COREDNS-LOADBALANCE(7)

NAME

loadbalance - randomizes the order of A, AAAA and MX records and optionally prefers specific subnets.

DESCRIPTION

The loadbalance will act as a round-robin DNS load balancer by randomizing the order of A, AAAA, and MX records in the answer.

See Wikipedia ⟨https://en.wikipedia.org/wiki/Round-robin_DNS⟩ about the pros and cons of this setup. It will take care to sort any CNAMEs before any address records, because some stub resolver implementations (like glibc) are particular about that.

SYNTAX

    loadbalance [round_robin | weighted WEIGHTFILE] {
                reload DURATION
                prefer CIDR [CIDR...]
    }

•   round_robin policy randomizes the order of A, AAAA, and MX records applying a uniform probability distribution. This is the default load balancing policy.

•   weighted policy assigns weight values to IPs to control the relative likelihood of particular IPs to be returned as the first (top) A/AAAA record in the answer. Note that it does not shuffle all the records in the answer, it is only concerned about the first A/AAAA record returned in the answer.

Additionally, the plugin supports subnet-based ordering using the prefer directive, which reorders A/AAAA records so that IPs from preferred subnets appear first.

•   WEIGHTFILE is the file containing the weight values assigned to IPs for various domain names. If the path is relative, the path from the root plugin will be prepended to it. The format is explained below in the Weightfile section.

•   DURATION interval to reload WEIGHTFILE and update weight assignments if there are changes in the file. The default value is 30s. A value of 0s means to not scan for changes and reload.

WEIGHTFILE

The generic weight file syntax:

    domain-name1
    ip11 weight11
    ip12 weight12
    ip13 weight13
    domain-name2
    ip21 weight21
    ip22 weight22

where ipXY is an IP address for domain-nameX and weightXY is the weight value associated with that IP. The weight values are in the range of [1,255].

The weighted policy selects one of the address record in the result list and moves it to the top (first) position in the list. The random selection takes into account the weight values assigned to the addresses in the weight file. If an address in the result list is associated with no weight value in the weight file then the default weight value "1" is assumed for it when the selection is performed.

EXAMPLES

Load balance replies coming back from Google Public DNS:

    . {
        loadbalance round_robin
        forward . 8.8.8.8 8.8.4.4
    }

Use the weighted strategy to load balance replies supplied by the file plugin. We assign weight vales 3, 1 and 2 to the IPs 100.64.1.1, 100.64.1.2 and 100.64.1.3, respectively. These IPs are addresses in A records for the domain name www.example.com defined in the ./db.example.com zone file. The ratio between the number of answers in which 100.64.1.1, 100.64.1.2 or 100.64.1.3 is in the top (first) A record should converge to 3 : 1 : 2. (E.g. there should be twice as many answers with 100.64.1.3 in the top A record than with 100.64.1.2). Corefile:

    example.com {
            file ./db.example.com {
                    reload 10s
            }
            loadbalance weighted ./db.example.com.weights {
                        reload 10s
            }
    }

weight file ./db.example.com.weights:

    www.example.com
    100.64.1.1 3
    100.64.1.2 1
    100.64.1.3 2

  SUBNET PRIORITIZATION

Prioritize IPs from 10.9.20.0/24 and 192.168.1.0/24:

    . {
        loadbalance round_robin {
            prefer 10.9.20.0/24 192.168.1.0/24
        }
        forward . 1.1.1.1
    }

If the DNS response includes multiple A/AAAA records, the plugin will reorder them to place the ones matching preferred subnets first.