There’s a paper from 2013 outlining a fragmentation attack on DNS that allows an off-path attacker to poison certain DNS results using IP fragmentation . I’ve been thinking about mitigation techniques and I’m interested in hearing what this group thinks. To paraphrase the attack: Find an authoritati…

The “Domain Validation++ For MitM-Resilient PKI” paper is public now. Has anyone read it? I’m only on page 3. (I think the same team has one or two other new papers too?)

I’ve read a pre-print version of the DV++ paper. It doesn’t expand on the fragmentation attack, but does explain it a little more clearly and documents how it applies to CAs. A big chunk of the paper is devoted to their proposal for a multi-VA style system. @cpu has been working on a scan of curren…

[image] mnordhoff: Has anyone read it? I’m only on page 3. Yup! I wanted to contextualize one portion of the paper where I think the author's have confused our intentions: After making our DV++ available in March 2017, a parallel similar direction was proposed by LetsEncrypt, called multi-V…

What about Unbound’s feature to send queries from random IPv6 addresses? If you route a whole netblock to the server, set e.g. “outgoing-interface: 2001:db8::/56” and probably “prefer-ip6: yes”, that would protect IPv6-capable stuff by making it infeasible to predict the source address and target th…

An interesting idea! Our experience so far with preferring IPv6 for HTTP and TLS connections has been that it’s very often flaky. Presumably Unbound’s fallback logic would be sufficient here, but I worry it would introduce a new source of flakiness in validations without a significant increase in se…

Yeah. I have no numbers but speculatively agree on all counts. Domains with totally broken IPv6 definitely happen. I don’t know how often. I hope that it’s rare enough, and Unbound’s fallback is aggressive enough, that it wouldn’t be a significant issue. While it’s probably true that most nameserv…

There’s the additional question: Since the attack depends on IP-level fragmentation, does TCP actually protect against it? If the authoritative resolver sets the DF (Don’t Fragment) bit on its TCP packets, then yes. But that flag is not universally set, and I don’t have good numbers on how common i…

Mitigating DNS fragmentation attack

Issuance Tech

cpu November 20, 2018, 4:24pm 9

Update: We have implemented the mitigation @jsha described: EDNS Buffer Size Changing to 512 Bytes

Topic		Replies	Views
EDNS Buffer Size Changing to 512 Bytes (In 2024: now 1232) API Announcements	2	7294	July 8, 2024
Why does Let's Encrypt send a large number of DNS TCP requests to authoritative resolution Issuance Tech	12	669	August 7, 2024
Minor change to validation networking API Announcements	2	1920	July 28, 2022
DNS problem - SERVFAIL for (seemingly) correctly replied names Help	39	5587	January 6, 2022
DNS-01 challenge fails since unbound 1.18. TXT records can be fetch using unbound 1.16 but not 1.18 1.19 Help	38	2763	February 21, 2024

Mitigating DNS fragmentation attack

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