coredns.io/content/plugins/bufsize.md

+++ title = "bufsize" description = "bufsize sizes EDNS0 buffer size to prevent IP fragmentation." weight = 7 tags = ["plugin", "bufsize"] categories = ["plugin"] date = "2020-02-06T12:07:03.877382" +++

Description

bufsize limits a requester's UDP payload size. It prevents IP fragmentation so that to deal with DNS vulnerability.

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.
• For now, if a client does not use EDNS, this plugin adds OPT RR.