Maps

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Revision as of 18:02, 3 April 2021 by Fmyhr (talk | contribs) (Rewrote intro section to try to explain map concept clearly.)
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An nftables map stores key-value pairs, like associative arrays / dictionaries / hashes do in many programming languages. In an nftables rule you can specify a packet field (e.g. tcp dport) and reference a map to search for the map element whose key matches the packet field's value, and return that map element's value (or failure if the map contains no matching element). Some examples below will help clarify this general description.

You can think of a map as a set that returns a value instead of just an "in set / not in set" result. Behind the scenes both sets and maps use the same generic set infrastructure and therefore share many of the same options and semantics.


Anonymous maps

The following rule uses a map to DNAT packets to different destination IP addresses depending on the packet's destination tcp port:

% nft add rule ip nat prerouting dnat tcp dport map { 80 : 192.168.1.100, 8888 : 192.168.1.101 }

This is a very concise and efficient way to perform classical port redirection when the real servers are located behind a firewall. It can be read as it follows:

  • If the TCP destination port is 80, then the packet is DNAT'ed to 192.168.1.100.
  • If the TCP destination port is 8888, then the packet is DNAT'ed to 192.168.1.101.


Named maps

You can also declare named maps, to which you can then add or delete elements at anytime. For example, with:

% nft add map nat porttoip  { type inet_service: ipv4_addr\; }
% nft add element nat porttoip { 80 : 192.168.1.100, 8888 : 192.168.1.101 }

we first define a map porttoip which looks up an IP address based on a UDP or TCP port number, and then add a couple of elements. The map itself only cares about the data types of its keys and values. It is up to us to use meaningful values for both, and to use the map appropriately in rules. In this example we use porttoip to map tcp destination ports to source IP addresses. Then we can perform SNAT using the rule:

% nft add rule ip nat postrouting snat tcp dport map @porttoip

Now outgoing packets to TCP/80 use source IP address 192.168.1.100, those to TCP/8888 are SNAT'ed to 192.168.1.101. We can easily adjust our SNAT at anytime simply by adding or deleting elements in porttoip.