Flowtables Firewall Configuration

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Overview

This section provides information on firewall configuration for flowtables.

set firewall flowtable …

To learn about the general traffic flow in VyOS firewalls, see Firewall.

- set firewall
    * flowtable
         - custom_flow_table
            + ...

Flowtables let you define a fastpath through the flowtable datapath. Flowtables support layer 3 (IPv4 and IPv6) and layer 4 (TCP and UDP) protocols.

../../_images/firewall-flowtable-packet-flow.webp

After the first packet successfully traverses the IP forwarding path (black circles path), you can offload subsequent packets to the flowtable through your ruleset. You specify when to add a flow to the flowtable during forward filtering (red circle number 6).

When a packet finds a matching entry in the flowtable (flowtable hit), the system transmits it to the output netdevice. This means packets bypass the classic IP forwarding path and use the Fast Path (orange circles path). As a result, you do not see these packets from any Netfilter hooks after ingress. If no matching entry exists in the flowtable (flowtable miss), the packet traverses the classic IP forwarding path.

Note

Flowtable Reference: https://docs.kernel.org/networking/nf_flowtable.html

Flowtable Configuration

To use flowtables, you need to configure the following:

Create a flowtable that includes the interfaces that are going to be used by the flowtable.

Create a firewall rule. Set the action to offload and use your desired flowtable for offload-target.

Creating a flow table:

set firewall flowtable <flow_table_name> interface <iface>

Specify interfaces to use in the flowtable.

set firewall flowtable <flow_table_name> description <text>

Provide a description for the flow table.

set firewall flowtable <flow_table_name> offload <hardware | software>

Specify the offload type the flowtable uses: hardware or software. The default is software offload.

Note

Hardware offload: Make sure your network interface controller (NIC) supports hardware offloading and that you have the necessary drivers

installed before enabling this option.

Creating rules for using flow tables:

set firewall [ipv4 | ipv6] forward filter rule <1-999999> action offload

Create a firewall rule in the forward chain with the action set to offload.

set firewall [ipv4 | ipv6] forward filter rule <1-999999> offload-target <flowtable>

Create a firewall rule in the forward chain and specify which flowtable to use. Only applicable if the action is offload.

Configuration Example

Consider the following in this setup:

This example uses two interfaces in the flowtables: eth0 and eth1.

The example provides a minimal firewall ruleset with filtering rules and rules for using flowtable offload capabilities.

The first packet is evaluated by the firewall path, so a desired connection should be explicitly accepted. The same should occur for traffic in reverse order. In most cases, state policies are used to accept a connection in the reverse path.

In the following example only traffic coming from interface eth0, TCP protocol, and destination port 1122 is accepted. All other traffic to the router is dropped.

Commands

set firewall flowtable FT01 interface 'eth0'
set firewall flowtable FT01 interface 'eth1'
set firewall ipv4 forward filter default-action 'drop'
set firewall ipv4 forward filter rule 10 action 'offload'
set firewall ipv4 forward filter rule 10 offload-target 'FT01'
set firewall ipv4 forward filter rule 10 state 'established'
set firewall ipv4 forward filter rule 10 state 'related'
set firewall ipv4 forward filter rule 20 action 'accept'
set firewall ipv4 forward filter rule 20 state 'established'
set firewall ipv4 forward filter rule 20 state 'related'
set firewall ipv4 forward filter rule 110 action 'accept'
set firewall ipv4 forward filter rule 110 destination address '192.0.2.100'
set firewall ipv4 forward filter rule 110 destination port '1122'
set firewall ipv4 forward filter rule 110 inbound-interface name 'eth0'
set firewall ipv4 forward filter rule 110 protocol 'tcp'

Explanation

Here’s what happens for a desired connection:

A packet arrives on eth0 with destination address 192.0.2.100, TCP protocol, and destination port 1122. Assume this address is reachable through interface eth1.

For this first packet, the connection state is new. Neither rule 10 nor rule 20 applies.

Rule 110 matches, so the connection is accepted.

When the server 192.0.2.100 replies, the connection state becomes established, and rule 20 accepts the reply.

The router receives the second packet for this connection. Because the connection state is established, rule 10 matches and adds a new entry in the flowtable FT01 for this connection.

Subsequent packets skip the traditional path and use the Fast Path for offloading.

Checks

Check the conntrack table to verify that the system accepted and properly offloaded connections.

vyos@FlowTables:~$ show firewall ipv4 forward filter
Ruleset Information

---------------------------------
ipv4 Firewall "forward filter"

Rule     Action    Protocol      Packets    Bytes  Conditions
-------  --------  ----------  ---------  -------  ----------------------------------------------------------------
10       offload   all                 8      468  ct state { established, related }  flow add @VYOS_FLOWTABLE_FT01
20       accept    all                 8      468  ct state { established, related }  accept
110      accept    tcp                 2      120  ip daddr 192.0.2.100 tcp dport 1122 iifname "eth0"  accept
default  drop      all                 7      420

vyos@FlowTables:~$ sudo conntrack -L | grep tcp
conntrack v1.4.6 (conntrack-tools): 5 flow entries have been shown.
tcp      6 src=198.51.100.100 dst=192.0.2.100 sport=41676 dport=1122 src=192.0.2.100 dst=198.51.100.100 sport=1122 dport=41676 [OFFLOAD] mark=0 use=2
vyos@FlowTables:~$