SUMMARY : UDP broadcast ...

From: Tim Chan (
Date: Tue May 11 1993 - 13:41:18 CDT

Hi everyone,

        Thanks for all those responded ... unfortunately due to some
unknown reason, I was mysteriously taken off the sun-managers list
for a couple of weeks, and not sure if I received all the responses.

        I have received a total of 7 responses for the subject :

                4 - yes
                2 - no
                1 - ask for summary

        But after a little experience, I think the answer is "no".
By default, if you have 2 interfaces, the Sun kernel will do
IP forwarding, you can turn off IP forwarding by putting :

options IPFORWARDING = "-1"

in your kernel config file or just patch your kernel by setting
"ip_forwarding" to "-1".

However, this is just IP forwarding, not broadcasting. The Sun kernel
does "DIRECTED_BROADCAST" by default also, see /sys/netinet/in_proto.c,
but not UDP broadcasting.

I've set up a Sun with 2 interfaces connected to 2 idle ethernets :

                Sun1----| |----Sun2
                        | |
                        |----- SunG ----|
                                 IP Forwarding

On Sun2, I did a etherfind to display all packets ...

Then on Sun1, I did "rpcinfo -b" ... but nothing being forwarded to Sun2's

There is no other host or router on either of the networks. So I think
my experience is quite conclusive.

============================== summary reply ===============================
original question:

        I hope this is not an FAQ ...

        I am wondering if the SunOS 4.1.3 kernel does UDP broadcast forwarding.
That is, if I use a Sun with 2 interfaces as a router, does it forward all
UDP broadcast packets from on net to the other ...

============================= summary responses ==========================
>From Wed Mar 10 16:26:03 1993

Yes, because UDP is on top of IP, just as TCP is on top of IP. NFS is
on top UDP and I run Suns as gateways and they do forward NFS,
therefore, all other types of UDP should go through too.

>From chris@rufus.state.COM.AU Wed Mar 10 18:00:18 1993

I really hate it when people do this to me, but please summarise any reponses.
You beat me to this question by about 4 hours :-)


>From Thu Mar 11 01:48:34 1993

I believe so, ie. if you don't do anything with /sys/netinet/in_prot.c


>From Thu Mar 11 10:40:21 1993

I am not an expert on the matter, but from experience
I think the answer is yes. we seem to have great
difficulty in keeping our SUNs from using a dual-
ethernet SUN as a gateway (since we have a real
Cisco router, that it what they should use.)

>From ups!kalli!kevin@fourx.Aus.Sun.COM Thu Mar 11 14:13:19 1993

No, and it shouldn't by default. If that were the case, then every UDP
broadcast would spread across all connected machines...

                l & h,

>From Mon Mar 15 17:15:25 1993

Yes, it does ... I have seen this with my own two eyes. SunOS 3.x does NOT.

>From tellab5!vpnet!trdlnk!mike@uunet.UU.NET Mon Mar 15 21:40:59 1993

No. Routers are not supposed to forward "all" broadcast packets
because that could lead to "broadcast loops" if there are two or more
routers connecting a pair of networks. For example, if you had two
routers that forward all broadcast packets connected to the same two
networks and a packet was broadcast on network 1, router a would
forward it to network 2, router b would then forward it back to network
1 and router a would forward it back to network 2 and it would go back
and forth until the time to live was exceeded, meanwhile flooding both

The RFC's do allow "directed" broadcasts to be forwarded. These are
broadcasts which specify a specific network address (rather than all
1's). These can not cause a broadcast loop problem because they will
only be routed in one direction (towards the specified network). I
don't know if the Sun kernel forwards directed broadcasts.

For more details on broadcasting, see RFC 919.

>From! Mon May 10 10:37:03 1993

you didn't summarize.


Tim Chan
================================ Address ===============================
Tim Chan, Staff Software Engineer, SynOptics Communications Inc.
    (408)-764-1706 fax: (408)-988-8548 4401 Great America Parkway,
email : P.O. Box 58185, Santa Clara, CA 95052-8185

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