FreeBSD Security Advisory: FreeBSD-SA-98:06.icmp

FreeBSD Security Officer security-officer at FreeBSD.ORG
Wed Jun 10 11:35:09 PDT 1998


FreeBSD-SA-98:06                                            Security Advisory
                                                                FreeBSD, Inc.

Topic:          smurf attack

Category:       core
Module:         kernel
Announced:      1998-06-10
Affects:        FreeBSD 2.2.*, FreeBSD-stable and FreeBSD-current 
		before 1998/05/26 suffer from this problem.
Corrected:      FreeBSD-current as of 1998/05/26
		FreeBSD-stable as of 1998/05/26
FreeBSD only:   yes


IMPORTANT MESSAGE: The FreeBSD security officer now uses the policy for sending out

I.   Background    

     As can be read in CERT advisory CA-98.01.smurf, there exists
     a denial of service attack called "smurfing". This attack sends
     ICMP echo requests to the broadcast address of a network. This
     results in the source address of the ICMP packets being flooded
     with ICMP echo replies. Of course, the source address is

II.  Problem Description

     A solution at the intermediate network being abused to generate
     the ICMP echo replies is to either block ICMP echo requests
     directed to a broadcast address or to configure the hosts on
     that network not to respond to such an ICMP request.  In the
     CERT advisory, the following was reported:

       In FreeBSD 2.2.5 and up, the tcp/ip stack does not respond
       to ICMP echo requests destined for broadcast and multicast
       addresses by default. This behavior can be changed via the
       sysctl command via mib net.inet.icmp.bmcastecho.

     Unfortunately, an error was made with the implementation of
     this functionality and, despite the text in the CERT
     advisory, the net.inet.icmp.bmcastecho sysctl variable default
     is to respond to ICMP packets sent to the networks broadcast
     address. You should explicitly run the command
	sysctl -w net.inet.icmp.bmcastecho=0
     to disable this.

III. Impact

     Your network can suffer performance degradation when a
     large amount of spoofed ICMP is sent to your broadcast address.

IV.  Workaround

     Block ICMP echo requests to broadcast addresses in your kernel
     using ipfw(8).  See CERT advisory CA-98.01.smurf for more

V.   Solution

     Apply the following patch:

     Patch for 3.0-current, 2.2-stable, 2.2.5 and 2.2.6 systems:

  Index: ip_icmp.c
  RCS file: /home/cvsup/freebsd/CVS/src/sys/netinet/ip_icmp.c,v
  retrieving revision 1.29
  retrieving revision 1.30
  diff -u -r1.29 -r1.30
  --- ip_icmp.c	1997/08/25 16:29:27	1.29
  +++ ip_icmp.c	1998/05/26 11:34:30	1.30
  @@ -375,8 +375,7 @@
   	case ICMP_ECHO:
   		if (!icmpbmcastecho
  -		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0
  -		    && IN_MULTICAST(ntohl(ip->ip_dst.s_addr))) {
  +		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0) {
  @@ -385,8 +384,7 @@
   	case ICMP_TSTAMP:
   		if (!icmpbmcastecho
  -		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0
  -		    && IN_MULTICAST(ntohl(ip->ip_dst.s_addr))) {
  +		    && (m->m_flags & (M_MCAST | M_BCAST)) != 0) {

FreeBSD, Inc.

Web Site:             
Confidential contacts:          security-officer at
Security notifications:         security-notifications at
Security public discussion:     freebsd-security at
PGP Key:      

Notice: Any patches in this document may not apply cleanly due to
        modifications caused by digital signature or mailer software.
        Please reference the URL listed at the top of this document
        for original copies of all patches if necessary.

Version: 2.6.3ia
Charset: noconv


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