A denial-of-service (DoS) or distributed denial-of-service (DDoS) attack is a malicious attempt to disrupt normal traffic of a targeted server, service, or network by overwhelming the target or its surrounding infrastructure with a flood of Internet traffic. There are several types of DDoS attacks, for example, HTTP flood, SYN flood, DNS amplification, etc.
Protection against DDoS
These rules are only an improvement for firewall, do not forget to properly secure your device: Building Your First Firewall!
/ip firewall address-list add list=ddos-attackersadd list=ddos-target /ip firewall filter add action=return chain=detect-ddos dst-limit=32,32,src-and-dst-addresses/10s add action=add-dst-to-address-list address-list=ddos-target address-list-timeout=10m chain=detect-ddos add action=add-src-to-address-list address-list=ddos-attackers address-list-timeout=10m chain=detect-ddos /ip firewall raw add action=drop chain=prerouting dst-address-list=ddos-target src-address-list=ddos-attackers
First, we will send every new connection to the specific firewall chain where we will detect DDoS:
/ip firewall filter add chain=forward connection-state=new action=jump jump-target=detect-ddos
In the newly created chain, we will add the following rule with the “dst-limit” parameter. This parameter is written in the following format: dst-limit=count[/time],burst,mode[/expire]. We will match 32 packets with 32 packet burst based on destination and source address flow, which renews every 10 seconds. The rule will work until a given rate is exceeded.
/ip firewall filter add chain=detect-ddos dst-limit=32,32,src-and-dst-addresses/10s action=return
So far all the legitimate traffic should go through the “action=return”, but in the case of DoS/DDoS “dst-limit” buffer will be fulfilled and a rule will not “catch” any new traffic. Here come the next rules, which will deal with the attack. Let`s start with creating a list for attackers and victims which we will drop:
ip firewall address-list add list=ddos-attackers ip firewall address-list add list=ddos-targets ip firewall raw add chain=prerouting action=drop src-address-list=ddos-attackers dst-address-list=dddos-targets
With the firewall filter section, we will add attackers in the “DDoS-attackers” and victims in list “ddos-targets” list:
/ip firewall filter add action=add-dst-to-address-list address-list=ddos-target address-list-timeout=10m chain=detect-ddos add action=add-src-to-address-list address-list=ddos-attackers address-list-timeout=10m chain=detect-ddos
An SYN flood is a form of DoS attack in which an attacker sends a succession of SYN requests to a target’s system in an attempt to consume enough server resources to make the system unresponsive to legitimate traffic. Fortunately, in RouterOS we have specific feature for such an attack:
/ip settings set tcp-syncookies=yes
The feature works with sending back ACK packets that contain a little cryptographic hash, which the responding client will echo back with as part of its SYN-ACK packet. If the kernel doesn’t see this “cookie” in the reply packet, it will assume the connection is bogus and drop it.
An SYN-ACK flood is an attack method that involves sending a target server spoofed SYN-ACK packet at a high rate. The server requires significant resources to process such packets out-of-order (not in accordance with the normal SYN, SYN-ACK, ACK TCP three-way handshake mechanism), it can become so busy handling the attack traffic, that it cannot handle legitimate traffic and hence the attackers achieve a DoS/DDoS condition. In RouterOS, we can configure similar rules from the previously mentioned example, but more specifically for SYN-ACK flood:
/ip firewall filter add action=return chain=detect-ddos dst-limit=32,32,src-and-dst-addresses/10s protocol=tcp tcp-flags=syn,ack