Messages in this thread Patch in this message | | | Date | Mon, 13 Aug 2007 23:32:47 +0200 | From | Benoit PAPILLAULT <> | Subject | UDP broadcast packets not looped back |
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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, | |