rrl

Name

rrl - provides BIND-like Response Rate Limiting to help mitigate DNS amplification attacks. rrl also allows request rate limiting.

Description

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 per-second allowance, and cannot be more negative than window * per-second allowance.

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

When limiting requests, the category of each request is determined by the prefix of the client IP (per the ipv4/6-prefix-length).

Syntax

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
    requests-per-second ALLOWANCE
    max-table-size SIZE
    report-only
}

• 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.

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

• 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.

• report-only - Do not drop requests/responses when rates are exceeded, only log metrics. Defaults to false.

Metrics

If monitoring is enabled (via the prometheus plugin) then the following metric are exported:

• coredns_rrl_responses_exceeded_total{client_ip} - Counter of responses exceeding QPS limit.
• coredns_rrl_requests_exceeded_total{client_ip} - Counter of requests exceeding QPS limit.

External Plugin

RRL is an external plugin, which means it is not included in CoreDNS releases. To use rrl, you'll need to build a CoreDNS image with rrl included (near the top of the plugin list). In a nutshell you'll need to:

• Clone https://github.com/coredns/coredns
• Add this plugin to plugin.cfg per instructions therein.
• make -f Makefile.release DOCKER=your-docker-repo release
• make -f Makefile.release DOCKER=your-docker-repo docker
• make -f Makefile.release DOCKER=your-docker-repo docker-push

Examples

Example 1

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

Bugs / Known Issues / Limitations

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.

In CoreDNS rrl wildcard responses are accounted for individually.

Additional References

A Quick Introduction to Response Rate Limiting

This Plugin's Design Spec