Hi there,
I wrote a small C program using the BSD socket API to send a UDP
broadcast packet to all interfaces of my machine. To do so, i sendto()
it to 255.255.255.255 after using setsockopt(SO_BINDTODEVICE) on the
correct interface. It works perfectly.
However, in the same program, either using the same socket or another on
the same UDP, I found out that I just received the packet i'm sending as
well. And do not want it. I perfectly understand that some program might
be interested in broadcast packets they sent, but i don't.
I have seen that in the multicast world, every program can choose this
behaviour throught the setsockopt IP_MULTICAST_LOOP. I do not want to
use multicast since I don't want my packets to be routed (if there was
ever a multicast router on the link). But I admit I could use a TTL=1
packet in this case.
Anyway, I dig five minutes into the kernel source code I have at hand
and wrote a small patch to add IP_MULTICAST_LOOP to broadcast packets.
Since it might be interesting to other people, I just submit it here. It
has been testing on Ubuntu and kernel 2.6.23-rc1 from the wireless.git
repository. It should be very easy to adapt to other kernel version.
I have not seen this feature as being standard throught the OpenGroup
specification
(http://www.opengroup.org/onlinepubs/009695399/functions/setsockopt.html),
but it might be usefull anyway.
Comments welcome,
Benoit
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index c9e2b5e..1052a6e 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -232,6 +232,9 @@ int ip_mc_output(struct sk_buff *skb)
skb->dev = dev;
skb->protocol = htons(ETH_P_IP);
+ printk("rt->rt_flags = %x mc_loop=%d\n",
+ rt->rt_flags, sk ? inet_sk(sk)->mc_loop : 0);
+
/*
* Multicasts are looped back for other local users
*/
@@ -266,10 +269,12 @@ int ip_mc_output(struct sk_buff *skb)
}
if (rt->rt_flags&RTCF_BROADCAST) {
+ if (!sk || inet_sk(sk)->mc_loop) {
struct sk_buff *newskb = skb_clone(skb, GFP_ATOMIC);
if (newskb)
NF_HOOK(PF_INET, NF_IP_POST_ROUTING, newskb, NULL,
newskb->dev, ip_dev_loopback_xmit);
+ }
}
return NF_HOOK_COND(PF_INET, NF_IP_POST_ROUTING, skb, NULL, skb->dev,
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