About the AnyConnect System Extension
AnyConnect uses a network system extension on macOS 11 (and later), bundled into an application named AnyConnect Socket Filter. The app controls the extension activation and deactivation and is installed under /Applications/Cisco.
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DNS proxy
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App/transparent proxy
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Content filter
AnyConnect requires its system extension and all its components to be active for proper operation, which implies that the mentioned components are all present and show as green (running) in the left pane of the macOS Network UI.
If a third-party product is employing a system extension with DNS proxy capabilities, either Cisco Secure Client or the third-party product (or both), may not function properly. macOS only supports one active DNS proxy extension at a time.