I'm looking for an adjective that will describe a software tool that is ready to be used out of the box without any additional configuration, but it may be configured to fit a specific case.
[adjective] TLS termination proxy.
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You used one yourself: out of the box. As an attributive adjective it would normally be hyphenated (see my emphasis in the quote).
Used to refer to the immediate usability or functionality of a newly purchased product, typically an electronic device or a piece of software.
‘most laptops come with wireless capability out of the box’
‘console games need to be good to go right out of the box’
‘the product is designed as an out-of-the-box portal for the medium to large enterprise’
When discussing such software solutions, you generally talk about turnkey systems:
: built, supplied, or installed complete and ready to operate • a turnkey nuclear plant • a turnkey computer system ; also : of or relating to a turnkey building or installation • a turnkey contract • turnkey vendors
A computer system that has been customized for a particular application. The term derives from the idea that the end user can just turn a key and the system is ready to go. Turnkey systems include all the hardware and software necessary for the particular application. They are usually developed by OEMs (original equipment manufacturers) who buy a computer from another company and then add software and devices themselves.
You can say that it's preconfigured:
preconfigure : tr.v : to configure (something) in advance; preconfigure the printer; offers preconfigured systems/solutions for small businesses