The rights of the several parties, plaintiff as well as defendant, may be put in issue, tried, and determined in such action, and 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 when service of the complaint 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 that when there are several unknown persons having an interest in the property their rights may be considered together in the action as not between themselves.