Merge pull request #49827 from robmry/warn_no_ext_nameservers

Warn when no external DNS nameservers are found

libnetwork/internal/resolvconf/resolvconf.go

@@ -85,10 +85,10 @@ type metadata struct {
 	SearchOverride  bool
 	OptionsOverride bool
 	NDotsFrom       string
-	UsedDefaultNS   bool
 	Transform       string
 	InvalidNSs      []string
 	ExtNameServers  []ExtDNSEntry
+	Warnings        []string
 }
 
 // Load opens a file at path and parses it as a resolv.conf file.
@@ -229,7 +229,7 @@ func (rc *ResolvConf) TransformForLegacyNw(ipv6 bool) {
 	if len(rc.nameServers) == 0 {
 		log.G(context.TODO()).Info("No non-localhost DNS nameservers are left in resolv.conf. Using default external servers")
 		rc.nameServers = defaultNSAddrs(ipv6)
-		rc.md.UsedDefaultNS = true
+		rc.md.Warnings = append(rc.md.Warnings, "Used default nameservers.")
 	}
 }
 
@@ -280,6 +280,9 @@ func (rc *ResolvConf) TransformForIntNS(
 	}
 
 	rc.md.Transform = "internal resolver"
+	if len(rc.md.ExtNameServers) == 0 {
+		rc.md.Warnings = append(rc.md.Warnings, "NO EXTERNAL NAMESERVERS DEFINED")
+	}
 	return append([]ExtDNSEntry(nil), rc.md.ExtNameServers...), nil
 }
 
@@ -325,8 +328,8 @@ options {{join . " "}}
 {{join . "\n"}}
 {{end}}{{if .Comments}}
 # Based on host file: '{{.Md.SourcePath}}'{{with .Md.Transform}} ({{.}}){{end}}
-{{if .Md.UsedDefaultNS -}}
-# Used default nameservers.
+{{range .Md.Warnings -}}
+# {{.}}
 {{end -}}
 {{with .Md.ExtNameServers -}}
 # ExtServers: {{.}}

libnetwork/internal/resolvconf/resolvconf_test.go

@@ -410,6 +410,10 @@ func TestRCTransformForIntNS(t *testing.T) {
 			reqdOptions:   []string{"ndots:0", "attempts:3", "edns0", "trust-ad"},
 			expExtServers: []ExtDNSEntry{mke("127.0.0.53", true)},
 		},
+		{
+			name:  "No config",
+			input: "",
+		},
 	}
 
 	for _, tc := range testcases {

libnetwork/internal/resolvconf/testdata/TestRCTransformForIntNS/No_config.golden

@@ -0,0 +1,5 @@
+nameserver 127.0.0.11
+
+# Based on host file: '/etc/resolv.conf' (internal resolver)
+# NO EXTERNAL NAMESERVERS DEFINED
+# Overrides: []

libnetwork/sandbox_dns_unix.go

@@ -219,6 +219,11 @@ func (sb *Sandbox) restoreHostsPath() {
 }
 
 func (sb *Sandbox) setExternalResolvers(entries []resolvconf.ExtDNSEntry) {
+	if len(entries) == 0 {
+		log.G(context.TODO()).WithField("cid", sb.ContainerID()).Warn("DNS resolver has no external nameservers")
+		sb.extDNS = nil
+		return
+	}
 	sb.extDNS = make([]extDNSEntry, 0, len(entries))
 	for _, entry := range entries {
 		sb.extDNS = append(sb.extDNS, extDNSEntry{