Link end points

GoJS
6

You’ll need to customize the LinkingTool:

      $(go.Diagram, . . .,
        {
          "linkingTool.findLinkablePort": function() {
            const diagram = this.diagram;
            let obj = this.startObject;
            if (obj === null) {
              obj = diagram.findObjectAt(diagram.firstInput.documentPoint, null, null);
            }
            if (obj === null) return null;
            const part = obj.part;
            if (!(part instanceof go.Node)) return null;
            return part.port;  // use the default port
          },

This assumes that every Node is “linkable”. If that might not be the case, due to LinkingTool.direction or GraphObject.fromLinkable or toLinkable, the override will have to be smarter.