Re: [PATCH 1/1] netfilter: connmark: introduce savedscp

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

Hi Kevin,

On Sun, Mar 24, 2019 at 02:23:45PM +0000, Kevin 'ldir' Darbyshire-Bryant wrote:
> savedscp is a method of storing the DSCP of an ip packet into conntrack
> mark.  In combination with a suitable tc filter action (conndscp but may
> end up being integrated into connmark) DSCP values are able to be stored
> on egress and restored on ingress across links that otherwise alter or
> bleach DSCP.
> 
> This is useful for qdiscs such as CAKE which are able to shape according
> to policies based on DSCP.
> 
> Ingress classification is traditionally a challenging task since
> iptables rules haven't yet run and tc filter/eBPF programs are pre-NAT
> lookups, hence are unable to see internal IPv4 addresses as used on the
> typical home masquerading gateway.
> 
> The ingress problem is solved by the tc filter, but the tc people didn't
> like the idea of tc setting conntrack mark values, though they are ok
> with reading conntrack values and hence restoring DSCP from conntrack
> marks.
> 
> x_tables CONNMARK with the new savedscp action solves the problem of
> storing the DSCP to the conntrack mark.
> 
> It accepts 2 parameters.  The mark is a 32bit value with usually one
> bit set.  This bit is set when savedscp saves the DSCP to the mark.
> This is useful to implement a 'one shot'
> iptables based classification where the 'complicated' iptables rules are
> only run once to classify the connection on initial (egress) packet and
> subsequent packets are all marked/restored with the same DSCP.  A mark
> of zero disables the setting of a status bit/s.
> 
> The mask is a 32bit value of at least 6 contiguous bits and represents
> the area where the DSCP will be stored.
> 
> e.g.
> 
> iptables -A QOS_MARK_eth0 -t mangle -j CONNMARK --savedscp-mark 0x01000000/0xfc000000
> 
> Would store the DSCP in the top 6 bits of the 32bit mark field, and use
> the LSB of the top byte as the 'DSCP has been stored' marker.

I'd prefer we explore how to express this in nftables, I think the
main problem here is that we do not support for datatype compatibility
/ casting yet, hence dscp cannot be assign to the mark. But we only
need to update userspace code to achieve this.

Thanks.