明明db.auth返回1，可是mongo -u admin -p就是失败，为啥呢？
andyliwr@andyliwrPC:~/文档/xiaodiFTP/nodejs/xiaodiTodoList$ mongo
MongoDB shell version: 2.6.10
connecting to: test
> use admin
switched to db admin
> db.auth("admin", "123456")
1
> db.auth("admin", "1234535")
Error: 18 { ok: 0.0, errmsg: "auth failed", code: 18 }
0
> exit
bye
andyliwr@andyliwrPC:~/文档/xiaodiFTP/nodejs/xiaodiTodoList$ mongo -u admin -p
MongoDB shell version: 2.6.10
Enter password:
connecting to: test
2016-07-31T22:41:33.520+0800 Error: 18 { ok: 0.0, errmsg: "auth failed", code: 18 } at src/mongo/shell/db.js:1287
exception: login failed你的用户是在admin这个数据库中的，而默认连接的数据库是test，自然认证会失败，你应该在使用mongo连接时指定数据库为admin，这样才能进行正确的认证：
mongo -u admin -p adminmongo localhost/admin -u admin -p
// or
mongo -u admin -p --authenticationDatabase admin
都是为了指定使用admin这个库中的用户来认证。有困难找文档，或者看一下--help。我机器上的是3.2，可能会和你的2.6有些出入。
$ mongo --help
MongoDB shell version: 3.2.6
usage: mongo [options] [db address] [file names (ending in .js)]
db address can be:
foo foo database on local machine
192.169.0.5/foo foo database on 192.168.0.5 machine
192.169.0.5:9999/foo foo database on 192.168.0.5 machine on port 9999
Options:
--shell run the shell after executing files
--nodb don't connect to mongod on startup - no
'db address' arg expected
--norc will not run the ".mongorc.js" file on
start up
--quiet be less chatty
--port arg port to connect to
--host arg server to connect to
--eval arg evaluate javascript
-h [ --help ] show this usage information
--version show version information
--verbose increase verbosity
--ipv6 enable IPv6 support (disabled by default)
--disableJavaScriptJIT disable the Javascript Just In Time
compiler
--enableJavaScriptProtection disable automatic JavaScript function
marshalling
--ssl use SSL for all connections
--sslCAFile arg Certificate Authority file for SSL
--sslPEMKeyFile arg PEM certificate/key file for SSL
--sslPEMKeyPassword arg password for key in PEM file for SSL
--sslCRLFile arg Certificate Revocation List file for SSL
--sslAllowInvalidHostnames allow connections to servers with
non-matching hostnames
--sslAllowInvalidCertificates allow connections to servers with invalid
certificates
--sslFIPSMode activate FIPS 140-2 mode at startup
Authentication Options:
-u [ --username ] arg username for authentication
-p [ --password ] arg password for authentication
--authenticationDatabase arg user source (defaults to dbname)
--authenticationMechanism arg authentication mechanism
--gssapiServiceName arg (=mongodb) Service name to use when authenticating
using GSSAPI/Kerberos
--gssapiHostName arg Remote host name to use for purpose of
GSSAPI/Kerberos authentication