coredns/plugin/bufsize/README.md

bufsize

Name

bufsize - sizes EDNS0 buffer size to prevent IP fragmentation.

Description

bufsize limits a requester's UDP payload size. It prevents IP fragmentation, mitigating certain DNS vulnerabilities. This will only affect queries that have an OPT RR.

Syntax

bufsize [SIZE]

[SIZE] is an int value for setting the buffer size. The default value is 512, and the value must be within 512 - 4096. Only one argument is acceptable, and it covers both IPv4 and IPv6.

Examples

Enable limiting the buffer size of outgoing query to the resolver (172.31.0.10):

. {
    bufsize 512
    forward . 172.31.0.10
    log
}

Enable limiting the buffer size as an authoritative nameserver:

. {
    bufsize 512
    file db.example.org
    log
}

Considerations

• Setting 1232 bytes to bufsize may avoid fragmentation on the majority of networks in use today, but it depends on the MTU of the physical network links.