Re: [PATCH 19/22] conf, fwd: Split notion of gateway/router from guest-visible host address

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 16 Aug 2024 15:40:00 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> The @gw fields in the ip4_ctx and ip6_ctx give the (host's) default
> gateway.  We use this for two quite distinct things: advertising the
> gateway that the guest should use (via DHCP, NDP and/or --config-net)
> and for a limited form of NAT.  So that the guest can access services
> on the host, we map the gateway address within the guest to the
> loopback address on the host.
> 
> Using the gateway address for this isn't necessarily the best choice
> for this purpose, certainly not for all circumstances.  So, start off
> by splitting the notion of these into two different values: @guest_gw
> which is the gateway address the guest should use and @nat_host_loopback,
> which is the guest visible address to remap to the host's loopback.
> 
> Usually nat_host_loopback will have the same value as guest_gw.  However
> when --no-map-gw is specified we leave them unspecified instead.  This
> means when we use nat_host_loopback, we don't need to separately check
> c->no_map_gw to see if it's relevant.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  conf.c  | 60 +++++++++++++++++++++++++++++----------------------------
>  dhcp.c  | 10 ++++++----
>  fwd.c   |  4 ++--
>  passt.h | 16 +++++++++------
>  pasta.c |  6 ++++--
>  5 files changed, 53 insertions(+), 43 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index b1c58d5b..26373584 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -410,12 +410,12 @@ static void add_dns_resolv(struct ctx *c, const char *nameserver,
>  		 * redirect
>  		 */
>  		if (IN4_IS_ADDR_LOOPBACK(&ns4)) {
> -			if (c->no_map_gw)
> +			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.nat_host_loopback))

If you change the command-line option name to use "map", it would be
good to also change these names.

-- 
Stefano