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Re: ICMP ECHO_REQUEST on BROADCAST--HOWTO Filter!
From: shields () CROSSLINK NET (Michael Shields)
Date: Thu, 24 Jul 1997 18:50:37 -0000


The real purpose of the `ip directed-broadcast' command is to
allow the filtering of server visibility and reachability
(for example, allowing departmentally-maintained BOOTP servers).

It does not prevent translation of a generic 'ping 1.2.3.255' to
an ethernet broadcast.

It prevents the Cisco from doing so, yes.  Here is an example, pinging
from one side of a Cisco (206.246.124.0/24) to another
(206.246.88.192/26).

ip directed-broadcast:

~$ ping 206.246.88.255
PING 206.246.88.255 (206.246.88.255): 56 data bytes
64 bytes from 206.246.124.1: icmp_seq=0 ttl=255 time=16.6 ms
64 bytes from 206.246.88.203: icmp_seq=0 ttl=254 time=17.4 ms (DUP!)
64 bytes from 206.246.88.230: icmp_seq=0 ttl=254 time=18.2 ms (DUP!)
64 bytes from 206.246.88.195: icmp_seq=0 ttl=63 time=18.5 ms (DUP!)
64 bytes from 206.246.88.202: icmp_seq=0 ttl=254 time=18.7 ms (DUP!)
64 bytes from 206.246.88.231: icmp_seq=0 ttl=254 time=19.0 ms (DUP!)

--- 206.246.88.255 ping statistics ---
1 packets transmitted, 1 packets received, +5 duplicates, 0% packet loss
round-trip min/avg/max = 16.6/18.0/19.0 ms

no ip directed-broadcast:

~$ ping 206.246.88.255
PING 206.246.88.255 (206.246.88.255): 56 data bytes
64 bytes from 206.246.124.1: icmp_seq=0 ttl=255 time=2.9 ms

--- 206.246.88.255 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 2.9/2.9/2.9 ms

Of course you can still launch an attack from a machine on local
ethernet.  Here's a Linux 2.0.30 patch to stop it from answering
broadcast pings.

Index: net/ipv4/icmp.c
===================================================================
RCS file: /usr/src/master/linux/net/ipv4/icmp.c,v
retrieving revision 1.1.1.8
retrieving revision 1.2
diff -u -r1.1.1.8 -r1.2
--- icmp.c      1997/07/08 21:55:18     1.1.1.8
+++ icmp.c      1997/07/23 00:25:13     1.2
@@ -1114,20 +1114,13 @@
                /*
                 *      RFC 1122: 3.2.2.6 An ICMP_ECHO to broadcast MAY be silently ignored (we don't as it is used
                 *      by some network mapping tools).
+                *      [But I've decided to ignore it anyway. --Shields 1997-07-22]
                 *      RFC 1122: 3.2.2.8 An ICMP_TIMESTAMP MAY be silently discarded if to broadcast/multicast.
                 */
                if (icmph->type != ICMP_ECHO)
-               {
                        icmp_statistics.IcmpInErrors++;
-                       kfree_skb(skb, FREE_READ);
-                       return(0);
-               }
-               /*
-                *      Reply the multicast/broadcast using a legal
-                *      interface - in this case the device we got
-                *      it from.
-                */
-               daddr=dev->pa_addr;
+               kfree_skb(skb, FREE_READ);
+               return(0);
        }

        len-=sizeof(struct icmphdr);

--
Shields, CrossLink.



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