FortiClient SSL-VPL Failed

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Failed to establish the VPN connection. This may be caused by a mismatch in the TLS version. Please check the TLS version settings in the Advanced of the Internet options. (-5029)

Using FortiClient to establish an SSL-VPN connection to the FortiGate can output a warning message.

Symptom

FortiClient Warning. Failed to establish the VPN connection. This may be caused by a mismatch in the TLS version. Please check the TLS version settings in the Advanced of the Internet options. (-5029)

Cause

fortinet

The now outdated cryptographic protocol TLS 1.0 is no longer enabled by default as of FortiOS 6.0. It is recommended to use at least TLS 1.1 (Cipher Suites) for authentication and data encryption. We are currently on TLS 1.3 which has been approved by the IETF (Internet Engineering Task Force).

Solution

If you want to continue to use older clients that are only ready for use later with TLS 1.2 or higher may if planned client migration via update rollout, TLS 1.0 can be activated on the FortiGate.

Check the current TLS setting from the FortiGate Console with CLI Command:

FG60E # get vpn ssl settings | grep tls
tlsv1-0             : disable
tlsv1-1             : enable
tlsv1-2             : enable
dtls-hello-timeout  : 10
dtls-tunnel         : enable

Activate in the CLI the Cipher Suite TLS 1.0.

config vpn ssl settings
set tlsv1-0 enable

How to enable TLS 1.0 on Windows


The TLS version 1.0 in the Microsoft Windows snap-in (inetcpl) Internet Options can also be activate.

Windows-Logo + R

Hit the key Win+R and enter inetcpl.cpl

Run inetcpl.cpl

In the opened Internet Options window Internet Properties click to Advanced tab and click Use TLS Version 1.0 to enable it.

Internet Options Advanced

More Solution

With older Windows versions, or with routers with PPPoE Internet connection, errors when establishing SSL-VPN connections can be eliminated as follows.

It appears the FortiClient error message:

Unable to establish the VPN connection. The VPN server may be unreachable (-5)

To do this, check the MTU size of the network interfaces with the following command from an open command prompt

netsh interface ipv4 show subinterface

The output might look something like this:

C:\> netsh interface ipv4 show subinterface

   MTU  Medienerkennungsstatus   Bytes eingehend  Bytes ausgehend  Schnittstelle
------  ---------------  ---------  ---------  -------------
  1500                5          0          0  Ethernet
  1500                1  598892209   19487894  WiFi
  1500                5          0          0  Mobilfunk
  1500                1       5248     144442  VMware Network Adapter VMnet1
  1500                5          0          0  LAN-Verbindung* 1
  1500                1          0     134436  VMware Network Adapter VMnet8
  1500                5          0          0  LAN-Verbindung* 5
4294967295                1          0      67869  Loopback Pseudo-Interface 1
  1500                5          0          0  Bluetooth-Netzwerkverbindung
  1500                5          0          0  Ethernet 3

Check the MTU size and adjust to 1400 if necessary. In a command prompt opened as an administrator, with running netsh.

netsh interface ip4 set subinterface Ethernet mtu=1400 store=persistent

Alternatively, call Regedit and navigate to the following key.

Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces

under the appropriate interface with the appropriate IP address, here {222e135b-d09c-47a3-9236-63a041a02ea6} change the key MTU with the value 578 hexadecimal.

regedit interfaces

After restarting the computer, the SSL-VPN connection can be established.

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