DDoS Detection and Blocking

    This is the way to prevent (D)DoS Attack from your users to attacked resources, and drop (D)DoS directed to your clients.
    First, we catch all new connections and send them to dedicated firewall chain:

    /ip firewall filter
    add chain=forward connection-state=new action=jump jump-target=detect-ddos

    Note: In RouterOS, any single UDP packet is considered to be new connection by Connection Tracking in any Firewall section (except NAT) until the packet in opposite direction is sent. It’s good behaviour, because legitimate traffic (like uTorrent’s or Skype’s UDP stream) is bidirectional, so it triggers ‘connection-state=new’ only once per stream; on the other side, any unidirectional flooding (the only exception I can imagine is NetFlow traffic) generates ‘connection-state=new’ per each packet even if all packets have the same Src and Dst Addresses and Ports, so it can be easily detected as DoS Attack.

    Then, for each “SrcIP:DstIP” pair we allow some number of new connections. One may want also add some exceptions (like DNS servers – it won’t be good if they will be blocked):

    /ip firewall filter
    add chain=detect-ddos dst-limit=32,32,src-and-dst-addresses/10s action=return
    add chain=detect-ddos src-address= action=return

    Note: At least up to version 5.6, ‘dst-limit’ matcher has two bugs:

    • ‘Expire’ value is 10 times lower than you set; so ’10s’ is actually 1 second
    • ‘dst-limit’ matches first ‘Burst’ packets (as it should be) plus one, and then skips packets for the first second; so if you have Rate set to 32 and Burst set to 0, and you start to flood packets, the rule will match 1 packet, and on 2nd packet it won’t match until 1sec passes – that’s why you need ‘Burst’ value at least as high as ‘Rate’ value

    Now, we have only packets which exceed our limits – and we add their source to ‘ddoser’ and the target to ‘ddosed’ address lists:

    /ip firewall filter
    add chain=detect-ddos action=add-dst-to-address-list address-list=ddosed address-list-timeout=10m
    add chain=detect-ddos action=add-src-to-address-list address-list=ddoser address-list-timeout=10m

    Then packet processing returns to ‘forward’ chain, where we block any packets from ddosers to ddosed resources:

    /ip firewall filter
    add chain=forward connection-state=new src-address-list=ddoser dst-address-list=ddosed action=drop

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