Installation and Configuration GuideX.509 FreeS/WAN Patch - Version 0.9.8Contents
1. SummaryThe X.509 patch supports RSA-based authentication using X.509 certificates between a Linux FreeS/WAN security gateway and an unlimited number of IPSec peers. Starting with version 0.9 of the patch certificate authority (CA) trust chains and certificate revocation lists (CRLs) are introduced, thereby eliminating the need to store peer certificates locally on the Linux host. Compatibility has successfully been tested with peers running the following IPsec clients:
Furthermore interoperability with the following VPN gateways has been demonstrated during the IPsec 2001 Conference in Paris:
Potentially any IPSec implementation with X.509 certificate support can be made to cooperate with X.509-enabled FreeS/WAN. 2. AcknowledgementsMajor contributions to the X.509 patch for Linux FreeS/WAN have come from Marco Bertossa, Andreas Hess, Patric Lichtsteiner, Andreas Schleiss, and Roger Wegmann, all present or former students of the Zurich University of Applied Sciences in Winterthur (Switzerland). The development of the patch is coordinated by Andreas Steffen, professor for Communications Systems at the ZHW. The X.509 patch also integrates the original contribution by Kai Martius supporting RSA based authentication using OpenPGP certificates and PGP's proprietary Key IDs. 3. Installation3.1 The X.509 distributionThe X.509 patch distribution contains the following files:
3.2. Installing the X.509 patchCopy the patch freeswan.diff to the FreeS/WAN directory and type:
This applies all necessary changes to the pluto directory as well as to the scripts utils/auto and utils/_confread. 3.3 Installing FreeS/WAN with X.509 on a fresh kernelIf you are installing the X.509 features simultaneously with FreeS/WAN on top of a fresh Linux Kernel, then after having applied the patch freeswan.diff as described in section 3.2 you can start the normal FreeS/WAN compilation and installation process by typing one of the commands
The X.509 functionality will be automatically included and everything will be copied to the right place by typing
3.4 Adding X.509 on top of a working FreeS/WAN IPsec stackIf you already have a FreeS/WAN enhanced Linux Kernel running and want to add X.509 functionality afterwards, then you must apply the patch freeswan.diff as described in section 3.2. In a next step you must recompile pluto by changing into the pluto directory and typing
followed by
Next you must manually copy utils/auto and utils/_confread into the directory where you keep your IPsec executables (usually /usr/local/lib/ipsec). As a last step you must restart IPsec to enable the X.509 features
4. Configuring the connections - ipsec.confUp to version 0.8.6 of the X.509 patch, the only choice you had was to use locally stored public keys or locally stored X.509 certificates. These traditional methods are described in section 4.5 and section 4.6, respectively. Starting with version 0.9 we recommend the use of the new authentication method based on CA certificates which is described in the following section 4.1. 4.1 Using CA certificatesStarting with version 0.9 of the X.509 patch, configuring IPSec connections that use X.509 certificates has become an easy task. If you want to define multiple connections then you can save typing work by grouping the definitions common to most connections into the %default section of the configuration file /etc/ipsec.conf:
By defining authby=rsasig you specify that you want to use peer authentication based on RSA public key signatures. With version 0.9 of the X.509 patch you expect the public key to be transmitted by the peer during phase 1 of the IKE protocol, embedded into a X.509 certificate that is signed by a trusted certificate authority (CA). If we assume that the FreeS/WAN security gateway is left and the peer is right (of course you could define the directions also the other way round), then the line
states that you expect the public key to be delivered by the peer in the form of an X.509 certificate. The FreeS/WAN security gateway actually does not need its public key when executing the IKE protocol. Only the private key stored in /etc/ipsec.secrets is required for signing the hash value computed over the ISAKMP parameters. But since pluto expects a leftrsasigkey definition and also out of symmetry reasons, e.g. when you want to use the same ipsec.conf file on both ends of a connection between two FreeS/WAN security gateways, we also define
We assume in our example that the FreeS/WAN host will present the same identity to all its peers and therefore we include its ID in the %defaults section. We recommend the use of a Fully Qualified Domain Name (FQDN) of the form
Important: The Linux host uses its X.509 certificate stored in /etc/x509cert.der to identify itself to its peers, When an FQDN identifier is used, it must be explicitly included as a so called subjectAltName of type dnsName (DNS:) in the certificate. For details on how to generate such certificates, refer to section 7.2. If you don't want to mess with subjectAltNames then you can use the subject's distinguished name instead, which is an identifier of type DER_ASN1_DN and that can be specified in the LDAP-type format
Additional whitespace can be added everywhere as desired since it will be automatically eliminated by the X.509 parser. An exception is the single whitespace between individual words , like e.g. in strongSec GmbH, which is preserved by the parser. The relative distinguished names can alternatively be separated by a slash ( '/') instead of a comma (',')
This is the representation extracted from the certificate by the OpenSSL command line option
In versions prior to 0.9.8 the distinguished name had to be preceded by an '@' character, as e.g. in
or in
This older notation is still valid, but its use is deprecated. The following X.501 attributes are supported by the X.509 patch
In order to maintain backwards compatibility with older versions of the X.509 patch, the subject can also be extracted from the FreeS/WAN certificate with the help of the fswcert tool:
which gives the binary form of the DER_ASN1_DN identifier written in hex format
Now we can proceed to define our connections. In many applications we might have dozens of mostly Windows-based road warriors connecting to a central FreeS/WAN security gateway. The following most simple statement:
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 FreeS/WAN security gateway puts explicit trust in. Additionally the signature during IKE main mode gives proof that the peer is in possession of the private RSA key matching the public key contained in the transmitted certificate. The ID by which a peer is identifying itself during IKE main mode can by any of the ID types IPV4_ADDR, FQDN, USER_FQDN 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. With the roadwarrior connection definition listed above, an IPsec SA for the FreeS/WAN security gateway pulpo.strongsec.com itself can be established. If any roadwarrior should be able to reach e.g. the two subnets 10.0.1.0/24 and 10.0.3.0/24 behind the security gateway then the following connection definitions will make this possible
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) as specified in section 4.3 or as an alternative, access can be controlled by explicitly putting a roadwarrior entry for each eligible peer into ipsec.conf: conn soggy If the IP address of a peer is known to be stable, it can be specified as well. This entry is mandatory when the FreeS/WAN host wants to act as the initiator an IPSec connection. conn soggy In the last example the ID types FQDN, USER_FQDN, DER_ASN1_DN and IPV4_ADDR, respectively, have been used. Of course all connection definitions presented so far have made use of the %defaults section specifying a common left side and the use of X.509 certificates indicated by the use of the %cert magic value. 4.2 Installing the CA certificatesX.509 certificates received by FreeS/WAN 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 the X.509 patch version 0.9 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
Multiple CAs are supported, but presently they just create a large pool of valid user or host certificates and cannot be assigned to specific connection definitions in /etc/ipsec.conf. 4.3 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, pluto searches the directory
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 IPSec SA will not be established. If no CRL is found in the crls directory 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. CRLs must be stored in either binary DER or base64 PEM format in the crls directory. Section 7.3 will explain in detail how CRLs can be created using OpenSSL. 4.4 Dynamic update of certificates and CRLsPluto reads certificates and CRLs from their respective files during system startup and keeps them in memory in the form of chained lists. X.509 certificates have a finite life span defined by their validity field. Therefore it must be possible to replace host and CA certificates kept in system memory without disturbing established ISAKMP 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 dynamic replacement of the various files:
4.5 Using locally stored public keysFreeS/WAN stores the public keys of its peers together with the corresponding IDs in the configuration file /etc/ipsec.conf. For details please consult the man page
Standard FreeS/WAN supports the ID types IPV4_ADDR, FQDN, and USER_FQDN, only. The X.509 patch adds to this the ID type DER_ASN1_DN, which stands for a distinguished name written in Abstract Syntax Notation #1 (ASN.1), e.g.
which is coded in a binary format called the Distinguished Encoding Rules (DER). Both the ASN.1 subject name and the corresponding public key can be extracted from a X.509 certificate with the help of the fswcert tool to be found here. Use the commands: for base64 encoded certificates, or for DER encoded binary certificates, or for PKCS#12 files. The output of fswcert has the form: The character '~' is used by the X.509 patch to tell pluto that the identification is of type DER_ASN1_DN. It is used in a similar way as the '#' character already defined by Kai Martius for the ID type KEY_ID. Do the same with the X.509 certificate of the FreeS/WAN security gateway: or or when the certificate is part of a PKCS#12 file. The output from fswcert has the form
Now we can proceed to make connection entries by pasting the fswcert output into /etc/ipsec.conf:
4.6 Using locally stored certificatesIf you don't want to use CA certificates as proposed in section 4.1 and decide to work with locally stored public keys instead as described in section 4.5, you can save a lot of cutting and pasting work by importing the peer certificates directly into pluto. Using the keywords leftcert and rightcert, the entries in /etc/ipsec.conf are reduced to If a certificate contains a subjectAltName extension, then this can be used to specify an alternative rightid or leftid type, respectively, as the example "conn soggy" shows. If no rightid or leftid entry is present then the subject distinguished name contained in the certificate is taken as the ID. All leftcert and rightcert certificates must be copied either in base64 PEM or binary DER format to the /etc/ipsec.d directory. Irrespective of the file suffix, pluto "automagically" determines the correct format. 5. Installing the private key - ipsec.secrets5.1 Loading private key files in PKCS#1 formatStarting with version 0.9.8 of the X.509 patch, Pluto has been enabled to load RSA private keys in the PKCS#1 file format. The key files can be optionally secured with a passphrase. RSA private key files are declared in /etc/ipsec.secrets using the syntax
The key file can be either in base64 PEM-format or binary DER-format. The actual coding is detected "automagically" by Pluto. The example
uses a relative pathname. In that case Pluto will look for the private key file in the directory
As an alternative an absolute pathname can be given as in
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 3DES using a symmetric transport key derived from a cryptographically strong passphrase.
Because of the weak security, key files protected by single DES will not be accepted by Pluto!!! Once on the security gateway the private key can either be permanently unlocked so that it can be used by Pluto without having to know a passphrase
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
Some CAs distribute private keys embedded in a PKCS#12 file. Since Pluto is not able yet to read this format directly, the private key part must first be extracted using the command
if the key file pulpoKey.pem is to be secured again by a passphrase, or
if the private key is to be stored unlocked. 5.2 Extracting private keys using fswcertWith X.509 patch releases prior to version 0.9.8 the private key must be extracted using thefswcert tool to be found here. If your private key is stored in the encrypted binary PKCS#12 format together with your X.509 certificate and quite often also containing the root CA certificate, use
to extract the private key. If your key is stored in the PKCS#1private key format generated e.g. by OpenSSL, use the command
to extract the private key. In both cases you will be prompted for the passphrase protecting the private key. The output of fswcert has the form
which can be pasted into the file /etc/ipsec.secrets. 6. Installing the certificate - x509cert.derCopy the DER encoded binary X.509 certificate of the FreeS/WAN security gateway into the file /etc/x509cert.der
If your certificate is in PEM format, then the following command converts it into binary DER format:
The content of this binary certificate file will be transmitted to the peer as part of ISAKMP messages #5 (FreeS/WAN as initiator) or message #6 (FreeS/WAN as responder) during IKE Main Mode. 7. Generating certificates and CRLs with OpenSSLThis section is not a full-blown tutorial on how to use OpenSSL. It just lists a few points that are relevant if you want to generate your own certificates and CRLs for use with FreeS/WAN. 7.1 Generating a CA certificateThe OpenSSL statement
creates a 2048 bit RSA private key caKey.pem and a self-signed CA certificate caCert.pem with a validity of 4 years (1460 days).
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 certificates to be in binary DER format then the following command achieves this transformation:
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, Pluto "automagically" determines the correct format. 7.2 Generating a host or user certificateThe OpenSSL statement
generates a 1024 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:
if you want to identify the host by its Fully Qualified Domain Name (FQDN ), or
if you want the ID to be of type IPV4_ADDR . Of course you could include both ID types with
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 USER_FQDN which can be specified as
or if the user's e-mail address is part of the subject's distinguished name
Now the certificate request can be signed by the CA with the command
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. Usually a Windows-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 file:
7.3 Generating a CRLAn empty CRL that is signed by the CA can be generated with the command
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
The directory /etc/ipsec.d/crls contains all CRLs either in binary DER or in base64 PEM format. Irrespective of the file suffix, pluto "automagically" determines the correct format. 7.4 Revoking a certificateA specific host certificate stored in the file host.pem is revoked with the command
Next the CRL file must be updated
The content of the CRL file can be listed with the command
in the case of a base64 CRL, or alternatively for a CRL in DER format
8. Configuring the clients8.1 FreeS/WANA FreeS/WAN to FreeS/WAN connection is symmetrical. Any of the four defined ID types can be used, even different types on either end of the connection, although this wouldn't make much sense.
8.2 PGPnetUse the file peerCert.p12 to import PGPnet's X.509 certificate, the CA certificate, plus the encrypted private key in binary PKCS#12 format into the PGPkey tool. You will be prompted for the passphrase securing the private key. Use the file myCert.pem to import the X.509 certificate of the FreeS/WAN security gateway into the PGPkey tool. The PGPkeyTool does not accept X.509 certificates in binary DER format, so it must be imported in base64 format:
Make sure that there is no human-readable listing of the X.509 certificate in front of the line
otherwise PGPnet will refuse to load the *.PEM file. Any surplus lines can either be deleted by loading the certificate into a text editor or you can apply the command
to achieve the same effect. With authentication based on X.509 certificates, PGPnet always sends the ID type DER_ASN1_DN, therefore rightid in the connection definition of the FreeS/WAN security gateway must be an ASN.1 distinguished name. In the receiving direction PGPnet accepts all four ID types from FreeS/WAN.
8.3 SafeNet/Soft-PKSafeNet/Soft-PK can be configured to send its identity either as DER_ASN1_DN, IPV4_ADDR, FQDN, or USER_FQDN. In the receiving direction SafeNet/Soft-PK accepts all four ID types coming from FreeS/WAN.
8.4 SSH SentinelSSH Sentinel sends its identity as DER_ASN1_DN if the subjectAltName field of its certificate is empty. If a subjectAltName field is present, then the corresponding type IPV4_ADDR, FQDN, or USER_FQDN is automatically chosen. With several subjectAltName entries, the precedence of the different ID types is not quite clear. In the receiving direction SSH Sentinel accepts all four ID types from FreeS/WAN.
8.5 Windows 2000Windows 2000 always sends the ID type DER_ASN1_DN, therefore rightid in the connection definition of the FreeS/WAN security gateway must be an ASN.1 distinguished name. In the receiving direction Windows 2000 accepts all four ID types from FreeS/WAN.
9 Monitoring functionsStarting with version 0.9.6 the X.509 patch offers the following monitoring functions:
This command lists all public keys currently installed in the chained list of public keys. These keys were statically loaded from ipsec.conf or aquired either from received certificates or retrieved from secure DNS servers using opportunistic mode. The public key listing has the following form: Dec 09 16:21:48 2001, keyID 9ad6 d35f d34f de4d,
RSA 1024, It consists of
A public key can be associated with several IDs, e.g. using subjectAltNames in certificates and an ID can possess several public keys, e.g. retrieved from a secure DNS server. The command
lists all CA certificates that have been loaded from /etc/ipsec.d/cacerts. The output has the form
and shows
The command
lists all CRLs that have been loaded from /etc/ipsec.d/crls. The output has the form
and shows
10 Using the patch with OpenPGP certificatesThe X.509 patch also supports RSA based authentication using OpenPGP certificates and PGP's proprietary KEY_ID. This specific authentication mode is automatically chosen when you put the OpenPGP certificate of the FreeS/WAN security gateway into the file
Since the file /etc/x509cert.der has precedence, it must not be present. The keyextractor tool written by Kai Martius and currently available from http://www.zengl.net/freeswan is required to extract public and private keys from OpenPGP certificates.
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X.509 FreeS/WAN patch: Copyright © 2000, Andreas Hess, Patric Lichtsteiner, Roger Wegmann PGPnet-RSA parts of patch: Copyright © 2000, Kai Martius <kai@secunet.de > fswcert utility: Copyright © 2000, Andreas Grünbacher <a.gruenbacher@computer.org> This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. See http://www.fsf.org/copyleft/gpl.txt. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
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