Feature: add support for dns search domains (#2597)

dns/resolver.go

@@ -39,6 +39,7 @@ type Resolver struct {
 	group                 singleflight.Group
 	lruCache              *cache.LruCache
 	policy                *trie.DomainTrie
+	searchDomains         []string
 }
 
 // LookupIP request with TypeA and TypeAAAA, priority return TypeA
@@ -285,16 +286,33 @@ func (r *Resolver) lookupIP(ctx context.Context, host string, dnsType uint16) ([
 	query := &D.Msg{}
 	query.SetQuestion(D.Fqdn(host), dnsType)
 
-	msg, err := r.Exchange(query)
+	msg, err := r.ExchangeContext(ctx, query)
 	if err != nil {
 		return nil, err
 	}
 
 	ips := msgToIP(msg)
-	if len(ips) == 0 {
+	if len(ips) != 0 {
+		return ips, nil
+	} else if len(r.searchDomains) == 0 {
 		return nil, resolver.ErrIPNotFound
 	}
-	return ips, nil
+
+	// query provided search domains serially
+	for _, domain := range r.searchDomains {
+		q := &D.Msg{}
+		q.SetQuestion(D.Fqdn(fmt.Sprintf("%s.%s", host, domain)), dnsType)
+		msg, err := r.ExchangeContext(ctx, q)
+		if err != nil {
+			return nil, err
+		}
+		ips := msgToIP(msg)
+		if len(ips) != 0 {
+			return ips, nil
+		}
+	}
+
+	return nil, resolver.ErrIPNotFound
 }
 
 func (r *Resolver) msgToDomain(msg *D.Msg) string {
@@ -336,6 +354,7 @@ type Config struct {
 	Pool           *fakeip.Pool
 	Hosts          *trie.DomainTrie
 	Policy         map[string]NameServer
+	SearchDomains  []string
 }
 
 func NewResolver(config Config) *Resolver {
@@ -345,10 +364,11 @@ func NewResolver(config Config) *Resolver {
 	}
 
 	r := &Resolver{
-		ipv6:     config.IPv6,
-		main:     transform(config.Main, defaultResolver),
-		lruCache: cache.New(cache.WithSize(4096), cache.WithStale(true)),
-		hosts:    config.Hosts,
+		ipv6:          config.IPv6,
+		main:          transform(config.Main, defaultResolver),
+		lruCache:      cache.New(cache.WithSize(4096), cache.WithStale(true)),
+		hosts:         config.Hosts,
+		searchDomains: config.SearchDomains,
 	}
 
 	if len(config.Fallback) != 0 {