Introduction

Firewall filters are used to allow or block specific packets forwarded to your local network, originated from your router, or destined to the router.

There are two methods on how to set up filtering:

  • allow specific traffic and drop everything else
  • drop only malicious traffic, everything else is allowed.

Both methods have pros and cons, for example, from a security point of view first method is much more secure, but requires administrator input whenever traffic for new service needs to be accepted. This strategy provides good control over the traffic and reduces the possibility of a breach because of service misconfiguration.

On the other hand, when securing a customer network it would be an administrative nightmare to accept all possible services that users may use. Therefore careful planning of the firewall is essential in advanced setups.

A firewall filter consists of three predefined chains that cannot be deleted:

  • input - used to process packets entering the router through one of the interfaces with the destination IP address which is one of the router's addresses. Packets passing through the router are not processed against the rules of the input chain
  • forward - used to process packets passing through the router
  • output - used to process packets originated from the router and leaving it through one of the interfaces. Packets passing through the router are not processed against the rules of the output chain

When processing a chain, rules are taken from the chain in the order they are listed there from top to bottom. If a packet matches the criteria of the rule, then the specified action is performed on it, and no more rules are processed in that chain (the exception is the passthrough action). If a packet has not matched any rule within the built-in chain, then it is accepted.  More detailed packet processing in RouterOS is described in the Packet Flow in RouterOS diagram.

Properties

PropertyDescription
action (action name; Default: accept)Action to take if a packet is matched by the rule:
  • accept - accept the packet. A packet is not passed to the next firewall rule.
  • add-dst-to-address-list - add destination address to address list specified by address-list parameter
  • add-src-to-address-list - add source address to address list specified by address-list parameter
  • drop - silently drop the packet
  • fasttrack-connection - process packets from a connection using FastPath by enabling FastTrack for the connection
  • jump - jump to the user-defined chain specified by the value of jump-target parameter
  • log - add a message to the system log containing the following data: in-interface, out-interface, src-mac, protocol, src-ip:port->dst-ip:port and length of the packet. After a packet is matched it is passed to the next rule in the list, similar as passthrough
  • passthrough - if a packet is matched by the rule, increase counter and go to next rule (useful for statistics)
  • reject - drop the packet and send an ICMP reject message
  • return - passes control back to the chain from where the jump took place
  • tarpit - captures and holds TCP connections (replies with SYN/ACK to the inbound TCP SYN packet)
address-list-timeout (none-dynamic | none-static | time; Default: none-dynamic)Time interval after which the address will be removed from the address list specified by address-list parameter. Used in conjunction with add-dst-to-address-list or add-src-to-address-list actions
  • Value of none-dynamic (00:00:00) will leave the address in the address list till reboot
  • Value of none-static will leave the address in the address list forever and will be included in configuration export/backup
chain (name; Default: )Specifies to which chain rule will be added. If the input does not match the name of an already defined chain, a new chain will be created
comment (string; Default: )Descriptive comment for the rule
chain (name; Default: )Specifies to which chain rule will be added. If the input does not match the name of an already defined chain, a new chain will be created
comment (string; Default: )Descriptive comment for the rule
connection-bytes (integer-integer; Default: )Matches packets only if a given amount of bytes has been transferred through the particular connection. 0 - means infinity, for example connection-bytes=2000000-0 means that the rule matches if more than 2MB has been transferred through the relevant connection
connection-limit (integer,netmask; Default: )Matches connections per address or address block after a given value is reached. Should be used together with connection-state=new and/or with tcp-flags=syn because matcher is very resource-intensive
connection-mark (no-mark | string; Default: )Matches packets marked via mangle facility with particular connection mark. If no-mark is set, the rule will match any unmarked connection
connection-nat-state (srcnat | dstnat; Default: )Can match connections that are srcnatted, distracted, or both. Note that connection-state=related connections connection-nat-state is determined by the direction of the first packet. and if connection tracking needs to use dst-nat to deliver this connection to the same hosts as the main connection it will be in connection-nat-state=dstnat even if there are no dst-nat rules at all
connection-rate (Integer 0..4294967295; Default: )Connection Rate is a firewall matcher that allows capturing traffic based on the present speed of the connection
connection-state (established | invalid | new | related | untracked; Default: )Interprets the connection tracking analytics data for a particular packet:
  • established - a packet that belongs to an existing connection
  • invalid - a packet that does not have a determined state in connection tracking (usually - severe out-of-order packets, packets with wrong sequence/ack number, or in case of a resource over usage on the router), for this reason, an invalid packet will not participate in NAT (as only connection-state=new packets do), and will still contain original source IP address when routed. We strongly suggest dropping all connection-state=invalid packets in firewall filter forward and input chains
  • new - the packet has started a new connection, or otherwise associated with a connection that has not seen packets in both directions.
  • related - a packet that is related to, but not parts of an existing connection, such as ICMP errors or a packet that begins FTP data connection
  • an untracked - packet which was set to bypass connection tracking in firewall RAW tables.
connection-type (ftp | h323 | irc | pptp | quake3 | sip | tftp; Default: )Matches packets from related connections based on information from their connection tracking helpers. A relevant connection helper must be enabled under the: /ip firewall service-port
content (string; Default: )Match packets that contain specified text
dscp (integer: 0..63; Default: )Matches DSCP IP header field.
dst-address (IP/netmask | IP range; Default: )Matches packets which destination is equal to specified IP or falls into specified IP range.
dst-address-list (name; Default: )Matches destination address of a packet against user-defined address-list.
dst-address-type (unicast | local | broadcast | multicast; Default: )Matches destination address type:
  • unicast - IP address used for point to point transmission
  • local - if dst-address is assigned to one of the router's interfaces
  • broadcast - packet is sent to all devices in a subnet
  • multicast - packet is forwarded to a defined group of devices
dst-limit (integer[/time],integer,dst-address | dst-port | src-address[/time]; Default: )Matches packets until a given rate is exceeded. Rate is defined as packets per time interval. As opposed to the limit matcher, every flow has its own limit. Flow is defined by a mode parameter. Parameters are written in the following format: rate[/time],burst,mode[/expire].
  • rate - packet count per time interval per-flow to match
  • time - specifies the time interval in which the packet count rate per flow cannot be exceeded (optional, 1s will be used if not specified)
  • burst - initial number of packets per flow to match: this number gets recharged by one every time/rate, up to this number
  • mode - this parameter specifies what unique fields define flow (src-address, dst-address, src-and-dst-address, dst-address-and-port, addresses-and-dst-port)
  • expire - specifies interval after which flow with no packets will be allowed to be deleted (optional)
dst-port (integer[-integer]: 0..65535; Default: )List of destination port numbers or port number ranges
fragment (yes|no; Default: )Matches fragmented packets. The first (starting) fragment does not count. If connection tracking is enabled there will be no fragments as the system automatically assembles every packet
hotspot (auth | from-client | http | local-dst | to-client; Default: )Matches packets received from HotSpot clients against various HotSpot matchers.
  • auth - matches authenticated HotSpot client packets
  • from-client - matches packets that are coming from the HotSpot client
  • http - matches HTTP requests sent to the HotSpot server
  • local-dst - matches packets that are destined to the HotSpot server
  • to-client - matches packets that are sent to the HotSpot client
icmp-options (integer:integer; Default: )Matches ICMP type: code fields
in-bridge-port (name; Default: )Actual interface the packet has entered the router if the incoming interface is a bridge. Works only if use-ip-firewall is enabled in bridge settings.
in-bridge-port-list (name; Default: )Set of interfaces defined in interface list. Works the same as in-bridge-port
in-interface (name; Default: )Interface the packet has entered the router
in-interface-list (name; Default: )Set of interfaces defined in interface list. Works the same as in-interface
ingress-priority (integer: 0..63; Default: )Matches the priority of an ingress packet. Priority may be derived from VLAN, WMM, DSCP, or MPLS EXP bit. read more
ipsec-policy (in | out, ipsec | none; Default: )Matches the policy used by IPsec. Value is written in the following format: direction, policy. The direction is Used to select whether to match the policy used for decapsulation or the policy that will be used for encapsulation.
  • in - valid in the PREROUTING, INPUT, and FORWARD chains
  • out - valid in the POSTROUTING, OUTPUT, and FORWARD chains
  • ipsec - matches if the packet is subject to IPsec processing;
  • none - matches packet that is not subject to IPsec processing (for example, IPSec transport packet).

For example, if a router receives an IPsec encapsulated Gre packet, then rule ipsec-policy=in,ipsec will match Gre packet, but a rule ipsec-policy=in,none will match the ESP packet.

ipv4-options (any | loose-source-routing | no-record-route | no-router-alert | no-source-routing | no-timestamp | none | record-route | router-alert | strict-source-routing | timestamp; Default: )Matches IPv4 header options.
  • any - match packet with at least one of the ipv4 options
  • loose-source-routing - match packets with a loose source routing option. This option is used to route the internet datagram based on information supplied by the source
  • no-record-route - match packets with no record route option. This option is used to route the internet datagram based on information supplied by the source
  • no-router-alert - match packets with no router alter option
  • no-source-routing - match packets with no source routing option
  • no-timestamp - match packets with no timestamp option
  • record-route - match packets with record route option
  • router-alert - match packets with router alter option
  • strict-source-routing - match packets with strict source routing option
  • timestamp - match packets with a timestamp
jump-target (name; Default: )Name of the target chain to jump to. Applicable only if action=jump
layer7-protocol (name; Default: )Layer7 filter name defined in layer7 protocol menu.
limit (integer,time,integer; Default: )Matches packets up to a limited rate (packet rate or bit rate). A rule using this matcher will match until this limit is reached. Parameters are written in the following format: rate[/time],burst:mode.
  • rate - packet or bit count per time interval to match
  • time - specifies the time interval in which the packet or bit rate cannot be exceeded (optional, 1s will be used if not specified)
  • burst - initial number of packets or bits to match: this number gets recharged every 10ms so burst should be at least 1/100 of a rate per second
  • mode - packet or bit mode
log (yes | no; Default: no)Add a message to the system log containing the following data: in-interface, out-interface, src-mac, protocol, src-ip:port->dst-ip:port, and length of the packet.
log-prefix (string; Default: )Adds specified text at the beginning of every log message. Applicable if action=log or log=yes configured.
nth (integer,integer; Default: )Matches every nth packet: nth=2,1 rule will match every first packet of 2, hence, 50% of all the traffic that is matched by the rule
out-bridge-port (name; Default: )Actual interface the packet is leaving the router if the outgoing interface is a bridge. Works only if use-ip-firewall is enabled in bridge settings.
out-bridge-port-list (name; Default: )Set of interfaces defined in interface list. Works the same as out-bridge-port
out-interface (; Default: )Interface the packet is leaving the router
out-interface-list (name; Default: )Set of interfaces defined in interface list. Works the same as out-interface
packet-mark (no-mark | string; Default: )Matches packets marked via mangle facility with particular packet mark. If no-mark is set, the rule will match any unmarked packet.
packet-size (integer[-integer]:0..65535; Default: )Matches packets of specified size or size range in bytes.
per-connection-classifier (ValuesToHash:Denominator/Remainder; Default: )PCC matcher allows dividing traffic into equal streams with the ability to keep packets with a specific set of options in one particular stream. Read more >>
port (integer[-integer]: 0..65535; Default: )Matches if any (source or destination) port matches the specified list of ports or port ranges. Applicable only if protocol is TCP or UDP
priority (integer: 0..63; Default:)Matches the packet's priority after a new priority has been set. Priority may be derived from VLAN, WMM, DSCP, MPLS EXP bit, or from the priority that has been set using the set-priority action. Read more
protocol (name or protocol ID; Default: tcp)Matches particular IP protocol specified by protocol name or number
psd (integer,time,integer,integer; Default: )Attempts to detect TCP and UDP scans. Parameters are in the following format WeightThreshold, DelayThreshold, LowPortWeight, HighPortWeight
  • WeightThreshold - total weight of the latest TCP/UDP packets with different destination ports coming from the same host to be treated as port scan sequence
  • DelayThreshold - delay for the packets with different destination ports coming from the same host to be treated as possible port scan subsequence
  • LowPortWeight - the weight of the packets with privileged (<1024) destination port
  • HighPortWeight - the weight of the packet with non-privileged destination port
random (integer: 1..99; Default: )Matches packets randomly with a given probability
reject-with (icmp-admin-prohibited | icmp-net-prohibited | icmp-protocol-unreachable | icmp-host-prohibited | icmp-network-unreachable | tcp-reset | icmp-host-unreachable | icmp-port-unreachable; Default: icmp-network-unreachable)Specifies ICMP error to be sent back if the packet is rejected. Applicable if action=reject
routing-table (string; Default: )Matches packets which destination address is resolved in specific a routing table.
routing-mark (string; Default: )Matches packets marked by mangle facility with particular routing mark
src-address (Ip/Netmask, Ip range; Default: )Matches packets which source is equal to specified IP or falls into a specified IP range
src-address-list (name; Default: )Matches source address of a packet against user-defined address list
src-address-type (unicast | local | broadcast | multicast; Default: )

{ta{tableMatches source address type:

  • unicast - IP address used for point to point transmission
  • local - if an address is assigned to one of the router's interfaces
  • broadcast - packet is sent to all devices in the subnet
  • multicast - packet is forwarded to a defined group of devices
src-port (integer[-integer]: 0..65535; Default: )List of source ports and ranges of source ports. Applicable only if a protocol is TCP or UDP
src-mac-address (MAC address; Default: )Matches source MAC address of the packet
tcp-flags (ack | cwr | ece | fin | psh | rst | syn | urg; Default: )Matches specified TCP flags
  • ack - acknowledging data
  • cwr - congestion window reduced
  • ece - ECN-echo flag (explicit congestion notification)
  • fin - close connection
  • psh - push function
  • rst - drop connection
  • syn - new connection
  • urg - urgent data
tcp-mss (integer[-integer]: 0..65535; Default: )Matches TCP MSS value of an IP packet
time (time-time,sat | fri | thu | wed | tue | mon | sun; Default: )Allows to create a filter based on the packets' arrival time and date or, for locally generated packets, departure time and date
tls-host (string; Default: )Allows matching HTTPS traffic based on TLS SNI hostname. Accepts GLOB syntax for wildcard matching. Note that the matcher will not be able to match hostname if the TLS handshake frame is fragmented into multiple TCP segments (packets).
ttl (integer: 0..255; Default: )Matches packets TTL value

A tls-host matcher is a powerful tool to block HTTPS-based websites, but for example, Youtube is using QUIC (UDP-based protocol) instead of normal HTTP/2 (TCP-based protocol). TLS-host does not work with QUIC as it depends on TCP connection.

Stats

To show additional read-only properties:

/ip firewall filter print stats


PropertyDescription
bytes (integer)The total amount of bytes matched by the rule
packets (integer)The total amount of packets matched by the rule
[admin@MikroTik] > ip firewall filter print stats 
Flags: X - disabled, I - invalid, D - dynamic 
 #    CHAIN                                                                                                                 ACTION                            BYTES         PACKETS
 0  D ;;; special dummy rule to show fasttrack counters
      forward                                                                                                               passthrough              50 507 925 242      50 048 246
 1    ;;; defconf: drop invalid
      forward                                                                                                               drop                            432 270           9 719
 2    ;;; defconf: drop invalid
      input                                                                                                                 drop                            125 943           2 434
 3    input                                                                                                                 accept                   20 090 211 549      20 009 864
 4    ;;; defconf: accept ICMP
      input                                                                                                                 accept                          634 926           7 648
 5    ;;; defconf: drop all not coming from LAN
      input                                                                                                                 drop                          4 288 079          83 428
 6    ;;; defconf: accept in ipsec policy
      forward                                                                                                               accept                                0               0
7    ;;; defconf: accept out ipsec policy
      forward                                                                                                               accept                                0               0
8    ;;; defconf: fasttrack
      forward                                                                                                               fasttrack-connection     28 505 528 775      31 504 682
9    ;;; defconf: accept established,related, untracked
      forward                                                                                                               accept                   28 505 528 775      31 504 682
10    ;;; defconf: drop all from WAN not DSTNATed
      forward                                                                                                               drop                                  0               0

Example

An example on how to properly secure your device you can find in our Building Your First Firewall article!

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