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Cisco ASA Series CLI Configuration Guide
Chapter 1 Configuring Connection Settings
Information About Connection Settings
TCP Sequence Randomization
Each TCP connection has two ISNs: one generated by the client and one generated by the server. The
ASA randomizes the ISN of the TCP SYN passing in both the inbound and outbound directions.
Randomizing the ISN of the protected host prevents an attacker from predecting the next ISN for a new
connection and potentially hijacking the new session.
TCP initial sequence number randomization can be disabled if required. For example:
•
If another in-line firewall is also randomizing the initial sequence numbers, there is no need for both
firewalls to be performing this action, even though this action does not affect the traffic.
•
If you use eBGP multi-hop through the ASA, and the eBGP peers are using MD5. Randomization
breaks the MD5 checksum.
•
You use a WAAS device that requires the ASA not to randomize the sequence numbers of
connections.
TCP Normalization
The TCP normalization feature identifies abnormal packets that the ASA can act on when they are
detected; for example, the ASA can allow, drop, or clear the packets. TCP normalization helps protect
the ASA from attacks. TCP normalization is always enabled, but you can customize how some features
behave.
The TCP normalizer includes non-configurable actions and configurable actions. Typically,
non-configurable actions that drop or clear connections apply to packets that are always bad.
Configurable actions (as detailed in
“Customizing the TCP Normalizer with a TCP Map” section on
) might need to be customized depending on your network needs.
See the following guidelines for TCP normalization:
•
The normalizer does not protect from SYN floods. The ASA includes SYN flood protection in other
ways.
•
The normalizer always sees the SYN packet as the first packet in a flow unless the ASA is in loose
mode due to failover.
TCP State Bypass
By default, all traffic that goes through the ASA is inspected using the Adaptive Security Algorithm and
is either allowed through or dropped based on the security policy. The ASA maximizes the firewall
performance by checking the state of each packet (is this a new connection or an established
connection?) and assigning it to either the session management path (a new connection SYN packet), the
fast path (an established connection), or the control plane path (advanced inspection). See the
Summary of Contents for 5505 - ASA Firewall Edition Bundle
Page 28: ...Glossary GL 24 Cisco ASA Series CLI Configuration Guide ...
Page 61: ...P A R T 1 Getting Started with the ASA ...
Page 62: ......
Page 219: ...P A R T 2 Configuring High Availability and Scalability ...
Page 220: ......
Page 403: ...P A R T 2 Configuring Interfaces ...
Page 404: ......
Page 499: ...P A R T 2 Configuring Basic Settings ...
Page 500: ......
Page 533: ...P A R T 2 Configuring Objects and Access Lists ...
Page 534: ......
Page 601: ...P A R T 2 Configuring IP Routing ...
Page 602: ......
Page 745: ...P A R T 2 Configuring Network Address Translation ...
Page 746: ......
Page 845: ...P A R T 2 Configuring AAA Servers and the Local Database ...
Page 846: ......
Page 981: ...P A R T 2 Configuring Access Control ...
Page 982: ......
Page 1061: ...P A R T 2 Configuring Service Policies Using the Modular Policy Framework ...
Page 1062: ......
Page 1093: ...P A R T 2 Configuring Application Inspection ...
Page 1094: ......
Page 1191: ...P A R T 2 Configuring Unified Communications ...
Page 1192: ......
Page 1333: ...P A R T 2 Configuring Connection Settings and QoS ...
Page 1334: ......
Page 1379: ...P A R T 2 Configuring Advanced Network Protection ...
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Page 1475: ...P A R T 2 Configuring Modules ...
Page 1476: ......
Page 1549: ...P A R T 2 Configuring VPN ...
Page 1550: ......
Page 1965: ...P A R T 2 Configuring Logging SNMP and Smart Call Home ...
Page 1966: ......
Page 2059: ...P A R T 2 System Administration ...
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Page 2098: ...1 8 Cisco ASA Series CLI Configuration Guide Chapter 1 Troubleshooting Viewing the Coredump ...
Page 2099: ...P A R T 2 Reference ...
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