The ruling in knocking doors of other people's

*Not facing the door when seeking permission to enter

كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم إِذَا أَتَى بَابَ قَوْمٍ لَمْ يَسْتَقْبِلِ الْبَابَ مِنْ تِلْقَاءِ وَجْهِهِ وَلَكِنْ مِنْ رُكْنِهِ الأَيْمَنِ أَوِ الأَيْسَرِ وَيَقُولُ ‏ “‏ السَّلاَمُ عَلَيْكُمُ السَّلاَمُ عَلَيْكُمْ ‏”‏ ‏.‏ وَذَلِكَ أَنَّ الدُّورَ لَمْ يَكُنْ عَلَيْهَا يَوْمَئِذٍ سُتُورٌ

Abdullah ibn Busr: When the Messenger of Allaah came to some people’s door, he didn’t face it squarely, but faced the right or left corner, and said: Aslaam Alaykum! Asalaam Alaykum!

That was because there were no curtains on the doors of the house at that time.

Source: Saheeh Sunan Abi Dawud (no. 5186)

Shaikh Abdul-Muhsin al-‘Abbad: This is from the beautiful manners and honorable characteristics!
That is: a person not facing the door (when desiring to enter another’s house) because facing the door will make a person see inside the house.

However, if he’s facing to the right or left, then he’ll be far from his eye seeing what the owner of the house doesn’t want that he sees or be shown.

Source: Sharh Sunan Abi Dawud (no. 588)