Access Control Lists
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• Control Code – Decimal number (representing a bit string) that specifies flag bits
in byte 14 of the TCP header. (Range: 0-63)
• Control Code Bit Mask – Decimal number representing the code bits to match.
The control bitmask is a decimal number (for an equivalent binary bit mask) that is
applied to the control code. Enter a decimal number, where the equivalent binary bit
“1” means to match a bit and “0” means to ignore a bit. The following bits may be
specified:
- 1 (fin) – Finish
- 2 (syn) – Synchronize
- 4 (rst) – Reset
- 8 (psh) – Push
- 16 (ack) – Acknowledgement
- 32 (urg) – Urgent pointer
For example, use the code value and mask below to catch packets with the following
flags set:
- SYN flag valid, use control-code 2, control bitmask 2
- Both SYN and ACK valid, use control-code 18, control bitmask 18
- SYN valid and ACK invalid, use control-code 2, control bitmask 18
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