[PATCH] net:ipv4: Packet is not forwarded when ingress interface is not configured with bc_forwarding

Allow an IPv4 packet with a destination address of broadcast to be forwarded 
if the ingress interface is not configured with bc_forwarding but the egress 
interface is. This is inline with Cisco's implementation of directed 
broadcast.

Henry Shen (1):
  net:ipv4: Fix pakcet not forwarded when ingress interface is not
    configured with bc_forwarding

 net/ipv4/route.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

-- 
2.30.1

When an IPv4 packet with a destination address of broadcast is received
on an ingress interface, it will not be forwarded out of the egress
interface if the ingress interface is not configured with bc_forwarding 
but the egress interface is. If both the ingress and egress interfaces
are configured with bc_forwarding, the packet can be forwarded
successfully.

This patch is to be inline with Cisco's implementation that packet can be 
forwarded if ingress interface is NOT configured with bc_forwarding, 
but egress interface is.

Signed-off-by: Henry Shen <redacted>
---
 net/ipv4/route.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 02d81d79deeb..d082b199b8c6 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c

@@ -2101,6 +2101,8 @@ static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 	struct rtable	*rth;
 	struct flowi4	fl4;
 	bool do_cache = true;
+	struct in_device *out_dev;
+	int		rcv_local = 0;
 
 	/* IP on this device is disabled. */
 

@@ -2176,12 +2178,20 @@ static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 	}
 
 	if (res->type == RTN_BROADCAST) {
+		out_dev = in_dev_get(FIB_RES_DEV(*res));
+		if (!out_dev)
+			goto out;
+
+		if (in_dev == out_dev)
+			rcv_local = 1;
+		in_dev_put(out_dev);
 		if (IN_DEV_BFORWARD(in_dev))
 			goto make_route;
 		/* not do cache if bc_forwarding is enabled */
 		if (IPV4_DEVCONF_ALL(net, BC_FORWARDING))
 			do_cache = false;
-		goto brd_input;
+		if (rcv_local)
+			goto brd_input;
 	}
 
 	if (res->type == RTN_LOCAL) {

@@ -2197,7 +2207,8 @@ static int ip_route_input_slow(struct sk_buff *skb, __be32 daddr, __be32 saddr,
 		goto no_route;
 	}
 	if (res->type != RTN_UNICAST)
-		goto martian_destination;
+		if (res->type != RTN_BROADCAST)
+			goto martian_destination;
 
 make_route:
 	err = ip_mkroute_input(skb, res, in_dev, daddr, saddr, tos, flkeys);

-- 
2.30.1

On 2/28/21 5:53 PM, Henry Shen wrote:

When an IPv4 packet with a destination address of broadcast is received
on an ingress interface, it will not be forwarded out of the egress
interface if the ingress interface is not configured with bc_forwarding 
but the egress interface is. If both the ingress and egress interfaces
are configured with bc_forwarding, the packet can be forwarded
successfully.

This patch is to be inline with Cisco's implementation that packet can be 
forwarded if ingress interface is NOT configured with bc_forwarding, 
but egress interface is.

In Linux, forwarding decisions are made based on the ingress device, not
the egress device.