[#11] Add dns, nns resolver

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

resolver/dns.go

@@ -0,0 +1,41 @@
+package resolver
+
+import (
+	"fmt"
+	"net"
+
+	cid "github.com/nspcc-dev/neofs-sdk-go/container/id"
+)
+
+const (
+	testnetDomain = ".containers.testnet.fs.neo.org"
+	mainnetDomain = ".containers.fs.neo.org"
+)
+
+// ResolveContainerTestnet request txt record name + '.containers.testnet.fs.neo.org' to default dns server.
+func ResolveContainerTestnet(name string) (*cid.ID, error) {
+	return ResolveContainerDomainName(name + testnetDomain)
+}
+
+// ResolveContainerMainnet request txt record name + '.containers.fs.neo.org' to default dns server.
+func ResolveContainerMainnet(name string) (*cid.ID, error) {
+	return ResolveContainerDomainName(name + mainnetDomain)
+}
+
+// ResolveContainerDomainName trys to resolve container domain name to container ID using system dns server.
+func ResolveContainerDomainName(domain string) (*cid.ID, error) {
+	results, err := net.LookupTXT(domain)
+	if err != nil {
+		return nil, err
+	}
+
+	cnrID := cid.New()
+	for _, res := range results {
+		if err = cnrID.Parse(res); err != nil {
+			continue
+		}
+		return cnrID, nil
+	}
+
+	return nil, fmt.Errorf("not found")
+}