Concepts

Prefixes

Advanced TCP Protection protects the IP prefixes you select from sophisticated TCP attacks. A prefix can be an IP address or an IP range in CIDR format. You must add prefixes to Advanced TCP Protection so that Cloudflare can analyze incoming packets and offer protection against sophisticated TCP DDoS attacks.

Prefixes added to Advanced TCP Protection must be one of the following:

• A prefix onboarded to Magic Transit.
• A subset of a prefix onboarded to Magic Transit.

You cannot add a prefix (or a subset of a prefix) that you have not onboarded to Magic Transit or whose status is still -Unapproved-. Contact your account team to get help with prefix approvals.

Allowlist

The Advanced TCP Protection allowlist is a list of prefixes that will bypass all configured Advanced TCP Protection rules.

For example, you could add prefixes used only by partners of your company to the allowlist so that they are exempt from packet inspection and mitigation actions performed by Advanced TCP Protection.

Important

Prefixes in the allowlist will be vulnerable to IP spoofing attacks. If an attacker can guess the source IP addresses you have allowlisted, their packets will be allowlisted.

Rule

A rule configures Advanced TCP Protection for a given scope, according to several settings: execution mode, burst sensitivity, and rate sensitivity.

Each system component (SYN flood protection and out-of-state TCP protection) has its own list of rules, and it should have at least one rule.

Filter

A filter modifies Advanced TCP Protection’s execution mode — monitoring, mitigation (enabled), or disabled — for all incoming packets matching an expression.

The filter expression can reference source and destination IP addresses and ports. Each system component (SYN flood protection and out-of-state TCP protection) should have one or more rules, but filters are optional.

Each Advanced TCP Protection system component has its own filters. You can configure a filter for each execution mode:

• Mitigation Filter: The system will drop packets matching the filter expression.
• Monitoring Filter: The system will log packets matching the filter expression.
• Off Filter: The system will ignore packets matching the filter expression.

When there is a match, a filter will alter the execution mode for all configured rules in a given system component (SYN flood protection or out-of-state TCP protection), including disabled rules.

For instructions on creating filters in the Cloudflare dashboard, refer to Create a filter. For API examples, refer to Common API calls.

Example use case

You can create a monitor filter for a new prefix that you are onboarding by using the expression to match against the prefix.

Your already onboarded prefixes can remain protected with one or more configured rules in mitigation mode.

When onboarding a new prefix, you would configure a monitoring filter for this prefix and then add it to Advanced TCP Protection.

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Determining the execution mode

When you have both rules and filters configured, the execution mode is determined according to the following:

1. If there is a match for one of the configured filters, use the filter’s execution mode. The filter evaluation order is based on their mode, in the following order:

   1. Mitigation filter (filter with enabled mode)
   2. Monitoring filter (filter with monitoring mode)
   3. Off filter (filter with disabled mode)

2. If no filter matched, use the execution mode determined by existing rules.

3. If no rules match, disable Advanced TCP Protection.