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fix(rewrite): fix cname target rewrite for CNAME chains (#7853)

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* fix(rewrite): fix cname target rewrite for CNAME chains

This fix corrects the cname target rewrite to handle CNAME chains:
- Preserves only the CNAME records before matching the rule
- Rewrites only the CNAME target that matches the rule
- Includes all records from the re-resolved upstream response

Signed-off-by: hide <hide@hide.net.eu.org>

* docs(rewrite): document how answer records are handled in CNAME target rewrite

Signed-off-by: hide <hide@hide.net.eu.org>

* fix(rewrite): simplify slice append per staticcheck S1011

Signed-off-by: hide <hide@hide.net.eu.org>

* docs(rewrite): add extra line between code and paragraph

Signed-off-by: hide <hide@hide.net.eu.org>

---------

Signed-off-by: hide <hide@hide.net.eu.org>
Co-authored-by: hide <hide@hide.net.eu.org>
This commit is contained in:
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2026-02-22 06:10:35 +09:00
committed by GitHub
parent 191a783e46
commit 78524a7921
3 changed files with 38 additions and 13 deletions
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11
plugin/rewrite/README.md
View File

@@ -503,9 +503,9 @@ If only some calls contain the `revert` flag, then the value in the response wil
## CNAME Field Rewrites
There might be a scenario where you want the `CNAME` target of the response to be rewritten. You can do this by using the `CNAME` field rewrite. This will generate new answer records according to the new `CNAME` target.
There might be a scenario where you want the `CNAME` target of the response to be rewritten. You can do this by using the `CNAME` field rewrite. Answer records preceding the `CNAME` target are kept unchanged, the `CNAME` target is rewritten, and the subsequent records are replaced with the lookup result of the rewritten `CNAME` target.
The syntax for the CNAME rewrite rule is as follows. The meaning of
The syntax for the `CNAME` rewrite rule is as follows. The meaning of
`exact|prefix|suffix|substring|regex` is the same as with the name rewrite rules.
An omitted type is defaulted to `exact`.
@@ -527,7 +527,8 @@ $ dig @10.1.1.1 my-app.com
;my-app.com. IN A
;; ANSWER SECTION:
my-app.com. 200 IN CNAME my-app.com.cdn.example.net.
my-app.com. 200 IN CNAME my-app.example.
my-app.example. 200 IN CNAME my-app.com.cdn.example.net.
my-app.com.cdn.example.net. 300 IN A 20.2.0.1
my-app.com.cdn.example.net. 300 IN A 20.2.0.2
```
@@ -541,7 +542,9 @@ $ dig @10.1.1.1 my-app.com
;my-app.com. IN A
;; ANSWER SECTION:
my-app.com. 200 IN CNAME my-app.com.other.cdn.com.
my-app.com. 200 IN CNAME my-app.example.
my-app.example. 200 IN CNAME my-app.com.other.cdn.com.
my-app.com.other.cdn.com. 100 IN A 30.3.1.2
```
Note that the answer will contain a completely different set of answer records after rewriting the `CNAME` target.

18
plugin/rewrite/cname_target.go
View File

@@ -91,21 +91,21 @@ func (r *cnameTargetRuleWithReqState) RewriteResponse(res *dns.Msg, rr dns.RR) {
}
var newAnswer []dns.RR
// iterate over first upstram response
// iterate over first upstream response
// add the cname record to the new answer
for _, rr := range res.Answer {
if cname, ok := rr.(*dns.CNAME); ok {
// change the target name in the response
cname.Target = toTarget
newAnswer = append(newAnswer, rr)
}
}
// iterate over upstream response received
for _, rr := range upRes.Answer {
if rr.Header().Name == toTarget {
// preserve CNAME records until the rewrite target
newAnswer = append(newAnswer, rr)
if cname.Target == fromTarget {
// change the target name in the response
cname.Target = toTarget
break
}
}
}
// add the upstream response to the new answer
newAnswer = append(newAnswer, upRes.Answer...)
res.Answer = newAnswer
// if not propagated, the truncated response might get cached,
// and it will be impossible to resolve the full response

22
plugin/rewrite/cname_target_test.go
View File

@@ -61,6 +61,12 @@ func (u *MockedUpstream) Lookup(ctx context.Context, state request.Request, name
}
m.Truncated = true
return m, nil
case "intermediate-2.staging.":
m.Answer = []dns.RR{
test.CNAME("intermediate-2.staging. 200 IN CNAME final.staging.net."),
test.A("final.staging.net. 120 IN A 5.6.7.8"),
}
return m, nil
}
return &dns.Msg{}, nil
}
@@ -76,6 +82,7 @@ func TestCNameTargetRewrite(t *testing.T) {
{[]string{"continue", "cname", "substring", "efgh", "zzzz.www"}, reflect.TypeFor[*cnameTargetRule]()},
{[]string{"continue", "cname", "regex", `(.*)\.web\.(.*)\.site\.`, `{1}.webapp.{2}.org.`}, reflect.TypeFor[*cnameTargetRule]()},
{[]string{"continue", "cname", "exact", "music.truncated.spotify.com.", "music.truncated.spotify.com."}, reflect.TypeFor[*cnameTargetRule]()},
{[]string{"continue", "cname", "suffix", "prod.", "staging."}, reflect.TypeFor[*cnameTargetRule]()},
}
rules := make([]Rule, 0, len(ruleset))
for i, r := range ruleset {
@@ -180,6 +187,21 @@ func doTestCNameTargetTests(t *testing.T, rules []Rule) {
},
true,
},
{"cname-chain.org.", dns.TypeA,
[]dns.RR{
test.CNAME("cname-chain.org. 200 IN CNAME intermediate-1.com"),
test.CNAME("intermediate-1.com 200 IN CNAME intermediate-2.prod."),
test.CNAME("intermediate-2.prod. 200 IN CNAME final.prod.net."),
test.A("final.prod.net. 120 IN A 1.2.3.4"),
},
[]dns.RR{
test.CNAME("cname-chain.org. 200 IN CNAME intermediate-1.com"),
test.CNAME("intermediate-1.com 200 IN CNAME intermediate-2.staging."),
test.CNAME("intermediate-2.staging. 200 IN CNAME final.staging.net."),
test.A("final.staging.net. 120 IN A 5.6.7.8"),
},
false,
},
}
ctx := context.TODO()
for i, tc := range tests {