Unify configuration of short-term storage

Add the related documentation to the README. Include a nullU2FStorage
handler to prevent nil pointer dereferences in U2F code.

README.md

@@ -40,6 +40,18 @@ The authentication server data model is based on the concept of a
 in LDAP, but it has to be told the specific details of how to find
 them and how to map the information there to what it needs.
 
+## Other Dependencies
+
+The auth-server can optionally use *memcached* to store short-term
+data with a relatively high probability of retrieval. This is used to
+store U2F challenges, and used OTP tokens for replay protection. If no
+memcache servers are configured, such functionality will be disabled
+but the auth-server will still run (useful for tests, or simpler
+deployments).
+
+It is possible to specify multiple memcached servers for HA purposes,
+with a *write-all / read-any* model.
+
 # Configuration
 
 The behavior of auth-server can be configured with a YAML file.
@@ -65,6 +77,8 @@ The YAML file should contain a dictionary with the following attributes:
     * `cert` is the path to the client certificate
     * `key` is the path to the client private key
     * `ca` is the path to the CA store to verify the server certificate
+* `memcache_servers` contains a list of memcached server addresses (in
+  host:port format)
 
 ## Rate limiting
 

server/authserver.go

@@ -206,10 +206,6 @@ func (c *ServiceConfig) notifyBlacklists(user *User, req *auth.Request, resp *au
 	}
 }
 
-type shortTermStorageConfig struct {
-	Servers []string `yaml:"memcache_servers"`
-}
-
 // Config for the authentication server.
 type Config struct {
 	// Global configuration for backends.
@@ -230,13 +226,8 @@ type Config struct {
 	// Configuration for the user-meta-server backend.
 	UserMetaDBConfig *clientutil.BackendConfig `yaml:"user_meta_server"`
 
-	// Configuration for the U2F short-term challenge storage
-	// (backed by memcached).
-	U2FShortTerm *shortTermStorageConfig `yaml:"u2f_short_term_storage"`
-
-	// Configuration for the OTP short-term replay protection
-	// storage (backed by memcached).
-	OTPShortTerm *shortTermStorageConfig `yaml:"otp_short_term_storage"`
+	// Memcache servers used for short-term storage.
+	MemcacheServers []string `yaml:"memcache_servers"`
 
 	// Runtime versions of the above. These objects are shared by
 	// all services, as they contain the actual map data.
@@ -404,12 +395,11 @@ func NewServer(config *Config) (*Server, error) {
 		config: config,
 	}
 
-	if config.U2FShortTerm != nil {
-		s.u2fShortTerm = newMemcacheU2FStorage(config.U2FShortTerm.Servers)
-	}
-	if config.OTPShortTerm != nil {
-		s.otpShortTerm = newMemcacheOTPStorage(config.OTPShortTerm.Servers)
+	if len(config.MemcacheServers) != nil {
+		s.u2fShortTerm = newMemcacheU2FStorage(config.MemcacheServers)
+		s.otpShortTerm = newMemcacheOTPStorage(config.MemcacheServers)
 	} else {
+		s.u2fShortTerm = &nullU2FStorage{}
 		s.otpShortTerm = &nullOTPStorage{}
 	}
 

server/u2f.go

@@ -71,3 +71,8 @@ func deserializeU2FChallenge(data []byte) (*u2f.Challenge, error) {
 	}
 	return &chal, nil
 }
+
+type nullU2FStorage struct{}
+
+func (s *nullU2FStorage) SetUserChallenge(user string, chal *u2f.Challenge) error { return nil }
+func (s *nullU2FStorage) GetUserChallenge(user string) (*u2f.Challenge, bool)     { return nil, false }