TCP Connections to a Broadcast Address on BSD-Based Systems

Type securityvulns
Reporter Securityvulns
Modified 2002-03-19T00:00:00


BSD-based TCP/IP code has a bug with respect to creating TCP connections to a broadcast address. This bug can potentially be a security vulnerability when firewall administrators assume that the TCP implementation works correctly and do not block broadcast addresses. If good security practices have been followed on a firewall host or gateway, the potential for exploitation is probably low.

FreeBSD and NetBSD both have the bug. OpenBSD appears to have it from examination of the code, but I have been unable to test it.

The Standard:

TCP connections are only valid when the destination address is a unicast address. That is, the destination must not be a multicast or broadcast address. One place this is clearly specified in the Standards is RFC 1122,  Remote Address Validation


        A TCP implementation MUST silently discard an incoming SYN
        segment that is addressed to a broadcast or multicast

The Bug:

Uncorrected BSD-based TCP implementations do not actually check if the destination IP address is a broadcast address. Rather, the packet's link layer address is checked. Here is the old code from FreeBSD's tcp_input.c,

             * RFC1122, p. 104: discard bcast/mcast SYN
             * in_broadcast() should never return true on a received
             * packet with M_BCAST not set.
             * Packets with a multicast source address should also
             * be discarded.
            if (m->m_flags & (M_BCAST|M_MCAST))
                    goto drop;

ifdef INET6

            if (isipv6) {
                    if (IN6_IS_ADDR_MULTICAST(&ip6->ip6_dst) ||
                            goto drop;
            } else


            if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr)) ||
                IN_MULTICAST(ntohl(ip->ip_src.s_addr)) ||
                ip->ip_src.s_addr == htonl(INADDR_BROADCAST))
                    goto drop;

The comment in the code reveals the reason for the mistake. The authors assume that no one would accidentally or maliciously break the rules. One can easily send packets with a unicast link-layer address, but containing an IP broadcast address. No check is made in the above code for such a situation.


There are many ways to take advantage of this bug. Here is one easy way that will not interfere with normal operations of the test network:

    1) On the victim machine add an alias for an unused network,

              # ifconfig if0 inet alias

    2) On the attack machine, which must be local to the victim on
       the interface configured in step (1), add a route(8),

            # route add <victim's IP>

    3) Now from the attacker, try to establish a TCP connection to
       the victim's broadcast address on any port that might be
       listening on the victim,

            # ssh <victim's IP>

The connection should succeed. Another slightly scarier attack, since it doesn't require any changes to the victim host, is to just change the attacker's idea of the netmask, making the network "bigger," so that the broadcast address of the network now looks like a unicast to the attacker. The attacker needs to manually add an entry in the ARP cache. But remember this might break some things on the attacker while the network is misconfigured.

The Vulnerability:

This creates a potential security vulnerability. The firewall administrator may assume that it is not possible to establish TCP connections to a broadcast address and therefore may not protect it adequately.

This vulnerability is mitigated by a number of factors:

  • If the firewall follows a more secure explicit-pass-default-deny policy, this probably will not be a problem.

  • An attacking or misconfigured host must be local to the victim.

  • The attacking host can only connect to broadcast addresses on the local interface (on FreeBSD, others not extensively tested if at all).

One issue may exacerbates the problem for ipfw(8) users on FreeBSD. The 'me' destination in an ipfw(8) rule does NOT match the interface's broadcast address. So,

deny tcp from any to me via if0

Would not block a TCP connection to the broadcast address on if0. Using rules like the above on a firewall machine meant to allow forwarded traffic through the external interface, but not allow direct connections to the firewall is probably not uncommon, but it creates a vulnerable configuration.

The Fix:

Adding in_broadcast() checks in tcp_input.c trivially fix the problem. If you cannot patch your kernel, make sure your firewall rules block TCP on broadcast addresses. If you are not firewalling, there is little additional security exposure due to this bug (why bother getting fancy and connecting to the broadcast address of the interface when you can connect to the regular, bound IP?). But it is still a bug.


I notified security-officer@{free,open,net} on Feburary 17th. From examining OpenBSD source code, it appears to have the flaw. I have confirmed that NetBSD is vulnerable. I have been unable to actually test the vulnerability on an operational OpenBSD system. I have not heard anything from either NetBSD or OpenBSD, and no changes related to this bug appear to have been committed to their code. Patches for NetBSD and OpenBSD are attached below.

I committed changes to FreeBSD 5-CURRENT on Feburary 25th (CVS revision 1.148) and to 4-STABLE on February 28th (revision After discussion with the FreeBSD security-officer@ team, these changes will not be incorporated into the RELENG_4_{3,4,5} security-fix branches nor will an advisory be released.

Greetz^W Acknowledgements:

Thanks to Igor M Podlesny <> for bringing this to our attention in FreeBSD PR misc/35022,

Thanks to Ruslan Ermilov <> for diggigng out the origins of the bug and pointing out that the RST-response should be blocked as well as dropping incoming SYNs.

Thanks to the Security Officer Team at FreeBSD for their quick response (especially

Thanks to the rest of the FreeBSD developers on freebsd-net@.

Patch for NetBSD (tested):

Index: src/sys/netinet/tcp_input.c

RCS file: /export/netbsd/ncvs/syssrc/sys/netinet/tcp_input.c,v retrieving revision diff -u -r1.108.4.10 tcp_input.c --- src/sys/netinet/tcp_input.c 24 Jan 2002 22:44:21 -0000 +++ src/sys/netinet/tcp_input.c 16 Mar 2002 23:14:14 -0000 @@ -677,7 +677,8 @@ * Make sure destination address is not multicast. * Source address checked in ip_input(). / - if (IN_MULTICAST(ip->ip_dst.s_addr)) { + if (IN_MULTICAST(ip->ip_dst.s_addr) || + in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) { / XXX stat / goto drop; } @@ -2183,6 +2184,11 @@ / if (tiflags & TH_RST) goto drop; + + if (IN_MULTICAST(ip->ip_dst.s_addr) || + in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) + goto drop; + { /* * need to recover version # field, which was overwritten on

Patch for OpenBSD (untested, problem not verified):

Index: src/sys/netinet/tcp_input.c

RCS file: /export/openbsd/ncvs/src/sys/netinet/tcp_input.c,v retrieving revision 1.109 diff -u -r1.109 tcp_input.c --- src/sys/netinet/tcp_input.c 15 Mar 2002 18:19:52 -0000 1.109 +++ src/sys/netinet/tcp_input.c 17 Mar 2002 01:08:35 -0000 @@ -1080,8 +1080,6 @@

             * RFC1122, p. 104: discard bcast/mcast SYN
    • in_broadcast() should never return true on a received
    • packet with M_BCAST not set. / if (m->m_flags & (M_BCAST|M_MCAST)) goto drop; @@ -1094,7 +1092,8 @@ break; #endif / INET6 */ case AF_INET:
  • if (IN_MULTICAST(ip->ip_dst.s_addr))
  • if (IN_MULTICAST(ip->ip_dst.s_addr) ||
  • in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) { goto drop; break; } @@ -2139,7 +2138,8 @@ break; #endif / INET6 / case AF_INET:
  • if (IN_MULTICAST(ip->ip_dst.s_addr))
  • if (IN_MULTICAST(ip->ip_dst.s_addr) ||
  • in_broadcast(ip->ip_dst, m->m_pkthdr.rcvif)) goto drop; } if (tiflags & TH_ACK) {

-- Crist J. Clark | | |