Installation and Configuration GuideX.509 FreeS/WAN- Version 1.1.3Contents
1. SummaryThe X.509 patch supports RSA-based authentication using X.509 or OpenPGP certificates between a Linux FreeS/WAN security gateway and an unlimited number of IPsec peers.
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. The latest addition has been the successful interoperability with the Check Point VPN-1 NG gateway. 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 support of Virtual IPs and the DHCP-over-IPsec protocol has been developped and coded by Mario Strasser, research assistant at the ZHW. Stephane Laroche from Colubris has contributed dynamic CRL fetching. Stephen J. Bevan has contributed the enforcement of port and protocol selectors on outbound traffic based on extended eroutes. 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. The development of the patch is coordinated by Andreas Steffen, professor for Communications and Security at the ZHW. 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 source 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 Enabling dynamic LDAP URL fetchingBy default LDAP support will not be compiled into Pluto. In order to enable dynamic LDAP URL fetching the following line must be uncommented in the programs/pluto/Makefile:
Compilation will be successful only if the OpenLDAP 2.x header files and the ldap library are present. http, ftp, and file URLs are fetched using the curl command line tool. LDAP source code support is not required for these types of URLs. 3.4 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
or
in the FreeS/WAN directory. The X.509 functionality will be automatically included and everything will be copied to the right place by typing
3.5 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 the userland progams by staying in the FreeS/WAN top source directory and typing
followed by
As a last step you must restart FreeS/WAN to enable the X.509 features
4. Configuring the connections - ipsec.conf4.1 Configuring my sideUsually the local side is the same for all connections. Therefore it makes sense to put the definitions characterizing the FreeS/WAN security gateway into the conn %default section of the configuration file /etc/ipsec.conf. If we assume throughout this document 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 we can write conn %default The X.509 certificate by which the FreeS/WAN security gateway will authenticate itself by sending it in binary form to its peers as part of the Internet Key Exchange (IKE) is specified by the line
The certificate can either be stored in base64 PEM-format or the binary DER-format. Irrespective of the file suffix, Pluto "automagically" determines the correct format. Therefore
or
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 FreeS/WAN's own certificates from locally stored trusted peer certificates (see section 4.7 for details), they could also be stored in a subdirectory within the default directory /etc/ipsec.d/certs, as e.g. in
Absolute pathnames are also possible, as in
As an ID for the VPN gateway we recommend the use of a Fully Qualified Domain Name (FQDN) of the form conn rw Important: When an 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 section 6.2. 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 Since the subject's DN is part of the certificate, the leftid does not have to be declared explicitly. Thus the entry conn rw 4.2 Multiple certificatesStarting with version 0.9.10, the X.509 patch supports multiple local host certificates and corresponding RSA private keys:conn rw1 conn rw2 4.3 Configuring the peer side using CA certificatesNow 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: conn rw 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. One of the two possible representations of a Distinguished Name (DN) is the LDAP-type format rightid="C=CH,O=strongSec GmbH, CN=wroclaw.strongsec.com" 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 (RDNs) 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
The following X.501 RDNs are supported by the X.509 patch
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 conn rw1 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 4.5 or as an alternative, access can be controlled by explicitly putting a roadwarrior entry for each eligible peer into ipsec.conf: conn soggy When 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, were used. Of course all connection definitions presented so far have included the lines in the conn %defaults section, comprising among others a left and leftcert entry, as well as a rightrsasigkey parameter set to the magic value %cert , signifying that the public key will be extracted from a X.509 certificates sent by the peer. 4.4 Handling Virtual IPs and wildcard subnetsOften roadwarriors are behind NAT-boxes with IPsec passthrough, 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 FreeS/WAN security gateway the following entries are required in /etc/ipsec.conf: conn rw1 With the new wildcard parameter rightsubnetwithin these three entries can be reduced to the single connection definition conn rw 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 brackets defined by the wildcard subnet definition (in our example 10.0.1.0/24). For each roadwarrior a connection instance tailored to the subnet of the particular client will be created, based on the generic rightsubnetwithin template. This new feature introduced with version 0.9.12 of the X.509 patch can also be helpful with VPN clients getting a dynamically assigned inner IP from a DHCP server located on the NAT router box. 4.5 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. 4.6 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 6.3 will explain in detail how CRLs can be created using OpenSSL. 4.7 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:
Starting with version 1.1.0 of the X.509 patch, CRLs can be automatically fetched from an HTTP or LDAP server using the CRL distribution points contained in X.509 certificates. The command
shows any pending fetch requests: Oct 31 00:29:53 2002, trials: 2 In the example above, a http and a ldap URL were extracted from a received end certificate. An independent thread then tries to fetch a CRL from the designated distribution points. The same thread also periodically checks if any loaded CRLs are about to expire. The check interval can be defined in the config setup section of the ipsec.conf file:
In our example the thread wakes up every 600 seconds or 10 minutes in order to check the validity of the CRLs or to retry any pending fetch requests: List of X.509 CRLs: The first trial to update a CRL is started 2*crlcheckinterval before the nextUpdate time, i.e. when less than 20 minutes are left in our practical example. When crlcheckinterval is set to 0 (this is also the default value when the parameter is not set in ipsec.conf) then the CRL checking and updating thread is not started and dynamic CRL fetching is disabled. 4.8 CRL PolicyBy default Pluto 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 this can be done by setting the strictcrlpolicy option in the config setup section of the ipsec.conf file:
config setup A certificate received from a peer will not be accepted if no corresponding CRL is present in /etc/ipsec.conf. And if an ISAKMP SA re-negotiation takes place after the nextUpdate deadline has been reached, the peer certificate will be declared invalid and the cached RSA 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 stand still will ensue. As mentioned earlier the default setting is strictcrlpolicy=no 4.9 Configuring the peer side using locally stored certificatesIf you don't want to use trust chains based on CA certificates as proposed in section 4.2, you can alternatively import trusted peer certificates directly into Pluto. Thus you do not have to rely on the certificate to be transmitted by the peer as part of the IKE protocol. With the conn %default section defined in section 4.1 and the use of the rightcert keyword for the peer side, the connection definitions in section 4.2 can alternatively be written as
conn soggy Using the same rules concerning pathnames that apply to FreeS/WAN's own certificates, the following two definitions are also valid for trusted peer certificates:
or
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 Multiple private keysStarting with version 0.9.10 of the X.509 patch, multiple private keys are supported. Since the connections defined in ipsec.conf can find the correct private key based on the public key contained in the certificate assigned by leftcert, default private key definitions without specific IDs can be used
6. 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. 6.1 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. 6.2 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:
6.3 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. 6.4 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
7. Configuring the clients7.1 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.
7.2 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.
7.3 SafeNet/Soft-PK/Soft-RemoteSafeNet/Soft-PK and SafeNet/Soft-Remote can be configured to send their identity either as DER_ASN1_DN, IPV4_ADDR, FQDN, or USER_FQDN. In the receiving direction SafeNet/Soft-PK or SafeNet/Soft-Remote accept all four ID types coming from FreeS/WAN.
7.4 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.
7.5 Windows 2000/XPWindows 2000 and Windows XP always send 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 or Windows XP accept all four ID types from FreeS/WAN.
8. Monitoring functionsThe 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:
Apr 10 00:10:31 2002, 1024 RSA Key AwEAAb8F/, It consists of
A public key can be associated with several IDs, e.g. using class=subjectAltNames in certificates and an ID can possess several public keys, e.g. retrieved from a secure DNS server. The commandipsec auto [--utc] --listcerts lists all local certificates, both FreeS/WAN's own and those of trusted peer loaded via leftcert and rightcert, respectively. The output has the formMay 01 07:09:47 2002, count: 2
The command
lists all CA certificates that have been loaded from /etc/ipsec.d/cacerts. The output has the form
May 01 07:09:47 2002, count: 1 and shows
The command
lists all CRLs that have been loaded from /etc/ipsec.d/crls. The output has the form Oct 30 22:57:51 2002, revoked certs: 37 and shows
is equivalent to
9. Firewall support functions9.1 Protocol and port restrictionsVersion 0.9.14 of the X.509 patch offers the possibility to restrict the protocol and
optionally the ports in an IPsec SA using the rightprotoport and
leftprotoport parameters. conn icmp Protocols and ports can be designated either by their numerical values or by their acronyms defined in /etc/services.
lists the following connection definitions: "icmp": 160.85.106.10[@pulpo.strongsec.com]:1/0...%any:1/0 Based on the protocol and port selectors, appropriate eroutes will be set up, so that only the specified payload types will pass through the IPsec tunnel. 9.2 Environment variables in the updown scriptThe X.509 patch makes the following environment variables available in the updown script indicated by the leftupdown parameter:
(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. 9.3 Sample updown script for iptablesThe template _updown.x509 that can be found in the utils directory of the FreeS/WAN distribution after applying the X.509 patch, can be used as an updown script to dynamically insert and delete iptables firewall rules. The 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 firewall vpn: 10. Using the patch with FreeS/WAN and raw RSA keysStandard FreeS/WAN as it is available from www.freeswan.org does public key authentication base on raw RSA public keys that are directly defined in /etc/ipsec.conf
When standard FreeS/WAN receives a certificate request (CR), it immediately drops the negotiation because it does not know how to answer the request. As a workaround X.509 enabled FreeS/WAN does not send a CR if the RSA key has been statically loaded using rightrsasigkey. A problem remains when roadwarriors initiate a connection. Since X.509 enabled FreeS/WAN does not know the identity of the initiating peer in advance, it will always send a CR, causing the rupture of the IKE negotiation if the peer is a standard FreeS/WAN host. To circumvent this problem the configuration parameter nocrsend can be set in the config setup section of /etc/ipsec.conf: config setup: With this entry no certificate request is sent in any connection. The default setting is nocrsend=no. 11. Using the patch with OpenPGP certificatesThe X.509 patch also supports RSA-based authentication using OpenPGP certificates and OpenPGP V3 fingerprints employed as KEY_ID identifiers. 11.1 OpenPGP certificatesOpenPGP certificates containing RSA public keys can now directly be loaded in ASCII armored PGP format using the leftcert and rightcert parameters in /etc/ipsec.conf: conn pgp The peer certificate must be stored locally (the default directory is /etc/ipsec.d/certs) since currently no trust can be established for PGP certificates received from a peer via the IKE protocol. 11.2 OpenPGP private keysPGP private keys in unencrypted form can now directly be loaded in ASCII armored PGP format via an entry in /etc/ipsec.secrets: : RSA gatewayKey.asc Existing IDEA-encrypted legacy RSA private keys can be unlocked with the help of GnuPG and the IDEA extension (see http://www.gnupg.org/gph/en/pgp2x.html) using the following commands
11.3 Monitoring functionsThe command ipsec auto --listcerts shows all loaded PGP certificates in the following format: Aug 28 09:51:55 2002, count: 1 The entries are
11.4 Suppression of certificate request messagesPGPnet configured to work with OpenPGP certificates aborts the IKE negotiation when it receives a X.509 certificate. Therefore it is recommended (mandatory for roadwarrior connections) to set config setup: in the config setup section of /etc/ipsec.conf.
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X.509 FreeS/WAN patch: Copyright © 2000, Andreas Hess, Patric Lichtsteiner, Roger Wegmann Dynamic CRL fetching Port and Protocol selectors for outbound
traffic PGPnet-RSA parts of patch: Copyright © 2000, Kai Martius <kai@secunet.de > 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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