Brocade FastIron Ethernet Switch Security Configuration Guide - Manual (Page 326)

• If the number of ICMP packets exceeds the burst-normal value, the excess ICMP packets are
dropped.
• If the number of ICMP packets exceeds the burst-max value, all ICMP packets are dropped for the
number of seconds specified by the lockup value. When the lockup period expires, the packet
counter is reset and measurement is restarted.
In the example, if the number of ICMP packets received per second exceeds 5,000, the excess
packets are dropped. If the number of ICMP packets received per second exceeds 10,000, the device
drops all ICMP packets for the next 300 seconds (5 minutes).
TCP SYN attacks
TCP SYN attacks exploit the process of how TCP connections are established to disrupt normal traffic
flow. When a TCP connection starts, the connecting host first sends a TCP SYN packet to the
destination host. The destination host responds with a SYN ACK packet, and the connecting host
sends back an ACK packet. This process, known as a "TCP three-way handshake," establishes the
TCP connection.
While waiting for the connecting host to send an ACK packet, the destination host keeps track of the
as-yet incomplete TCP connection in a connection queue. When the ACK packet is received,
information about the connection is removed from the connection queue. Usually there is not much
time between the destination host sending a SYN ACK packet and the source host sending an ACK
packet, so the connection queue clears quickly.
In a TCP SYN attack, an attacker floods a host with TCP SYN packets that have random source IP
addresses. For each of these TCP SYN packets, the destination host responds with a SYN ACK
packet and adds information to the connection queue. However, because the source host does not
exist, no ACK packet is sent back to the destination host, and an entry remains in the connection
queue until it ages out (after approximately a minute). If the attacker sends enough TCP SYN packets,
the connection queue can fill up, and service can be denied to legitimate TCP connections.
To protect against TCP SYN attacks, you can configure the Brocade device to drop TCP SYN packets
when excessive numbers are encountered. You can set threshold values for TCP SYN packets that
are targeted at the router itself or passing through an interface, and drop them when the thresholds
are exceeded.
For example, to set threshold values for TCP SYN packets targeted at the router, enter the following
command in global CONFIG mode.
device(config)#ip tcp burst-normal 10 burst-max 100 lockup 300
To set threshold values for TCP SYN packets received on interface 3/11, enter the following
commands.
device(config)#interface ethernet 3/11
device(config-if-e1000-3/11)#ip tcp burst-normal 10 burst-max 100 lockup 300
For Layer 3 router code, if the interface is part of a VLAN that has a router VE, you must configure
TCP/SYN attack protection at the VE level. Otherwise, you can configure this feature at the interface
level as shown in the previous example. WhenTCP/SYN attack protection is configured at the VE
level, it will apply to routed traffic only. It will not affect switched traffic.
NOTE
You must configure VLAN information for the port before configuring TCP/SYN attack protection. You
cannot change the VLAN configuration for a port on which TCP/SYN attack protection is enabled.
TCP SYN attacks
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