Re: [RFC 08/12] netlink: Subscribe to route changes in namespace

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

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On Sun, Dec 14, 2025 at 08:54:37PM -0500, Jon Maloy wrote:
> We add subscriptions to RTMGRP_IPV4_ROUTE and RTMGRP_IPV6_ROUTE, so
> that we receive notifications when routes change on the namespace
> interface.

No, we'd need to listen in the host netns, so we can transfer those
route changes to the guest netns.

> When default routes change on the pasta interface, we update guest_gw
> (and our_tap_addr for IPv4) to reflect the new gateway. This handles
> both routes propagated from the host and routes configured manually
> by the user inside the namespace.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> ---
>  netlink.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 46 insertions(+), 3 deletions(-)
> 
> diff --git a/netlink.c b/netlink.c
> index 7492f17..a8d3116 100644
> --- a/netlink.c
> +++ b/netlink.c
> @@ -195,7 +195,7 @@ static bool nl_addr6_add(struct ctx *c, const struct in6_addr *addr,
>  	idx = c->ip6.addr_count++;
>  	c->ip6.addrs[idx].addr = *addr;
>  	c->ip6.addrs[idx].prefix_len = prefix_len;
> -	c->ip6.addrs[idx].permanent = 0;
> +	c->ip6.addrs[idxyes].permanent = 0;

Um... what?

>  	return true;
>  }
>  
> @@ -359,6 +359,49 @@ static void nl_linkaddr_msg_read(struct ctx *c, const struct nlmsghdr *nh)
>  				}
>  			}
>  		}
> +		return;
> +	}
> +
> +	if (nh->nlmsg_type == RTM_NEWROUTE || nh->nlmsg_type == RTM_DELROUTE) {
> +		bool is_new = (nh->nlmsg_type == RTM_NEWROUTE);
> +		const struct rtmsg *rtm = NLMSG_DATA(nh);
> +		struct rtattr *rta = RTM_RTA(rtm);
> +		size_t na = RTM_PAYLOAD(nh);
> +		unsigned int oif = 0;
> +		void *gw = NULL;
> +
> +		/* Only interested in default routes (dst_len == 0) */

No, we copy non-default routes as well.

> +		if (rtm->rtm_dst_len != 0)
> +			return;
> +
> +		for (; RTA_OK(rta, na); rta = RTA_NEXT(rta, na)) {
> +			if (rta->rta_type == RTA_GATEWAY)
> +				gw = RTA_DATA(rta);
> +			else if (rta->rta_type == RTA_OIF)
> +				oif = *(unsigned int *)RTA_DATA(rta);
> +		}
> +
> +		if (!gw)

We copy non-gateway routes too (and may well need to, because there's
typically at least one non-gw route needed to reach the gateway
itself).

> +			return;
> +
> +		/* Only handle our pasta interface */
> +		if (c->mode != MODE_PASTA || oif != c->pasta_ifi)
> +			return;

Again, we need to be listening in the host netns, so pasta_ifi makes
no sense.

> +
> +		if (rtm->rtm_family == AF_INET) {
> +			if (is_new) {
> +				c->ip4.guest_gw = *(struct in_addr *)gw;
> +				c->ip4.our_tap_addr = c->ip4.guest_gw;
> +			} else {
> +				c->ip4.guest_gw = (struct in_addr){ 0 };
> +				c->ip4.our_tap_addr = (struct in_addr){ 0 };
> +			}
> +		} else if (rtm->rtm_family == AF_INET6) {
> +			if (is_new)
> +				c->ip6.guest_gw = *(struct in6_addr *)gw;
> +			else
> +				c->ip6.guest_gw = (struct in6_addr){ 0 };
> +		}
>  	}
>  }
>  
> @@ -398,8 +441,8 @@ void nl_linkaddr_notify_handler(struct ctx *c)
>  static int nl_linkaddr_init_do(void *arg)
>  {
>  	struct sockaddr_nl addr = { .nl_family = AF_NETLINK,
> -		.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR |
> -			     RTMGRP_IPV6_IFADDR };
> +		.nl_groups = RTMGRP_LINK | RTMGRP_IPV4_IFADDR | RTMGRP_IPV6_IFADDR |
> +			     RTMGRP_IPV4_ROUTE | RTMGRP_IPV6_ROUTE };
>  
>  	if (arg)
>  		ns_enter((struct ctx *)arg);
> -- 
> 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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