Peace be upon you. According to Grand Ayatollah Sayyid Sistani (may Allah protect him), it is obligatory to wipe the surface of the foot. The source is the Short Messages for Legal Inquiries - Qom Office. Additionally, Grand Ayatollah Sayyid Khamenei (may Allah protect him) states that the obligation in wiping the feet is to wipe their surface from the tips of the toes to the ankle, without wiping the underside of the toes. The source is a special inquiry. May you be successful in all good.