Manual:IP/Firewall/Filter

Summary

Sub-menu: /ip firewall filter

The firewall implements packet filtering and thereby provides security functions that are used to manage data flow to, from and through the router. Along with the Network Address Translation it serves as a tool for preventing unauthorized access to directly attached networks and the router itself as well as a filter for outgoing traffic.

Network firewalls keep outside threats away from sensitive data available inside the network. Whenever different networks are joined together, there is always a threat that someone from outside of your network will break into your LAN. Such break-ins may result in private data being stolen and distributed, valuable data being altered or destroyed, or entire hard drives being erased. Firewalls are used as a means of preventing or minimizing the security risks inherent in connecting to other networks. Properly configured firewall plays a key role in efficient and secure network infrastrure deployment.

MikroTik RouterOS has very powerful firewall implementation with features including:

stateful packet inspection

Layer-7 protocol detection

peer-to-peer protocols filtering

traffic classification by:

source MAC address

IP addresses (network or list) and address types (broadcast, local, multicast, unicast)

port or port range

IP protocols

protocol options (ICMP type and code fields, TCP flags, IP options and MSS)

interface the packet arrived from or left through

internal flow and connection marks

DSCP byte

packet content

rate at which packets arrive and sequence numbers

packet size

packet arrival time

and much more!

Chains

The firewall operates by means of firewall rules. Each rule consists of two parts - the matcher which matches traffic flow against given conditions and the action which defines what to do with the matched packet.

Firewall filtering rules are grouped together in chains. It allows a packet to be matched against one common criterion in one chain, and then passed over for processing against some other common criteria to another chain. For example a packet should be matched against the IP address:port pair. Of course, it could be achieved by adding as many rules with IP address:port match as required to the forward chain, but a better way could be to add one rule that matches traffic from a particular IP address, e.g.: /ip firewall filter add src-address=1.1.1.2/32 jump-target="mychain" and in case of successfull match passes control over the IP packet to some other chain, id est mychain in this example. Then rules that perform matching against separate ports can be added to mychain chain without specifying the IP addresses.

There are three predefined chains, which 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

Packet flow diagrams illustrate how packets are processed in RouterOS.

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 chain, then it is accepted.

Properties

Property	Description
action (action name; Default: accept)	Action to take if packet is matched by the rule: accept - accept the packet. Packet is not passed to 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 jump - jump to the user defined chain specified by the value of jump-target parameter log - add a message to the system log containing following data: in-interface, out-interface, src-mac, protocol, src-ip:port->dst-ip:port and length of the packet. After packet is matched it is passed to next rule in the list, similar as passthrough passthrough - ignore this rule and go to next one (useful for statistics). reject - drop the packet and send an ICMP reject message tarpit - captures and holds TCP connections (replies with SYN/ACK to the inbound TCP SYN packet)
address-list (string; Default: )	Name of the address list to be used. Applicable if action is add-dst-to-address-list or add-src-to-address-list
address-list-timeout (time; Default: 00:00:00)	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 00:00:00 will leave the address in the address list forever
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 transfered through the particular connection. 0 - means infinity, for example connection-bytes=2000000-0 means that the rule matches if more than 2MB has been transfered through the relevant connection
connection-limit (integer,netmaks; Default: )	Restrict connection limit per address or address block/td>
connection-mark (string; Default: )	Matches packets marked via mangle facility with particular connection mark
connection-rate (; Default: )
connection-state (estabilished | invalid | new | related; Default: )	Interprets the connection tracking analysis data for a particular packet: established - a packet which belongs to an existing connection invalid - a packet which could not be identified for some reason new - a packet which begins a new connection related - a packet which is related to, but not part of an existing connection, such as ICMP errors or a packet which begins FTP data connection
connection-type (; Default: )
content (; Default: )
dscp (; Default: )
dst-address (; Default: )
dst-address-list (; Default: )
dst-address-type (; Default: )
dst-limit (; Default: )
dst-port (; Default: )
fragment (; Default: )
hotspot (; Default: )
icmp-options (; Default: )
in-bridge-port (; Default: )
in-interface (; Default: )
ingress-priority (; Default: )
ipv4-options (; Default: )
jump-target (; Default: )
layer7-protocol (; Default: )
limit (; Default: )
log-prefix (; Default: )
nth (; Default: )
out-bridge-port (; Default: )
out-interface (; Default: )
p2p (; Default: )
packet-mark (; Default: )
packet-size (; Default: )
per-connection-classifier (; Default: )
port (; Default: )
protocol (; Default: )
psd (; Default: )
random (; Default: )
reject-with (; Default: )
routing-mark (; Default: )
src-address (; Default: )
src-address-list (; Default: )
src-address-type (; Default: )
src-port (; Default: )
tcp-flags (; Default: )
tcp-mss (; Default: )
time (; Default: )
ttl (; Default: )

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Basic examples