Re: [RFC 11/12] netlink: Prevent host route events from overwriting guest-configured gateway

[David Gibson <david@gibson.dropbear.id.au>]

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On Sun, Dec 14, 2025 at 08:54:40PM -0500, Jon Maloy wrote:
> We add a new context flag 'guest_gw_from_ns' to track if the gateway
> was configured from the guest side, something that might have happened
> either via the -g option or by a namespace-side netlink event. When set,
> host side route events will not be permitted to overwrite the guest's
> gateway configuration.

If -g is set, I don't think we should even subscribe to the routing
updates in the first place.

If --config-net is *not* set, we shouldn't be touching the guest
config anyway.

If --config-net *is* set, that's essentially saying that we own the
guest's network configuration.  We don't need to go to these lengths
to stop it from shooting itself by making conflicting config updates.
> 
> This gives any gateway setting from the guest side precedence over any
> ditto coming from the host side.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  conf.c    |  2 ++
>  netlink.c | 15 +++++++++++++--
>  passt.h   |  4 ++++
>  3 files changed, 19 insertions(+), 2 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index 0a4a28a..f26637c 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -1891,6 +1891,7 @@ void conf(struct ctx *c, int argc, char **argv)
>  			    !IN6_IS_ADDR_LOOPBACK(&c->ip6.guest_gw)) {
>  				if (c->mode == MODE_PASTA)
>  					c->ip6.no_copy_routes = true;
> +				c->ip6.guest_gw_from_ns = true;
>  				break;
>  			}
>  
> @@ -1900,6 +1901,7 @@ void conf(struct ctx *c, int argc, char **argv)
>  			    !IN4_IS_ADDR_LOOPBACK(&c->ip4.guest_gw)) {
>  				if (c->mode == MODE_PASTA)
>  					c->ip4.no_copy_routes = true;
> +				c->ip4.guest_gw_from_ns = true;
>  				break;
>  			}
>  
> diff --git a/netlink.c b/netlink.c
> index d049239..de04fb7 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -490,6 +490,9 @@ static void nl_linkaddr_host_msg_read(struct ctx *c, const struct nlmsghdr *nh)
>  		if (rtm->rtm_family == AF_INET) {
>  			char buf[INET_ADDRSTRLEN];
>  
> +			if (c->ip4.guest_gw_from_ns)
> +				return;
> +
>  			if (!is_new) {
>  				c->ip4.guest_gw = (struct in_addr){ 0 };
>  				c->ip4.our_tap_addr = (struct in_addr){ 0 };
> @@ -503,6 +506,9 @@ static void nl_linkaddr_host_msg_read(struct ctx *c, const struct nlmsghdr *nh)
>  		} else if (rtm->rtm_family == AF_INET6) {
>  			char buf[INET6_ADDRSTRLEN];
>  
> +			if (c->ip6.guest_gw_from_ns)
> +				return;
> +
>  			if (!is_new) {
>  				c->ip6.guest_gw = (struct in6_addr){ 0 };
>  				return;
> @@ -658,15 +664,20 @@ static void nl_linkaddr_msg_read(struct ctx *c, const struct nlmsghdr *nh)
>  			if (is_new) {
>  				c->ip4.guest_gw = *(struct in_addr *)gw;
>  				c->ip4.our_tap_addr = c->ip4.guest_gw;
> +				c->ip4.guest_gw_from_ns = true;
>  			} else {
>  				c->ip4.guest_gw = (struct in_addr){ 0 };
>  				c->ip4.our_tap_addr = (struct in_addr){ 0 };
> +				c->ip4.guest_gw_from_ns = false;
>  			}
>  		} else if (rtm->rtm_family == AF_INET6) {
> -			if (is_new)
> +			if (is_new) {
>  				c->ip6.guest_gw = *(struct in6_addr *)gw;
> -			else
> +				c->ip6.guest_gw_from_ns = true;
> +			} else {
>  				c->ip6.guest_gw = (struct in6_addr){ 0 };
> +				c->ip6.guest_gw_from_ns = false;
> +			}
>  		}
>  	}
>  }
> diff --git a/passt.h b/passt.h
> index 70ccaf1..5e7bc99 100644
> --- a/passt.h
> +++ b/passt.h
> @@ -82,6 +82,7 @@ enum passt_modes {
>   * @ifname_out:		Optional interface name to bind outbound sockets to
>   * @no_copy_routes:	Don't copy all routes when configuring target namespace
>   * @no_copy_addrs:	Don't copy all addresses when configuring namespace
> + * @guest_gw_from_ns:	Gateway was set from namespace (config or ns event)
>   */
>  struct ip4_ctx {
>  	/* PIF_TAP addresses */
> @@ -104,6 +105,7 @@ struct ip4_ctx {
>  
>  	bool no_copy_routes;
>  	bool no_copy_addrs;
> +	bool guest_gw_from_ns;
>  };
>  
>  /**
> @@ -125,6 +127,7 @@ struct ip4_ctx {
>   * @ifname_out:		Optional interface name to bind outbound sockets to
>   * @no_copy_routes:	Don't copy all routes when configuring target namespace
>   * @no_copy_addrs:	Don't copy all addresses when configuring namespace
> + * @guest_gw_from_ns:	Gateway was set from namespace (config or ns event)
>   */
>  struct ip6_ctx {
>  	/* PIF_TAP addresses */
> @@ -148,6 +151,7 @@ struct ip6_ctx {
>  
>  	bool no_copy_routes;
>  	bool no_copy_addrs;
> +	bool guest_gw_from_ns;
>  };
>  
>  #include <netinet/if_ether.h>
> -- 
> 2.51.1
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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