I've configured an otherwise empty GNUPGHOME with dirmngr.conf containing only no-use-tor (to be able to diagnose more easily -- this problem might exist with tor as well). /etc/resolv.conf contains nameserver 127.0.0.53 (this refers to a locally-running stub resolver, implemented by systemd-resolved)
In that environment, i did:
gpg --keyserver keys.riseup.net --search $KEYID
and i captured packets on the loopback to see what was being sent. here's what tshark shows:
1 0.000000000 127.0.0.1 → 127.0.0.53 DNS 93 Standard query 0xcf2f SRV _pgpkey-http._tcp.keys.riseup.net 2 0.000296446 127.0.0.53 → 127.0.0.1 DNS 93 Standard query response 0xcf2f No such name SRV _pgpkey-http._tcp.keys.riseup.net 3 0.000666257 127.0.0.1 → 127.0.0.53 DNS 94 Standard query 0xc7bd Unused _pgpkey-http._tcp.keys.riseup.net 4 0.000770855 127.0.0.53 → 127.0.0.1 DNS 54 Standard query response 0xc7bd Format error 5 5.011447229 127.0.0.1 → 127.0.0.53 DNS 94 Standard query 0xc7bd Unused _pgpkey-http._tcp.keys.riseup.net 6 5.011711829 127.0.0.53 → 127.0.0.1 DNS 54 Standard query response 0xc7bd Format error 7 10.021579705 127.0.0.1 → 127.0.0.53 DNS 75 Standard query 0xe09f A keys.riseup.net 8 10.022275031 127.0.0.53 → 127.0.0.1 DNS 91 Standard query response 0xe09f A keys.riseup.net A 216.66.15.2 9 10.022776210 127.0.0.1 → 127.0.0.53 DNS 75 Standard query 0x4dbb AAAA keys.riseup.net 10 10.026563845 127.0.0.53 → 127.0.0.1 DNS 75 Standard query response 0x4dbb AAAA keys.riseup.net 11 10.026950565 127.0.0.1 → 127.0.0.53 DNS 76 Standard query 0x377f Unused keys.riseup.net 12 10.027121863 127.0.0.53 → 127.0.0.1 DNS 54 Standard query response 0x377f Format error 13 15.035047005 127.0.0.1 → 127.0.0.53 DNS 76 Standard query 0x377f Unused keys.riseup.net 14 15.035285597 127.0.0.53 → 127.0.0.1 DNS 54 Standard query response 0x377f Format error 15 20.100402262 127.0.0.1 → 127.0.0.53 DNS 75 Standard query 0xe063 A keys.riseup.net 16 20.100684943 127.0.0.53 → 127.0.0.1 DNS 91 Standard query response 0xe063 A keys.riseup.net A 216.66.15.2 17 20.100897027 127.0.0.1 → 127.0.0.53 DNS 75 Standard query 0xaa6b AAAA keys.riseup.net 18 20.122736194 127.0.0.53 → 127.0.0.1 DNS 75 Standard query response 0xaa6b AAAA keys.riseup.net 19 20.122959015 127.0.0.1 → 127.0.0.53 DNS 76 Standard query 0x9084 Unused keys.riseup.net 20 20.123046323 127.0.0.53 → 127.0.0.1 DNS 54 Standard query response 0x9084 Format error 21 25.130848494 127.0.0.1 → 127.0.0.53 DNS 76 Standard query 0x9084 Unused keys.riseup.net 22 25.131098687 127.0.0.53 → 127.0.0.1 DNS 54 Standard query response 0x9084 Format error
My biggest concern here is the "Format error" responses. on further inspection, it looks like the query itself is actually malformed. I think it's an extra zero-byte trailing the query name.
I'll upload the pcapng if i can figure out how to do that.