# 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**](http://wiki.strongswan.org). ## 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 "" /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 "" /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 "" /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 "" /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 "" /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 "" /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 "" /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 "" /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: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 ### 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=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. ### 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 "" 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 | 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. ### 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