20.2 Broadcast Addresses
If we denote an IPv4 address as {subnetid, hostid}, where subnetid represents the bits that are covered
by the network mask (or the CIDR prefix) and hostid represents the bits that are not, then
we have two types of broadcast addresses. We denote a field
containing all one bits as 鈥?.
-
Subnet-directed broadcast address:
{subnetid, 鈥?}鈥擳his addresses all
the interfaces on the specified subnet. For example, if we have the
subnet 192.168.42/24, then 192.168.42.255 would be the
subnet-directed broadcast address for all interfaces on the
192.168.42/24 subnet.
Normally, routers do not forward these
broadcasts (pp. 226鈥?27 of TCPv2). In Figure 20.2, we show a router connected to the two
subnets 192.168.42/24 and 192.168.123/24.
The router receives a unicast IP datagram on the
192.168.123/24 subnet with a destination address of 192.168.42.255
(the subnet-directed broadcast address of another interface). The
router normally does not forward the datagram on to the
192.168.42/24 subnet. Some systems have a configuration option that
allows subnet-directed broadcasts to be forwarded (Appendix E of
TCPv1).
Forwarding subnet-directed broadcasts enables a
class of denial-of-service attacks called "amplification" attacks;
for instance, sending an ICMP echo request to a subnet-directed
broadcast address can cause multiple replies to be sent for a
single request. Combined with a forged IP source address, this
results in a bandwidth utilization attack against the victim
system, so it's advisable to leave this configuration option
off.
For this reason, it's inadvisable to design an
application that relies on forwarding of subnet-directed broadcasts
except in a controlled environment, where you know it's safe to
turn them on.
-
Limited broadcast address: {鈥?, 鈥?, 鈥?} or
255.255.255.255鈥擠atagrams destined to this address must never be
forwarded by a router.
255.255.255.255 is to be used as the destination
address during the bootstrap process by applications such as BOOTP
and DHCP, which do not yet know the node's IP address.
The question is: What does a host do when an
application sends a UDP datagram to 255.255.255.255? Most hosts
allow this (assuming the process has set the SO_BROADCAST
socket option) and convert the destination address to the
subnet-directed broadcast address of the outgoing interface. It is
often necessary to access the datalink directly (Chapter 29) to send a packet
to 255.255.255.255.
Another question is: What does a multihomed host
do when the application sends a UDP datagram to 255.255.255.255?
Some systems send a single broadcast on the primary interface (the
first interface that was configured) with the destination IP
address set to the subnet-directed broadcast address of that
interface (p. 736 of TCPv2). Other systems send one copy of the
datagram out from each broadcast-capable interface. Section 3.3.6
of RFC 1122 [Braden 1989] "takes no stand" on this issue. For
portability, however, if an application needs to send a broadcast
out from all broadcast-capable interfaces, it should obtain the
interface configuration (Section 17.6) and
do one sendto for each broadcast-capable interface with
the destination set to that interface's broadcast address.
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