The firewall implements stateful (by utilizing connection tracking) and stateless 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 (NAT), 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. A properly configured firewall plays a key role in efficient and secure network infrastructure deployment.
MikroTik RouterOS has a very powerful firewall implementation with features including:
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.
RouterOS utilizes 5 sub-facilities of the firewall:
To completely understand firewall rules, first, you have to understand various states which might apply to a particular network packet. There are five connection states in RouterOS:
Let's look at the basic firewall setup to protect the router. By default RouterOS firewall accepts everything, blocking is achieved by adding a filter rule to drop everything at the end of all rules. For out router we want to allow only ICMP, ssh, and Winbox and drop the rest:
RouterOS also allows to filter packets before connection tracking and selectively send only specific traffic to connection tracking. This allows us to significantly reduce the load on the CPU and mitigate DOS/DDoS attacks. Configuration of such rules is done in the RAW filtering table.
Additional /ip firewall filter configuration examples find under the Building Your First Firewall section.
Connection tracking allows the kernel to keep track of all logical network connections or sessions, and thereby relate all of the packets which may make up that connection. NAT relies on this information to translate all related packets in the same way. Because of connection tracking, you can use stateful firewall functionality even with stateless protocols such as UDP.
A list of tracked connections can be seen in the /ip firewall connection for ipv4 and /ipv6 firewall connection for IPv6.
Based on connection table entries arrived packet can get assigned one of the connection states: new, invalid, established, related, or untracked.
There are two different methods when the packet is considered new. The first one is in the case of stateless connections (like UDP) when there is no connection entry in the connection table. The other one is in the case of a stateful protocol (TCP). In this case, a new packet that starts a new connection is always a TCP packet with an SYN flag.
If a packet is not new it can belong to either an established or related connection or not belong to any connection making it invalid. A packet with an established state, as most of you already guessed, belongs to an existing connection from the connection tracking table. A related state is very similar, except that packet belongs to a connection that is related to one of the existing connections, for example, ICMP error packets or FTP data connection packets.
Connection state notrack is a special case when RAW firewall rules are used to exclude connection from connection tracking. This one rule would make all forwarded traffic bypass the connection tracking engine speeding packet processing through the device.
Any other packet is considered invalid and in most cases should be dropped.
Based on this information we can set a basic set of filter rules to speed up packet filtering and reduce the load on the CPU by accepting established/related packets, dropping invalid packets, and working on more detailed filtering only for new packets.
Such a rule set must not be applied on routers with asymmetric routing, because asymmetrically routed packets may be considered invalid and dropped.
This section lists protocols and ports used by various MikroTik RouterOS services. It helps you to determine why your MikroTik router listens to certain ports, and what you need to block/allow in case you want to prevent or grant access to certain services.
The default services are:
|www||Webfig http service|
|www-ssl||Webfig HTTPS service|
|winbox||Responsible for Winbox tool access, as well as Tik-App smartphone app and Dude probe|
|api-ssl||API over SSL service|
Note that it is not possible to add new services, only existing service modifications are allowed.
|address (IP address/netmask | IPv6/0..128; Default: )||List of IP/IPv6 prefixes from which the service is accessible.|
|certificate (name; default: none)||The name of the certificate used by a particular service. Applicable only for services that depend on certificates (www-ssl, api-ssl)|
|name (name; default: none)||Service name|
|port (integer: 1..65535; Default: )||The port particular service listens on|
To restrict Winbox service access to the device only from the 192.168.88.0/24 subnet, we have to configure the following:
We recommend disabling unused services.
Firewall address lists allow a user to create lists of IP addresses grouped together under a common name. Firewall filter, Mangle, and NAT facilities can then use those address lists to match packets against them. The address list records can also be updated dynamically via the action=add-src-to-address-list or action=add-dst-to-address-list items found in NAT, Mangle, and Filter facilities.
Firewall rules with action add-src-to-address-list or add-dst-to-address-list works in passthrough mode, which means that the matched packets will be passed to the next firewall rules. A basic example of a dynamically created address-list:
Layer7-protocol is a method of searching for patterns in ICMP/TCP/UDP streams. It collects the first 10 packets of a connection or the first 2KB of a connection and searches for the pattern in the collected data. If the pattern is not found in the collected data, the matcher stops inspecting further. Allocated memory is freed and the protocol is considered unknown. You should take into account that a lot of connections will significantly increase memory and CPU usage. To avoid this, add regular firewall matches to reduce the amount of data passed to layer-7 filters repeatedly.
An additional requirement is that the layer7 matcher must see both directions of traffic (incoming and outgoing). To satisfy this requirement l7 rules should be set in the forward chain. If a rule is set in the input/prerouting chain then the same rule must be also set in the output/postrouting chain, otherwise, the collected data may not be complete resulting in an incorrectly matched pattern.
In this example, we will use a pattern to match RDP packets.
When a user uses HTTPS, Layer7 rules will not be able to match this traffic. Only unencrypted HTTP can be matched.