Is it permissible according to Sharia to name one's son "Al-Khattab," knowing that the one named Al-Khattab was the father of our master Umar, and is this name considered bad?

There is no objection to naming a newborn "Al-Khattab" or "Khattab." The basic rule for names is permissibility and lawfulness. The mention that the name is associated with someone who was an enemy of Islam does not justify prohibiting it. Some Companions had names that were identical to those of some polytheists, and there is no record that the Prophet (peace be upon him) ordered them to change their names. Rather, this is similar to the disbelievers of Quraysh who died before the Prophet's mission. Naming a child "Al-Khattab" or "Khattab" is permissible, but it is preferable to name children with names that have been encouraged, such as Abdullah and Abdur-Rahman.