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ACK scan DOS attack – How to mitigate

by | Feb 26, 2021

Wondering how to mitigate ACK scan DOS attack? We can help you.

ACK scans are generally used to identify ports or hosts that may be filtered and resistant to any other form of scanning. An adversary uses TCP ACK segments to gather information about firewall or ACL configuration.

Attackers scan our router or send unwanted traffic/requests like SYN, ACK, FIN to specific UDP/TCP Port. Sometimes, they also send continuous unwanted traffic into particular open port/s. This can crash our router and make it out of service.

Here at Bobcares we often handle DOS attack as a part of our Server Management Services.

Today let’s see some of the steps which our Support Engineers follow to mitigate this issue.

How does ACK scan DOS attack work?

A target system is presented with a packet with the ACK flag set with a sequence number of zero to an interesting port.

Generally, if the sequence number is not zero, there is a violation of TCP rules associated with that parameter.

And the target sends back an RST. The presence of the RST provides an attacker a good indication that the host is alive but behind some form of filtering like a firewall, a router, or even some proxies.

A TCP ACK segment when sent to a closed port or sent out-of-sync to a listening port, the expected behavior is for the device to respond with an RST. This helps the attacker to get an idea about the type of firewall.

When combined with SYN techniques an attacker will get a clear picture of the types of packets that get through to a host and can understand the firewall rule-set.

ACK scanning, when combined with SYN scanning, also allows the adversary to analyze whether a firewall is stateful or non-stateful.

Two potential rules for detecting this behavior are:

alert tcp any -> any (flags:A; ack:0; msg: “Potential Ack Scan”; sid: 10001;)


alert tcp any -> any (flags:AR; msg: “Ack and RST detected-Potential Ack Scan”; sid: 10002;)

In the first rule, the assumption is that the Acknowledgment flag will be set and the sequence value will be set to “0”.  This will make the target return an “RST”.

The second rule looks for the existence of an “RST” with the Acknowledgment flag set rather than looking for the existence of a zero-sequence value. The existence of these two flags together simultaneously can also be an indicator of an ACK scan being used for reconnaissance purposes, or “firewalking”.

Any SYN-ACK responses are possible connections: an RST(reset) response means the port is closed, but there is a live computer here. No responses indicate SYN is filtered on the network.

An attacker can scan the router or send unwanted traffic/request like SYN,ACK,FIN to specific UDP/TCP Port.

Generally, if the router is accessible from outside of the network, the attacker can access it by brute force. A typical Probe response is given below:

Probe Response - Assigned State
TCP RST response - unfiltered
No response received (even after retransmissions) - filtered
ICMP unreachable error - filtered

An example of a typical ACK scan:

# nmap -sA -T4 <target>

Starting Nmap ( )
Nmap scan report for target
Not shown: 994 filtered ports
22/tcp unfiltered ssh
25/tcp unfiltered smtp
53/tcp unfiltered domain
70/tcp unfiltered gopher
80/tcp unfiltered http
113/tcp unfiltered auth
Nmap done: 1 IP address (1 host up) scanned in 4.01 seconds


Steps to Mitigate

1. Set up a firewall to filter scan attempts.

We can use the following commands to filter scan attempts with iptables.

iptables -A INPUT -p tcp –tcp-flags SYN,ACK SYN,ACK -m state –state NEW -j DROP
iptables -A INPUT -p tcp –tcp-flags ALL NONE -j DROP

iptables -A INPUT -p tcp –tcp-flags SYN,FIN SYN,FIN -j DROP
iptables -A INPUT -p tcp –tcp-flags SYN,RST SYN,RST -j DROP
iptables -A INPUT -p tcp –tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP

iptables -A INPUT -p tcp –tcp-flags FIN,RST FIN,RST -j DROP
iptables -A INPUT -p tcp –tcp-flags ACK,FIN FIN -j DROP
iptables -A INPUT -p tcp –tcp-flags ACK,PSH PSH -j DROP
iptables -A INPUT -p tcp –tcp-flags ACK,URG URG -j DROP

2. Reset the IP if it’s dynamic. Just turn the router off for the DHCP lease time or spoof a different MAC address.

3. Appropriate rules can be applied to DROP PKT like: burst limit/rate, Source limit, destination limit, connection limit, length, etc.

We can use the following IPtables commands for this:

$ sudo iptables --append INPUT --source --jump DROP


# iptables -A INPUT -m state --state NEW -j DROP


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In short, we saw how the ACK scan DOS attack works along with the steps which our Support Engineers follow to mitigate this.


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