gollub / strongswan

strongSwan - IPsec for Linux

strongSwan Configuration

Overview

strongSwan is an OpenSource IPsec-based VPN solution.

This document is just a short introduction, for more detailed information consult the man pages and our wiki.

Quickstart

In the following examples we assume, for reasons of clarity, that left designates the local host and that right is the remote host.

Certificates for users, hosts and gateways are issued by a fictitious strongSwan CA. How to generate private keys and certificates using OpenSSL or the strongSwan PKI tool will be explained in one of the sections below. The CA certificate strongswanCert.pem must be present on all VPN endpoints in order to be able to authenticate the peers.

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/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/moonCert.pem

/etc/ipsec.secrets:

    : RSA moonKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn net-net
        leftsubnet=10.1.0.0/16
        leftcert=moonCert.pem
        right=192.168.0.2
        rightsubnet=10.2.0.0/16
        rightid="C=CH, O=strongSwan, CN=sun.strongswan.org"
        auto=start

Configuration on gateway sun:

/etc/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/sunCert.pem

/etc/ipsec.secrets:

    : RSA sunKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn net-net
        leftsubnet=10.2.0.0/16
        leftcert=sunCert.pem
        right=192.168.0.1
        rightsubnet=10.1.0.0/16
        rightid="C=CH, O=strongSwan, CN=moon.strongswan.org"
        auto=start

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/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/moonCert.pem

/etc/ipsec.secrets:

    : RSA moonKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn host-host
        leftcert=moonCert.pem
        right=192.168.0.2
        rightid="C=CH, O=strongSwan, CN=sun.strongswan.org"
        auto=start

Configuration on host sun:

/etc/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/sunCert.pem

/etc/ipsec.secrets:

    : RSA sunKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn host-host
        leftcert=sunCert.pem
        right=192.168.0.1
        rightid="C=CH, O=strongSwan, CN=moon.strongswan.org"
        auto=start

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/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/moonCert.pem

/etc/ipsec.secrets:

    : RSA moonKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn rw
        leftsubnet=10.1.0.0/16
        leftcert=moonCert.pem
        right=%any
        auto=add

Configuration on roadwarrior carol:

/etc/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/carolCert.pem

/etc/ipsec.secrets:

    : RSA carolKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn home
        leftcert=carolCert.pem
        right=192.168.0.1
        rightsubnet=10.1.0.0/16
        rightid="C=CH, O=strongSwan, CN=moon.strongswan.org"
        auto=start

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 parameter

rightsourceip=10.3.0.0/16

to the gateway's ipsec.conf. 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

leftsourceip=%config

Configuration on gateway moon:

/etc/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/moonCert.pem

/etc/ipsec.secrets:

    : RSA moonKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn rw
        leftsubnet=10.1.0.0/16
        leftcert=moonCert.pem
        right=%any
        rightsourceip=10.3.0.0/16
        auto=add

Configuration on roadwarrior carol:

/etc/ipsec.d/cacerts/strongswanCert.pem

/etc/ipsec.d/certs/carolCert.pem

/etc/ipsec.secrets:

    : RSA carolKey.pem "<optional passphrase>"

/etc/ipsec.conf:

    conn home
        leftsourceip=%config
        leftcert=carolCert.pem
        right=192.168.0.1
        rightsubnet=10.1.0.0/16
        rightid="C=CH, O=strongSwan, CN=moon.strongswan.org"
        auto=start

Generating certificates and CRLs

This section is not a full-blown tutorial on how to use OpenSSL or 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 OpenSSL statement

openssl req -x509 -days 1460 -newkey rsa:4096 \
            -keyout strongswanKey.pem -out strongswanCert.pem

creates a 4096 bit RSA private key strongswanKey.pem and a self-signed CA certificate strongswanCert.pem with a validity of 4 years (1460 days).

openssl x509 -in cert.pem -noout -text

lists the properties of a X.509 certificate cert.pem. It allows you to verify whether the configuration defaults in openssl.cnf have been inserted correctly.

If you prefer the CA certificate to be in binary DER format then the following command achieves this transformation:

 openssl x509 -in strongswanCert.pem -outform DER -out strongswanCert.der

The statements

ipsec pki --gen -s 4096 > strongswanKey.der
ipsec pki --self --ca --lifetime 1460 --in strongswanKey.der \
          --dn "C=CH, O=strongSwan, CN=strongSwan Root CA" \
          > strongswanCert.der
ipsec pki --print --in strongswanCert.der

achieve about the same with the strongSwan PKI tool. Unlike OpenSSL the tool stores keys and certificates in the binary DER format by default. The --outform option may be used to write PEM encoded files.

The directory /etc/ipsec.d/cacerts 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.

Generating a host or user certificate

The OpenSSL statement

 openssl req -newkey rsa:2048 -keyout hostKey.pem \
             -out hostReq.pem

generates a 2048 bit RSA private key hostKey.pem and a certificate request hostReq.pem which has to be signed by the CA.

If you want to add a subjectAltName field to the host certificate you must edit the OpenSSL configuration file openssl.cnf and add the following line in the [ usr_cert ] section:

 subjectAltName=DNS:moon.strongswan.org

if you want to identify the host by its Fully Qualified Domain Name (FQDN), or

 subjectAltName=IP:192.168.0.1

if you want the ID to be of type IPV4_ADDR. Of course you could include both ID types with

 subjectAltName=DNS:moon.strongswan.org,IP:192.168.0.1

but the use of an IP address for the identification of a host should be discouraged anyway.

For user certificates the appropriate ID type is RFC822_ADDR which can be specified as

 subjectAltName=email:[e-mail]

or if the user's e-mail address is part of the subject's distinguished name

 subjectAltName=email:copy

Now the certificate request can be signed by the CA with the command

 openssl ca -in hostReq.pem -days 730 -out hostCert.pem -notext

If you omit the -days option then the default_days value (365 days) specified in openssl.cnf is used. The -notext option avoids that a human readable listing of the certificate is prepended to the Base64 encoded certificate body.

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 certificates. This can be done in the [ usr_cert ] section of the openssl.cnf configuration file:

crlDistributionPoints=@crl_dp

[ crl_dp ]

URI.1="http://crl.strongswan.org/strongswan.crl"
URI.2="ldap://ldap.strongswan.org/cn=strongSwan Root CA, o=strongSwan,
       c=CH?certificateRevocationList"

If you have only a single HTTP distribution point then the short form

crlDistributionPoints="URI:http://crl.strongswan.org/strongswan.crl"

also works.

Again the statements

ipsec pki --gen > moonKey.der
ipsec pki --pub --in moonKey.der | ipsec pki --issue --lifetime 730 \
          --cacert strongswanCert.der --cakey strongswanKey.der \
          --dn "C=CH, O=strongSwan, CN=moon.strongswan.org" \
          --san moon.strongswan.org --san 192.168.0.1 \
          --crl http://crl.strongswan.org/strongswan.crl > moonCert.der

do something similar using the strongSwan PKI tool.

Usually, a Windows or Mac OS X (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

Generating a CRL

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

 openssl ca -gencrl -crldays 15 -out crl.pem

If you omit the -crldays option then the default_crl_days value (30 days) specified in openssl.cnf is used.

If you prefer the CRL to be in binary DER format then this conversion can be achieved with

 openssl crl -in crl.pem -outform DER -out cert.crl

The strongSwan PKI tool provides the --signcrl command to sign CRLs.

The directory /etc/ipsec.d/crls contains all CRLs either in binary DER or in Base64 PEM format, irrespective of the file suffix the correct format will be determined.

Revoking a certificate

A specific host certificate stored in the file host.pem is revoked with the command

 openssl ca -revoke host.pem

Next the CRL file must be updated

 openssl ca -gencrl -crldays 60 -out crl.pem

The content of the CRL file can be listed with the command

 openssl crl -in crl.pem -noout -text

in the case of a Base64 CRL, or alternatively for a CRL in DER format

 openssl crl -inform DER -in cert.crl -noout -text

Again the --signcrl command of the strongSwan PKI tool may also be used to create new CRLs containing additional certificates.

Configuring the connections - ipsec.conf

Configuring my side

Usually the local side is the same for all connections. Therefore it makes sense to put the definitions characterizing the strongSwan security gateway into the conn %default section of the configuration file /etc/ipsec.conf. If we assume throughout this document that the strongSwan security gateway is left and the peer is right then we can write

conn %default
     leftcert=moonCert.pem
     # load connection definitions automatically
     auto=add

The X.509 certificate by which the strongSwan security gateway will authenticate itself by sending it in binary form to its peers as part of the Internet Key Exchange (IKE) is specified in the line

 leftcert=moonCert.pem

The certificate can either be stored in Base64 PEM-format or in the binary DER-format. Irrespective of the file suffix the correct format will be determined. Therefore leftcert=moonCert.der or leftcert=moonCert.cer would also be valid alternatives.

When using relative pathnames as in the examples above, the certificate files must be stored in in the directory /etc/ipsec.d/certs. In order to distinguish strongSwan's own certificates from locally stored trusted peer certificates (see below for details), they could also be stored in a subdirectory below /etc/ipsec.d/certs as e.g. in

leftcert=mycerts/moonCert.pem

Absolute pathnames are also possible as in

leftcert=/usr/ssl/certs/moonCert.pem

As an ID for the VPN gateway we recommend the use of a Fully Qualified Domain Name (FQDN) of the form

conn rw
     right=%any
     leftid=moon.strongswan.org

Important: When a FQDN identifier is used it must be explicitly included as a so called subjectAltName of type dnsName (DNS:) in the certificate indicated by leftcert. For details on how to generate certificates with subjectAltNames, please refer to the sections above.

If you don't want to mess with subjectAltNames, you can use the certificate's Distinguished Name (DN) instead, which is an identifier of type DER_ASN1_DN and which can be written e.g. in the LDAP-type format

conn rw
     right=%any
     leftid="C=CH, O=strongSwan, CN=moon.strongswan.org"

Since the subject's DN is part of the certificate, the leftid does not have to be declared explicitly. Thus the entry

conn rw
     right=%any

automatically assumes the subject DN of leftcert to be the host ID.

Multiple certificates

strongSwan supports multiple local host certificates and corresponding RSA private keys:

conn rw1
     right=%any
     rightid=peer1.domain1
     leftcert=myCert1.pem
     # leftid is DN of myCert1

conn rw2
     right=%any
     rightid=peer2.domain2
     leftcert=myCert2.pem
     # leftid is DN of myCert2

When peer1 initiates a connection then strongSwan will send myCert1 and will sign with myKey1 defined in /etc/ipsec.secrets (see below) whereas myCert2 and myKey2 will be used in a connection setup started from peer2.

Configuring the peer side using CA certificates

Now we can proceed to define our connections. In many applications we might have dozens of road warriors connecting to a central strongSwan security gateway. The following most simple statement:

conn rw
     right=%any

defines the general roadwarrior case. The line right=%any literally means that any IPsec peer is accepted, regardless of its current IP source address and its ID, as long as the peer presents a valid X.509 certificate signed by a CA the strongSwan security gateway puts explicit trust in. Additionally, the signature during IKE gives proof that the peer is in possession of the private key matching the public key contained in the transmitted certificate.

The ID by which a peer is identifying itself during IKE can by any of the ID types _IPV[46]ADDR, FQDN, RFC822_ADDR or DER_ASN1_DN. If one of the first three ID types is used, then the accompanying X.509 certificate of the peer must contain a matching subjectAltName field of the type ipAddress (IP:), dnsName (DNS:) or rfc822Name (email:), respectively. With the fourth type, DER_ASN1_DN, the identifier must completely match the subject field of the peer's certificate. One of the two possible representations of a Distinguished Name (DN) is the LDAP-type format

 rightid="C=CH, O=strongSwan IPsec, CN=sun.strongswan.org"

Additional whitespace can be added everywhere as desired since it will be automatically eliminated by the parser. An exception is the single whitespace between individual words, like e.g. in strongSwan IPsec, which is preserved.

The Relative Distinguished Names (RDNs) can alternatively be separated by a slash / instead of a comma ,

 rightid="/C=CH/O=strongSwan IPsec/CN=sun.strongswan.org"

This is the representation extracted from the certificate by the OpenSSL -subject command line option

 openssl x509 -in sunCert.pem -noout -subject

The following RDNs are supported by strongSwan

Name	Description
DC	Domain Component
C	Country
ST	State or province
L	Locality or town
O	Organization
OU	Organizational Unit
CN	Common Name
ND	NameDistinguisher, used with CN
N	Name
G	Given name
S	Surname
I	Initials
T	Personal title
E	E-mail
Email	E-mail
emailAddress	E-mail
SN	Serial number
serialNumber	Serial number
D	Description
ID	X.500 Unique Identifier
UID	User ID
TCGID	[Siemens] Trust Center Global ID
UN	Unstructured Name
unstructuredName	Unstructured Name
UA	Unstructured Address
unstructuredAddress	Unstructured Address
EN	Employee Number
employeeNumber	Employee Number
dnQualifier	DN Qualifier

With the roadwarrior connection definition listed above, an IPsec SA for the strongSwan security gateway moon.strongswan.org itself can be established. If the roadwarriors should be able to reach e.g. the two subnets 10.1.0.0/24 and 10.1.3.0/24 behind the security gateway then the following connection definitions will make this possible

conn rw1
     right=%any
     leftsubnet=10.1.0.0/24

conn rw3
     right=%any
     leftsubnet=10.1.3.0/24

For IKEv2 connections this can even be simplified by using

leftsubnet=10.1.0.0/24,10.1.3.0/24

If not all peers in possession of a X.509 certificate signed by a specific certificate authority shall be given access to the Linux security gateway, then either a subset of them can be barred by listing the serial numbers of their certificates in a certificate revocation list (CRL) or as an alternative, access can be controlled by explicitly putting a roadwarrior entry for each eligible peer into ipsec.conf:

conn sun
     right=%any
     rightid=sun.strongswan.org

conn carol
     right=%any
     rightid=[e-mail]

conn dave
     right=%any
     rightid="C=CH, O=strongSwan, CN=[e-mail]"

When the IP address of a peer is known to be stable, it can be specified as well. This entry is mandatory when the strongSwan host wants to act as the initiator of an IPsec connection.

conn sun
     right=192.168.0.2
     rightid=sun.strongswan.org

conn carol
     right=192.168.0.100
     rightid=[e-mail]

conn dave
     right=192.168.0.200
     rightid="C=CH, O=strongSwan, CN=[e-mail]"

conn venus
     right=192.168.0.50

In the last example the ID types FQDN, RFC822_ADDR, DER_ASN1_DN and IPV4_ADDR, respectively, were used. Of course all connection definitions presented so far have included the lines in the conn %defaults section, comprising among other a leftcert entry.

Handling Virtual IPs and narrowing

Often roadwarriors are behind NAT-boxes, which causes the inner IP source address of an IPsec tunnel to be different from the outer IP source address usually assigned dynamically by the ISP. Whereas the varying outer IP address can be handled by the right=%any construct, the inner IP address or subnet must always be declared in a connection definition. Therefore for the three roadwarriors rw1 to rw3 connecting to a strongSwan security gateway the following entries are required in /etc/ipsec.conf:

conn rw1
     right=%any
     righsubnet=10.4.0.5/32

conn rw2
     right=%any
     rightsubnet=10.4.0.47/32

conn rw3
     right=%any
     rightsubnet=10.4.0.128/28

Because the charon daemon uses narrowing (even for IKEv1) these three entries can be reduced to the single connection definition

conn rw
     right=%any
     rightsubnet=10.4.0.0/24

Any host will be accepted (of course after successful authentication based on the peer's X.509 certificate only) if it declares a client subnet lying totally within the boundaries defined by the subnet definition (in our example 10.4.0.0/24).

This strongSwan feature can also be helpful with VPN clients getting a dynamically assigned inner IP from a DHCP server located on the NAT router box.

Since the private IP address of roadwarriors will often not be known they are usually assigned virtual IPs from a predefined pool. This also makes routing traffic back to the roadwarriors easier. For example, to assign each client an IP address from the 10.5.0.0/24 subnet conn rw can be defined as

conn rw
     right=%any
     rightsourceip=10.5.0.0/24

Protocol and Port Selectors

strongSwan offers the possibility to restrict the protocol and optionally the ports in an IPsec SA using the rightprotoport and leftprotoport parameters. For IKEv2 multiple such restrictions can also be configured in leftsubnet and rightsubnet.

Some examples:

conn icmp
     right=%any
     rightprotoport=icmp
     leftid=moon.strongswan.org
     leftprotoport=icmp

conn http
     right=%any
     rightprotoport=6
     leftid=moon.strongswan.org
     leftprotoport=6/80

conn l2tp
     right=%any
     # with port wildcard for interoperability with certain L2TP clients
     rightprotoport=17/%any
     leftid=moon.strongswan.org
     leftprotoport=17/1701

conn dhcp
     right=%any
     rightprotoport=udp/bootpc
     leftid=moon.strongswan.org
     leftsubnet=0.0.0.0/0  #allows DHCP discovery broadcast
     leftprotoport=udp/bootps

Protocols and ports can be designated either by their numerical values or by their acronyms defined in /etc/services.

Based on the protocol and port selectors appropriate policies will be set up, so that only the specified payload types will pass through the IPsec tunnel.

IPsec policies based on wildcards

In large VPN-based remote access networks there is often a requirement that access to the various parts of an internal network must be granted selectively, e.g. depending on the group membership of the remote access user. strongSwan makes this possible by applying wildcard filtering on the VPN user's distinguished name (ID_DER_ASN1_DN).

Let's make a practical example:

An organization has a sales department (OU=Sales) and a research group (OU=Research). In the company intranet there are separate subnets for Sales (10.0.0.0/24) and Research (10.0.1.0/24) but both groups share a common web server (10.0.2.100). The VPN clients use Virtual IP addresses that are either assigned statically or from a dynamic pool. The sales and research departments use IP addresses from separate address pools (10.1.0.0/24) and (10.1.1.0/24), respectively. An X.509 certificate is issued to each employee, containing in its subject distinguished name the country (C=CH), the company (O=ACME), the group membership (OU=Sales or OU=Research) and the common name (e.g. CN=Bart Simpson).

The IPsec policy defined above can now be enforced with the following three IPsec security associations:

conn sales
     right=%any
     rightid="C=CH, O=ACME, OU=Sales, CN=*"
     rightsourceip=10.1.0.0/24       # Sales IP range
     leftsubnet=10.0.0.0/24          # Sales subnet

conn research
     right=%any
     rightid="C=CH, O=ACME, OU=Research, CN=*"
     rightsourceip=10.1.1.0/24       # Research IP range
     leftsubnet=10.0.1.0/24          # Research subnet

conn web
     right=%any
     rightid="C=CH, O=ACME, OU=*, CN=*"
     rightsubnet=10.1.0.0/23         # Remote access IP range
     leftsubnet=10.0.2.100/32        # Web server
     rightprotoport=tcp              # TCP protocol only
     leftprotoport=tcp/http          # TCP port 80 only

The * character is used as a wildcard in relative distinguished names (RDNs). In order to match a wildcard template, the ID_DER_ASN1_DN of a peer must contain the same number of RDNs (selected from the list given earlier) appearing in the exact order defined by the template.

"C=CH, O=ACME, OU=Research, OU=Special Effects, CN=Bart Simpson"

matches the templates

"C=CH, O=ACME, OU=Research, OU=*, CN=*"
"C=CH, O=ACME, OU=*, OU=Special Effects, CN=*"
"C=CH, O=ACME, OU=*, OU=*, CN=*"

but not the template

"C=CH, O=ACME, OU=*, CN=*"

which doesn't have the same number of RDNs.

IPsec policies based on CA certificates

As an alternative to the wildcard based IPsec policies described above, access to specific client host and subnets can be controlled on the basis of the CA that issued the peer certificate

conn sales
     right=%any
     rightca="C=CH, O=ACME, OU=Sales, CN=Sales CA"
     rightsourceip=10.1.0.0/24       # Sales IP range
     leftsubnet=10.0.0.0/24          # Sales subnet

conn research
     right=%any
     rightca="C=CH, O=ACME, OU=Research, CN=Research CA"
     rightsourceip=10.1.1.0/24       # Research IP range
     leftsubnet=10.0.1.0/24          # Research subnet

conn web
     right=%any
     rightca="C=CH, O=ACME, CN=ACME Root CA"
     rightsubnet=10.1.0.0/23         # Remote access IP range
     leftsubnet=10.0.2.100/32        # Web server
     rightprotoport=tcp              # TCP protocol only
     leftprotoport=tcp/http          # TCP port 80 only

In the example above, the connection sales can be used by peers presenting certificates issued by the Sales CA, only. In the same way, the use of the connection research is restricted to owners of certificates issued by the Research CA. The connection web is open to both "Sales" and "Research" peers because the required ACME Root CA is the issuer of the Research and Sales intermediate CAs. If no rightca parameter is present then any valid certificate issued by one of the trusted CAs in /etc/ipsec.d/cacerts can be used by the peer.

The leftca parameter usually doesn't have to be set explicitly because by default it is set to the issuer field of the certificate loaded via leftcert. The statement

 rightca=%same

sets the CA requested from the peer to the CA used by the left side itself as e.g. in

conn sales
     right=%any
     rightca=%same
     leftcert=mySalesCert.pem

Configuring certificates and CRLs

Installing the CA certificates

X.509 certificates received by strongSwan during the IKE protocol are automatically authenticated by going up the trust chain until a self-signed root CA certificate is reached. Usually host certificates are directly signed by a root CA, but strongSwan also supports multi-level hierarchies with intermediate CAs in between. All CA certificates belonging to a trust chain must be copied in either binary DER or Base64 PEM format into the directory

 /etc/ipsec.d/cacerts/

Installing optional certificate revocation lists (CRLs)

By copying a CA certificate into /etc/ipsec.d/cacerts/, automatically all user or host certificates issued by this CA are declared valid. Unfortunately, private keys might get compromised inadvertently or intentionally, personal certificates of users leaving a company have to be blocked immediately, etc. To this purpose certificate revocation lists (CRLs) have been created. CRLs contain the serial numbers of all user or host certificates that have been revoked due to various reasons.

After successful verification of the X.509 trust chain, strongSwan searches its list of CRLs, either obtained by loading them from the /etc/ipsec.d/crls/ directory, or fetching them dynamically from a HTTP or LDAP server, for the presence of a CRL issued by the CA that has signed the certificate.

If the serial number of the certificate is found in the CRL then the public key contained in the certificate is declared invalid and the IKE SA will not be established. If no CRL is found or if the deadline defined in the nextUpdate field of the CRL has been reached, a warning is issued but the public key will nevertheless be accepted (this behavior can be changed, see below). CRLs must be stored either in binary DER or Base64 PEM format in the crls directory.

Dynamic update of certificates and CRLs

strongSwan reads certificates and CRLs from their respective files during system startup and keeps them in memory. X.509 certificates have a finite life span defined by their validity field. Therefore it must be possible to replace CA or OCSP certificates kept in system memory without disturbing established IKE SAs. Certificate revocation lists should also be updated in the regular intervals indicated by the nextUpdate field in the CRL body. The following interactive commands allow the manual replacement of the various files:

Command	Action
ipsec rereadaacerts	reload all files in /etc/ipsec.d/aacerts/
ipsec rereadacerts	reload all files in /etc/ipsec.d/acerts/
ipsec rereadcacerts	reload all files in /etc/ipsec.d/cacerts/
ipsec rereadcrls	reload all files in /etc/ipsec.d/crls/
ipsec rereadocspcerts	reload all files in /etc/ipsec.d/ocspcerts/
ipsec rereadsecrets	reload /etc/ipsec.secrets and configured keys
ipsec rereadall	all the commands above combined
ipsec purgecerts	purge all cached certificates
ipsec purgecrl	purge all cached CRLs
ipsec purgeocsp	purge the OCSP cache

CRLs can also be automatically fetched from an HTTP or LDAP server by using the CRL distribution points contained in X.509 certificates.

Local caching of CRLs

The ipsec.conf option

config setup
     cachecrls=yes

activates the local caching of CRLs that were dynamically fetched from an HTTP or LDAP server. Cached copies are stored in /etc/ipsec.d/crls 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 is about to expire it is automatically replaced with an updated CRL fetched from one of the defined CRL distribution points.

Online Certificate Status Protocol (OCSP)

The Online Certificate Status Protocol is defined by RFC 2560. It can be used to query an OCSP server about the current status of an X.509 certificate and is often used as a more dynamic alternative to a static Certificate Revocation List (CRL). Both the OCSP request sent by the client and the OCSP response messages returned by the server are transported via a standard TCP/HTTP connection.

In the simplest OCSP setup, a default URI under which the OCSP server for a given CA can be accessed is defined in ipsec.conf:

ca strongswan
     cacert=strongswanCert.pem
     ocspuri=http://ocsp.strongswan.org:8880
     auto=add

The HTTP port can be freely chosen.

OpenSSL implements an OCSP server that can be used in conjunction with an OpenSSL-based Public Key Infrastructure. The OCSP server is started with the following command:

openssl ocsp -index index.txt -CA strongswanCert.pem -port 8880 \
             -rkey ocspKey.pem -rsigner ocspCert.pem \
             -resp_no_certs -nmin 60 -text

The command consists of the parameters

-index   index.txt is a copy of the OpenSSL index file containing the list
         of all issued certificates.  The certificate status in index.txt
         is designated either by V for valid or R for revoked.  If a new
         certificate is added or if a certificate is revoked using the
         openssl ca command, the OCSP server must be restarted in order for
         the changes in index.txt to take effect.

-CA      the CA certificate

-port    the HTTP port the OCSP server is listening on.

-rkey    the private key used to sign the OCSP response.  The use of the
         sensitive CA private key is not recommended since this could
         jeopardize the security of your production PKI if the OCSP
         server is hacked.  It is much better to generate a special
         RSA private key just for OCSP signing use instead.

-rsigner the certificate of the OCSP server containing a public key which
         matches the private key defined by -rkey and which can be used by
         the client to check the trustworthiness of the signed OCSP
         response.

-resp_no_certs  With this option the OCSP signer certificate defined by
                -rsigner is not included in the OCSP response.

-nmin    the validity interval of an OCSP response given in minutes.

-text    this option activates a verbose logging output, showing the
         contents of both the received OCSP request and sent OCSP response.

The OCSP signer certificate can either be put into the default directory

/etc/ipsec.d/ocspcerts

or alternatively strongSwan can receive it as part of the OCSP response from the remote OCSP server. In order to verify that the server is indeed authorized by a CA to deal out certificate status information an extended key usage attribute must be included in the OCSP server certificate. Just insert the parameter

extendedKeyUsage=OCSPSigner

in the [ usr_cert ] section of your openssl.cnf configuration file before the CA signs the OCSP server certificate.

For a given CA the corresponding ca section in ipsec.conf (see below) allows to define the URI of a single OCSP server. As an alternative an OCSP URI can be embedded into each host and user certificate by putting the line

authorityInfoAccess = OCSP;URI:http://ocsp.strongswan.org:8880

into the [ usr_cert ] section of your openssl.cnf configuration file. If an OCSP authorityInfoAccess extension is present in a certificate then this record overrides the default URI defined by the ca section.

CRL Policy

By default strongSwan is quite tolerant concerning the handling of CRLs. It is not mandatory for a CRL to be present in /etc/ipsec.d/crls and if the expiration date defined by the nextUpdate field of a CRL has been reached just a warning is issued but a peer certificate will always be accepted if it has not been revoked.

If you want to enforce a stricter CRL policy then you can do this by setting the strictcrlpolicy option. This is done in the config setup section of the ipsec.conf file:

config setup
     strictcrlpolicy=yes
      ...

A certificate received from a peer will not be accepted if no corresponding CRL or OCSP response is available. And if an IKE SA re-negotiation takes place after the nextUpdate deadline has been reached, the peer certificate will be declared invalid and the cached public key will be deleted, causing the connection in question to fail. Therefore if you are going to use the strictcrlpolicy=yes option, make sure that the CRLs will always be updated in time. Otherwise a total standstill might ensue.

As mentioned earlier the default setting is strictcrlpolicy=no.

Configuring the peer side using locally stored certificates

If you don't want to use trust chains based on CA certificates as proposed above you can alternatively import trusted peer certificates directly.

With the conn %default section defined above and the use of the rightcert keyword for the peer side, the connection definitions presented earlier can alternatively be written as

conn sun
      right=%any
      rightid=sun.strongswan.org
      rightcert=sunCert.cer

 conn carol
      right=192.168.0.100
      rightcert=carolCert.der

If the peer certificates are loaded locally then there is no need to send any certificates to the other end via the IKE protocol. Especially if self-signed certificates are used which wouldn't be accepted anyway by the other side. In these cases it is recommended to add

leftsendcert=never

to the connection definition(s) in order to avoid the sending of the host's own certificate. The default value is

leftsendcert=ifasked

which causes certificates to only be sent if a certificate request is received. If a peer does not send a certificate request then the setting

leftsendcert=always

may be used to force sending of the certificate to the other peer.

If a peer certificate contains a subjectAltName extension, then an alternative rightid type can be used, as the example conn sun shows. If no rightid entry is present then the subject distinguished name contained in the certificate is taken as the ID.

Using the same rules concerning pathnames that apply to the gateway's own certificates, the following two definitions are also valid for trusted peer certificates:

rightcert=peercerts/carolCert.der

or

rightcert=/usr/ssl/certs/carolCert.der

Configuring the private keys - ipsec.secrets

Loading private key files

strongSwan is able to load RSA (or ECDSA) private keys in the PKCS#1 or PKCS#8 file formats, or from PKCS#12 containers. The key files can optionally be secured with a passphrase.

RSA private key files are declared in /etc/ipsec.secrets using the syntax

: RSA <my keyfile> "<optional passphrase>"

The key file can be either in Base64 PEM-format or binary DER-format. The actual coding is detected automatically. The example

: RSA moonKey.pem

uses a pathname relative to the default directory

/etc/ipsec.d/private

As an alternative an absolute pathname can be given as in

: RSA /usr/ssl/private/moonKey.pem

In both cases make sure that the key files are root readable only.

Often a private key must be transported from the Certification Authority where it was generated to the target security gateway where it is going to be used. In order to protect the key it can be encrypted with a symmetric cipher using a transport key derived from a cryptographically strong passphrase.

Once on the security gateway the private key can either be permanently unlocked so that it can be used by the IKE daemon without having to know a passphrase

openssl rsa -in moonKey.pem -out moonKey.pem

or as an option the key file can remain secured. In this case the passphrase unlocking the private key must be added after the pathname in /etc/ipsec.secrets

: RSA moonKey.pem "This is my passphrase"

Some CAs distribute private keys embedded in a PKCS#12 file. strongSwan can read private keys directly from such a file (end-entity and CA certificates are also extracted):

: P12 moonCert.p12 "This is my passphrase"

Entering passphrases interactively

On a VPN gateway you would want to put the passphrase protecting the private key file right into /etc/ipsec.secrets as described in the previous section, so that the gateway can be booted in unattended mode. The risk of keeping unencrypted secrets on a server can be minimized by putting the box into a locked room. As long as no one can get root access on the machine the private keys are safe.

On a mobile laptop computer the situation is quite different. The computer can be stolen or the user may leave it unattended so that unauthorized persons can get access to it. In theses cases it would be preferable not to keep any passphrases openly in /etc/ipsec.secrets but to prompt for them interactively instead. This is easily done by defining

: RSA moonKey.pem %prompt

Since strongSwan is usually started during the boot process, usually no interactive console windows is available which can be used to prompt for the passphrase. This must be initiated by the user by typing

ipsec secrets

which actually is an alias for the existing command

ipsec rereadsecrets

and which causes a passphrase prompt to appear. To abort entering a passphrase enter just a carriage return.

Configuring CA properties - ipsec.conf

Besides the definition of IPsec connections the ipsec.conf file can also be used to configure a few properties of the certification authorities needed to establish the X.509 trust chains. The following example shows some of the parameters that are currently available:

ca strongswan
    cacert=strongswanCert.pem
    ocspuri=http://ocsp.strongswan.org:8880
    crluri=http://crl.strongswan.org/strongswan.crl'
    crluri2="ldap://ldap.strongswan.org/O=strongSwan, C=CH?certificateRevocationList"
    auto=add

In a similar way as conn sections are used for connection definitions, an arbitrary number of optional ca sections define the basic properties of CAs.

Each ca section is named with a unique label

ca strongswan

The only mandatory parameter is

cacert=strongswanCert.pem

which points to the CA certificate which usually resides in the default directory /etc/ipsec.d/cacerts/ but could also be retrieved via an absolute path name.

The OCSP URI

ocspuri=http://ocsp.strongswan.org:8880

allows to define an individual OCSP server per CA. Also up to two additional CRL distribution points (CDPs) can be defined

crluri=http://crl.strongswan.org/strongswan.crl'
crluri2="ldap://ldap.strongswan.org/O=strongSwan, C=CH?certificateRevocationList"

which are added to any CDPs already present in the received certificates themselves.

With the auto=add statement the ca definition is automatically loaded during startup. Setting auto=ignore will ignore the ca section.

Any parameters which appear in several ca definitions can be put in a common ca %default section

ca %default
    crluri=http://crl.strongswan.org/strongswan.crl'

Monitoring functions

strongSwan offers the following monitoring functions:

Command	Action
ipsec listaacerts	list all Authorization Authority certificates loaded from /etc/ipsec.d/aacerts/
ipsec listacerts	list all X.509 attribute certificates loaded from /etc/ipsec.d/acerts/
ipsec listalgs	list cryptographic algorithms for IKE
ipsec listcacerts	list all CA certificates loaded from /etc/ipsec.d/cacerts/ or received via IKE
ipsec listcainfos	list all properties defined in ca sections in ipsec.conf
ipsec listcerts	list all certificates loaded via leftcert and rightcert
ipsec listcounters	list global or connection specific counter values
ipsec listcrls	list all CLRs loaded from /etc/ipsec.d/crls/
ipsec listocsp	list contents of the OCSP response cache
ipsec listocspcerts	list all OCSP signer certificates loaded from /etc/ipsec.d/ocspcerts/ or received in OCSP responses
ipsec listplugins	list all loaded plugin features
ipsec listpubkeys	list all raw public keys e.g. loaded via leftsigkey and rightsigkey
ipsec listall	all the above commands combined
ipsec status	list concise status information on established connections
ipsec statusall	list detailed status information on connections

Firewall support functions

Environment variables in the updown script

strongSwan makes the following environment variables available in the updown script indicated by the leftupdown option:

Variable	Example	Comment
$PLUTO_PEER_ID	[e-mail]	RFC822_ADDR (1)
$PLUTO_PEER_PROTOCOL	17	udp (2)
$PLUTO_PEER_PORT	68	bootpc (3)
$PLUTO_MY_ID	moon.strongswan.org	FQDN (1)
$PLUTO_MY_PROTOCOL	17	udp (2)
$PLUTO_MY_PORT	67	bootps (3)

(1) $PLUTO_PEER_ID/$PLUTO_MY_ID contain the IDs of the two ends of an established connection. In our examples these correspond to the strings defined by rightid and leftid, respectively.

(2) $PLUTO_PEER_PROTOCOL/$PLUTO_MY_PROTOCOL contain the protocol defined by the rightprotoport and leftprotoport options, respectively. Both variables contain the same protocol value. The variables take on the value '0' if no protocol has been defined.

(3) $PLUTO_PEER_PORT/$PLUTO_MY_PORT contain the ports defined by the rightprotoport and leftprotoport options, respectively. The variables take on the value '0' if no port has been defined.

There are several more, refer to the provided default script for a documentation of them.

Automatic insertion and deletion of iptables firewall rules

The default _updown script automatically inserts and deletes dynamic iptables firewall rules upon the establishment or teardown, respectively, of an IPsec security association. This feature is activated with the line

leftfirewall=yes

If you define a leftsubnet with a netmask larger than /32 then the automatically inserted FORWARD iptables rules will not allow clients to access the internal IP address of the gateway even if it is part of that subnet definition. If you want additional INPUT and OUTPUT iptables rules to be inserted, so that the host itself can be accessed then add the following line:

lefthostaccess=yes

The _updown script also features a logging facility which will register the creation (+) and the expiration (-) of each successfully established VPN connection in a special syslog file in the following concise and easily readable format:

Jul 19 18:58:38 moon vpn:
    + carol.strongswan.org  192.168.0.100 -- 192.168.0.1 == 10.1.0.0/16
Jul 19 22:15:17 moon vpn:
    - carol.strongswan.org  192.168.0.100 -- 192.168.0.1 == 10.1.0.0/16