Mastering Proxmox - Third Edition

The TCP flags filter

In simple terms, TCP flags are control bits that indicate how TCP packets should be handled by the client. These control bits or indicators reside in the TCP header. There are a total of nine control bits with one bit for each flag. The full description of how exactly these TCP flags work is beyond the scope of this book since TCP is a vast subject of various complexities. Here, we will only see what those TCP flags are and how the Proxmox firewall handles TCP flag filtering. The following table is a summary of the TCP flags and their functions:

TCP flag	Function
URG—1 bit	This indicates that the TCP packet is urgent.
ACK—1 bit	This indicates the acknowledgment field. After the initial SYN for all packets, ...