| 知乎专栏 ｜ 多维度架构 |
只允许通过192.168.2.1,192.168.2.2 访问本机
# vim /etc/hosts.allow sshd:192.168.2.1,192.168.2.2
禁止所有人访问本机
# vim /etc/hosts.deny sshd:ALL
上面使白名单策略，你也可以采用黑名单策略。
# vi /etc/ssh/sshd_config
连接后2m没有任何键盘输入以及屏幕输出，将自动切换SSH连接。
LoginGraceTime 2m
禁止root用户登录(disable root SSH login)
PermitRootLogin no
限制SSH验证重试次数(maximum number of authentication)：
MaxAuthTries 6
config /etc/ssh/sshd_config
$ sudo vim /etc/ssh/sshd_config AuthorizedKeysFile %h/.ssh/authorized_keys $ sudo /etc/init.d/ssh reload
ssh-keygen
ssh-keygen -dmaster server
[netkiller@master ~]$ ssh-keygen -d Generating public/private dsa key pair. Enter file in which to save the key (/home/netkiller/.ssh/id_dsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/netkiller/.ssh/id_dsa. Your public key has been saved in /home/netkiller/.ssh/id_dsa.pub. The key fingerprint is: bf:a9:21:2c:82:77:2d:71:33:12:20:10:93:5f:cb:74 netkiller@master [netkiller@master ~]$ [netkiller@master ~]$ cp .ssh/id_dsa.pub .ssh/authorized_keys [netkiller@master ~]$ chmod 600 .ssh/authorized_keys [netkiller@master ~]$ ls -l .ssh/ total 12 -rw------- 1 netkiller netkiller 612 Mar 27 15:31 authorized_keys -rw------- 1 netkiller netkiller 736 Mar 27 15:24 id_dsa -rw-r--r-- 1 netkiller netkiller 612 Mar 27 15:24 id_dsa.pub [netkiller@master ~]$
backup server
[netkiller@backup ~]$ ssh-keygen -d Generating public/private dsa key pair. Enter file in which to save the key (/home/netkiller/.ssh/id_dsa): Created directory '/home/netkiller/.ssh'. Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/netkiller/.ssh/id_dsa. Your public key has been saved in /home/netkiller/.ssh/id_dsa.pub. The key fingerprint is: c5:2f:0e:4e:b0:46:47:ec:19:30:be:9c:20:ad:9c:51 netkiller@backup [netkiller@backup ~]$ cp .ssh/id_dsa.pub .ssh/authorized_keys [netkiller@backup ~]$ chmod 600 .ssh/authorized_keys [netkiller@backup ~]$ ls -l .ssh/ total 16 -rw------- 1 netkiller netkiller 609 Mar 27 15:31 authorized_keys -rw------- 1 netkiller netkiller 736 Mar 27 15:27 id_dsa -rw-r--r-- 1 netkiller netkiller 609 Mar 27 15:27 id_dsa.pub
交换公钥证书
master => backup
[netkiller@master ~]$ scp .ssh/id_dsa.pub netkiller@backup.example.org:.ssh/master.pub netkiller@backup.example.org's password: id_dsa.p 100% 612 0.6KB/s 00:00 [netkiller@master ~]$ [netkiller@backup ~]$ cat .ssh/master.pub >> .ssh/authorized_keys
test
[netkiller@master ~]$ ssh backup.example.org Enter passphrase for key '/home/netkiller/.ssh/id_dsa': Last login: Tue Mar 27 15:26:35 2007 from master.example.org [netkiller@backup ~]$
master <= backup
[netkiller@backup ~]$ scp .ssh/id_dsa.pub netkiller@master.example.org:.ssh/backup.pub netkiller@master.example.org's password: id_dsa.pub 100% 609 0.6KB/s 00:00 [netkiller@backup ~]$ [netkiller@master ~]$ cat .ssh/backup.pub >> .ssh/authorized_keys
test
[netkiller@backup ~]$ ssh master.example.org Enter passphrase for key '/home/netkiller/.ssh/id_dsa': Last login: Tue Mar 27 15:44:37 2007 from backup.example.org [netkiller@master ~]$
注意：authorized_keys权限必须为600，否则可能登陆的时候还会让你输入密码，但是一旦改成600以后并且成功登陆，此问题不再出现。
script
ssh-keygen -d cp .ssh/id_dsa.pub .ssh/authorized_keys chmod 600 .ssh/authorized_keys ls -l .ssh/
![[提示]](/graphics/tip.png) | 提示 |
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禁止证书登陆 PubkeyAuthentication no; 或者 AuthorizedKeysFile /dev/null |
建议你使用证书登录，并禁用密码认证 PasswordAuthentication yes，这样更安全，且不会骇客穷举你的口令。
PasswordAuthentication no
GSSAPI (Generic Security Services Application Programming Interface) 是一套类似Kerberos 5 的通用网络安全系统接口. 该接口是对各种不同的客户端服务器安全机制的封装,以消除安全接口的不同,降低编程难度. 但该接口在目标主机无域名解析时会有如下问题
GSSAPI 基本用不到建议关闭
#GSSAPIAuthentication no GSSAPIAuthentication yes #GSSAPICleanupCredentials yes GSSAPICleanupCredentials yes #GSSAPIStrictAcceptorCheck yes #GSSAPIKeyExchange no
debug1: Host '10.1.1.17' is known and matches the RSA host key. debug1: Found key in /home/neo/.ssh/known_hosts: 1 debug1: ssh_rsa_verify: signature correct debug1: SSH 2 _MSG_NEWKEYS sent debug1: expecting SSH 2 _MSG_NEWKEYS debug1: SSH 2 _MSG_NEWKEYS received debug1: SSH 2 _MSG_SERVICE_REQUEST sent debug1: SSH 2 _MSG_SERVICE_ACCEPT received debug1: Authentications that can continue: publickey,gssapi-with-mic,password debug1: Next authentication method: gssapi-with-mic debug1: An invalid name was supplied Cannot determine realm for numeric host address debug1: An invalid name was supplied Cannot determine realm for numeric host address debug1: An invalid name was supplied debug1: Next authentication method: publickey debug1: Trying private key: /home/neo/.ssh/identity debug1: Trying private key: /home/neo/.ssh/id_rsa debug1: Trying private key: /home/neo/.ssh/id_dsa debug1: Next authentication method: password ====> 事实上,正是从gssapi-with-mic这一行开始,开始耗时间: 找到 GSSAPIAuthentication yes 改为 GSSAPIAuthentication no
关闭 GSSAPI
GSSAPIAuthentication no #GSSAPIAuthentication yes #GSSAPICleanupCredentials yes #GSSAPICleanupCredentials yes #GSSAPIStrictAcceptorCheck yes #GSSAPIKeyExchange no
/etc/ssh/sshd_config
IgnoreUserKnownHosts yes
修改配置文件“~/.ssh/config”, 加入下面配置项
StrictHostKeyChecking no UserKnownHostsFile /dev/null
ssh登录服务器时总是要停顿等待一下才能连接上,这是因为OpenSSH服务器有一个DNS查找选项(UseDNS)默认是打开的. UseDNS选项打开状态下,当客户端试图登录OpenSSH服务器时,服务器端先根据客户端的IP地址进行DNS PTR反向查询,查询出客户端的host name,然后根据查询出的客户端host name进行DNS正向A记录查询,验证与其原始IP地址是否一致,这是防止客户端欺骗的一种手段.
vim /etc/ssh/sshd_config =====> 增加 UseDNS no 打开这个参数ssh在连接sever如果无法进行dns解析的时候会出现如下卡顿现象(ssh 加 -v参数)： debug1: Found key in /home/neo/.ssh/known_hosts:71 debug1: ssh_rsa_verify: signature correct debug1: SSH2_MSG_NEWKEYS sent debug1: expecting SSH2_MSG_NEWKEYS debug1: SSH2_MSG_NEWKEYS received debug1: SSH2_MSG_SERVICE_REQUEST sent debug1: SSH2_MSG_SERVICE_ACCEPT received <---- delay 4-5 seconds------> debug1: Authentications that can continue: publickey,password debug1: Next authentication method: publickey debug1: Trying private key: /home/neo/.ssh/identity
证书登陆非常安全，但是很有可能正常用户在你不知道情况下，给你安装了一个证书，他随时都可能进入你的系统
任何一个有权限的用户都能很方便的植入一个证书到 .ssh/authorized_keys 文件中
PubkeyAuthentication no AuthorizedKeysFile /dev/null
是不是自相矛盾？ 这个跟上面讲的正好相反，这里只允许使用key文件登陆。
PasswordAuthentication no
这种方式比起密码要安全的多，唯一要注意的地方就是证书被拷贝 ，建议你给证书加上 passphrase。
证书的 passphrase 是可以通过openssl工具将其剥离的，SSH证书我没有试过，但是原理都差不多。
当你使用XShell, Xftp, WinSCP, SecureCRT, SecureFX ......等等软件登录时，该软件都提供记住密码的功能，使你下次再登陆的时候无须输入密码就可以进入系统。这样做的确非常方便，
但是你是否想过你的电脑一旦丢失或者被其他人进入，那有多么危险。我之前每天背着笔记本电脑上班，上面安装着XShell并且密码全部记忆在里面。这使我意识到一点电脑丢失，有多么可怕。
禁止SSH客户端记住密码，你不要要求别人那么做。你也无法控制，最终我找到了一种解决方案。
ChallengeResponseAuthentication yes
每次登陆都回提示你输入密码。密码保存也无效。
在 Linux 中允许指定用户使用 SSH，将指定的用户添加 /etc/ssh/sshd_config 文件中即可，多个用户用空格分割他们。
# echo "AllowUsers myuser" >> /etc/ssh/sshd_config
运行下列命令检查是否添加成功。
# cat /etc/ssh/sshd_config | grep -i allowusers AllowUsers myuser
重启生效
# systemctl restart sshd
测试一下效果
# ssh test@192.168.1.4 test@192.168.1.4's password: Permission denied, please try again. 日志输出: Mar 29 02:00:35 CentOS7 sshd[4900]: User test from 192.168.1.6 not allowed because not listed in AllowUsers Mar 29 02:00:35 CentOS7 sshd[4900]: input_userauth_request: invalid user test [preauth] Mar 29 02:00:40 CentOS7 unix_chkpwd[4902]: password check failed for user (test) Mar 29 02:00:40 CentOS7 sshd[4900]: pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=192.168.1.6 user=test Mar 29 02:00:43 CentOS7 sshd[4900]: Failed password for invalid user test from 192.168.1.6 port 42568 ssh2
使用用户 myuser 正常登陆，因为他在允许的用户列表中。
# ssh myuser@192.168.1.4 myuser@192.168.1.4's password: 输出: Mar 29 02:01:13 CentOS7 sshd[4939]: Accepted password for myuser from 192.168.1.6 port 42590 ssh2 Mar 29 02:01:13 CentOS7 sshd[4939]: pam_unix(sshd:session): session opened for user myuser by (uid=0)
# echo "DenyUsers user1" >> /etc/ssh/sshd_config 运行下列命令检查是否添加成功。 # cat /etc/ssh/sshd_config | grep -i denyusers DenyUsers user1 重启 ssh 服务生效 # systemctl restart sshd
# echo "AllowGroups wheel" >> /etc/ssh/sshd_config
确认是否添加成功
# cat /etc/ssh/sshd_config | grep -i AllowGroups AllowGroups wheel # getent group wheel wheel:x:1005:user1,user2,user3
# echo "DenyGroups wheel" >> /etc/ssh/sshd_config
# cat /etc/ssh/sshd_config | grep -i denygroups DenyGroups wheel # getent group wheel wheel:x:1005:user1,user2,user3