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.
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.
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
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
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
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
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.
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.
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:carol@strongswan.org
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
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.
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.
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.
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.
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 | |
| emailAddress | |
| 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=carol@strongswan.org
conn dave
right=%any
rightid="C=CH, O=strongSwan, CN=dave@strongswan.org"
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=carol@strongswan.org
conn dave
right=192.168.0.200
rightid="C=CH, O=strongSwan, CN=dave@strongswan.org"
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.
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
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.
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.
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
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/
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.
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.
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.
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.
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.
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
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"
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.
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'
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 |
strongSwan makes the following environment variables available
in the updown script indicated by the leftupdown option:
| Variable | Example | Comment |
|---|---|---|
| $PLUTO_PEER_ID | carol@strongswan.org | 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.
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