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OpenConnect VPN client

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# Fortinet SSL VPN

Experimental support for Fortinet SSL VPN was added to OpenConnect in March 2021. It is also known as FortiGate in some documentation. It is a PPP-based protocol using the native PPP support which was merged into the 9.00 release.

Fortinet mode is requested by adding `--protocol=fortinet` to the command line:

      openconnect --protocol=fortinet fortigate.example.com

Since TCP over TCP is very suboptimal, OpenConnect tries to always use PPP-over-DTLS, and will only fall over to the PPP-over-TLS tunnel if that fails, or if disabled via the `--no-dtls` argument.

## Authentication

OpenConnect currently supports basic username/password, optional TLS client certificate, and optional multifactor authentication token entry via the two known challenge/response mechanisms: plaintext/"tokeninfo" (issue #225) and HTML forms (issue #332).

If you have access to a Fortinet VPN which uses other types of authentication, please send information to the mailing list so that we can add support to OpenConnect.

## Quirks and Issues

FortiGate server versions prior to v6.2.1 do not allow the post-authentication cookie (as output by `--authenticate`) to be used to reestablish a dropped connection. This means that if the client loses its connection to the gateway (for example, due to a network outage, or after roaming to a different physical adapter) a new authentication will always be required. This is a substantial design flaw which is not present in any of the other protocols supported by OpenConnect.

Starting with FortiOS 6.2.1, an optional server-side setting (`tun-connect-without-reauth`) appears intended to support reconnection, but still doesn't work very well (see discussion on issue #297). Please send reports on success and failure with Fortinet reconnection to the mailing list so we can understand it better.