rrl - provides response rate limiting to help mitigate DNS amplification attacks.

The rrl plugin tracks response rates per category of response. The category of a given response consists of the following:

• Prefix of the client IP (per the ipv4/6-prefix-length)
• Requested name (qname) excluding response type of error (see response type below)
• Requested type (qtype) excluding response type of error (see response type below)
• Response type (each corresponding to the configurable per-second allowances)
  • response - for positive responses that contain answers
  • nodata - for NODATA responses
  • nxdomain - for NXDOMAIN responses
  • referrals - for referrals or delegations
  • error - for all DNS errors (except NXDOMAIN)

To better protect against attacks using invalid requests, requested name and type are not categorized separately for error type requests. In other words, all error responses are limited collectively per client, regardless of qname or qtype.

Each category has an account balance which is credited at a rate of the configured per-second allowance for that response type, and debited each time a response in that category would be sent to a client. When an account balance is negative, responses in the category are dropped until the balance goes non-negative. Account balances cannot be more positive than window, and cannot be more negative than window * per-second allowance.

This implementation intends to replicate the behavior of BIND 9's response rate limiting feature.

rrl [ZONES...] {
    window SECONDS
    ipv4-prefix-length LENGTH
    ipv6-prefix-length LENGTH
    responses-per-second ALLOWANCE
    nodata-per-second ALLOWANCE
    nxdomains-per-second ALLOWANCE
    referrals-per-second ALLOWANCE
    errors-per-second ALLOWANCE
    max-table-size SIZE
}

• window SECONDS - the rolling window in SECONDS during which response rates are tracked. Default 15.

• ipv4-prefix-length LENGTH - the prefix LENGTH in bits to use for identifying a ipv4 client. Default 24.

• ipv6-prefix-length LENGTH - the prefix LENGTH in bits to use for identifying a ipv6 client. Default 56.

• responses-per-second ALLOWANCE - the number of positive responses allowed per second. An ALLOWANCE of 0 disables rate limiting of positive responses. Default 0.

• nodata-per-second ALLOWANCE - the number of NODATA responses allowed per second. An ALLOWANCE of 0 disables rate limiting of NODATA responses. Defaults to responses-per-second.

• nxdomains-per-second ALLOWANCE - the number of NXDOMAIN responses allowed per second. An ALLOWANCE of 0 disables rate limiting of NXDOMAIN responses. Defaults to responses-per-second.

• referrals-per-second ALLOWANCE - the number of referral responses allowed per second. An ALLOWANCE of 0 disables rate limiting of referral responses. Defaults to responses-per-second.

• errors-per-second ALLOWANCE - the number of error responses allowed per second (excluding NXDOMAIN). An ALLOWANCE of 0 disables rate limiting of error responses. Defaults to responses-per-second.

• max-table-size SIZE - the maximum number of responses to be tracked at one time. When exceeded, rrl stops rate limiting new responses. Defaults to 100000.

Example 1

. {
  rrl . {
    responses-per-second 10
  }
}

BIND9's implementation of Response Rate Limiting will rate limit all wildcard generated records in one account per the base domain of the wild card. e.g. Both a.dom.com. and b.dom.com. would be accounted for as dom.com., if they are generated from the wildcard record *.dom.com.

Per the BIND 9.11 ARM...

Responses generated from local wildcards are counted and limited as if they were for the parent domain name. This controls flooding using random.wild.example.com.

CoreDNS can serve plugins that can be authoritative for wildcard sub domains, but getting rrl to be aware of these is complex. Therfore in CoreDNS rrl, wildcard generated records are accounted for individually.

A Quick Introduction to Response Rate Limiting

This Plugin's Design Spec