Filter

Document revision:	2.7 (Fri Nov 04 16:04:37 GMT 2005)
Applies to:	V2.9

General Information

Summary

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 serve as a tool for preventing unauthorized access to directly attached networks and the router itself as well as a filter for outgoing traffic.

Quick Setup Guide

Specifications

Packages required: system
License required: Level1 (P2P filters limited to 1) , Level3
Submenu level: /ip firewall filter
Standards and Technologies: IP, RFC2113
Hardware usage: Increases with filtering rules count

Related Documents

Firewall Filter

Submenu level: /ip firewall filter

Description

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.

General Filtering Principles

The firewall operates by means of firewall rules. A rule is a definitive form expression that tells the router what to do with a particular IP packet. Each rule consists of two parts that are the matcher which matches traffic flow against given conditions and the action which defines what to do with the mathched packets. Rules are organized in chains for better management.

The filter facility has three default chains: input, forward and output that are responsible for traffic coming from, through and to the router, respectively. New user-defined chains can be added, as necessary. Since these chains have no default traffic to match, rules with action=jump and relevant jump-target should be added to one or more of the three default chains.

Filter Chains

As mentioned before, the 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.

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.

Property Description

action (accept | add-dst-to-address-list | add-src-to-address-list | drop | jump | log | passthrough | reject | return | tarpit; default: accept) - action to undertake if the packet matches the rule

address-list (name) - specifies the name of the address list to collect IP addresses from rules having action=add-dst-to-address-list or action=add-src-to-address-list actions. These address lists could be later used for packet matching

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

chain (forward | input | output | name) - specifies the chain to put a particular rule into. As the different traffic is passed through different chains, always be careful in choosing the right chain for a new rule. If the input does not match the name of an already defined chain, a new chain will be created

comment (text) - a descriptive comment for the rule. A comment can be used to identify rules form scripts

connection-bytes (integer-integer) - matches packets only if a given amount of bytes has been transfered through the particular connection

connection-limit (integer,netmask) - restrict connection limit per address or address block

connection-mark (name) - matches packets marked via mangle facility with particular connection mark

connection-state (estabilished | invalid | new | related) - interprets the connection tracking analysis data for a particular packet

connection-type (ftp | gre | h323 | irc | mms | pptp | quake3 | tftp) - 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 (text) - the text packets should contain in order to match the rule

dst-address (IP address/netmask | IP address-IP address) - specifies the address range an IP packet is destined to. Note that console converts entered address/netmask value to a valid network address, i.e.:1.1.1.1/24 is converted to 1.1.1.0/24

dst-address-list (name) - matches destination address of a packet against user-defined address list

dst-address-type (unicast | local | broadcast | multicast) - matches destination address type of the IP packet, one of the:

dst-limit (integer/time{0,1},integer,dst-address | dst-port | src-address{+},time{0,1}) - limits the packet per second (pps) rate on a per destination IP or per destination port base. As opposed to the limit match, every destination IP address / destination port has it's own limit. The options are as follows (in order of appearance):

dst-port (integer: 0..65535-integer: 0..65535{*}) - destination port number or range

hotspot (multiple choice: from-client | auth | local-dst | http) - matches packets received from clients against various Hot-Spot. All values can be negated

icmp-options (integer:integer) - matches ICMP Type:Code fields

in-interface (name) - interface the packet has entered the router through

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) - match ipv4 header options

jump-target (forward | input | output | name) - name of the target chain to jump to, if the action=jump is used

limit (integer/time{0,1},integer) - restricts packet match rate to a given limit. Usefull to reduce the amount of log messages

log-prefix (text) - all messages written to logs will contain the prefix specified herein. Used in conjunction with action=log

nth (integer,integer: 0..15,integer{0,1}) - match a particular Nth packet received by the rule. One of 16 available counters can be used to count packets

out-interface (name) - interface the packet will leave the router through

p2p (all-p2p | bit-torrent | blubster | direct-connect | edonkey | fasttrack | gnutella | soulseek | warez | winmx) - matches packets from various peer-to-peer (P2P) protocols

packet-mark (text) - matches packets marked via mangle facility with particular packet mark

packet-size (integer: 0..65535-integer: 0..65535{0,1}) - matches packet of the specified size or size range in bytes

phys-in-interface (name) - matches the bridge port physical input device added to a bridge device. It is only useful if the packet has arrived through the bridge

phys-out-interface (name) - matches the bridge port physical output device added to a bridge device. It is only useful if the packet will leave the router through the bridge

protocol (ddp | egp | encap | ggp | gre | hmp | icmp | idrp-cmtp | igmp | ipencap | ipip | ipsec-ah | ipsec-esp | iso-tp4 | ospf | pup | rdp | rspf | st | tcp | udp | vmtp | xns-idp | xtp | integer) - matches particular IP protocol specified by protocol name or number. You should specify this setting if you want to specify ports

psd (integer,time,integer,integer) - attempts to detect TCP and UDP scans. It is advised to assign lower weight to ports with high numbers to reduce the frequency of false positives, such as from passive mode FTP transfers

random (integer: 1..99) - matches packets randomly with given propability

reject-with (icmp-admin-prohibited | icmp-echo-reply | icmp-host-prohibited | icmp-host-unreachable | icmp-net-prohibited | icmp-network-unreachable | icmp-port-unreachable | icmp-protocol-unreachable | tcp-reset | integer) - alters the reply packet of reject action

routing-mark (name) - matches packets marked by mangle facility with particular routing mark

src-address (IP address/netmask | IP address-IP address) - specifies the address range an IP packet is originated from. Note that console converts entered address/netmask value to a valid network address, i.e.:1.1.1.1/24 is converted to 1.1.1.0/24

src-address-list (name) - matches source address of a packet against user-defined address list

src-address-type (unicast | local | broadcast | multicast) - matches source address type of the IP packet, one of the:

src-mac-address (MAC address) - source MAC address

src-port (integer: 0..65535-integer: 0..65535{*}) - source port number or range

tcp-flags (ack | cwr | ece | fin | psh | rst | syn | urg) - tcp flags to match

tcp-mss (integer: 0..65535) - matches TCP MSS value of an IP packet

time (time-time,sat | fri | thu | wed | tue | mon | sun{+}) - allows to create filter based on the packets' arrival time and date or, for locally generated packets, departure time and date

tos (max-reliability | max-throughput | min-cost | min-delay | normal) - specifies a match for the value of Type of Service (ToS) field of an IP header

Notes

Because the NAT rules are applied first, it is important to hold this in mind when setting up firewall rules, since the original packets might be already modified by the NAT

Filter Applications

Protect your RouterOS router

To protect your router, you should not only change admin's password but also set up packet filtering. All packets with destination to the router are processed against the ip firewall input chain. Note, that the input chain does not affect packets which are being transferred through the router.

/ 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=udp action=accept \
	comment="Allow UDP"  
add chain=input protocol=icmp action=accept \
	comment="Allow ICMP" 
add chain=input src-address=192.168.0.0/24 action=accept \
	comment="Allow access to router from known network" 
add chain=input action=drop comment="Drop anything else"  
			

Protecting the Customer's Network

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 IP addreses called "bogons":

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=224.0.0.0/3 action=drop 
add chain=forward dst-address=224.0.0.0/3 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="drop invalid connections"  
add chain=icmp protocol=icmp icmp-options=3:0 action=accept \
 	comment="allow established connections"  
add chain=icmp protocol=icmp icmp-options=3:1 action=accept \
 	comment="allow already established connections"  
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"