---
layout:      post
title:       "Linux SSH 服务的安装和使用"
subtitle:    ""
description: "本文简要的对 Linux 下 SSH 服务的安装、配置和使用进行说明。"
excerpt:     "用于指导 Linux 下 SSH 服务的配置。"
date:        2020-05-20 12:00:00
author:      "Rick Chan"
tags:        ["Applications", "SSH"]
categories:  ["Software"]
published:   true
---

## 安装 SSH Server

```bash
# Ubuntu
sudo apt install openssh-server
# Manjaro
sudo pacman -S openssh
# 生成 Host Key
ssh-keygen -A
```

## 开启 SSH 服务

当目标机上有 SSHD 的情况下，可以开启 SSHD 服务。编写 sshd_config 文件，放置到 /etc/ssh 下，内容如下：

```ini
# Package generated configuration file
# See the sshd_config(5) manpage for details

# What ports, IPs and protocols we listen for
Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::
#ListenAddress 0.0.0.0
Protocol 2
# HostKeys for protocol version 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_dsa_key
HostKey /etc/ssh/ssh_host_ecdsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
#Privilege Separation is turned on for security
UsePrivilegeSeparation yes

# Lifetime and size of ephemeral version 1 server key
KeyRegenerationInterval 3600
ServerKeyBits 1024

# Logging
SyslogFacility AUTH
LogLevel INFO

# Authentication:
LoginGraceTime 120
#PermitRootLogin prohibit-password
PermitRootLogin yes
StrictModes yes

RSAAuthentication yes
PubkeyAuthentication yes
#AuthorizedKeysFile %h/.ssh/authorized_keys

# Don't read the user's ~/.rhosts and ~/.shosts files
IgnoreRhosts yes
# For this to work you will also need host keys in /etc/ssh_known_hosts
RhostsRSAAuthentication no
# similar for protocol version 2
HostbasedAuthentication no
# Uncomment if you don't trust ~/.ssh/known_hosts for RhostsRSAAuthentication
#IgnoreUserKnownHosts yes

# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no

# Change to yes to enable challenge-response passwords (beware issues with
# some PAM modules and threads)
ChallengeResponseAuthentication no

# Change to no to disable tunnelled clear text passwords
#PasswordAuthentication yes

# Kerberos options
#KerberosAuthentication no
#KerberosGetAFSToken no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes

# GSSAPI options
#GSSAPIAuthentication no
#GSSAPICleanupCredentials yes

X11Forwarding yes
X11DisplayOffset 10
PrintMotd no
TCPKeepAlive yes
#UseLogin no

#MaxStartups 10:30:60
#Banner /etc/issue.net

# Allow client to pass locale environment variables
AcceptEnv LANG LC_*

#Subsystem sftp /usr/lib/openssh/sftp-server
Subsystem sftp internal-sftp

# Set this to 'yes' to enable PAM authentication, account processing,
# and session processing. If this is enabled, PAM authentication will
# be allowed through the ChallengeResponseAuthentication and
# PasswordAuthentication.  Depending on your PAM configuration,
# PAM authentication via ChallengeResponseAuthentication may bypass
# the setting of "PermitRootLogin without-password".
# If you just want the PAM account and session checks to run without
# PAM authentication, then enable this but set PasswordAuthentication
# and ChallengeResponseAuthentication to 'no'.
UsePAM yes
```

注意以上配置使用了内置 SFTP 服务（“Subsystem sftp internal-sftp”）。之后既可以使用 SCP 上传和下载文件，也可以使用 SFTP 上传下载文件。

之后在目标机上

```bash
# 开机自运行
sudo systemctl enable sshd.service
# 启动 sshd 服务
sudo systemctl start sshd.service
# 查看 sshd 服务状态
sudo systemctl status sshd.service
# 或（ubuntu12）
sudo /etc/init.d/ssh restart
```

如果没有 systemd，可以使用如下命令：

```bash
sudo service ssh start
```

## SSH 登陆

在开发机上

```bash
ssh <User Name>@<IP> -p <port>
```

不加 -p 参数，则默认使用 22 端口。

## SCP 上传和下载

SCP 上传

```bash
scp <Local Path or File> <User Name>@<IP>:<Path or File>
```

SCP 下载

```bash
scp <User Name>@<IP>:<Path or File> <Local Path or File>
```

SCP 一次传输多个文件

```bash
scp <User Name>@<IP>:<Path>/\{file1,file2\} .
# 还可以使用正则表达式，如：
scp <User Name>@<IP>:<Path>/* .
scp <User Name>@<IP>:<Path>/*.txt .
```

## 免密访问

ssh 登陆或 scp 的时候每次都输入密码很麻烦，如果用于访问 SSH 服务器的客户端值得信任的话可以授权该主机用户免密登陆。做法是先在客户端生成密钥对，如果已有密钥对可跳过本步骤：  

```bash
ssh-keygen
```

之后在客户端使用：  

```bash
ssh-copy-id <User Name>@<IP>
# 后续按照提示进行操作即可
```

命令将客户端的 ssh public key 提供给服务器，之后该客户端的授权账号（拥有该密钥对的账号）就可以免密访问服务器了。  

在服务器的 \<User Home\>/.ssh/authorized_keys 文件中可以看到已授权客户端公钥，删除公钥可取消对应客户端账户的免密访问权限。

## Client SSH Config 文件

Client SSH Config 一般为 ~/.ssh/config，可用于 Host 配置，包括设置授权方式，设置 Host 用户名和端口等。

### 增加 ssh-rsa 授权方式

当出现以下问题：

```bash
no matching host key type found. Their offer: ssh-rsa
```

可通过修改 ~/.ssh/config 来解决：

```bash
Host *
  HostkeyAlgorithms +ssh-rsa
  PubkeyAcceptedKeyTypes +ssh-rsa
```

### 设置 Host 用户名和端口

```bash
Host <addr>
  HostName <name>
  User <usr>
  Port <port>
```

### 关闭授权验证

SSH 客户端每连接一台新主机都要进行授权验证，并询问用户是否同一，并将主机信息记录到 known_hsots 文件里。有些时候需要使用自动化流程，此时无法处理授权验证过程，此时可修改 ~/.ssh/config 文件（不存在则手动创建），增加如下两行：

```bash
StrictHostKeyChecking no
UserKnownHostsFile /dev/null
```

*关闭授权验证将增加 SSH 的安全隐患。*