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The @researchfairy noticed [1] that something's wrong with PubMed so I did a little investigating with the help of my favourite command line tools, host(1), traceroute(1) and RIPE's BGPPlay tool.

The hostname for pubmed is pubmed.ncbi.nlm.nih.gov. The DNS zone is ncbi.nlm.nih.gov.

DNS zones have serial numbers. That's how secondary nameservers can figure out if something has changed and they should fetch a new copy of the zone to serve. They figure this out using a serial number which, by convention, is a date and a sequence number.

% host -t soa ncbi.nlm.nih.gov
ncbi.nlm.nih.gov has SOA record dns1-ncbi.ncbi.nlm.nih.gov. systems.ncbi.nlm.nih.gov. 2025022701 10800 5400 2419200 82800

This suggests that the zone was last changed a few days ago. So it's not a DNS change that led to this problem.

That zone has seven nameservers. Rather a lot, but not unusual for an old government system,

$ host -t ns ncbi.nlm.nih.gov
ncbi.nlm.nih.gov name server ns.nih.gov.
ncbi.nlm.nih.gov name server ns2.nih.gov.
ncbi.nlm.nih.gov name server ns3.nih.gov.
ncbi.nlm.nih.gov name server lhcns1.nlm.nih.gov.
ncbi.nlm.nih.gov name server lhcns2.nlm.nih.gov.
ncbi.nlm.nih.gov name server dns1-ncbi.ncbi.nlm.nih.gov.
ncbi.nlm.nih.gov name server dns2-ncbi.ncbi.nlm.nih.gov.

Asking these nameservers directly for the address of pubmed, we find that the ones ending with nlm.nih.gov work fine,

$ host -4 -t a pubmed.ncbi.nlm.nih.gov lhcns1.nlm.nih.gov.
pubmed.ncbi.nlm.nih.gov has address 34.107.134.59
28 min

but asking any of the first three does not work:

$ host -4 -t a pubmed.ncbi.nlm.nih.gov ns.nih.gov.
;; communications error to 128.231.128.251#53: timed out

What is wrong with the NIH nameservers?

To be continued...

[1] https://scholar.social/@researchfairy/114089685773663683