenarios the CA certificate strongswanCert.pem must be present on all VPN endpoints in order to be able to authenticate the peers. For your particular VPN application you can either use certificates from any third-party CA or generate the needed private keys and certificates yourself with the strongSwan pki tool, the use of which will be explained in one of the sections following below.

Site-to-Site Case

In this scenario two security gateways moon and sun will connect the two subnets moon-net and sun-net with each other through a VPN tunnel set up between the two gateways:

10.1.0.0/16 -- | 192.168.0.1 | === | 192.168.0.2 | -- 10.2.0.0/16
  moon-net          moon                 sun           sun-net

Configuration on gateway moon:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/moonCert.pem
/etc/swanctl/private/moonKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        net-net {
            remote_addrs = 192.168.0.2

            local {
                auth = pubkey
                certs = moonCert.pem
            }
            remote {
                auth = pubkey
                id = "C=CH, O=strongSwan, CN=sun.strongswan.org"
            }
            children {
                net-net {
                    local_ts  = 10.1.0.0/16
                    remote_ts = 10.2.0.0/16
                    start_action = trap
                }
            }
        }
    }

Configuration on gateway sun:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/sunCert.pem
/etc/swanctl/private/sunKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        net-net {
            remote_addrs = 192.168.0.1

            local {
                auth = pubkey
                certs = sunCert.pem
            }
            remote {
                auth = pubkey
                id = "C=CH, O=strongSwan, CN=moon.strongswan.org"
            }
            children {
                net-net {
                    local_ts  = 10.2.0.0/16
                    remote_ts = 10.1.0.0/16
                    start_action = trap
                }
            }
        }
    }

The local and remote identities used in this scenario are the subjectDistinguishedNames contained in the end entity certificates. The certificates and private keys are loaded into the charon daemon with the command

swanctl --load-creds

whereas

swanctl --load-conns

loads the connections defined in swanctl.conf. With start_action = trap the IPsec connection is automatically set up with the first plaintext payload IP packet wanting to go through the tunnel.

Host-to-Host Case

This is a setup between two single hosts which don't have a subnet behind them. Although IPsec transport mode would be sufficient for host-to-host connections we will use the default IPsec tunnel mode.

| 192.168.0.1 | === | 192.168.0.2 |
     moon                sun

Configuration on host moon:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/moonCert.pem
/etc/swanctl/private/moonKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        host-host {
            remote_addrs = 192.168.0.2

            local {
                auth=pubkey
                certs = moonCert.pem
            }
            remote {
                auth = pubkey
                id = "C=CH, O=strongSwan, CN=sun.strongswan.org"
            }
            children {
                net-net {
                    start_action = trap
                }
            }
        }
    }

Configuration on host sun:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/sunCert.pem
/etc/swanctl/private/sunKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        host-host {
            remote_addrs = 192.168.0.1

            local {
                auth = pubkey
                certs = sunCert.pem
            }
            remote {
                auth = pubkey
                id = "C=CH, O=strongSwan, CN=moon.strongswan.org"
            }
            children {
                host-host {
                    start_action = trap
                }
            }
        }
    }

Roadwarrior Case

This is a very common case where a strongSwan gateway serves an arbitrary number of remote VPN clients usually having dynamic IP addresses.

10.1.0.0/16 -- | 192.168.0.1 | === | x.x.x.x |
  moon-net          moon              carol

Configuration on gateway moon:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/moonCert.pem
/etc/swanctl/private/moonKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        rw {
            local {
                auth = pubkey
                certs = moonCert.pem
                id = moon.strongswan.org
            }
            remote {
                auth = pubkey
            }
            children {
                net-net {
                    local_ts  = 10.1.0.0/16
                }
            }
        }
    }

Configuration on roadwarrior carol:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/carolCert.pem
/etc/swanctl/private/carolKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        home {
            remote_addrs = moon.strongswan.org

            local {
                auth = pubkey
                certs = carolCert.pem
                id = carol@strongswan.org
            }
            remote {
                auth = pubkey
                id = moon.strongswan.org
            }
            children {
                home {
                    local_ts  = 10.1.0.0/16
                    start_action = start
                }
            }
        }
    }

For remote_addrs the hostname moon.strongswan.org was chosen which will be resolved by DNS at runtime into the corresponding IP destination address. In this scenario the identity of the roadwarrior carol is the email address carol@strongswan.org which must be included as a subjectAlternativeName in the roadwarrior certificate carolCert.pem.

Roadwarrior Case with Virtual IP

Roadwarriors usually have dynamic IP addresses assigned by the ISP they are currently attached to. In order to simplify the routing from moon-net back to the remote access client carol it would be desirable if the roadwarrior had an inner IP address chosen from a pre-defined pool.

10.1.0.0/16 -- | 192.168.0.1 | === | x.x.x.x | -- 10.3.0.1
  moon-net          moon              carol       virtual IP

In our example the virtual IP address is chosen from the address pool 10.3.0.0/16 which can be configured by adding the section

pools {
    rw_pool {
        addrs = 10.3.0.0/16
    }
}

to the gateway's swanctl.conf from where they are loaded into the charon daemon using the command

swanctl --load-pools

To request an IP address from this pool a roadwarrior can use IKEv1 mode config or IKEv2 configuration payloads. The configuration for both is the same

vips = 0.0.0.0

Configuration on gateway moon:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/moonCert.pem
/etc/swanctl/private/moonKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        rw {
            pools = rw_pool

            local {
                auth = pubkey
                certs = moonCert.pem
                id = moon.strongswan.org
            }
            remote {
                auth = pubkey
            }
            children {
                net-net {
                    local_ts  = 10.1.0.0/16
                }
            }
        }
    }

    pools {
        rw_pool {
            addrs = 10.30.0.0/16
        }
    }

Configuration on roadwarrior carol:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/carolCert.pem
/etc/swanctl/private/carolKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        home {
            remote_addrs = moon.strongswan.org
            vips = 0.0.0.0

            local {
                auth = pubkey
                certs = carolCert.pem
                id = carol@strongswan.org
            }
            remote {
                auth = pubkey
                id = moon.strongswan.org
            }
            children {
                home {
                    local_ts  = 10.1.0.0/16
                    start_action = start
                }
            }
        }
    }

Roadwarrior Case with EAP Authentication

This is a very common case where a strongSwan gateway serves an arbitrary number of remote VPN clients which authenticate themselves via a password based Extended Authentication Protocol as e.g. EAP-MD5 or EAP-MSCHAPv2.

10.1.0.0/16 -- | 192.168.0.1 | === | x.x.x.x |
  moon-net          moon              carol

Configuration on gateway moon:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/moonCert.pem
/etc/swanctl/private/moonKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        rw {
            local {
                auth = pubkey
                certs = moonCert.pem
                id = moon.strongswan.org
            }
            remote {
                auth = eap-md5
            }
            children {
                net-net {
                    local_ts  = 10.1.0.0/16
                }
            }
            send_certreq = no
        }
    }

The swanctl.conf file additionally contains a secrets section defining all client credentials

    secrets {
        eap-carol {
            id = carol@strongswan.org
            secret = Ar3etTnp
        }
        eap-dave {
            id = dave@strongswan.org
            secret = W7R0g3do
        }
    }

Configuration on roadwarrior carol:

/etc/swanctl/x509ca/strongswanCert.pem

/etc/swanctl/swanctl.conf:

    connections {
        home {
            remote_addrs = moon.strongswan.org

            local {
                auth = eap
                id = carol@strongswan.org
            }
            remote {
                auth = pubkey
                id = moon.strongswan.org
            }
            children {
                home {
                    local_ts  = 10.1.0.0/16
                    start_action = start
                }
            }
        }
    }

    secrets {
        eap-carol {
            id = carol@strongswan.org
            secret = Ar3etTnp
        }
    }

Roadwarrior Case with EAP Identity

Often a client EAP identity is exchanged via EAP which differs from the external IKEv2 identity. In this example the IKEv2 identity defaults to the IPv4 address of the client.

10.1.0.0/16 -- | 192.168.0.1 | === | x.x.x.x |
  moon-net          moon              carol

Configuration on gateway moon:

/etc/swanctl/x509ca/strongswanCert.pem
/etc/swanctl/x509/moonCert.pem
/etc/swanctl/private/moonKey.pem

/etc/swanctl/swanctl.conf:

    connections {
        rw {
            local {
                auth = pubkey
                certs = moonCert.pem
                id = moon.strongswan.org
            }
            remote {
                auth = eap-md5
                eap_id = %any
            }
            children {
                net-net {
                    local_ts  = 10.1.0.0/16
                }
            }
            send_certreq = no
        }
    }

    secrets {
        eap-carol {
            id = carol
            secret = Ar3etTnp
        }
        eap-dave {
            id = dave
            secret = W7R0g3do
        }
    }

Configuration on roadwarrior carol:

/etc/swanctl/x509ca/strongswanCert.pem

/etc/swanctl/swanctl.conf:

    connections {
        home {
            remote_addrs = moon.strongswan.org

            local {
                auth = eap
                eap_id = carol
            }
            remote {
                auth = pubkey
                id = moon.strongswan.org
            }
            children {
                home {
                    local_ts  = 10.1.0.0/16
                    start_action = start
                }
            }
        }
    }

    secrets {
        eap-carol {
            id = carol
            secret = Ar3etTnp
        }
    }

Generating Certificates and CRLs

This section is not a full-blown tutorial on how to use the strongSwan pki tool. It just lists a few points that are relevant if you want to generate your own certificates and CRLs for use with strongSwan.

Generating a CA Certificate

The pki statement

pki --gen --type ed25519 --outform pem > strongswanKey.pem

generates an elliptic Edwards-Curve key with a cryptographic strength of 128 bits. The corresponding public key is packed into a self-signed CA certificate with a lifetime of 10 years (3652 days)

pki --self --ca --lifetime 3652 --in strongswanKey.pem \
           --dn "C=CH, O=strongSwan, CN=strongSwan Root CA" \
           --outform pem > strongswanCert.pem

which can be listed with the command

pki --print --in strongswanCert.pem

subject:  "C=CH, O=strongSwan, CN=strongSwan Root CA"
issuer:   "C=CH, O=strongSwan, CN=strongSwan Root CA"
validity:  not before May 18 08:32:06 2017, ok
           not after  May 18 08:32:06 2027, ok (expires in 3651 days)
serial:    57:e0:6b:3a:9a:eb:c6:e0
flags:     CA CRLSign self-signed
subjkeyId: 2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d
pubkey:    ED25519 256 bits
keyid:     a7:e1:6a:3f:e7:6f:08:9d:89:ec:23:92:a9:a1:14:3c:78:a8:7a:f7
subjkey:   2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d

If you prefer the CA private key and X.509 certificate to be in binary DER format then just omit the --outform pem option. The directory /etc/swanctl/x509ca contains all required CA certificates either in binary DER or in Base64 PEM format. Irrespective of the file suffix the correct format will be determined by strongSwan automagically.

Generating a Host or User End Entity Certificate

Again we are using the command

pki --gen --type ed25519 --outform pem > moonKey.pem

to generate an Ed25519 private key for the host moon. Alternatively you could type

pki --gen --type rsa --size 3072 > moonKey.der

to generate a traditional 3072 bit RSA key and store it in binary DER format. As an alternative a TPM 2.0 Trusted Platform Module available on every recent Intel platform could be used as a virtual smartcard to securely store an RSA or ECDSA private key. For details, refer to the TPM 2.0 HOWTO.

In a next step the command

pki --req --type priv --in moonKey.pem \
          --dn "C=CH, O=strongswan, CN=moon.strongswan.org" \
          --san moon.strongswan.org --outform pem > moonReq.pem

creates a PKCS#10 certificate request that has to be signed by the CA. Through the [multiple] use of the --san parameter any number of desired subjectAlternativeNames can be added to the request. These can be of the form

--san sun.strongswan.org     # fully qualified host name
--san carol@strongswan.org   # RFC822 user email address
--san 192.168.0.1            # IPv4 address
--san fec0::1                # IPv6 address

Based on the certificate request the CA issues a signed end entity certificate with the following command

pki --issue --cacert strongswanCert.pem --cakey strongswanKey.pem \
            --type pkcs10 --in moonReq.pem --serial 01 --lifetime 1826 \
            --outform pem > moonCert.pem

If the --serial parameter with a hexadecimal argument is omitted then a random serial number is generated. Some third party VPN clients require that a VPN gateway certificate contains the TLS Server Authentication Extended Key Usage (EKU) flag which can be included with the following option

--flag serverAuth

If you want to use the dynamic CRL fetching feature described in one of the following sections then you may include one or several crlDistributionPoints in your end entity certificates using the --crl parameter

--crl  http://crl.strongswan.org/strongswan.crl
--crl "ldap://ldap.strongswan.org/cn=strongSwan Root CA, o=strongSwan,c=CH?certificateRevocationList"

The issued host certificate can be listed with

pki --print --in moonCert.pem

subject:  "C=CH, O=strongSwan, CN=moon.strongswan.org"
issuer:   "C=CH, O=strongSwan, CN=strongSwan Root CA"
validity:  not before May 19 10:28:19 2017, ok
           not after  May 19 10:28:19 2022, ok (expires in 1825 days)
serial:    01
altNames:  moon.strongswan.org
flags:     serverAuth
CRL URIs:  http://crl.strongswan.org/strongswan.crl
authkeyId: 2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d
subjkeyId: 60:9d:de:30:a6:ca:b9:8e:87:bb:33:23:61:19:18:b8:c4:7e:23:8f
pubkey:    ED25519 256 bits
keyid:     39:1b:b3:c2:34:72:1a:01:08:40:ce:97:75:b8:be:ce:24:30:26:29
subjkey:   60:9d:de:30:a6:ca:b9:8e:87:bb:33:23:61:19:18:b8:c4:7e:23:8f

Usually, a Windows, OSX, Android or iOS based VPN client needs its private key, its host or user certificate and the CA certificate. The most convenient way to load this information is to put everything into a PKCS#12 container:

openssl pkcs12 -export -inkey carolKey.pem \
               -in carolCert.pem -name "carol" \
               -certfile strongswanCert.pem -caname "strongSwan Root CA" \
               -out carolCert.p12

The strongSwan pki tool currently is not able to create PKCS#12 containers so that openssl must be used.

Generating a CRL

An empty CRL that is signed by the CA can be generated with the command

pki --signcrl --cacert strongswanCert.pem --cakey strongswanKey.pem \
              --lifetime 30 > strongswan.crl

If you omit the --lifetime option then the default value of 15 days is used. CRLs can either be uploaded to a HTTP or LDAP server or put in binary DER or Base64 PEM format into the /etc/swanctl/x509crl directory from where they are loaded into the charon daemon with the command

swanctl --load-creds

Revoking a Certificate

A specific end entity certificate is revoked with the command

pki --signcrl --cacert strongswanCert.pem --cakey strongswanKey.pem \
              --lifetime 30 --lastcrl strongswan.crl \
              --reason key-compromise --cert moonCert.pem > new.crl

Instead of the certificate file (in our example moonCert.pem), the serial number of the certificate to be revoked can be indicated using the --serial parameter. The pki --signcrl --help command documents all possible revocation reasons but the --reason parameter can also be omitted. The content of the new CRL file can be listed with the command

pki --print --type crl --in new.crl

issuer:   "C=CH, O=strongSwan, CN=strongSwan Root CA"
update:    this on May 19 11:13:01 2017, ok
           next on Jun 18 11:13:01 2017, ok (expires in 29 days)
serial:    02
authKeyId: 2b:95:14:5b:c3:22:87:de:d1:42:91:88:63:b3:d5:c1:92:7a:0f:5d
1 revoked certificate:
  01: May 19 11:13:01 2017, key compromise

Local Caching of CRLs

The strongswan.conf option

charon {
    cache_crls = yes
}

activates the local caching of CRLs that were dynamically fetched from an HTTP or LDAP server. Cached copies are stored in /etc/swanctl/x509crl using a unique filename formed from the issuer's subjectKeyIdentifier and the suffix .crl.

With the cached copy the CRL is immediately available after startup. When the local copy has become stale, an updated CRL is automatically fetched from one of the defined CRL distribution points during the next IKEv2 authentication.