Re: Simplifying DNAT Rules using Maps

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Wed, Jun 03, 2020 at 12:08:50PM -0400, Max Ehrlich wrote:
> Hi,
> 
> I'm switching from iptables to nftables, specifically from a high
> level translator (awall) to using nftables directly since the
> scripting environment is so expressive.
> 
> I have quite a few ipv4 DNAT rules that I need to translate, and they
> all have a similar form like the following for a web service:
> 
> table ip nat {
>   chain prerouting {
>     ip daddr != 10.0.0.0/8 fib daddr type local tcp dport http dnat
> 10.1.1.112:8080
>   }
> 
>   chain postrouting {
>     ip saddr 10.0.0.0/8 ip daddr 10.1.1.112 tcp dport 8080 masquerade
>   }
> }
> 
> table ip filter {
>   chain forward {
>     ip daddr 10.1.1.112 tcp dport 8080 accept
>   }
> }
> 
> I want to simplify this using a map so that I can add services to the
> map instead of having to copy all three rules every time. Something
> like this
> 
> table ip nat {
>   map dnat_services {
>     type inet_service: ipv4_addr . inet_service
>     elements = {
>       http: 10.1.1.112 . 8080
>     }
>   }
> 
>   chain prerouting {
>     ip daddr != 10.0.0.0/8 fib daddr type local dnat tcp dport map
> @dnat_services
>   }
> ...
> 
> would be great but it seems like the dnat target doesnt accept
> concatenations. I get that this can be done with two maps but it makes
> it quite ugly to write although there are performance benefits. Also I
> have no idea what to do about the filter and masquerade rules. For
> example
> 
> chain postrouting {
>   ip saddr 10.0.0.0/8 ip daddr tcp dport map @dnat_services masquerade
> }
> 
> doesn't parse (my assumption was this would have been that the ip
> daddr would be the result of looking up the tcp dport in the given
> map, it matches the dnat syntax)
> 
> So is there a cleaner way to write these rules using maps?

This is supported since nftables >= 0.9.4

# cat ruleset.nft
table ip nat {
        map destinations {
                type ipv4_addr . inet_service : ipv4_addr . inet_service
        }

        chain f {
                type nat hook postrouting priority srcnat; policy accept;
                snat ip addr . port to ip daddr . tcp dport map @destinations
        }
}
# nft -f ruleset.nft

Then, you can add elements to the `destinations' map that contains the
mapping.

nft add element ip nat destinations { 1.1.1.1 . 80 : 2.2.2.2 . 443 }