Changes between Version 16 and Version 17 of HOWTO


Ignore:
Timestamp:
07/25/14 15:22:04 (10 years ago)
Author:
Samuli Seppänen
Comment:

Continued fixing formatting

Legend:

Unmodified
Added
Removed
Modified

  • HOWTO

    v16 v17  
    10941094== Larger symmetric keys ==
    10951095
    1096 By default OpenVPN uses Blowfish, a 128 bit symmetrical cipher.
     1096By default OpenVPN uses '''Blowfish''', a 128 bit symmetrical cipher.
    10971097
    10981098OpenVPN automatically supports any cipher which is supported by the OpenSSL library, and as such can support ciphers which use large key sizes. For example, the 256-bit version of AES (Advanced Encryption Standard) can be used by adding the following to both server and client configuration files:
    1099 
    1100     cipher AES-256-CBC
    1101 
    1102 Keep the root key (ca.key) on a standalone machine without a network connection
     1099{{{
     1100cipher AES-256-CBC
     1101}}}
     1102== Keep the root key (ca.key) on a standalone machine without a network connection ==
    11031103
    11041104One of the security benefits of using an X509 PKI (as OpenVPN does) is that the root CA key (ca.key) need not be present on the OpenVPN server machine. In a high security environment, you might want to specially designate a machine for key signing purposes, keep the machine well-protected physically, and disconnect it from all networks. Floppy disks can be used to move key files back and forth, as necessary. Such measures make it extremely difficult for an attacker to steal the root key, short of physical theft of the key signing machine.
    1105 Revoking Certificates
    1106 
    1107 Revoking a certificate means to invalidate a previously signed certificate so that it can no longer be used for authentication purposes.
     1105
     1106= Revoking Certificates =
     1107
     1108''Revoking a certificate'' means to invalidate a previously signed certificate so that it can no longer be used for authentication purposes.
    11081109
    11091110Typical reasons for wanting to revoke a certificate include:
    11101111
    1111    The private key associated with the certificate is compromised or stolen.
    1112    The user of an encrypted private key forgets the password on the key.
    1113    You want to terminate a VPN user's access.
    1114 
    1115 Example
    1116 
    1117 As an example, we will revoke the client2 certificate, which we generated above in the "key generation" section of the HOWTO.
    1118 
    1119 First open up a shell or command prompt window and cd to the easy-rsa directory as you did in the "key generation" section above. On Linux/BSD/Unix:
    1120 
    1121     . ./vars
    1122     ./revoke-full client2
    1123 
     1112 * The private key associated with the certificate is compromised or stolen.
     1113 * The user of an encrypted private key forgets the password on the key.
     1114 * You want to terminate a VPN user's access.
     1115
     1116== Example ==
     1117
     1118As an example, we will revoke the '''client2''' certificate, which we generated above in the "key generation" section of the HOWTO.
     1119
     1120First open up a shell or command prompt window and cd to the '''easy-rsa''' directory as you did in the "key generation" section above. On Linux/BSD/Unix:
     1121{{{
     1122. ./vars
     1123./revoke-full client2
     1124}}}
    11241125On Windows:
    1125 
    1126     vars
    1127     revoke-full client2
    1128 
     1126{{{
     1127vars
     1128revoke-full client2
     1129}}}
    11291130You should see output similar to this:
    1130 
    1131     Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf
    1132     DEBUG[load_index]: unique_subject = "yes"
    1133     Revoking Certificate 04.
    1134     Data Base Updated
    1135     Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf
    1136     DEBUG[load_index]: unique_subject = "yes"
    1137     client2.crt: /C=KG/ST=NA/O=OpenVPN-TEST/CN=client2/emailAddress=me@myhost.mydomain
    1138     error 23 at 0 depth lookup:certificate revoked
    1139 
     1131{{{
     1132Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf
     1133DEBUG[load_index]: unique_subject = "yes"
     1134Revoking Certificate 04.
     1135Data Base Updated
     1136Using configuration from /root/openvpn/20/openvpn/tmp/easy-rsa/openssl.cnf
     1137DEBUG[load_index]: unique_subject = "yes"
     1138client2.crt: /C=KG/ST=NA/O=OpenVPN-TEST/CN=client2/emailAddress=me@myhost.mydomain
     1139error 23 at 0 depth lookup:certificate revoked
     1140}}}
    11401141Note the "error 23" in the last line. That is what you want to see, as it indicates that a certificate verification of the revoked certificate failed.
    11411142
    1142 The revoke-full script will generate a CRL (certificate revocation list) file called crl.pem in the keys subdirectory. The file should be copied to a directory where the OpenVPN server can access it, then CRL verification should be enabled in the server configuration:
    1143 
    1144     crl-verify crl.pem
    1145 
     1143The '''revoke-full''' script will generate a CRL (certificate revocation list) file called '''crl.pem''' in the '''keys''' subdirectory. The file should be copied to a directory where the OpenVPN server can access it, then CRL verification should be enabled in the server configuration:
     1144{{{
     1145crl-verify crl.pem
     1146}}}
    11461147Now all connecting clients will have their client certificates verified against the CRL, and any positive match will result in the connection being dropped.
    1147 CRL Notes
    1148 
    1149     When the crl-verify option is used in OpenVPN, the CRL file will be re-read any time a new client connects or an existing client renegotiates the SSL/TLS connection (by default once per hour). This means that you can update the CRL file while the OpenVPN server daemon is running, and have the new CRL take effect immediately for newly connecting clients. If the client whose certificate you are revoking is already connected, you can restart the server via a signal (SIGUSR1 or SIGHUP) and flush all clients, or you can telnet to the management interfaceand explicitly kill the specific client instance object on the server without disturbing other clients.
    1150 
    1151      
    1152     While the crl-verify directive can be used on both the OpenVPN server and clients, it is generally unnecessary to distribute a CRL file to clients unless a server certificate has been revoked. Clients don't need to know about other client certificates which have been revoked because clients shouldn't be accepting direct connections from other clientsin the first place.
    1153 
    1154      
    1155     The CRL file is not secret, and should be made world-readable so that the OpenVPN daemon can read it after root privileges have been dropped.
    1156 
    1157      
    1158     If you are using the chrootdirective, make sure to put a copy of the CRL file in the chroot directory, since unlike most other files which OpenVPN reads, the CRL file will be read after the chroot call is executed, not before.
    1159 
    1160      
    1161     A common reason why certificates need to be revoked is that the user encrypts their private key with a password, then forgets the password. By revoking the original certificate, it is possible to generate a new certificate/key pair with the user's original common name.
    1162 
    1163      
    1164 
    1165 Important Note on possible "Man-in-the-Middle" attack if clients do not verify the certificate of the server they are connecting to.
     1148== CRL Notes ==
     1149
     1150 * When the crl-verify option is used in OpenVPN, the CRL file will be re-read any time a new client connects or an existing client renegotiates the SSL/TLS connection (by default once per hour). This means that you can update the CRL file while the OpenVPN server daemon is running, and have the new CRL take effect immediately for newly connecting clients. If the client whose certificate you are revoking is already connected, you can restart the server via a signal (SIGUSR1 or SIGHUP) and flush all clients, or you can telnet to the management interfaceand explicitly kill the specific client instance object on the server without disturbing other clients.
     1151 * While the crl-verify directive can be used on both the OpenVPN server and clients, it is generally unnecessary to distribute a CRL file to clients unless a server certificate has been revoked. Clients don't need to know about other client certificates which have been revoked because clients shouldn't be accepting direct connections from other clientsin the first place.
     1152 * The CRL file is not secret, and should be made world-readable so that the OpenVPN daemon can read it after root privileges have been dropped.
     1153 * If you are using the chrootdirective, make sure to put a copy of the CRL file in the chroot directory, since unlike most other files which OpenVPN reads, the CRL file will be read after the chroot call is executed, not before.
     1154 * A common reason why certificates need to be revoked is that the user encrypts their private key with a password, then forgets the password. By revoking the original certificate, it is possible to generate a new certificate/key pair with the user's original common name.
     1155
     1156= Important Note on possible "Man-in-the-Middle" attack if clients do not verify the certificate of the server they are connecting to =
    11661157
    11671158To avoid a possible Man-in-the-Middle attack where an authorized client tries to connect to another client by impersonating the server, make sure to enforce some kind of server certificate verification by clients. There are currently five different ways of accomplishing this, listed in the order of preference:
    11681159
    1169     [OpenVPN 2.1 and above]Build your server certificates with specific key usage and extended key usage. The RFC3280 determine that the following attributes should be provided for TLS connections:
    1170 
    1171      
    1172     Mode        Key usage       Extended key usage
    1173     Client      digitalSignature        TLS Web Client Authentication
    1174     keyAgreement
     11601. OpenVPN 2.1 and above: Build your server certificates with specific key usage and extended key usage.
     1161
     1162The RFC3280 determine that the following attributes should be provided for TLS connections:
     1163
     1164{{{#!html
     1165<table border="1" cellspacing="0" cellpadding="8">
     1166<tbody>
     1167<tr><th>Mode</th><th>Key usage</th><th>Extended key usage</th></tr>
     1168<tr>
     1169<td rowspan="3">Client</td>
     1170<td>digitalSignature</td>
     1171<td rowspan="3">TLS Web Client Authentication</td>
     1172</tr>
     1173<tr>
     1174<td>keyAgreement</td>
     1175</tr>
     1176<tr>
     1177<td>digitalSignature, keyAgreement</td>
     1178</tr>
     1179<tr>
     1180<td rowspan="2">Server</td>
     1181<td>digitalSignature, keyEncipherment</td>
     1182<td rowspan="2">TLS Web Server Authentication</td>
     1183</tr>
     1184<tr>
     1185<td>digitalSignature, keyAgreement</td>
     1186</tr>
     1187</tbody>
     1188</table>
     1189}}}
     1190
     1191Mode Key usage Extended key usage
     1192Client digitalSignature TLS Web Client Authentication
     1193keyAgreement
    11751194    digitalSignature, keyAgreement
    11761195    Server      digitalSignature, keyEncipherment       TLS Web Server Authentication