This article describes how to restrict the SSL ciphers provided by FortiGate for DNS over TLS communications when using FortiGate as a DNS server.
Scope
FortiOS 7.0.6+, 7.2.0+.
Solution
Starting from FortiOS 7.0.6 and 7.2.0, disabling ssl-static-key-ciphers will prevent all TLS sessions that are terminated by FortiGate from using static keys (AES128-SHA, AES256-SHA, AES128-SHA256, AES256-SHA256).
config system global
set ssl-min-proto-version {SSLv3 | TLSv1 | TLSv1-1 | TLSv1-2* | TLSv1-3}
set ssl-static-key-ciphers {enable* | disable}
set strong-crypto {enable* | disable}
end
Note: The default setting has been marked with *.
It is possible to see the effect of the above commands by running an SSL scan. See below:
[+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: DHE-RSA-AES256-GCM-SHA384 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: ECDHE-RSA-CHACHA20-POLY1305 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: DHE-RSA-CHACHA20-POLY1305 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: ECDHE-RSA-AES128-GCM-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: DHE-RSA-AES128-GCM-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: ECDHE-RSA-AES256-SHA384 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: DHE-RSA-AES256-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: DHE-RSA-CAMELLIA256-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: ECDHE-RSA-AES128-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: DHE-RSA-AES128-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: DHE-RSA-CAMELLIA128-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: AES256-GCM-SHA384 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: AES128-GCM-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: AES256-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: CAMELLIA256-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: AES128-SHA256 [+] 192.168.2.10:853 - Connected with SSL Version: TLSv1.2, Cipher: CAMELLIA128-SHA256
Unlike other TLS services on FortiGate such as SSL VPN, currently, it is not possible to allow or ban a specific cipher suite for DNS over TLS (DoT) but the SSL static ciphers highlighted above in red are restricted by FortiGate when ssl-static-key-ciphers are disabled.