maven-sync-setup/sync/settings.xml

<settings>
  <profiles>
    <profile>
      <id>central</id>
    </profile>
  </profiles>
  <mirrors>
    <mirror>
      <id>central</id>
      <mirrorOf>*</mirrorOf>
      <url>http://repo/releases</url>
    </mirror>
  </mirrors>
</settings>