Date: Wed, 26 Jun 2002 13:48:38 -0400 From: Anthony Volodkin <anthonyv@brainlink.com> To: freebsd-net@freebsd.org Subject: Forwarding UDP packets Message-ID: <3D19FE76.1030404@brainlink.com>
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Hi,
Recently I've been faced with an odd problem. I setup a pptp link to my
network from my friend's XP machine. While the link functions fine
(both ends can ping each other, etc), there is one problem with it. I
cannot get any broadcast packets through the link. I receive them on
the tun0 interface, but no matter what I try i can't get them out of the
fxp0 interface. I cannot get them to go the other way either. I know
this is against standards, as they suggest routers should not forward
broadcast packets, but I would still like to have this ability.
Did anyone ever write a patch of some sort or maybe found a tool that
does this type of thing?
(many people suggested natd, and after playing with that i was able to
redirect some bcast packets from tun0 to 1 host on my lan. I was not
able to do that in the other direction, however.)
I've found an old post on the hackers list by Jonathan Chen that
included a patch to enable this kind of functionality. I applied it to
my 4.6-RELEASE kernel and it didn't do anything but add a sysctl
variable. Any help would be greatly appreciated.
Here is that post:
---------------------------------------------------
On FreeBSD -CURRENT and -STABLE, packets to broadcast addresses are not
forwarded. For instance, if I have a FreeBSD router with interfaces
192.168.1.1 and 192.168.2.1, and I send packets from 192.168.1.2 to
192.168.2.255, the packets are dropped to the floor. IMO, this is wrong...
but I haven't consulted all the RFC's so I'm not sure if some standard out
there calls for it. In any case, the following patch creates a sysctl knob
to turn on or off this feature (since it can be considered a security risk
by some). I just want to ask around in case I turned out to be doing
something incredibly evil. Comments?
-Jon
Index: in.h
===================================================================
RCS file: /export/ncvs/src/sys/netinet/in.h,v
retrieving revision 1.55
diff -u -r1.55 in.h
--- in.h 2001/06/15 00:37:27 1.55
+++ in.h 2001/08/09 15:12:19
@@ -452,7 +452,8 @@
#define IPCTL_FASTFORWARDING 14 /* use fast IP forwarding code */
#define IPCTL_KEEPFAITH 15 /* FAITH IPv4->IPv6 translater ctl */
#define IPCTL_GIF_TTL 16 /* default TTL for gif encap packet */
-#define IPCTL_MAXID 17
+#define IPCTL_FORWARD_BROADCAST 18 /* forward broadcast packets */
+#define IPCTL_MAXID 18
#define IPCTL_NAMES { \
{ 0, 0 }, \
Index: ip_input.c
===================================================================
RCS file: /export/ncvs/src/sys/netinet/ip_input.c,v
retrieving revision 1.174
diff -u -r1.174 ip_input.c
--- ip_input.c 2001/06/23 17:17:58 1.174
+++ ip_input.c 2001/08/09 15:33:59
@@ -103,6 +103,10 @@
SYSCTL_INT(_net_inet_ip, IPCTL_FORWARDING, forwarding, CTLFLAG_RW,
&ipforwarding, 0, "Enable IP forwarding between interfaces");
+int ipforward_broadcast = 0;
+SYSCTL_INT(_net_inet_ip, IPCTL_FORWARD_BROADCAST, forward_broadcast,
CTLFLAG_RW,
+ &ipforward_broadcast, 0, "Enable broadcast packets when forwarding
IP packets");
+
static int ipsendredirects = 1; /* XXX */
SYSCTL_INT(_net_inet_ip, IPCTL_SENDREDIRECTS, redirect, CTLFLAG_RW,
&ipsendredirects, 0, "Enable sending IP redirects");
@@ -1684,7 +1688,8 @@
}
error = ip_output(m, (struct mbuf *)0, &ipforward_rt,
- IP_FORWARDING, 0);
+ IP_FORWARDING|
+ (ipforward_broadcast?IP_ALLOWBROADCAST:0), 0);
if (error)
ipstat.ips_cantforward++;
else {
--
Anthony Volodkin
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