Policy Routing

Policy routing is the method to steer traffic matching certain criteria to a certain gateway. This can be used to force some customers or specific protocols from the servers (for example HTTP traffic) to always be routed to a certain gateway. It can even be used to steer local and overseas traffic to different gateways. RouterOS implements several components that can be used to achieve said task:
  • routing tables
  • routing rules
  • firewall mangle marking

Routing Tables

A router can have multiple routing tables with its own set of routes routing the same destination to different gateways. Tables can be seen and configured from the /routing/table menu. By default, RouterOS has only the ‘main‘ routing table:
[admin@rack1_b33_CCR1036] /routing/table> printFlags: D - dynamic; X - disabled, I - invalid; U - used0 D name="main" fib
If a custom routing table is required, it should be defined in this menu prior to using it anywhere in the configuration. Let’s consider a basic example where we have two gateways and and we want to resolve only in the routing table named ‘myTable‘ to the gateway
/routing table add name=myTable fib/ip route add dst-address= gateway= route add dst-address= gateway= routing-table=myTable
For a user-created table to be able to resolve the destination, the main routing table should be able to resolve the destination too. In our example, the main routing table should also have a route to destination or at least a default route, since the default route is dynamically added by the DHCP for safety reasons it is better to add also in the main table.
[admin@rack1_b33_CCR1036] /ip/route> print detail Flags: D - dynamic; X - disabled, I - inactive, A - active;c - connect, s - static, r - rip, b - bgp, o - ospf, d - dhcp, v - vpn, m - modem, y - copy; H - hw-offloaded; + - ecmp   DAd   dst-address= routing-table=main pref-src="" gateway=          immediate-gw= distance=1 scope=30 target-scope=10          vrf-interface=ether8 suppress-hw-offload=no  0  As   dst-address= routing-table=main pref-src="" gateway=          immediate-gw= distance=1 scope=30 target-scope=10 suppress-hw-offload=no     DAc   dst-address= routing-table=main gateway=ether8 immediate-gw=ether8          distance=0 scope=10 suppress-hw-offload=no local-address=     DAc   dst-address= routing-table=main gateway=ether7 immediate-gw=ether7          distance=0 scope=10 suppress-hw-offload=no local-address=      1  As   dst-address= routing-table=myTable pref-src="" gateway=          immediate-gw= distance=1 scope=30 target-scope=10 suppress-hw-offload=no
But configuration above is not enough, we need a method to force the traffic to actually use our newly created table. RouterOS gives you two options to choose from:
  • firewall mangle – it gives more control over the criteria to be used to steer traffic, for example, per connection or per packet balancing, etc. For more info on how to use mangle marking see Firewall Marking examples.
  • routing rules – a basic set of parameters that can be used to quickly steer traffic. This is the method we are going to use for our example.

Routing Rules

Routing rules allow steering traffic based on basic parameters like a source address, a destination address, or in-interface as well as other parameters. For our example, we want to select traffic with destination and do not fall back to the main table:
/routing rule add dst-address= action=lookup-only-in-table table=myTable
Lets’s say that we know that customer is connected to ether4 and we want only that customer to route to a specific gateway. We can use the following rule:
/routing rule add dst-address= action=lookup-only-in-table table=myTable interface=ether4
If for some reason the gateway used in our table goes down, the whole lockup will fail and the destination will not be reachable. In active-backup setups we want the traffic to be able to fall back to the main table. To do that change the action from lookup-only-in-table to lookup. Also, routing rules can be used as a very “basic firewall”. Let’s say we do not want to allow a customer connected to ether4 to be able to access the network:
/routing rule add dst-address= interface=ether4 action=drop
List of all the parameters that can be used by routing rules:
Property Description
action (drop | lookup | lookup-only-in-table | unreachable) An action to take on the matching packet:drop – silently drop the packet.lookup – perform a lookup in routing tables.lookup-only-in-table – perform lookup only in the specified routing table (see table parameter).unreachable – generate ICMP unreachable message and send it back to the source.
comment (string)
disabled (yes | no) The disabled rule is not used.
dst-address() The destination address of the packet to match.
interface (string) Incoming interface to match.
min-prefix (integer [0..4294967295]) Equivalent to Linux IP rule suppress_prefixlength . For example to suppress the default route in the routing decision set the value to 0.
routing-mark (string) Match specific routing mark.
src-address (string) The source address of the packet to match.
table (name) Name of the routing table to use for lookup.
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