Meters

Meters

Dynamic sets/maps or meters are a way to use maps with stateful objects. They used to be known as flow tables before nft v0.8.1 and Linux kernel 4.3.

Among other things, they provide a native replacement for the hashlimit match in iptables. However, meters are a lot more flexible since you can use any selector, one or many through concatenations.

Note that the meter keyword is obsolete, the dynamic set and map syntax is now preferred for consistency.

Using meters

The following example shows to how ratelimit inbound TCP connections to port 22 per source IP address:

table ip filter {
      set my_ssh_ratelimit {
             type ipv4_addr
             timeout 60s
             flags dynamic
      }

      chain input {
             type filter hook input priority 0; policy drop;

             ct state new tcp dport 22 update @my_ssh_ratelimit { ip saddr limit rate 3/minute } accept
      }
}

For each packet matching this rule, it adds an element to the set whose key is 'ip saddr' and it attaches a ratelimiter to such element. If the element already exists, the ratelimiter is applied and the timeout is refreshed to 60 seconds. After 60 seconds of no use, the element expires and the ratelimiter is released.

You can also use concatenations to ratelimit the inbound connections per IP source address and TCP destination port:

table ip filter {
      set my_ssh_ratelimit {
             type ipv4_addr . inet_service
             timeout 60s
             flags dynamic
      }

      chain input {
             type filter hook input priority 0; policy drop;

             ct state new update @my_ssh_ratelimit { ip saddr . tcp dport limit rate 3/minute } accept
      }
}

Doing connlimit with nft

Since 4.18, there is a new ct count selector that allows you to count the number of existing connections. This extension uses the information available in the Connection Tracking System table, therefore, the counting of connection is based on the existing entries in the table.

The following example shows how to do connlimit from nftables:

table ip my_filter_table {
       set my_connlimit {
               type ipv4_addr
               size 65535
               flags dynamic
       }

       chain my_output_chain {
               type filter hook output priority filter; policy accept;
               ct state new add @my_connlimit { ip daddr ct count over 20 } counter packets 0 bytes 0 drop
       }
}

The example above dynamically populates the set connlimit from the packet path. For the first packet of each connection (ie. packets matching ct state new), this adds an entry into connlimit set, this entry uses the IPv4 destination address as a key. If the number of connections goes over 20, then packets are dropped.

Since connlimit is a set, you can perform any operation on it, such as listing and flushing its content.

Caveats:

• Do not define a set with a timeout. There is a garbage collector that removes the set element whenever ct count becomes zero to improve memory usage.
• For the same reason, you cannot use the update set statement which allows to refresh the timeout of your set element. Therefore, the ct count statement can only be used with the add set statement.

If you define a set with a timeout or you use the update set statement, then you will hit the "Operation is not supported" error.

Doing iptables hashlimit with nft

Meters replace iptables hashlimit in nft. From iptables v1.6.2 onward, you can use the tool iptables-translate to see how to translate hashlimit rules.

Almost all hashlimit options are available in nft, starting with --hashlimit-mode, it is replaced by the selector in a meter. All modes are available except no mode, a meter demands a selector, an iptables rule without hashlimit-mode isn't supported in nft. A simple rule translation is:

$ iptables-translate -A INPUT -m tcp -p tcp --dport 80 -m hashlimit --hashlimit-above 200/sec --hashlimit-mode srcip,dstport --hashlimit-name http1 -j DROP
nft add rule ip filter INPUT tcp dport 80 meter http1 { tcp dport . ip saddr limit rate over 200/second } counter drop

Notice that a meter is named, like hashlimit, and using multiple hashlimit-modes is similar to using a concatenation of selectors. Also, --hashlimit-above is translated to limit rate over, to simulate --hashlimit-upto just omit or replace over with until in the rule.

The options --hashlimit-burst and --hashlimit-htable-expire are translated to burst and timeout in a meter:

$ iptables-translate -A INPUT -m tcp -p tcp --dport 80 -m hashlimit --hashlimit-above 200kb/s --hashlimit-burst 1mb --hashlimit-mode srcip,dstport --hashlimit-name http2 --hashlimit-htable-expire 3000 -j DROP
nft add rule ip filter INPUT tcp dport 80 meter http2 { tcp dport . ip saddr timeout 3s limit rate over 200 kbytes/second burst 1 mbytes} counter drop

This rule shows how timeout and burst are used in a meter, also notice that meters, similarly to hashlimit, accepts limiting rates by bytes frequency instead of packets.

Another hashlimit option is to limit the traffic rate on subnets, of IP source or destination addresses, using the options --hashlimit-srcmask and --hashlimit-dstmask. This feature is available in nft by attaching a subnet mask to a meter selector, attach to ip saddr for source address and to ip daddr for destination adress:

$ iptables-translate -A INPUT -m tcp -p tcp --dport 80 -m hashlimit --hashlimit-upto 200 --hashlimit-mode srcip --hashlimit-name http3 --hashlimit-srcmask 24 -j DROP
nft add rule ip filter INPUT tcp dport 80 meter http3 { ip saddr and 255.255.255.0 limit rate 200/second } counter drop

This rule will limit packets rate, grouping subnets determined by the first 24 bits of the IP source address, from the incoming packets on port 80.

The remaining options, --hashlimit-htable-max, --hashlimit-htable-size and --hashlimit-htable-gcinterval don't apply to meters.