Rate limiting : DoS prevention : Configuring application-layer DoS protection : Limiting TCP connections per IP address by session cookie
 
Limiting TCP connections per IP address by session cookie
You can limit the number of TCP connections per HTTP session. This can prevent TCP connection floods from clients operating behind a shared IP with innocent clients.
Excessive numbers of TCP connections per session can occur if a web application or client is malfunctioning, or if an attacker is attempting to waste socket resources to produce a DoS.
This feature is similar to DoS Protection > Network > TCP Flood Prevention. However, this feature counts TCP connections per session cookie, while TCP Flood Prevention counts only TCP connections per IP address. Because it uses session cookies at the application layer instead of only TCP/IP connections at the network layer, this feature can differentiate multiple clients that may be behind the same source IP address, such as when the source IP address hides a subnet that uses network address translation (NAT). However, in order to work, the client must support cookies.
If the count exceeds the limit, the FortiWeb appliance executes the Action.
 
This scan is bypassed if the client’s source IP is a known search engine and you have enabled Allow Known Search Engines.
To configure a TCP connection limit per session
1. Go to DoS Protection > Application > Malicious IPs.
To access this part of the web UI, your administrator’s account access profile must have Read and Write permission to items in the Web Protection Configuration category. For details, see “Permissions”.
2. Click Create New.
A dialog appears.
3. Configure these settings:
Setting name
Description
Name
Type a unique name that can be referenced in other parts of the configuration. Do not use spaces or special characters. The maximum length is 35 characters.
TCP Connection Number Limit
Type the maximum number of TCP connections allowed with a single HTTP client.
The valid range is from 1 to 1,024. The default is 1. Fortinet suggests an initial value of 100. See also “Reducing false positives”.
Action
Select which action the FortiWeb appliance will take when it detects a violation of the rule:
• Alert — Accept the request and generate an alert email and/or log message.
• Alert & Deny — Block the request (or reset the connection) and generate an alert email and/or log message.
You can customize the web page that FortiWeb returns to the client with the HTTP status code. See “Customizing error and authentication pages (replacement messages)”.
• Period Block — Block subsequent requests from the client for a number of seconds. Also configure Block Period.
You can customize the web page that FortiWeb returns to the client with the HTTP status code. See “Customizing error and authentication pages (replacement messages)”.
Tip: For improved performance during a confirmed DDoS, select this option. Attackers participating in the DoS will then be blocked at the IP layer, conserving FortiWeb resources that would otherwise be consumed by scanning each attacker’s request at the HTTP layer, compounding the effects of the DDoS.
Note: If FortiWeb is deployed behind a NAT load balancer, when using this option, you must also define an X-header that indicates the original client’s IP (see “Defining your proxies, clients, & X-headers”). Failure to do so may cause FortiWeb to block all connections when it detects a violation of this type.
The default value is Alert.
Caution: This setting will be ignored if Monitor Mode is enabled.
Note: Because the new active appliance does not know previous session history, after an HA failover, for existing sessions, FortiWeb will not be able to enforce actions for this feature. See “Sessions & FortiWeb HA”.
Note: Logging and/or alert email will occur only if enabled and configured. See “Logging” and “Alert email”.
Note: If you will use this rule set with auto-learning, you should select Alert. If Action is Alert & Deny, or any other option that causes the FortiWeb appliance to terminate or modify the request or reply when it detects an attack attempt, the interruption will cause incomplete session information for auto-learning.
Block Period
Type the number of seconds that you want to block subsequent requests from the client after the FortiWeb appliance detects that the client has violated the rule.
This setting is available only if Action is set to Period Block. The valid range is from 1 to 10,000 (2.78 hours). The default value is 0. See also “Monitoring currently blocked IPs”.
Severity
When rule violations are recorded in the attack log, each log message contains a Severity Level (severity_level) field. Select which severity level the FortiWeb appliance will use when it logs a violation of the rule:
• Low
• Medium
• High
The default value is High.
Trigger Action
Select which trigger, if any, that the FortiWeb appliance will use when it logs and/or sends an alert email about a violation of the rule. See “Configuring triggers”.
4. Click OK.
5. Group the rule in a DoS protection policy (see “Grouping DoS protection rules”) that is used by a protection profile.
6. Enable the Session Management option in the protection profile.
Attack log messages contain DoS Attack: Malicious IPs Violation when this feature detects a TCP flood with the same HTTP session cookie. See also “Log rate limits”.
Example: TCP connection per session limit
If you set 10 as the connection limit, here are two scenarios:
• A client opens 5 TCP connections. Each connection has a different source port. Because each connection has a valid session cookie, and does not exceed the connection limit, the FortiWeb appliance allows them.
• A client opens 11 TCP connections. The FortiWeb appliance blocks the last connection because it exceeds the limit of 10.
See also
• Limiting TCP connections per IP address