Some people want to build a house just from a single log (no additional insulation on the exterior). In such case between the logs we use a thin strip to stop any draft.
In previous times we used flax, today we usually use polyurethane foam strips. In the corners where two logs intersect we use a square polyurethane foam to effectively stop any draft. Also note that after the logs settle all gaps are closed and these strips lose their function, no draft will be possible anyway.