What do the "Low" and "Standard" cipher options mean for a Backend Service that uses SSL, and which should I choose?

Short answer

The default value of the Ciphers field is Standard: TR7 does not write a custom cipher list, and the secure default list is used; it is the right choice for modern servers. Low is a compatibility option that also enables weak and old ciphers (including RC4 and unencrypted NULL ciphers); it is considered only for an old server that cannot be connected to in any other way. Low does not lower the TLS version; for a server that requires TLS 1.0/1.1, the right way is a Backend SSL Security Profile with the minimum version and the ciphers explicitly set. When a profile is selected, the Ciphers field has no effect. The fields appear only while SSL Service is on.

How Does It Work?

• The Ciphers field determines the cipher list that TR7 offers when connecting to the server on a Backend Service with SSL Service on.
• Standard: TR7 does not write a custom cipher list; the secure default list is used.
• Low: By lowering the TLS security level to the minimum, all ciphers, including those without encryption and without authentication, are allowed.
• Precedence: If a Backend SSL Security Profile is selected on the Backend Service, the cipher list, the minimum/maximum TLS version and the curves come from the profile; the Ciphers field is ignored.

Comparison

	Standard	Low	Backend SSL Security Profile
Cipher list	Secure default list	All, including weak and old ciphers	Those selected in the profile
TLS version	Default	Does not change	Minimum / maximum version in the profile
Security	High	Low	Depends on the profile
Does the health check inherit it	-	No	Yes (minimum TLS version and cipher list)
When	Always, by default	When no other way remains, temporarily	Old server or corporate TLS policy

Which One Should I Choose?

Your situation	Choice
The server is up to date (TLS 1.2 / 1.3)	Standard
The server speaks only TLS 1.0 / 1.1	A Backend SSL Security Profile with the minimum version set accordingly
The corporate policy mandates specific TLS versions and ciphers	Backend SSL Security Profile
The server supports only a very old cipher and the profile is not enough	Low, until the server is renewed

In TR7

• Location: Load Balancer → Backend Services → Edit → SSL Service on → Ciphers or Backend SSL Security Profile; it is saved with the Edit button at the bottom of the form. In a Backend Group, it is at the group level.
• Profile: It is created with Certificates & PKI → TLS Security Profiles → Add; only the minimum and maximum SSL version, the cipher list and the curve settings are applied to the Backend Service connection (TLS Security Profiles · The Profile in the Backend Service Connection).
• Cluster: While Clustering is on and Manual Sync Mode is off (default), the setting is transferred to the peer device automatically.

Common Misconceptions

• "Low makes it possible to connect to TLS 1.0 servers." Low only extends the cipher list; it does not change the minimum TLS version.
• "If traffic passes with Low, the health check also passes." The health check does not inherit the Low setting; while traffic passes, an HTTPS health check may fail. When a profile is used, the minimum TLS version and the cipher list of the profile are also transferred to the health checks that poll over TLS (such as HTTPS and TCPS); this way, the health check can also complete the handshake with the old server.
• "While a profile is selected, the Ciphers field is also applied." When a profile is selected, the cipher list comes only from the profile.

How it is done in the documentation

• Backend Services · Ciphers
• Backend Services · Backend SSL Security Profile
• TLS Security Profiles · Min. SSL Version

Related questions

• What does the SSL Service option do; what is needed to connect to the backend over an encrypted connection?
• What is a TLS Security Profile, and what does it configure?
• How do I disable weak ciphers?