Most modern web browser no longer support RC4 encrytion as it is officially declared unsecured by IETF memo RFC7465 published on February 2015. (See Workaround below to find an older browser to get around this error)
Thus if RC4-Only encryption is enabled in SonicOS, it would block access to the Firewall Management Portal and will display “err_ssl_version_or_cipher_mismatch” error on the browser. Google Chrome and Firefox were among the first ones to unsupport RC4 encryption, while IE was the last one to end its support on RC4.
In SonicOS 5.9.x and above firmware, an option to enable only RC4 ciphers has been introduced. Enabling this option would force SONICWALL to negotiate SSL connections using RC4-SHA1 or RC4-MD5.
A common workaround is to disable “RC4-only” Option on your Sonicwall.
Procedure: Note that this change will require a firewall reboot!
- Login to the SONICWALL management GUI (using an older browser or use older versions of Firefox Portable. Skip to the end of this document for links)
- Change to the Diag page at https://<interface IP address>/diag.html
- Under Encryption Settings, enable check box Enable RC4-Only Cipher Suite Support
- Click on Accept at the top to save the change.
- Restart for the change to take effect.
Quick Workaround to access the firewall is to download an older browser. I recommend to use Firefox Portable from 2014 or older. You can download one from here.