From the UT3 mailing list:

"Thanks for your patience, everyone.

The Linux server is now available for download. It is built from the 1.1
patch, and is network compatible with all retail UT3 clients.

We feel pretty good about this build, but it should be considered beta for
now, as it hasn't had the full wrath of the Internet applied to it yet.

Grab it from here:

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