fix: locate the last answer-section record for finalCacheOnly - #14
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parseAnswer() receives the flattened Answer + Authority + Additional sections, so the last entry of `answers` is not necessarily an answer. A responder may append an EDNS(0) OPT record to its response even when the query carried no OPT of its own, and that record then sits at the end of the list. Because the finalCacheOnly branch was entered only when the last entry of the whole list matched qtype, such a response skipped the branch altogether. The chain-tail records, which are owned by the canonical name rather than by the queried name, were then dropped by the name filter below and the lookup failed with "empty record received", even though the response carried a complete and valid CNAME chain. Scan for the last Answer section record instead, and require at least two of them, so the decision no longer depends on what a responder appends after the answers.
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📝 WalkthroughWalkthroughThe DNS client now resolves final-cache-only CNAME chains with cycle detection, terminal-record checks, minimum TTL handling, and selective record removal. Regression tests cover Answer and Additional section behavior, chain ordering, unrelated records, missing terminals, aliases, and cycles. ChangesCNAME cache parsing
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to The change improves final-cache-only CNAME handling, but a cyclic CNAME response can still cause a requested-type record at the repeated name to be accepted and cached, creating incorrect resolution for that response; merge should wait for this edge case to be fixed or explicitly accepted. 🚥 Pre-merge checks | ✅ 5 | ❌ 1❌ Failed checks (1 warning)
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Inline comments:
In `@src/resty/dns/client.lua`:
- Around line 668-675: Update the answer-counting logic around the loop over
answers so records with nil section are treated as SECTION_AN when the list
contains no section metadata, preserving existing behavior for explicitly
sectioned records. Apply the same fallback to the filtering logic around lines
684-690, and add a regression test covering a CNAME chain whose records omit
section fields.
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Keying the branch on "the last answer-section record matches qtype" was still too broad. An SRV response carries SRV records in the Answer section and A glue for its targets in the Additional section, so it now entered the branch, and the truncation at the end of the branch dropped the glue before the sorting loop below could cache it, costing one extra lookup per SRV target. Explicit CNAME queries, which the untyped resolve() path issues as part of its type order, lost their additional records the same way. The branch only ever needs to run when the queried name is an alias and the tail of the chain has to be renamed to it, so test for that directly: a CNAME record in the Answer section owned by the queried name, plus a last answer-section record matching qtype. A CNAME query asks for the alias record itself and is excluded. Both shapes are covered by new cases in the finalCacheOnly spec block.
Requiring the *last* answer-section record to match qtype still read a record's position as a signal, and a responder is free to order the answer section however it likes. Two shapes reproduced the original failure: an answer section listing the tail RRset before the CNAME that leads to it, and a signed answer whose RRSIG follows the records it covers. Both left the queried name unresolved with "empty record received". Look for the two things the collapse actually needs -- the queried name aliased away by a CNAME, and something of the requested type to collapse onto -- and ignore where in the section they sit. Drive the OPT case through the untyped resolve() path while here: that is the only path that asks for an additional section, so it is the only one that meets an OPT record in production.
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[P1] Validate that the qtype RRset belongs to the CNAME chain
The new has_cname && has_qtype check only proves that the Answer section contains a CNAME for the queried name and some RR of the requested type. It does not prove that the RR belongs to the CNAME target chain. For example, alias.example CNAME target.example, unrelated.example A 203.0.113.66, followed by an RRSIG makes the new branch keep the unrelated A record, rename it to alias.example, return it, and cache it. The previous code would not enter the branch when the trailing record was not A, so this widens the accepted response set and can cache an unrelated address under the queried alias.
Please follow the CNAME owner/target chain and only collapse the qtype RRset whose owner is the terminal canonical name; ignore or reject other same-type Answer records. Please add a regression test with a direct CNAME, an unrelated same-type RR, and a trailing RRSIG or OPT, asserting that the unrelated record is neither returned nor cached.
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Fixed in 8ac287a — you are right. Renaming a foreign address onto the queried name and caching it there is a correctness problem on its own; "master has the same hole whenever the record happens to be last" is not a reason to make the window bigger. The branch now walks the aliases from the queried name to the name the chain ends at, and collapses only the records that name owns: local target, chain_ttl = check_qname, math.huge
if qtype ~= _M.TYPE_CNAME then
-- bounded by the record count, so a cyclic chain cannot spin here
for _ = 1, #answers do
local link
for i = 1, #answers do
local answer = answers[i]
if answer.section == SECTION_AN
and answer.type == _M.TYPE_CNAME
and string_lower(answer.name) == target then
link = answer
break
end
end
if not link then
break
end
target = string_lower(link.cname)
chain_ttl = math_min(link.ttl, chain_ttl)
end
endOff-chain records of the requested type are now excluded by construction rather than by where they sit, so the accepted response set is narrower than Regression test added with exactly the shape you described — a direct CNAME, an unrelated same-type A, and a trailing RRSIG — asserting that neither the returned answer nor the entry cached under the alias carries the foreign address. It fails against the previous revision of this branch and passes now.
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Asking whether the answer section holds a CNAME for the queried name and some record of the requested type does not establish that the two have anything to do with each other. A response carrying an alias alongside a same-type record owned by an unrelated name would have had that record renamed onto the queried name, returned as the answer and cached under it -- a foreign address served for the queried name. Follow the aliases from the queried name to the name they end at, and collapse only the records that name owns. The walk is bounded by the record count, so a cyclic chain cannot spin. The TTL now comes from the links actually traversed plus the records actually kept, rather than from every record in the answer section.
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Truncating the answers to the collapsed records took a decision that was never this option's to take. `finalCacheOnly` exists to keep intermediate results out of the cache, and the intermediate results are the links of the chain; everything else in the response is for the loop below to sort and cache under its own name, as it does for every response that carries no chain at all. Truncating it away cost a chained SRV response the glue for its targets -- one extra lookup each -- which is the very cost the previous commit set out to avoid, fixed there only for responses that are not a chain. Rename the records the chain ends at, remove the links, leave the rest. Stop following the chain at a name already visited, too. Bounding the walk by the record count kept it from spinning, but left the name it stops at depending on how many records the response happens to carry: the same cyclic chain resolved or failed according to whether an OPT record was appended. Reading a response that way is what this series has been removing.
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Inline comments:
In `@src/resty/dns/client.lua`:
- Around line 680-704: Update the alias-chain walk around chain and followed so
it records when termination occurs because chain[target] is already present,
then skip the terminal-record collapse for that cycle case. Preserve normal
terminal-record scanning for non-cyclic chains, and add a regression case where
a requested-type record exists on the repeated target.
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Stopping the walk at a name already visited kept it from spinning and made where it stops independent of the response's shape, but the name it stops on is a name the chain passed through, not a name the chain ends at. A response looping back to an earlier link, with an address parked on that link, therefore had the address renamed onto the queried name and returned as its answer -- something `master` refuses. A chain that does not end resolves to nothing, so leave those responses to the loop below like any other that carries nothing to collapse. The walk already records the names it has passed, so the test is whether it stopped on one of them. The cyclic case in the spec looped straight back to the queried name, where the collapse was skipped for an unrelated reason and the case passed either way. Point it at a loop further along the chain, which is the shape that actually failed.
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Tip For best results, initiate chat on the files or code changes. 🧩 Analysis chain🏁 Script executed: #!/bin/bash
set -euo pipefail
ast-grep outline src/resty/dns/client.lua --match parseAnswer --view expanded
ast-grep outline spec/client_cache_spec.lua --items all --match 'cyclic|cycle|CNAME' --view expanded || true
printf '\n--- parseAnswer context ---\n'
rg -n -A120 -B20 'local function parseAnswer|parseAnswer\s*=' src/resty/dns/client.lua
printf '\n--- cyclic-chain spec context ---\n'
rg -n -i -A80 -B20 'cyclic|cycle|CNAME b|CNAME c' spec/client_cache_spec.lua
printf '\n--- current PR changes for relevant files ---\n'
git diff --unified=40 origin/master...HEAD -- src/resty/dns/client.lua spec/client_cache_spec.luaLength of output: 43543
The regression test covers the relevant non-query-name cycle. It also verifies both result exclusion and alias-cache exclusion. The Answer-section condition remains present, which is correct for records produced by The mocked test is sufficient for this ✏️ Learnings added
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Problem
When
finalCacheOnlyis enabled, a CNAME chain response can fail to resolve withdns client error: 101 empty record received, even though the response carries a complete and valid chain ending in the requested record type.It happens whenever the responder appends an EDNS(0) OPT record to the Additional section. Some resolvers do this unconditionally, i.e. also for queries that carried no OPT of their own (which RFC 6891 §7 forbids, but it does occur in the wild), so the client cannot avoid it by not sending EDNS(0).
An OPT record is only the most visible trigger. Anything that leaves a non-
qtyperecord at the end of the answer list has the same effect, including an answer section that simply is not ordered along the chain, or a signed answer whose RRSIG follows the records it covers.Response as parsed by
resty.dns.resolverforA? alias.example.com:Expected: the two A records are returned for
alias.example.com.Actual:
101 empty record received.Note that the Additional section only reaches
parseAnswer()whenadditional_sectionis set, whichresolve()does when it is called without anr_optstable. Lookups that pass explicit options never see the OPT record and are unaffected.Root cause
parseAnswer()receives the flattened Answer + Authority + Additional sections, so the last entry ofanswersis not necessarily an answer. The branch was entered only when that last entry matchedqtype:With an OPT record (type 41, section 3) trailing the list, this is never true, so the branch is skipped entirely. The A records — owned by the canonical name, not by the queried name — are then dropped by the name filter below,
answersends up empty, and the lookup reports an empty record.The Additional section fix in #11 added a
section == 1check inside the loop, but the branch condition itself still looks at the last entry of the whole list, so responses like the one above never reach that loop.The same shape of bug applies to any record a responder places after the answers, not just OPT.
Fix
The block exists to collapse a CNAME chain: rename the records the chain ends at to the queried name, give them the shortest TTL along the way, and keep the links themselves out of the cache — the intermediate results
finalCacheOnlyis named after. So do exactly that and nothing more.Follow the aliases from the queried name, taking only Answer section records, and stop at a name already seen:
Then, if the Answer section holds anything of the requested type owned by
target, rename those records, remove the links, and leave everything else alone:What that buys, point by point:
101 empty record received. And not through the record count either: stopping the walk at a name already seen means a cyclic chain ends where the response's shape does not decide it. Bounding the walk by#answerswould have kept it from spinning, but the same cycle then resolved or failed according to whether an OPT record was appended.resolve()path issues CNAME queries as part of its type order.The TTL comes from the links actually traversed plus the records actually kept, rather than from every record in the Answer section. For a well-formed chain that is the same value; for a response carrying unrelated records it is the correct one.
Answer lists that carry no
sectionfield at all — whichresty.dns.resolvernever produces, sinceparse_section()stamps every record — skip the block. The outcome is unchanged frommaster, as the name filter below drops those records too.The walk costs one extra pass over the answer records plus two small tables, both only when
finalCacheOnlyis on.Verification
Beyond the unit cases, the whole failure was replayed end to end against a stub server modelled on a real capture: a three-link CNAME chain, a responder that answers
QTYPE=CNAMEfor one link with NODATA, and an OPT record on every response. Resolution is driven repeatedly with the record TTLs expiring in between, which is what puts the client into the state where it starts from the cached successful type.The fix also holds under the pessimistic variant where the responder returns only the first link of the chain for an A query. Some CNAME queries are still issued there, but the one that gets NODATA no longer is: the link whose A response can be collapsed now records A as its successful type, so the client never asks it for a CNAME again.
Tests
Nine cases added to the
finalCacheOnlyblock inspec/client_cache_spec.lua:masterThe three that pass on
masterare regression guards, each verified by mutation to fail against an earlier revision of this branch or against a targeted defect — please keep them rather than pruning them as redundant. Several cases assert on the cache rather than on the returned records, because a record that silently stops being cached is invisible to a result assertion.The case with an out-of-order Answer section also uses a mixed-case CNAME owner, pinning the case-insensitive comparison. The OPT case runs through the untyped
resolve()path, which is the one that setsadditional_sectionand therefore the only one that meets an OPT record in production; the remaining cases pass an explicitqtype, following the existing cases in this block, and the input shape reachingparseAnswer()is the same either way.The three existing
finalCacheOnlycases from #11 are unaffected. Fullbustedrun: the set of failing tests is identical tomaster(external-DNS cases that do not run in this environment).Summary by CodeRabbit
Bug Fixes
Tests
Summary by CodeRabbit