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[translated]How to secure SSH login with one-time passwords on Linux
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[bazz222222222]
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How to secure SSH login with one-time passwords on Linux
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================================================================================
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As someone says, security is a not a product, but a process. While SSH protocol itself is cryptographically secure by design, someone can wreak havoc on your SSH service if it is not administered properly, be it weak passwords, compromised keys or outdated SSH client.
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As far as SSH authentication is concerned, [public key authentication][1] is in general considered more secure than password authentication. However, key authentication is actually not desirable or even less secure if you are logging in from a public or shared computer, where things like stealth keylogger or memory scraper can always a possibility. If you cannot trust the local computer, it is better to use something else. This is when "one-time passwords" come in handy. As the name implies, each one-time password is for single-use only. Such disposable passwords can be safely used in untrusted environments as they cannot be re-used even when they are stolen.
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One way to generate disposable passwords is [Google Authenticator][2]. In this tutorial, I am going to demonstrate another way to create one-time passwords for SSH login: [OTPW][3], a one-time password login package. Unlike Google Authenticator, you do not rely on any third party for one-time password generation and verification.
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### What is OTPW? ###
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OTPW consists of one-time password generator and PAM-integrated verification routines. In OTPW, one-time passwords are generated apriori with the generator, and carried by a user securely (e.g., printed in a paper sheet). Cryptographic hash of the generated passwords are then stored in the SSH server host. When a user logs in with a one-time password, OTPW's PAM module verifies the password, and invalidates it to prevent re-use.
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### Step One: Install and Configure OTPW on Linux ###
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#### Debian, Ubuntu or Linux Mint ####
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Install OTPW packages with apt-get.
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$ sudo apt-get install libpam-otpw otpw-bin
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Open a PAM configuration file for SSH (/etc/pam.d/sshd) with a text editor, and comment out the following line (to disable password authentication).
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#@include common-auth
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and add the following two lines (to enable one-time password authentication):
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auth required pam_otpw.so
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session optional pam_otpw.so
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#### Fedora or CentOS/RHEL ####
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OTPW is not available as a prebuilt package on Red Hat based systems. So let's install OTPW by building it from the source.
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First, install prerequites:
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$ sudo yum git gcc pam-devel
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$ git clone https://www.cl.cam.ac.uk/~mgk25/git/otpw
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$ cd otpw
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Open Makefile with a text editor, and edit a line that starts with "PAMLIB=" as follows.
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On 64-bit system:
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PAMLIB=/usr/lib64/security
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On 32-bit system:
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PAMLIB=/usr/lib/security
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Compile and install it. Note that installation will automatically restart an SSH server. So be ready to be disconnected if you are on an SSH connection.
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$ make
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$ sudo make install
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Now you need to update SELinux policy since /usr/sbin/sshd tries to write to user's home directory, which is not allowed by default SELinux policy. The following commands will do. If you are not using SELinux, skip this step.
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$ sudo grep sshd /var/log/audit/audit.log | audit2allow -M mypol
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$ sudo semodule -i mypol.pp
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Next, open a PAM configuration file for SSH (/etc/pam.d/sshd) with a text editor, and comment out the following line (to disable password authentication).
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#auth substack password-auth
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and add the following two lines (to enable one-time password authentication):
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auth required pam_otpw.so
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session optional pam_otpw.so
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#### Step Two: Configure SSH Server for One-time Passwords ####
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The next step is to configure an SSH server to accept one-time passwords.
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Open /etc/ssh/sshd_config with a text editor, and set the following three parameters. Make sure that you do not add these lines more than once, because that will cause an SSH server to fail.
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UsePrivilegeSeparation yes
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ChallengeResponseAuthentication yes
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UsePAM yes
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You also need to disable default password authentication. Optionally, enable public key authentication, so that you can fall back to key-based authentication in case you do not have one-time passwords.
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PubkeyAuthentication yes
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PasswordAuthentication no
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Now restart SSH server.
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Debian, Ubuntu or Linux Mint:
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$ sudo service ssh restart
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Fedora or CentOS/RHEL 7:
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$ sudo systemctl restart sshd
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#### Step Three: Generate One-time Passwords with OTPW ####
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As mentioned earlier, you need to create one-time passwords beforehand, and have them stored on the remote SSH server host. For this, run otpw-gen tool as the user you will be logging in as.
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$ cd ~
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$ otpw-gen > temporary_password.txt
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It will ask you to set a prefix password. When you later log in, you need to type this prefix password AND one-time password. Essentially the prefix password is another layer of protection. Even if the password sheet falls into the wrong hands, the prefix password forces them to brute-force.
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Once the prefix password is set, the command will generate 280 one-time passwords, and store them in the output text file (e.g., temporary_password.txt). Each password (length of 8 characters by default) is preceded by a three-digit index number. You are supposed to print the file in a sheet and carry it with you.
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You will also see ~/.otpw file created, where cryptographic hashs of these passwords are stored. The first three digits in each line indicate the index number of the password that will be used for SSH login.
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$ more ~/.otpw
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----------
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OTPW1
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280 3 12 8
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191ai+:ENwmMqwn
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218tYRZc%PIY27a
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241ve8ns%NsHFmf
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055W4/YCauQJkr:
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102ZnJ4VWLFrk5N
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2273Xww55hteJ8Y
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1509d4b5=A64jBT
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168FWBXY%ztm9j%
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000rWUSdBYr%8UE
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037NvyryzcI+YRX
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122rEwA3GXvOk=z
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### Test One-time Passwords for SSH Login ###
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Now let's login to an SSH server in a usual way:
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$ ssh user@remote_host
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If OTPW is successfully set up, you will see a slightly different password prompt:
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Password 191:
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Now open up your password sheet, and look for index number "191" in the sheet.
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023 kBvp tq/G 079 jKEw /HRM 135 oW/c /UeB 191 fOO+ PeiD 247 vAnZ EgUt
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According to sheet above, the one-time password for number "191" is "fOO+PeiD". You need to prepend your prefix password to it. For example, if your prefix password is "000", the actual one-time password you need to type is "000fOO+PeiD".
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Once you successfully log in, the password used is automatically invalidated. If you check ~/.otpw, you will notice that the first line is replaced with "---------------", meaning that password "191" has been voided.
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OTPW1
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280 3 12 8
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---------------
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218tYRZc%PIY27a
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241ve8ns%NsHFmf
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055W4/YCauQJkr:
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102ZnJ4VWLFrk5N
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2273Xww55hteJ8Y
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1509d4b5=A64jBT
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168FWBXY%ztm9j%
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000rWUSdBYr%8UE
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037NvyryzcI+YRX
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122rEwA3GXvOk=z
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### Conclusion ###
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In this tutorial, I demonstrated how to set up one-time password login for SSH using OTPW package. You may realized that a print sheet can be considered a less fancy version of security token in two-factor authentication. Yet, it is simpler and you do not rely on any third-party for its implementation. Whatever mechanism you are using to create disposable passwords, they can be helpful when you need to log in to an SSH server from an untrusted public computer. Feel free to share your experience or opinion on this topic.
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/secure-ssh-login-one-time-passwords-linux.html
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作者：[Dan Nanni][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://xmodulo.com/author/nanni
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[1]:http://xmodulo.com/how-to-force-ssh-login-via-public-key-authentication.html
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[2]:http://xmodulo.com/two-factor-authentication-ssh-login-linux.html
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[3]:http://www.cl.cam.ac.uk/~mgk25/otpw.html
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使用 SSH 和一次性密码安全登录 Linux
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================================================================================
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有人说，安全不是一个产品，而是一个过程（LCTT 注：安全公司 McAfee 认为，安全风险管理是一个方法论，而不是安全产品的堆叠）。虽然 SSH 协议被设计成使用加密技术来确保安全，但如果使用不当，别人还是能够破坏你的系统：比如弱密码、密钥泄露、使用的 SSH 客户端过时等，都能引发安全问题。
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在考虑 SSH 认证方案时，大家普遍认为[公钥认证][1]比密码认证更安全。然而，公钥认证技术并不是为公共环境设置的，如果你在一台公用电脑上使用公钥认证登录 SSH 服务器，你的服务器已经毫无安全可言了，公用的电脑可能会记录你的公钥，或从你的内存中读取公钥。如果你不信任本地电脑，那你最好还是使用其他方式登录服务器。现在就是“一次性密码”派上用场的时候了，就像名字所示，一次性密码只能被使用一次。这种一次性密码非常合适在不安全的环境下发挥作用，就算它被窃取，也无法再次使用。
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有个一次性密码方案叫[谷歌认证][2]，但在本文中，我要介绍的是另一种 SSH 登录方案：[OTPW][3]，它是个一次性密码登录的软件包。不像谷歌认证，OTPW 不需要依赖任何第三方库。
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### OTPW 是什么 ###
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OTPW 由一次性密码生成器和 PAM 认证规则组成。在 OTPW 中一次性密码由生成器事先生成，然后由用户以某种安全的方式获得（比如打印到纸上）。另一方面，这些密码会通过 Hash 加密保存在 SSH 服务器端。当用户使用一次性密码登录系统时，OTPW 的 PAM 模块认证这些密码，并且保证它们不能再次使用。
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### 步骤1：OTPW 的安装和配置 ###
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#### 在 Debian, Ubuntu 或 Linux Mint 发行版上 ####
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使用 apt-get 安装：
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$ sudo apt-get install libpam-otpw otpw-bin
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打开针对 SSH 服务的 PAM 配置文件（/etc/pam.d/sshd），注释掉下面这行（目的是禁用 PAM 的密码认证功能）：
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#@include common-auth
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添加下面两行（用于打开一次性密码认证功能）：
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auth required pam_otpw.so
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session optional pam_otpw.so
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#### 在 Fedora 或 CentOS/RHEL 发行版上 ####
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在基于 RedHat 的发行版中没有编译好的 OTPW，所以我们需要使用源代码来安装它。
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首先，安装编译环境：
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$ sudo yum git gcc pam-devel
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$ git clone https://www.cl.cam.ac.uk/~mgk25/git/otpw
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$ cd otpw
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打开 Makefile 文件，编辑以“PAMLIB＝”开头的那行配置：
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64 位系统：
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PAMLIB=/usr/lib64/security
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32 位系统：
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PAMLIB=/usr/lib/security
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编译安装。需要注意的是安装过程会把 SSH 服务重启一下，所以如果你是使用 SSH 连接到服务器，做好被断开连接的准备吧。
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$ make
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$ sudo make install
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现在你需要更新 SELinux 策略，因为 /usr/sbin/sshd 会往你的 home 目录写数据，而 SELinux 默认是不允许这么做的。如果你使用了 SELinux 服务（LCTT 注：使用 getenforce 命令查看结果，如果是 enforcing，就是打开了 SELinux 服务），如果没有使用 SELinux 服务，请跳过这一步。
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$ sudo grep sshd /var/log/audit/audit.log | audit2allow -M mypol
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$ sudo semodule -i mypol.pp
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接下来打开 PAM 配置文件（/etc/pam.d/sshd），注释下面这行（为了禁用密码认证）：
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#auth substack password-auth
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添加下面两行（用于打开一次性密码认证功能）：
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auth required pam_otpw.so
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session optional pam_otpw.so
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#### 步骤2：配置 SSH 服务器，使用一次性密码 ####
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打开 /etc/ssh/sshd_config 文件，设置下面三个参数。你要确保下面的参数不会重复存在，否则 SSH 服务器可能会出现异常。
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UsePrivilegeSeparation yes
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ChallengeResponseAuthentication yes
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UsePAM yes
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你还需要禁用默认的密码认证功能。另外可以选择开启公钥认证功能，那样的话你就可以在没有一次性密码的时候使用公钥进行认证。
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PubkeyAuthentication yes
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PasswordAuthentication no
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重启 SSH 服务器。
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Debian, Ubuntu 或 Linux Mint 发行版：
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$ sudo service ssh restart
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Fedora 或 CentOS/RHEL 7 发行版：
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$ sudo systemctl restart sshd
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#### 步骤3：使用 OTPW 产生一次性密码 ####
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之前提到过，你需要事先创建一次性密码，并保存起来。使用 otpw-gen 命令创建密码：
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$ cd ~
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$ otpw-gen > temporary_password.txt
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这个命令会让你输入密码前缀，当你以后登录的时候，你需要同时输入这个前缀以及一次性密码。密码前缀是另外一层保护，就算你的一次性密码表被泄漏，别人也无法通过暴力破解你的 SSH 密码。
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设置好密码前缀后，这个命令会产生 280 个一次性密码，并将它们保存在一个文本文件中（如 temporary_password.txt）。每个密码（默认是 8 个字符）由一个 3 位十进制数索引。你需要将这个密码表打印出来，并随身携带。
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查看 ./.otpw 文件，它存放了一次性密码的 HASH 值。头 3 位十进制数与你随身携带的密码表的索引一一对应，在你登录 SSH 服务器的时候会被用到。
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$ more ~/.otpw
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----------
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OTPW1
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280 3 12 8
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191ai+:ENwmMqwn
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218tYRZc%PIY27a
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241ve8ns%NsHFmf
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055W4/YCauQJkr:
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102ZnJ4VWLFrk5N
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2273Xww55hteJ8Y
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1509d4b5=A64jBT
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168FWBXY%ztm9j%
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000rWUSdBYr%8UE
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037NvyryzcI+YRX
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122rEwA3GXvOk=z
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### 测试一次性密码登录 SSH 服务器 ###
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使用普通的方式登录 SSH 服务器：
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$ ssh user@remote_host
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如果 OTPW 成功运行，你会看到一点与平时登录不同的地方：
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Password 191:
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现在打开你的密码表，找到索引号为 191 的密码。
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023 kBvp tq/G 079 jKEw /HRM 135 oW/c /UeB 191 fOO+ PeiD 247 vAnZ EgUt
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从上表可知，191 号密码是“fOO+PeiD”。你需要加上密码前缀，比如你设置的前缀是“000”，则你实际需要输入的密码是“000fOO+PeiD”。
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成功登录后，你这次输入的密码自动失效。查看 ~/.otpw 文件，你会发现第一行变成“---------------”，这表示 191 号密码失效了。
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OTPW1
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280 3 12 8
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---------------
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218tYRZc%PIY27a
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241ve8ns%NsHFmf
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055W4/YCauQJkr:
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102ZnJ4VWLFrk5N
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2273Xww55hteJ8Y
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1509d4b5=A64jBT
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168FWBXY%ztm9j%
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000rWUSdBYr%8UE
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037NvyryzcI+YRX
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122rEwA3GXvOk=z
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### 总结 ###
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在这个教程中，我介绍了如何使用 OTPW 工具来设置一次性登录密码。你也许意识到了在这种两个因子的认证方式中，打印一张密码表让人感觉好 low，但是这种方式是最简单的，并且不用依赖任何第三方软件。无论你用哪种方式创建一次性密码，在你需要在一个不被信任的环境登录 SSH 服务器的时候，它们都很有用。你可以就这个主题来分享你的经验和观点。
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/secure-ssh-login-one-time-passwords-linux.html
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作者：[Dan Nanni][a]
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译者：[bazz2](https://github.com/bazz2)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://xmodulo.com/author/nanni
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[1]:http://xmodulo.com/how-to-force-ssh-login-via-public-key-authentication.html
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[2]:http://xmodulo.com/two-factor-authentication-ssh-login-linux.html
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[3]:http://www.cl.cam.ac.uk/~mgk25/otpw.html
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