1. Interface A Awd10 New Connection Detected A Malware F 2
2. Interface A Awd10 New Connection Detected A Malware F Test

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Warning: This manual is moved to https://help.mikrotik.com/docs/display/ROS/Firewall

Summary

Sub-menu:/ip firewall nat

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.

Masquerade

Firewall NAT action=masquerade is unique subversion of action=srcnat, it was designed for specific use in situations when public IP can randomly change, for example DHCP-server changes it, or PPPoE tunnel after disconnect gets different IP, in short - when public IP is dynamic.

Every time interface disconnects and/or its IP address changes, router will clear all masqueraded connection tracking entries that send packet out that interface, this way improving system recovery time after public ip address change.

Unfortunately this can lead to some issues when action=masquerade is used in setups with unstable connections/links that get routed over different link when primary is down. In such scenario following things can happen:

• on disconnect, all related connection tracking entries are purged;
• next packet from every purged (previously masqueraded) connection will come into firewall as connection-state=new, and, if primary interface is not back, packet will be routed out via alternative route (if you have any) thus creating new connection;
• primary link comes back, routing is restored over primary link, so packets that belong to existing connections are sent over primary interface without being masqueraded leaking local IPs to a public network.

You can workaround this by creating blackhole route as alternative to route that might disappear on disconnect).

When action=srcnat is used instead, connection tracking entries remain and connections can simply resume.

Properties

Stats

/ip firewall nat print stats will show additional read-only properties

By default print is equivalent to print static and shows only static rules.

To print also dynamic rules use print all.

Or to print only dynamic rules use print dynamic

Menu specific commands

Basic examples

Source NAT

Masquerade

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).

Source nat to specific address

If you have multiple public IP addresses, source nat can be changed to specific IP, for example, one local subnet can be hidden behind first IP and second local subnet is masqueraded behind second IP.

Destination NAT

Forward all traffic to internal host

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 initiate connections to outside with given Public IP you should use source address translation, too.

Add Public IP to Public interface:

Add rule allowing access to the internal server from external networks:

Add rule allowing the internal server to initate connections to the outer networks having its source address translated to 10.5.8.200:

Port mapping/forwarding

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

Port forwarding to internal FTP server

As you can see from illustration above FTP uses more than one connection, but only command channel should be forwarded by Destination nat. Data channel is considered as related connection and should be accepted with 'accept related' rule if you have strict firewall. Note that for related connections to be properly detected FTP helper has to be enabled.

Note that active FTP might not work if client is behind dumb firewall or NATed router, because data channel is initiated by the server and cannot directly access the client.

If client is behind Mikrotik router, then make sure that FTP helper is enabled

1:1 mapping

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.

Same can be written using different address notation, that still have to match with the described network

Carrier-Grade NAT (CGNAT) or NAT444

To combat IPv4 address exhaustion, new RFC 6598 was deployed. The idea is to use shared 100.64.0.0/10 address space inside carrier's network and performing NAT on carrier's edge router to sigle public IP or public IP range.

Because of nature of such setup it is also called NAT444, as opposed to a NAT44 network for a 'normal' NAT environment, three different IPv4 address spaces are involved.

CGNAT configuration on RouterOS does not differ from any other regular source NAT configuration:

Where:

• 2.2.2.2 - public IP address,
• public_if - interface on providers edge router connected to internet

The advantage of NAT444 is obvious, less public IPv4 addresses used. But this technique comes with mayor drawbacks:

• The service provider router performing CGNAT needs to maintain a state table for all the address translations: this requires a lot of memory and CPU resources.
• Console gaming problems. Some games fail when two subscribers using the same outside public IPv4 address try to connect to each other.
• Tracking of users for legal reasons means extra logging, as multiple households go behind one public address.
• Anything requiring incoming connections is broken. While this already was the case with regular NAT, end users could usually still set up port forwarding on their NAT router. CGNAT makes this impossible. This means no web servers can be hosted here, and IP Phones cannot receive incoming calls by default either.
• Some web servers only allow a maximum number of connections from the same public IP address, as a means to counter DoS attacks like SYN floods. Using CGNAT this limit is reached more often and some services may be of poor quality.
• 6to4 requires globally reachable addresses and will not work in networks that employ addresses with limited topological span.

More on things that can break can be read in this article [1]

Packets with Shared Address Space source or destination addresses MUST NOT be forwarded across Service Provider boundaries. Service Providers MUST filter such packets on ingress links.In RouterOS this can be easily done with firewall filters on edge routers:

Service providers may be required to do logging of MAPed addresses, in large CGN deployed network that may be a problem.Fortunately RFC 7422 suggests a way to manage CGN translations in such a way as to significantly reduce the amount of logging required while providing traceability for abuse response.

RFC states that instead of logging each connection, CGNs could deterministically map customer private addresses (received on the customer-facing interface of the CGN, a.k.a., internal side) to public addresses extended with port ranges.

In RouterOS described algorithm can be done with few script functions.Lets take an example:

Instead of writing NAT mappings by hand we could write a function which adds such rules automatically.

After pasting above script in the terminal function 'addNatRules' is available. If we take our example, we need to map 6 shared network addresses to be mapped to 2.2.2.2 and each address uses range of 100 ports starting from 2000. So we run our function:

Interface A Awd10 New Connection Detected A Malware F 2

Now you should be able to get set of rules:

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Interface A Awd10 New Connection Detected A Malware F Test