SSH安全加固

 

配置文件: /etc/ssh/sshd_config 

# This file is automatically generated at startup
KexAlgorithms [e-mail],diffie-hellman-group-exchange-sha256
Port 22
Protocol 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
Compression delayed
ClientAliveInterval 30
PermitRootLogin yes
# Login via Key or Password
ChallengeResponseAuthentication yes
PasswordAuthentication yes
PubkeyAuthentication yes
UseDNS no
LoginGraceTime 30s
VersionAddendum none
AllowAgentForwarding no
X11Forwarding no
Ciphers [e-mail],[e-mail],[e-mail],aes256-ctr,aes192-ctr,aes128-ctr
MACs [e-mail],[e-mail],[e-mail],[e-mail],hmac-sha2-512,hmac-sha2-256,hmac-ripemd160,[e-mail]
# override default of no subsystems
Subsystem       sftp    /usr/libexec/sftp-server

 

 

下面提到的參數，需要對應修改的文件是/etc/ssh/sshd_config

1. 禁用RootLogin

將參數做以下更改：
PermitRootLogin no

2. 在自己電腦生成私鑰，添加公鑰到服務器

這一步已經做過了。這樣就可以不使用密碼登陸。具體參考如何設置ssh密鑰

https://www.digitalocean.com/community/tutorials/how-to-set-up-ssh-keys--2

About SSH Keys

Secure Shell (better known as SSH) is a cryptographic network protocol which allows users to securely perform a number of network services over an unsecured network. SSH keys provide a more secure way of logging into a server with SSH than using a password alone. While a password can eventually be cracked with a brute force attack, SSH keys are nearly impossible to decipher by brute force alone.

Generating a key pair provides you with two long string of characters: a public and a private key. You can place the public key on any server, and then unlock it by connecting to it with a client that already has the private key. When the two match up, the system unlocks without the need for a password. You can increase security even more by protecting the private key with a passphrase.

Within some of the commands found in this tutorial, you will notice some highlighted values. These are variables, and you should substitute them with your own values.

 

Step One—Create the RSA Key Pair

The first step is to create the key pair on the client machine (there is a good chance that this will just be your computer):

• ssh-keygen -t rsa

 

Step Two—Store the Keys and Passphrase

Once you have entered the Gen Key command, you will get a few more questions:

Enter file in which to save the key (/home/demo/.ssh/id_rsa): 

You can press enter here, saving the file to the user home (in this case, my example user is called demo).

Enter passphrase (empty for no passphrase):

It’s up to you whether you want to use a passphrase. Entering a passphrase does have its benefits: the security of a key, no matter how encrypted, still depends on the fact that it is not visible to anyone else. Should a passphrase-protected private key fall into an unauthorized users possession, they will be unable to log in to its associated accounts until they figure out the passphrase, buying the hacked user some extra time. The only downside, of course, to having a passphrase, is then having to type it in each time you use the key pair.

The entire key generation process looks like this:

• ssh-keygen -t rsa

Output

Generating public/private rsa key pair.
Enter file in which to save the key (/home/demo/.ssh/id_rsa): Enter passphrase (empty for no passphrase): Enter same passphrase again: Your identification has been saved in /home/demo/.ssh/id_rsa. Your public key has been saved in /home/demo/.ssh/id_rsa.pub. The key fingerprint is: 4a:dd:0a:c6:35:4e:3f:ed:27:38:8c:74:44:4d:93:67 demo@a The key's randomart image is: +--[ RSA 2048]----+ | .oo. | | . o.E | | + . o | | . = = . | | = S = . | | o + = + | | . o + o . | | . o | | | +-----------------+ 

The public key is now located in /home/demo/.ssh/id_rsa.pub. The private key (identification) is now located in /home/demo/.ssh/id_rsa.

 

Step Three—Copy the Public Key

Once the key pair is generated, it’s time to place the public key on the server that we want to use.

You can copy the public key into the new machine’s authorized_keys file with the ssh-copy-id command. Make sure to replace the example username and IP address below.

• ssh-copy-id demo@198.51.100.0

Note: If you are a Mac user, ssh-copy-id will not be installed on your machine. You can, however, install it using Homebrew:

• brew install ssh-copy-id

Alternatively, you can paste in the keys using SSH:

• cat ~/.ssh/id_rsa.pub | ssh demo@198.51.100.0 "mkdir -p ~/.ssh && chmod 700 ~/.ssh && cat >> ~/.ssh/authorized_keys"

No matter which command you chose, you may see something like:

The authenticity of host '198.51.100.0 (198.51.100.0)' can't be established.
RSA key fingerprint is b1:2d:33:67:ce:35:4d:5f:f3:a8:cd:c0:c4:48:86:12.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '198.51.100.0' (RSA) to the list of known hosts.
user@198.51.100.0's password: 

This message helps us to make sure that we haven’t added extra keys that you weren’t expecting.

Now you can go ahead and log into your user profile and you will not be prompted for a password. However, if you set a passphrase when creating your SSH key, you will be asked to enter the passphrase at that time (and whenever else you log in in the future).

 

Optional Step Four—Disable the Password for Root Login

Once you have copied your SSH keys onto your server and ensured that you can log in with the SSH keys alone, you can go ahead and restrict the root login to only be permitted via SSH keys.

In order to do this, open up the SSH config file:

• sudo nano /etc/ssh/sshd_config

Within that file, find the line that includes PermitRootLogin and modify it to ensure that users can only connect with their SSH key:

/etc/ssh/sshd_config

PermitRootLogin without-password 

Save and close the file when you are finished.

To put these changes into effect:

• sudo systemctl reload sshd.service

DigitalOcean Addendum

The DigitalOcean control panel allows you to add public keys to your new Droplets when they’re created. You can generate the SSH Key in a convenient location, such as the computer, and then upload the public key to the SSH key section.

Then, when you create a new Droplet, you can choose to include that public key on the server. No root password will be emailed to you and you can log in to your new server from your chosen client. If you created a passphrase, you will be prompted to enter that upon login.

By Etel Sverdlov

 

3. 禁止密碼登陸

參考鏈接，需要進行以下配置：

http://www.unixlore.net/articles/five-minutes-to-more-secure-ssh.html

A.這一句是禁止使用密碼登陸，只能夠使用第2步配置的key登陸。

PasswordAuthentication no

B.Challenge Password

在配合PAM的時候，Challenge Password會繞過PasswordAuthentication的配置。所以Challenge Password也需要配置為no。
ChallengeResponseAuthentication no

Challenge-response mechanism即問(密碼)-答(密碼)的機制，如果開着的話，攻擊者會通過此機制輸入text密碼。
注意如果B和C都是yes，那么A里面的配置是沒有用的。所以B和C中只要一個設置為NO就可以，我們在這里把B設為No。

C.UsePAM:PAM是Unix的登陸機制，平時最好開着

UsePAM yes

4. 重啟ssh,使配置生效

service ssh restart

 

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