When a sale of the premises is necessary, the title must be ascertained by proof to the satisfaction of the court before the judgment of sale can be made, and where service of the summons has been made by publication, like proof must be required of the right of the absent or unknown parties before such judgment is rendered, except where there are several unknown parties having an interest in the property, their rights may be considered together in the action and not as between themselves.
§ 70-29-203, MCA