config security dos tcp-slowdata-attack-protection

A Slow Data attack sends legitimate application layer requests but reads responses very slowly. With that, it may attempt to exhaust the target’ s connection pool. Slow reading advertises a very small number for the TCP Receive Window size and at the same time by emptying the client’s TCP receive buffers slowly. That ensures a very low data flow rate.

The purpose of the attack is to consume the system resources (memory, CPU time) slowly. We can disable the connection when it fails to send probe packages within the zero-window timer.

Syntax

config security dos tcp-slowdata-attack-protection

edit <name>

set probe-interval-time <integer>

set probe-count <integer>

set action [ pass | deny | block-period]

set block-period <integer>

set severity [ high | medium | low ]

set log [enable | disable]

next

end

CLI specification

CLI Parameter	Help message	Type	Scope	Default	Must
probe-interval-time	Probe internal timer for zero-window probe	char	0-256	30	No
probe-count	Max count for zero-window probe	char	0-256	5	No
action	Action taken when probe count exceeds limit and still no >0 windows packet received	choice	Pass deny block-period	deny	No
block-period	Number of seconds to block the connection action if you choose block-period as action	integer	1-3600	60	No
severity	Severity of the Log	choice	info low medium high	high	No
log	Record log message	choice	enable disable	disable	No

Function description

CLI Parameter	Description
probe-interval-time	Set probe interval time for the TCP zero window timer. After receiving a zero window packet, FortiADC will probe the peer side periodically until it receives a >0 window, or probe count exceeds the max probe-count.
probe-count	Max consecutive zero window probe count
action	Action taken after exceeding max probe count Pass –if the probe count exceeds probe-count, FortiADC stops the probe and passes all the packets in both direction. Deny – deny the connection with RST Block-period – deny the connection, and block any new connection from the peer side for a period of time
block-period	Block the new connection from peer side for a period. During this period, the new connection will abort.
severity	Log severity level
log	Enable or disable log

Example

configure security dos tcp-slowdata-attack-protection

edit zero-window-limit

set probe-interval-time 30

set probe-count 5

set action block-period

set block-period 20

set log enable

set severity medium

next

end