The 4th digit (the leading 4 in 4755) changes who the computer thinks is running the file the moment it executes.
Here is exactly how it affects execution:
1. Normal Execution (No "4" / 0755)
- Action: You double-click or run the command.
- Result: The program starts and has your identity and your limited permissions.
- Outcome for Wireshark: It tries to access the network card, the system asks "Are you Root?", the program says "No, I'm just Labex," and the system denies it.
2. SUID Execution (The "4" / 4755)
- Action: You double-click or run the command.
- Result: The "4" tells the Linux kernel: "Ignore the person who started this; run this program as if the Owner (Root) started it."
- Outcome for Wireshark: The program (
dumpcap) starts, the system asks "Are you Root?", the program says "Yes, I have the SUID bit," and the system allows it to capture network traffic.
In Short:
The 4th digit (SUID) creates a temporary "Identity Theft" for the sake of utility.
- Without the 4: The file executes as YOU.
- With the 4: The file executes as the OWNER (which is Root).
This is why you can capture packets in the lab without having to type sudo every single time you open Wireshark! You've essentially "pre-authorized" that one specific tool to act with high privileges on your behalf.
