Re: OK, IPv4 vs IPv6 is driving me crazy

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

Hi Stephen,

On Fri, Jul 23, 2021 at 08:09:31AM -0700, Stephen Satchell wrote:
> At one point, a member here -- when asked what the difference in defining
> rules in nftables between the two systems -- said "they are the same."
> 
> As I read the documentation on wiki.nftables.org:  NO!
> 
> The hooker here is the requirement thatt IPv6 header examination requires
> "nexthdr" to examine tcp, udp, and icmp packets.  How about other protocols:
> do I need to do something like this?

I suggest you to use:

        meta l4proto

it provides an abstraction that is independent from the layer 3 header
representation, ie. ip protocol and ip6 nexthdr.

> > nexthdr inet protocol {gre, esp, ah} jump other_protocols
> 
> If this is the case, than the "inet" combined table is useless, as my
> filters will need to be in separate "ip" and "ip6" tables.
> 
> Fortunately, I'm building a parameter-based firewall generator, so details
> like this can be hidden from the person specifying the pinholes for the
> firewall, if this is the case.
> 
> Or does nft(8) do the smart thing and, for IPv6, put the "nexthop" in the v6
> rules for you?

I have just slightly extended this section to document meta l4proto:

https://wiki.nftables.org/wiki-nftables/index.php/Matching_packet_headers#Matching_transport_protocol

> Maybe this excerpt from wiki.nftables.org answers my question:
>
> > inet
> > Tables of this family see both IPv4 and IPv6 traffic/packets, simplifying dual stack support.
> > 
> > Within a table of inet family, both IPv4 and IPv6 packets traverse the same rules. Rules for IPv4 packets don't affect IPv6 packets. Rules for both L3 protocols affect both.
> > 
> > Examples:
> > 
> > # This rule affects only IPv4 packets:
> > add rule inet filter input ip saddr 1.1.1.1 counter accept
> > 
> > # This rule affects only IPv6 packets:
> > add rule inet filter input ip6 daddr fe00::2 counter accept
> > 
> > # These rules affect both IPv4 and IPv6 packets:
> > add rule inet filter input ct state established,related counter accept
> > add rule inet filter input udp dport 53 accept
> 
> The thing is, the specification of "inet" is shorthand for inserting the
> same rule into two tables, "ip" and "ip6".  So, if I'm constructing a table
> I need to separate the "inet" table into two separate tables, "ip" and
> "ip6".
> 
> Someone please disabuse me of any incorrect notions.

I found this excerpt here:

https://wiki.nftables.org/wiki-nftables/index.php/Nftables_families#inet

and I have extended it to refer to meta l4proto too.

Thanks for your feedback.