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 and our wiki.
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.
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/priv/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/priv/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.
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/priv/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/priv/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
}
}
}
}
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/priv/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/priv/carolKey.pem
/etc/swanctl/swanctl.conf:
connections {
home {
remote_addrs = moon.strongswan.org
local {
auth = pubkey
certs = carolCert.pem
id = [email protected]
}
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
[email protected] which must be included as a subjectAlternativeName in
the roadwarrior certificate carolCert.pem.
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/rsa/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/priv/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 = [email protected]
}
remote {
auth = pubkey
id = moon.strongswan.org
}
children {
home {
local_ts = 10.1.0.0/16
start_action = start
}
}
}
}
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/priv/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 = [email protected]
secret = Ar3etTnp
}
eap-dave {
id = [email protected]
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 = [email protected]
}
remote {
auth = pubkey
id = moon.strongswan.org
}
children {
home {
local_ts = 10.1.0.0/16
start_action = start
}
}
}
}
secrets {
eap-carol {
id = [email protected]
secret = Ar3etTnp
}
}
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/priv/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
}
}
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.
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.
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 [email protected] # 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.
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
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
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.