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
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 (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,netmask; Default: )
Restrict connection limit per address or address block
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.
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:
unicast - IP address used for point to point transmission
local - if dst-address is assigned to one of router's interfaces
broadcast - packet is sent to all devices in subnet
multicast - packet is forwarded to defined group of devices
Matches packets if 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: count,time,burst,mode,expire.
count - maximum average packet rate measured in packets per time interval
time - specifies the time interval in which the packet rate is measured
burst - number of packets which are not counted by packet rate
mode - the classifier for packet rate limiting
expire - specifies interval after which recored ip address /port will be deleted
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
any - match packet with at least one of the ipv4 options
loose-source-routing - match packets with 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 timestamp
jump-target (name; Default: )
Name of the target chain to jump to. Applicable only if action=jump
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 is TCP or UDP
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 WeightThreshold, DelayThreshold, LopPortWeight, 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 - weight of the packets with privileged (<=1024) destination port
HighPortWeight - weight of the packet with non-priviliged destination port
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 action=reject
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:
unicast - IP address used for point to point transmission
local - if address is assigned to one of router's interfaces
broadcast - packet is sent to all devices in subnet
multicast - packet is forwarded to defined group of devices
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
ack - acknowledging data
cwr - congestion window reduced
ece - ECN-echo flag (explicit congestion notification)
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.
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:
