Re: [PATCH net-next v3 4/4] ipv4: replace obsolete routing comment

[Ido Schimmel <idosch@nvidia.com>]

On Thu, Nov 12, 2026 at 11:59:32PM +0000, liamsmith827 wrote:
> From: Liam Smith <liamsmith827@proton.me>
> 
> Replace an existing comment which describes obsolete fib_lookup(),
> MSG_DONTROUTE and IPv6 behavior.
> 
> Signed-off-by: Liam Smith <liamsmith827@proton.me>
> ---
>  net/ipv4/route.c | 27 +++++++++++----------------
>  1 file changed, 11 insertions(+), 16 deletions(-)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 03db105303b4..d862b5ecad71 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -2856,24 +2856,19 @@ struct rtable *ip_route_output_key_hash_rcu(struct net *net, struct flowi4 *fl4,
>  		if (fl4->flowi4_oif &&
>  		    (ipv4_is_multicast(fl4->daddr) ||
>  		     (!fl4->flowi4_l3mdev && err == -ESRCH))) {
> -			/* Apparently, routing tables are wrong. Assume,
> -			 * that the destination is on link.
> +			/* When oif is specified, fib_lookup() is called
> +			 * primarily to determine if the destination is
> +			 * accessible directly or through a gateway. It also
> +			 * checks whether there are any traffic restrictions
> +			 * (blackhole, prohibit, unreachable), or if there's a
> +			 * routing lookup miss.
>  			 *
> -			 * WHY? DW.
> -			 * Because we are allowed to send to iface
> -			 * even if it has NO routes and NO assigned
> -			 * addresses. When oif is specified, routing

The new comment should include the motivation for the fallback.
Something like [1].

> -			 * tables are looked up with only one purpose:
> -			 * to catch if destination is gatewayed, rather than
> -			 * direct. Moreover, if MSG_DONTROUTE is set,
> -			 * we send packet, ignoring both routing tables
> -			 * and ifaddr state. --ANK
> -			 *
> -			 *
> -			 * We could make it even if oif is unknown,
> -			 * likely IPv6, but we do not.
> +			 * This fallback path is always taken for multicast,
> +			 * regardless of any restrictions or misses. Non-vrf
> +			 * unicast does honor restrictions, but falls back if
> +			 * there's a miss. The destination is assumed on-link
> +			 * for both.
>  			 */
> -
>  			if (fl4->saddr == 0)
>  				fl4->saddr = inet_select_addr(dev_out, 0,
>  							      RT_SCOPE_LINK);

[1]
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 18cb8bfc5bfc..0b5f6e9cc437 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2859,24 +2859,18 @@ struct rtable *ip_route_output_key_hash_rcu(struct net *net, struct flowi4 *fl4,
 		if (fl4->flowi4_oif &&
 		    (ipv4_is_multicast(fl4->daddr) ||
 		     (!fl4->flowi4_l3mdev && err == -ESRCH))) {
-			/* Apparently, routing tables are wrong. Assume,
-			 * that the destination is on link.
+			/* We are allowed to send packets via the specified
+			 * interface even if it does not have any routes.
+			 * Upon a lookup miss, assume that the destination is
+			 * on-link, unless the interface is a VRF device or
+			 * enslaved to one.
 			 *
-			 * WHY? DW.
-			 * Because we are allowed to send to iface
-			 * even if it has NO routes and NO assigned
-			 * addresses. When oif is specified, routing
-			 * tables are looked up with only one purpose:
-			 * to catch if destination is gatewayed, rather than
-			 * direct. Moreover, if MSG_DONTROUTE is set,
-			 * we send packet, ignoring both routing tables
-			 * and ifaddr state. --ANK
-			 *
-			 *
-			 * We could make it even if oif is unknown,
-			 * likely IPv6, but we do not.
+			 * Unreachable, blackhole and prohibit routes / rules
+			 * are honored, but not for multicast packets.
+			 * Multicast in a VRF relies on this fallback since
+			 * the lookup hits the unreachable default route in
+			 * the VRF table.
 			 */
-
 			if (fl4->saddr == 0)
 				fl4->saddr = inet_select_addr(dev_out, 0,
 							      RT_SCOPE_LINK);