Network Address Translation is an Internet standard that allows hosts on local area networks to use one set of IP addresses for internal communications and another set of IP addresses for external communications. A LAN that uses NAT is referred as natted network. For NAT to function, there should be a NAT gateway in each natted network. The NAT gateway (NAT router) performs IP address rewriting on the way a packet travel from/to LAN.
There are two types of NAT:
source NAT or srcnat. This type of NAT is performed on packets that are originated from a natted network. A NAT router replaces the private source address of an IP packet with a new public IP address as it travels through the router. A reverse operation is applied to the reply packets travelling in the other direction.
destination NAT or dstnat. This type of NAT is performed on packets that are destined to the natted network. It is most comonly used to make hosts on a private network to be acceesible from the Internet. A NAT router performing dstnat replaces the destination IP address of an IP packet as it travel through the router towards a private network.
Hosts behind a NAT-enabled router do not have true end-to-end connectivity. Therefore some Internet protocols might not work in scenarios with NAT. Services that require the initiation of TCP connection from outside the private network or stateless protocols such as UDP, can be disrupted. Moreover, some protocols are inherently incompatible with NAT, a bold example is AH protocol from the IPsec suite.
To overcome these limitations RouterOS includes a number of so-called NAT helpers, that enable NAT traversal for various protocols.
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 NAT 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
dst-nat - replaces destination address and/or port of an IP packet to values specified by to-addresses and to-ports parameters
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
masquerade - replace source address of an IP packet to IP determined by routing facility.
netmap - creates a static 1:1 mapping of one set of IP addresses to another one. Often used to distribute public IP addresses to hosts on private networks
passthrough - ignore this rule and go to next one (useful for statistics).
redirect - replaces destination port of an IP packet to one specified by to-ports parameter
return - passes control back to the chain from where the jump took place
same - gives a particular client the same source/destination IP address from supplied range for each connection. This is most frequently used for services that expect the same client address for multiple connections from the same client
src-nat - replaces source address of an IP packet to values specified by to-addresses and to-ports parameters
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 (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.
routing-mark (string; Default: )
Matches packets marked by mangle facility with particular routing mark
same-not-by-dst (yes | no; Default: )
Specifies whether to take into account or not destination IP address when selecting a new source IP address. Applicable if action=same
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)
If you want to "hide" the private LAN 192.168.0.0/24 "behind" one address 10.5.8.109 given to you by the ISP, you should use the source network address translation (masquerading) feature of the MikroTik router. The masquerading will change the source IP address and port of the packets originated from the network 192.168.0.0/24 to the address 10.5.8.109 of the router when the packet is routed through it.
To use masquerading, a source NAT rule with action 'masquerade' should be added to the firewall configuration:
All outgoing connections from the network 192.168.0.0/24 will have source address 10.5.8.109 of the router and source port above 1024. No access from the Internet will be possible to the Local addresses. If you want to allow connections to the server on the local network, you should use destination Network Address Translation (NAT).
Destination NAT
If you want to link Public IP 10.5.8.200 address to Local one 192.168.0.109, you should use destination address translation feature of the MikroTik router. Also if you want allow Local server to talk with outside with given Public IP you should use source address translation, too.
If you want to link Public IP subnet 11.11.11.0/24 to local one 2.2.2.0/24, you should use destination address translation and source address translation features with action=netmap.
If you would like to direct requests for a certain port to an internal machine (sometimes called opening a port, port mapping), you can do it like this:
This rule translates to: when an incoming connection requests TCP port 1234, use the DST-NAT action and redirect it to local address 192.168.1.1 and the port 1234
