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/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!
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=126.96.36.199/32 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.
|action (action name; Default: accept)||Action to take if packet is matched by the rule:
|address-list (string; Default: )||Name of the address list to be used. Applicable if action is
|address-list-timeout (time; Default: 00:00:00)||Time interval after which the address will be removed from the address list specified by
|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-limit (integer,netmask; Default: )||Restrict connection limit per address or address block up to and including given value|
|connection-mark (no-mark | string; Default: )||Matches packets marked via mangle facility with particular connection mark. If no-mark is set, rule will match any unmarked connection.|
|connection-rate (Integer 0..4294967295; Default: )||Connection Rate is a firewall matcher that allow to capture traffic based on present speed of the connection. Read more >>|
|connection-state (estabilished | invalid | new | related; Default: )||Interprets the connection tracking analysis data for a particular packet:
|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 /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:
|dst-limit (integer[/time],integer,dst-address | dst-port | src-address[/time]; Default: )||Matches packets until a given pps limit is exceeded. As opposed to the limit matcher, every destination IP address / destination port has it's own limit. Parameters are written in following format:
|dst-port (integer[-integer]: 0..65535; Default: )||List of destination port numbers or port number ranges|
|fragment (yes|no; Default: )||Matches fragmented packets. First (starting) fragment does not count. If connection tracking is enabled there will be no fragments as system automatically assembles every packet|
|hotspot (auth | from-client | http | local-dst | to-client; Default: )|
|icmp-options (integer:integer; Default: )||Matches ICMP type:code fileds|
|in-bridge-port (name; Default: )||Actual interface the packet has entered the router, if incoming interface is bridge. Works only if use-ip-firewall is enabled in bridge settings.|
|in-interface (name; Default: )||Interface the packet has entered the router|
|ingress-priority (integer: 0..63; Default: )||Matches ingress priority of the packet. Priority may be derived from VLAN, WMM or MPLS EXP bit. Read more>>|
|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.
|jump-target (name; Default: )||Name of the target chain to jump to. Applicable only if
|layer7-protocol (name; Default: )||Layer7 filter name defined in layer7 protocol menu.|
|limit (integer,time,integer; Default: )||Matches packets until a given pps limit is exceeded. Parameters are written in following format:
|log-prefix (string; Default: )||Adds specified text at the beginning of every log message. Applicable if
|nth (integer,integer; Default: )||Matches every nth packet. Read more >>|
|out-bridge-port (name; Default: )||Actual interface the packet is leaving the router, if outgoing interface is bridge. Works only if use-ip-firewall is enabled in bridge settings.|
|out-interface (; Default: )||Interface the packet is leaving the router|
|p2p (all-p2p | bit-torrent | blubster | direct-connect | edonkey | fasttrack | gnutella | soulseek | warez | winmx; Default: )||Matches packets from various peer-to-peer (P2P) protocols. Does not work on encrypted p2p packets.|
|packet-mark (no-mark | string; Default: )||Matches packets marked via mangle facility with particular packet mark. If no-mark is set, 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 to divide traffic into equal streams with ability to keep packets with 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 (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 following format
|random (integer: 1..99; Default: )||Matches packets randomly with given probability.|
|reject-with (; Default: )||Specifies error to be sent back if packet is rejected. Applicable if
|routing-mark (string; Default: )||Matches packets marked by mangle facility with particular routing mark|
|src-address (Ip/Netmaks, Ip range; Default: )||Matches packets which source is equal to specified IP or falls into 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: )||
Matches source address type:
|src-port (integer[-integer]: 0..65535; Default: )||List of source ports and ranges of source ports. Applicable only if 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
|tcp-mss (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 filter based on the packets' arrival time and date or, for locally generated packets, departure time and date|
|ttl (integer: 0..255; Default: )||Matches packets TTL value|
/ip firewall filter print stats will show additional read-only properties
|bytes (integer)||Total amount of bytes matched by the rule|
|packets (integer)||Total amount of packets matched by the rule|
By default print is equivalent to print static and shows only static rules.
[admin@dzeltenais_burkaans] /ip firewall mangle> print stats Flags: X - disabled, I - invalid, D - dynamic # CHAIN ACTION BYTES PACKETS 0 prerouting mark-routing 17478158 127631 1 prerouting mark-routing 782505 4506
To print also dynamic rules use print all.
[admin@dzeltenais_burkaans] /ip firewall mangle> print all stats Flags: X - disabled, I - invalid, D - dynamic # CHAIN ACTION BYTES PACKETS 0 prerouting mark-routing 17478158 127631 1 prerouting mark-routing 782505 4506 2 D forward change-mss 0 0 3 D forward change-mss 0 0 4 D forward change-mss 0 0 5 D forward change-mss 129372 2031
Or to print only dynamic rules use print dynamic
[admin@dzeltenais_burkaans] /ip firewall mangle> print stats dynamic Flags: X - disabled, I - invalid, D - dynamic # CHAIN ACTION BYTES PACKETS 0 D forward change-mss 0 0 1 D forward change-mss 0 0 2 D forward change-mss 0 0 3 D forward change-mss 132444 2079
Menu specific commands
|reset-counters (id)||Reset statistics counters for specified firewall rules.|
|reset-counters-all ()||Reset statistics counters for all firewall rules.|
Lets say our private network is 192.168.0.0/24 and public (WAN) interface is ether1. We will set up firewall to allow connections to router itself only from our local network and drop the rest. Also we will allow ICMP protocol on any interface so that anyone can ping your router from internet.
/ip firewall filter add chain=input connection-state=invalid action=drop \ comment="Drop Invalid connections" add chain=input connection-state=established action=accept \ comment="Allow Established connections" add chain=input protocol=icmp action=accept \ comment="Allow ICMP" add chain=input src-address=192.168.0.0/24 action=accept \ in-interface=!ether1 add chain=input action=drop comment="Drop everything else"
To protect the customer's network, we should check all traffic which goes through router and block unwanted. For icmp, tcp, udp traffic we will create chains, where will be droped all unwanted packets:
/ip firewall filter add chain=forward protocol=tcp connection-state=invalid \ action=drop comment="drop invalid connections" add chain=forward connection-state=established action=accept \ comment="allow already established connections" add chain=forward connection-state=related action=accept \ comment="allow related connections"
Block "bogon" IP addresses
add chain=forward src-address=0.0.0.0/8 action=drop add chain=forward dst-address=0.0.0.0/8 action=drop add chain=forward src-address=127.0.0.0/8 action=drop add chain=forward dst-address=127.0.0.0/8 action=drop add chain=forward src-address=188.8.131.52/3 action=drop add chain=forward dst-address=184.108.40.206/3 action=drop
Make jumps to new chains:
add chain=forward protocol=tcp action=jump jump-target=tcp add chain=forward protocol=udp action=jump jump-target=udp add chain=forward protocol=icmp action=jump jump-target=icmp
Create tcp chain and deny some tcp ports in it:
add chain=tcp protocol=tcp dst-port=69 action=drop \ comment="deny TFTP" add chain=tcp protocol=tcp dst-port=111 action=drop \ comment="deny RPC portmapper" add chain=tcp protocol=tcp dst-port=135 action=drop \ comment="deny RPC portmapper" add chain=tcp protocol=tcp dst-port=137-139 action=drop \ comment="deny NBT" add chain=tcp protocol=tcp dst-port=445 action=drop \ comment="deny cifs" add chain=tcp protocol=tcp dst-port=2049 action=drop comment="deny NFS" add chain=tcp protocol=tcp dst-port=12345-12346 action=drop comment="deny NetBus" add chain=tcp protocol=tcp dst-port=20034 action=drop comment="deny NetBus" add chain=tcp protocol=tcp dst-port=3133 action=drop comment="deny BackOriffice" add chain=tcp protocol=tcp dst-port=67-68 action=drop comment="deny DHCP"
Deny udp ports in udp chain:
add chain=udp protocol=udp dst-port=69 action=drop comment="deny TFTP" add chain=udp protocol=udp dst-port=111 action=drop comment="deny PRC portmapper" add chain=udp protocol=udp dst-port=135 action=drop comment="deny PRC portmapper" add chain=udp protocol=udp dst-port=137-139 action=drop comment="deny NBT" add chain=udp protocol=udp dst-port=2049 action=drop comment="deny NFS" add chain=udp protocol=udp dst-port=3133 action=drop comment="deny BackOriffice"
Allow only needed icmp codes in icmp chain:
add chain=icmp protocol=icmp icmp-options=0:0 action=accept \ comment="echo reply" add chain=icmp protocol=icmp icmp-options=3:0 action=accept \ comment="net unreachable" add chain=icmp protocol=icmp icmp-options=3:1 action=accept \ comment="host unreachable" add chain=icmp protocol=icmp icmp-options=3:4 action=accept \ comment="host unreachable fragmentation required" add chain=icmp protocol=icmp icmp-options=4:0 action=accept \ comment="allow source quench" add chain=icmp protocol=icmp icmp-options=8:0 action=accept \ comment="allow echo request" add chain=icmp protocol=icmp icmp-options=11:0 action=accept \ comment="allow time exceed" add chain=icmp protocol=icmp icmp-options=12:0 action=accept \ comment="allow parameter bad" add chain=icmp action=drop comment="deny all other types"
other ICMP codes are found here.