Fortinet IPS manual SYN flood attacks, What is a SYN flood attack?, How SYN floods work

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SYN flood attacks	What is a SYN flood attack?

SYN flood attacks

This section describes:

•What is a SYN flood attack?

•How SYN floods work

•The FortiGate IPS Response to SYN flood attacks

•Configuring SYN flood protection

•Suggested settings for different network conditions

What is a SYN flood attack?

A SYN flood is a type of Denial of Service (DoS) attack. DoS is a class of attacks in which an attacker attempts to prevent legitimate users from accessing an internet service, for example, a web server. Using SYN floods, an attacker attempts to disable an Internet service by flooding a server with TCP/IP connection requests which consume all the available slots in the server’s TCP connection table. When the connection table is full, it is not possible to establish any new connections, and the web site on the server becomes inaccessible.

This section provides information about SYN flood attacks and the FortiGate IPS methods of preventing such attacks.

How SYN floods work

SYN floods work by exploiting the structure of the TCP/IP protocol. An attacker floods a server with connection attempts but never acknowledges the server’s replies to open the TCP/IP connection.

The TCP/IP protocol uses a three-step process to establish a network connection.

Figure 15: Establishing a TCP/IP connection

1The originator of the connection sends a SYN packet (a packet with the SYN flag set in the TCP header) to initiate the connection.

2The receiver sends a SYN/ACK packet (a packet with the SYN and ACK flags set in the TCP header) back to the originator to acknowledge the connection attempt.

3The originator then sends an ACK packet (a packet with the ACK flag set in the TCP header) back to the receiver to open the connection.

FortiGate IPS User Guide Version 3.0 MR7
01-30007-0080-20080916	51

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Contents E R G U I D E Trademarks Contents SYN flood attacks Protocol decodersIPS sensors DoS sensorsFortiGate IPS IntroductionTypographic conventions About this documentFortinet documentation Document conventionsFortiGate Pptp VPN User Guide Customer service and technical support Fortinet Knowledge CenterComments on Fortinet technical documentation IPS overview and general configuration IPS settings and controlsThis section contains the following topics Config ips global Set fail-open enable disable end When to use IPSDefault signature and anomaly settings Default fail open settingControlling sessions Setting the buffer sizeConfiguring logging and alert email Monitoring the network and dealing with attacksAttack log messages Signature FortiGuard Center AnomalySelect Create New Using IPS sensors in a protection profileCreating a protection profile that uses IPS sensors Adding protection profiles to firewall policiesAdding protection profiles to user groups Using IPS sensors in a protection profile Predefined signatures IPS predefined signaturesViewing the predefined signature list Clear All Filters SettingsEnable ColumnCreate a sensor and add IPS filters to it Viewing the predefined signature list Custom signatures IPS custom signaturesViewing the custom signature list Command syntax pattern Custom signature configurationAdding custom signatures using the web-based manager Adding custom signatures using the CLICreating custom signatures Custom signature fieldsShows the valid characters for custom signature fields Srcport Custom signature syntaxAttackid Name BufferOverflowContent keywords Keyword and value Description Deprecated, see pattern and context keywords Context host Pattern GETContext uri Pattern yahoo.comUri !uristr PcreRegex/mdelim RegexdelimismxAEGRUIP header keywords Keyword and Value Description Protocol tcpTCP header keywords Keyword and Value Description Tcpflags S,12 Tcpflags APUDP header keywords Keyword and Value Description Icmp keywords Keyword and Value UsageOther keywords Keyword and Value Description Example 1 signature to block access to example.com Example custom signaturesSbid --name Block.example.com Sbid --name Block.example.com Example 2 signature to block the Smtp ‘vrfy’ command Sbid --name Block.SMTP.VRFY.CMDSbid --name Block.SMTP.VRFY.CMD --pattern vrfy Creating custom signatures Protocol decoders Protocol decodersUpgrading the IPS protocol decoder list Protocol decoder list Protocols Protocol decoder names Port Viewing the protocol decoder listViewing the IPS sensor list AlldefaultAlldefaultpass IPS sensorsProtectemailserver Configuring IPS sensorsAdding an IPS sensor ProtectclientIPS sensor filters IPS sensor attributesDelete and Edit Delete or edit the filter Icons Configuring filtersReset IPS sensor overridesApplication Configuring pre-defined and custom overridesSource Exempt IPDoS sensors Appears, and select OK Configuring DoS sensorsViewing the DoS sensor list Sequence in which the sensors examine network trafficWill appear in the DoS sensor list Anomaly configurationDoS sensor attributes Name Enter or change the DoS sensor name CommentsUnderstanding the anomalies Udpsrcsession Anomaly Description TcpdstsessionUdpflood UdpscanUnderstanding the anomalies What is a SYN flood attack? SYN flood attacksHow SYN floods work How IPS works to prevent SYN floods What is SYN threshold?What is SYN proxy? FortiGate IPS Response to SYN flood attacksIPS operation before synflood threshold is reached Configuring SYN flood protection Suggested settings for different network conditionsConfigure the options for tcpsynflood Select OK FortiGate IPS response to Icmp sweep attacks What is an Icmp sweep?Icmp sweep attacks How Icmp sweep attacks workPredefined Icmp signatures Icmp sweep anomalies Configuring Icmp sweep protection FortiGate Version 3.0 MR7 IPS User Guide IndexTechnical support