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plugin/etcd: avoid apex fallback on backend errors (#8465)

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Signed-off-by: houyuwushang <liuluoqianqiu@outlook.com>
This commit is contained in:
houyuwushang
2026-08-20 17:24:12 +08:00
committed by GitHub
parent 992d4c20db
commit 4b26cced32
3 changed files with 78 additions and 16 deletions
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5
plugin/backend_lookup.go
View File

@@ -523,12 +523,11 @@ func checkForApex(ctx context.Context, b ServiceBackend, zone string, state requ
state.Req.Question[0].Name = dnsutil.Join("apex.dns", zone)
services, err := b.Services(ctx, state, false, opt)
if err == nil {
state.Req.Question[0].Name = old
state.Req.Question[0].Name = old
if err == nil || !b.IsNameError(err) {
return services, err
}
state.Req.Question[0].Name = old
return b.Services(ctx, state, false, opt)
}

61
plugin/backend_lookup_test.go
View File

@@ -2,6 +2,7 @@ package plugin
import (
"context"
"errors"
"fmt"
"net"
"testing"
@@ -18,12 +19,13 @@ import (
var _ ServiceBackend = &mockBackend{}
type mockBackend struct {
mockServices func(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error)
mockReverse func(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error)
mockLookup func(ctx context.Context, state request.Request, name string, typ uint16) (*dns.Msg, error)
mockRecords func(ctx context.Context, state request.Request, exact bool) ([]msg.Service, error)
minTTL uint32
serial uint32
mockServices func(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error)
mockReverse func(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error)
mockLookup func(ctx context.Context, state request.Request, name string, typ uint16) (*dns.Msg, error)
mockRecords func(ctx context.Context, state request.Request, exact bool) ([]msg.Service, error)
mockNameError func(err error) bool
minTTL uint32
serial uint32
}
func (m *mockBackend) Serial(_state request.Request) uint32 {
@@ -52,7 +54,10 @@ func (m *mockBackend) Lookup(ctx context.Context, state request.Request, name st
return m.mockLookup(ctx, state, name, typ)
}
func (m *mockBackend) IsNameError(_err error) bool {
func (m *mockBackend) IsNameError(err error) bool {
if m.mockNameError != nil {
return m.mockNameError(err)
}
return false
}
@@ -569,13 +574,17 @@ func TestCheckForApex(t *testing.T) {
}
func TestCheckForApexFallback(t *testing.T) {
errName := errors.New("name not found")
calls := 0
b := &mockBackend{
mockServices: func(_ctx context.Context, state request.Request, _exact bool, _opt Options) ([]msg.Service, error) {
calls++
if state.QName() == "apex.dns.example.org." {
return nil, dns.ErrRcode
return nil, errName
}
return []msg.Service{{Host: "::1", TTL: 20}}, nil
},
mockNameError: func(err error) bool { return errors.Is(err, errName) },
}
req := new(dns.Msg)
req.SetQuestion("example.org.", dns.TypeA)
@@ -587,6 +596,42 @@ func TestCheckForApexFallback(t *testing.T) {
if len(services) != 1 || services[0].Host != "::1" {
t.Fatalf("expected fallback services, got %+v", services)
}
if calls != 2 {
t.Fatalf("expected apex and fallback lookups, got %d calls", calls)
}
if state.QName() != "example.org." {
t.Fatalf("query name was not restored: %s", state.QName())
}
}
func TestCheckForApexBackendError(t *testing.T) {
errBackend := context.DeadlineExceeded
calls := 0
b := &mockBackend{
mockServices: func(_ctx context.Context, state request.Request, _exact bool, _opt Options) ([]msg.Service, error) {
calls++
if state.QName() != "apex.dns.example.org." {
t.Fatalf("unexpected fallback lookup for %s", state.QName())
}
return nil, errBackend
},
}
req := new(dns.Msg)
req.SetQuestion("example.org.", dns.TypeA)
state := request.Request{Req: req, W: &test.ResponseWriter{}}
services, err := checkForApex(context.Background(), b, "example.org.", state, Options{})
if !errors.Is(err, errBackend) {
t.Fatalf("expected backend error, got %v", err)
}
if len(services) != 0 {
t.Fatalf("expected no services, got %+v", services)
}
if calls != 1 {
t.Fatalf("expected one backend lookup, got %d", calls)
}
if state.QName() != "example.org." {
t.Fatalf("query name was not restored: %s", state.QName())
}
}
func TestIsDuplicate(t *testing.T) {

28
plugin/etcd/README.md
View File

@@ -80,6 +80,24 @@ find entries like `/skydns/test/skydns/mx1`.
This causes two lookups from CoreDNS to etcd in certain cases.
### Backend load
The *etcd* plugin does not watch or poll etcd when data changes. Record lookups are driven by
incoming DNS queries. An uncached lookup may issue multiple etcd requests when an exact-key fallback
or an internal CNAME lookup is needed.
Non-exact lookups use an etcd prefix range and return the complete subtree by design. Querying a name
high in the hierarchy can therefore read many keys. In particular, an A or AAAA query for a configured
zone name first looks below `apex.dns.ZONE`. If that entry does not exist, CoreDNS falls back to the
zone's root prefix for compatibility with older SkyDNS layouts. On a large zone, that fallback scans
the entire zone subtree.
Store zone-apex address records below the `dns/apex` path, as shown in the examples below, to avoid
the zone-wide fallback. The *cache* plugin reduces repeated backend reads for the same DNS question,
but does not reduce the size of the first prefix response. Use the *log* plugin to identify the client
and question name that trigger a lookup, and `coredns_dns_requests_total` from the *prometheus* plugin
to measure query volume by zone and type.
## Examples
This is the default SkyDNS setup, with everything specified in full:
@@ -170,7 +188,7 @@ entries to the ETCD path of your zone. If your zone is named `skydns.local` for
create an `A` record for this zone as follows:
~~~
% etcdctl put /skydns/local/skydns/ '{"host":"1.1.1.1","ttl":60}'
% etcdctl put /skydns/local/skydns/dns/apex/x1 '{"host":"1.1.1.1","ttl":60}'
~~~
If you query the zone name itself, you will receive the created `A` record:
@@ -182,8 +200,8 @@ If you query the zone name itself, you will receive the created `A` record:
If you would like to use DNS RR for the zone name, you can set the following:
~~~
% etcdctl put /skydns/local/skydns/x1 '{"host":"1.1.1.1","ttl":60}'
% etcdctl put /skydns/local/skydns/x2 '{"host":"1.1.1.2","ttl":60}'
% etcdctl put /skydns/local/skydns/dns/apex/x1 '{"host":"1.1.1.1","ttl":60}'
% etcdctl put /skydns/local/skydns/dns/apex/x2 '{"host":"1.1.1.2","ttl":60}'
~~~
If you query the zone name now, you will get the following response:
@@ -198,8 +216,8 @@ If you query the zone name now, you will get the following response:
If you would like to use `AAAA` records for the zone name too, you can set the following:
~~~
% etcdctl put /skydns/local/skydns/x3 '{"host":"2003::8:1","ttl":60}'
% etcdctl put /skydns/local/skydns/x4 '{"host":"2003::8:2","ttl":60}'
% etcdctl put /skydns/local/skydns/dns/apex/x3 '{"host":"2003::8:1","ttl":60}'
% etcdctl put /skydns/local/skydns/dns/apex/x4 '{"host":"2003::8:2","ttl":60}'
~~~
If you query the zone name for `AAAA` now, you will get the following response: