COREDNS-BUFSIZE(7)

💅 COREDNS-BUFSIZE(7) — the bufsize plugin, explained by your resident Unix bestie

okay so real talk, this one's a CoreDNS plugin page, and it's short but it matters, so don't skip it 🫶

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✨ NAME ✨

bufsize — it caps your EDNS0 buffer size so you don't get IP fragmentation. literally that's the whole job, and she does it well 👑

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🌸 DESCRIPTION 🌸

So here's the vibe: bufsize limits a requester's UDP payload size to within a maximum value. That's it, that's the plot 💅

How she decides:

• if a request with an OPT RR has a bufsize greater than the limit → the bufsize of the request gets reduced 📉
• otherwise → the request is unaffected, she doesn't even look at it 🙈

Why do we care? Because this prevents IP fragmentation, which mitigates certain DNS vulnerabilities. no cap, it's a whole safety thing 🛡️

⚠️ important caveat bestie: it cannot increase the UDP size requested by the client. It can be reduced only. She's a limiter, not a hype woman — she never talks your buffer size up 😭

Also, this will only affect queries that have an OPT RR (that's EDNS(0), the spec lives at https://www.rfc-editor.org/info/rfc6891/ — go read it if you're feeling academic 🎀).

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🦋 SYNTAX 🦋

bufsize [SIZE]

• [SIZE] — an int value for setting the buffer size ✨
• default value is 1232 (yes, that's the magic number, more on that below 👀)
• the value must be within 512 - 4096
• only one argument is acceptable, and it covers both IPv4 and IPv6 — one size, both stacks, iconic 🌈

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🎀 EXAMPLES 🎀

Example 1: enable limiting the buffer size of outgoing query to the resolver (172.31.0.10):

. {
    bufsize 1100
    forward . 172.31.0.10
    log
}

Example 2: enable limiting the buffer size as an authoritative nameserver:

. {
    bufsize 1220
    file db.example.org
    log
}

look at her go 🥹 both of these are literally in the original, don't @ me about the numbers.

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💜 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. so like, it's a really good default, not a universal truth 💫

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and that's the whole page, bestie — go forth and not fragment 💖

this lil love letter to the docs is from CoreDNS, dated October 2026. slay, stay caffeinated, and remember: reduce only, never inflate 🎀✨