[1/1] netfilter: connmark: introduce set-dscpmark

set-dscpmark is a method of storing the DSCP of an ip packet into
conntrack mark.  In combination with a suitable tc filter action
(act_ctinfo) DSCP values are able to be stored in the mark 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.

x_tables CONNMARK set-dscpmark target solves the problem of storing the
DSCP to the conntrack mark in a way suitable for the new act_ctinfo tc
action to restore.

The set-dscpmark option accepts 2 parameters, a 32bit 'dscpmask' and a
32bit 'statemask'.  The dscp mask must be 6 contiguous bits and
represents the area where the DSCP will be stored in the connmark.  The
state mask is a minimum 1 bit length mask that must not overlap with the
dscpmask.  It represents a flag which is set when the DSCP has been
stored in the conntrack 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 state
mask of zero disables the setting of a status bit/s.

example syntax with a suitably modified iptables user space application:

iptables -A QOS_MARK_eth0 -t mangle -j CONNMARK --set-dscpmark 0xfc000000/0x01000000

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.

|----0xFC----conntrack mark----000000---|
| Bits 31-26 | bit 25 | bit24 |~~~ Bit 0|
| DSCP       | unused | flag  |unused   |
|-----------------------0x01---000000---|
      ^                   ^
      |                   |
      ---|             Conditional flag
         |             set this when dscp
|-ip diffserv-|        stored in mark
| 6 bits      |
|-------------|

an identically configured tc action to restore looks like:

tc filter show dev eth0 ingress
filter parent ffff: protocol all pref 10 u32 chain 0
filter parent ffff: protocol all pref 10 u32 chain 0 fh 800: ht divisor 1
filter parent ffff: protocol all pref 10 u32 chain 0 fh 800::800 order 2048 key ht 800 bkt 0 flowid 1: not_in_hw
  match 00000000/00000000 at 0
	action order 1: ctinfo zone 0 pipe
	 index 2 ref 1 bind 1 dscp 0xfc000000/0x1000000

	action order 2: mirred (Egress Redirect to device ifb4eth0) stolen
	index 1 ref 1 bind 1

|----0xFC----conntrack mark----000000---|
| Bits 31-26 | bit 25 | bit24 |~~~ Bit 0|
| DSCP       | unused | flag  |unused   |
|-----------------------0x01---000000---|
      |                   |
      |                   |
      ---|             Conditional flag
         v             only restore if set
|-ip diffserv-|
| 6 bits      |
|-------------|

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
---
 include/uapi/linux/netfilter/xt_connmark.h | 10 ++++
 net/netfilter/xt_connmark.c                | 57 ++++++++++++++++++----
 2 files changed, 58 insertions(+), 9 deletions(-)

> On 3 Dec 2019, at 16:06, Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk> wrote:
> 
> set-dscpmark is a method of storing the DSCP of an ip packet into
> conntrack mark.  In combination with a suitable tc filter action
> (act_ctinfo) DSCP values are able to be stored in the mark on egress and
> restored on ingress across links that otherwise alter or bleach DSCP.
> 
<snip>

Well no one has yet screamed.  I do think I’ve done this wrong:

> 
> +enum {
> +	XT_CONNMARK_VALUE = BIT(0),
> +	XT_CONNMARK_DSCP = BIT(1)
> +};

and they should be = (1 << bit_number) instead.

I’ll send a v2 with that change (to nf-next) but anything else? or is it all too horrid
to contemplate :-)

Cheers,

Kevin