Dell PowerConnect B-RX BigIron RX Series Configuration Guide v02.8.00 - Page 1074

34 Protecting against TCP SYN attacks BigIron RX(config)# access-list 101 permit tcp any any match-all +syn BigIron RX(config)# int e 3/11 BigIron RX(config-if-e100-3/11)# dos-attack-prevent 101 burst-normal 5000000 burst-max 1000 lockup 300 TCP security enhancement TCP security enhancement improves upon the handling of TCP inbound segments. The enhancement eliminates or minimizes the possibility of a TCP reset attack, in which a perpetrator attempts to prematurely terminate an active TCP session, and a data injection attack, wherein an attacker injects or manipulates data in a TCP connection. In both cases, the attack is blind, meaning the perpetrator does not have visibility into the content of the data stream between two devices, but blindly injects traffic. Also, the attacker does not see the direct effect, the continuing communications between the devices and the impact of the injected packet, but may see the indirect impact of a terminated or corrupted session. The TCP security enhancement prevents and protects against the following three types of attacks: • Blind TCP reset attack using the reset (RST) bit. • Blind TCP reset attack using the synchronization (SYN) bit • Blind TCP packet injection attack The TCP security enhancement is automatically enabled. If necessary, you can disable this feature. Refer to "Disabling the TCP security enhancement" on page 999. Protecting against a blind TCP reset attack using the RST bit In a blind TCP reset attack using the RST bit, a perpetrator attempts to guess the RST segments in order to prematurely terminate an active TCP session. To prevent a user from using the RST bit to reset a TCP connection, the RST bit is subject to the following rules when receiving TCP segments: • If the RST bit is set and the sequence number is outside the expected window, the device silently drops the segment. • If the RST bit is exactly the next expected sequence number, the device resets the connection. • If the RST bit is set and the sequence number does not exactly match the next expected sequence value, but is within the acceptable window, the device sends an acknowledgement. This TCP security enhancement is enabled by default. To disable it, refer to "Disabling the TCP security enhancement" on page 999. Protecting against a blind TCP reset attack using the SYN bit In a blind TCP reset attack, a perpetrator attempts to guess the SYN bits to prematurely terminate an active TCP session. To prevent a user from using the SYN bit to tear down a TCP connection, the SYN bit is subject to the following rules when receiving TCP segments: • If the SYN bit is set and the sequence number is outside the expected window, the device sends an acknowledgement (ACK) back to the peer. 998 BigIron RX Series Configuration Guide 53-1002253-01