Connection required?

With xtrabackup 2.2 (and up through 2.3.0-alpha), I could backup a database with only the datadir. Now with xtrabackup (2.3.1-beta and onwards), it appears it needs a connection. Is it no longer possible to backup a database using only the datadir?

New versions:

# xtrabackup --backup --datadir=/data --target-dir=/tmp/foo
160419 20:41:15 version_check Connecting to MySQL server with DSN 'dbi:mysql:;mysql_read_default_group=xtrabackup;my sql_socket=/var/lib/mysql/mysql.sock' (using password: NO).
Failed to connect to MySQL server: DBI connect(';mysql_read_default_group=xtrabackup;mysq l_socket=/var/lib/mysql/mysql.sock','',...) failed: Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2) at - line 1314.
Connecting to MySQL server host: (null), user: (null), password: not set, port: 0, socket: /var/lib/mysql/mysql.sock
Failed to connect to MySQL server: Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2).

Old versions:

# xtrabackup --backup --datadir=/data --target-dir=/tmp/foo xtrabackup version 2.3.0alpha1 based on MySQL server 5.6.21 Linux (x86_64) (revision id: )
xtrabackup: uses posix_fadvise().
xtrabackup: cd to /data
xtrabackup: open files limit requested 0, set to 8192
xtrabackup: using the following InnoDB configuration:
xtrabackup: innodb_data_home_dir = ./
xtrabackup: innodb_data_file_path = ibdata1:10M:autoextend
xtrabackup: innodb_log_group_home_dir = ./
xtrabackup: innodb_log_files_in_group = 2
xtrabackup: innodb_log_file_size = 50331648
>> log scanned up to (10214186)
[01] Copying ./ibdata1 to /tmp/foo/ibdata1
[01] ...done