Changes between Initial Version and Version 1 of Concepts-Authentication


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Timestamp:
01/05/14 17:35:34 (8 years ago)
Author:
JoshC
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  • Concepts-Authentication

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     1= Authentication =
     2
     3[[TOC(depth=3)]]
     4
     5This page discusses the concepts of authentication in OpenVPN.
     6
     7== Authentication basics ==
     8
     9OpenVPN needs to verify the authenticity of the remote side it is connecting to, otherwise there's no security provided at all. You don't want to let any random system connect to your VPN. Both ends authenticate the other, and the authentication must pass on both sides for a valid connection to be made.
     10
     11Depending on the mode and configuration, the following are possible methods to authenticate connections:
     12
     13* keypairs and certificates
     14* username + password
     15
     16With TLS, the server always has its own key, an issued certificate, and the CA certificate; all clients must have a copy of this CA certificate as well. Clients can be authenticated using their own certificates, user credentials, or both at once as a form of 2-factor authentication.
     17
     18Note that the process of authenticating connections is available only in TLS modes. The other basic mode OpenVPN has is with a pre-shared key (PSK), and here the authentication is provided only by ownership of the symmetric key. The remainder of this page discusses authentication in TLS mode specifically.
     19
     20== Overview of the auth process ==
     21
     22The process of connecting and authenticating in TLS mode is described below.
     23
     24In short, the client & server exchange keypairs (although the client may be configured not to have one) and verify the presented certs. Any user credentials provided are then processed by the server. Finally, ccd files are checked and acted on. If all enabled authentication steps pass, the connection is allowed. Otherwise the connection is dropped or rejected somewhere during this process.
     25
     26== Authentication process details ==
     27
     281. A client initiates a connection to the server
     292. The server responds by presenting its own cert chain to the client for verification:
     30   A. The client uses its own copy of the CA cert to verify the received cert from the server
     31   B. If this check fails, the connection is terminated
     32   C. If a CRL is used by the client, the presented cert must not be listed as revoked
     333. If the client has its own keypair, then:
     34   A. The client presents its cert chain to the server
     35   B. The server verifies this cert against its own copy of the CA cert
     36   C. If the server's verification of the client cert fails, the connection is terminated
     37   D. If a CRL is used by the server, the presented cert must not be listed as revoked
     384. If the client is configured to send user credentials to the server, it does so over the now-authenticated TLS link
     395. If the server is configured to process user credentials, then:
     40   A. Credentials must have been supplied by the client unless configured to make this optional
     41   B. The program or plugin responsible for checking the user credentials is invoked
     42   C. If the check fails, the client is told the auth was rejected and the connection is terminated
     436. If a ccd (client-config-dir) is used on the server, then:
     44   A. If the named client is listed as disabled, the connection is terminated
     45   B. If `ccd-exclusive` is used, then the namaed client must have a ccd entry or the connection is terminated
     467. At this point authentication by both sides is complete
     47
     48== Certificates vs. usernames ==
     49
     50Certificates are cryptographically signed by the CA, so these provide a strong level of security and authentication. By contrast, usernames are somewhat less secure given the types of passphrases often used, and the prolific re-use of same or similar passphrase.
     51
     52When the server is configured not to check client certificates, the entire security of the VPN relies on the security of the user credentials and the program checking them. When combined together, both valid certificates and valid credentials are required, which improves security.
     53
     54== Advanced authentication options ==
     55
     56Some advanced directives can be used in addition to the normal process. These include:
     57
     58=== tls-verify ===
     59
     60The `tls-verify` directive allows an additional program to review the presented cert's details. When `tls-export-cert` is also used, the program has access to the cert itself for any additional verification steps. Exiting non-zero causes the connection to be terminated.
     61
     62=== client-connect ===
     63
     64The `client-connect` script can also terminate a connection by exiting with a non-zero code. This script is normally used to do dynamic on-connect tasks or populate options to be pushed to the client, but it can be used as a form of authentication control using the exit code as well.