嵌入式Linux下Dropbear SSH配置优化

1). 简介

嵌入式 Linux  由于运行平台通常资源受限同时对稳定性要求高，因此需要比较精简，那么针对 SSH 服务器/客户端应用，通常也不使用庞大的 OpenSSH，而是采用十分精简的 Dropbear SSH工具。Dropbear 是一个基于 MIT License 的开源软件，其一些基本信息可以参考如下软件发布页面：

https://matt.ucc.asn.au/dropbear/dropbear.html

本文所演示的平台来自于Toradex Apalis iMX8 嵌入式平台，基于 NXP iMX8 系列 ARM 处理器，核心为 Cortex-A52/A53 。

2). 硬件准备

a).  Apalis iMX8 ARM 核心版配合 Apalis Eva Board 载板，并连接调试串口和网口以便测试。

3). 具体配置说明

a). Apalis iMX8 模块标准 Ycoto Linux BSP 中已经包含 Dropbear 相关软件，不过由于默认配置为了开发测试方便，默认使能了 debug-tweaks 功能（比如这样可以允许 root 账户无密码登录），这样如下面 Ycoto Project/Openembedded 相关文件说明也就同时也使能了 weak ciphers

./ layers/meta-toradex-demos/recipes-core/dropbear/dropbear_%.bbappend

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# THE Eclipse RSE system explorer uses a ssh client which cannot cope with the

# dropbear ssh server if weak ciphers are disabled.

# If debug-tweaks is set in IMAGE_FEATURES then enable also weak ciphers.

# With debug-tweaks we allow password less root access, enforcing strong

# ciphers is pointless anyway.

PACKAGECONFIG = "${@bb.utils.contains("IMAGE_FEATURES", "debug-tweaks", "", "disable-weak-ciphers",d)}"

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b). 为了使 Dropbear SSH安全性更高，可以在 Ycoto 编译环境下参考如下 patch 文件修改关闭 debug-tweaks 和 weak ciphers。因为同时这样也关闭了 root 用户无密码登录，因此也需要给 root 用户配置默认密码。

./ local.conf 文件修改 patch

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--- a/build/conf/local.conf 2023-05-30 12:16:33.780891419 +0800

+++ b/build/conf/local.conf 2023-05-31 10:55:36.841801362 +0800

@@ -277,3 +277,9 @@

 include conf/machine/include/${MACHINE}.inc

 # DO NOT SET THE MACHINE AFTER THE ABOVE INCLUDE

+# accept the Freescale EULA

+ACCEPT_FSL_EULA = "1"

+# add root password

+EXTRA_IMAGE_FEATURES = "allow-root-login package-management"

+INHERIT += "extrausers"

+EXTRA_USERS_PARAMS = "usermod -P Abcd1234 root"

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./ 参考这里的说明将上述修改下重新编译生成的 Ycoto Linux Image 通过 Toradex Easy Installer 更新到 Apalis iMX8 模块，此时测试无论本地串口登录还是远程SSH登录 root 用户都需要输入预设的密码了，增强了安全性。

c). 为了进一步提高 SSH 安全性，可以创建普通 user 用户用于远程登录，而禁止 root 用户 SSH 远程登录。这样也可以通过限制 user 用户的权限来提高系统安全性。

./ 创建新的 user 用户

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root@apalis-imx8-07308034:~# useradd testuser

root@apalis-imx8-07308034:~# passwd testuser

New password:

Retype new password:

passwd: password updated successfully

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./ 禁止 root 用户 SSH 登录，参考如下 patch 修改 /etc/default/dropbear 文件

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--- a/etc/default/dropbear

+++ b/etc/default/dropbear

@@ -1,2 +1,2 @@

 # Disallow root logins by default

-DROPBEAR_EXTRA_ARGS=""

+DROPBEAR_EXTRA_ARGS=" -w"

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./ 测试使用 testuser 用户远程 SSH 登录成功， root 用户登录失效

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### root login ###

$ ssh root@10.20.1.168

root@10.20.1.168's password:

Permission denied, please try again.

### testuser login ###

$ ssh testuser@10.20.1.168

testuser@10.20.1.168's password:

mkdir: cannot create directory '/run/user/1000': Permission denied

chmod: cannot access '/run/user/1000': No such file or directory

apalis-imx8-07308034:~$

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./ 另外，如果需要本地串口 testuser 或者 root 用户自动登录，可以参考如下 patch 修改

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--- a/lib/systemd/system/serial-getty@.service

+++ b/lib/systemd/system/serial-getty@.service

@@ -30,7 +30,7 @@

 [Service]

 Environment="TERM=xterm"

-ExecStart=-/sbin/agetty -8 -L %I 115200 $TERM

+ExecStart=-/sbin/agetty -8 -a testuser -L %I 115200 $TERM

 Type=idle

 Restart=always

 UtmpIdentifier=%I

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d). 远程 SSH 除了默认的密码登录方式外，还可以开启安全等级更高的通过 public key 来无密码登录

./ 在需要远程登录 Apalis iMX8 设备的 PC 主机环境下通过 ssh-keygen 工具生成 SSH private key/public key pair

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### generate 4096-bits key pair ###

$ ssh-keygen -b 4096

Generating public/private rsa key pair.

Enter file in which to save the key (/home/simon/.ssh/id_rsa): /home/simon/local/tmp/ssh-test/id_rsa

Enter passphrase (empty for no passphrase):

Enter same passphrase again:

Your identification has been saved in /home/simon/local/tmp/ssh-test/id_rsa.

Your public key has been saved in /home/simon/local/tmp/ssh-test/id_rsa.pub.

The key fingerprint is:

SHA256:Pr5PQjzRuPMVS3Rrkdtq+7pDVOFMGumBLpFGkjGSEs0 simon@simon-Latitude-5300

The key's randomart image is:

+---[RSA 4096]----+

|   .+..++.. o.++.|

|   . E..o* o +Bo.|

|    .   + + +.+* |

|       . + o =o .|

|        S . o. . |

|       o + .  +  |

|        + o  o . |

|       . +    o  |

|        oo.   o=.|

+----[SHA256]-----+

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./ 通过 SSH 远程命令将生成的 public key 写入到 Apalis iMX8 dropbear authorized_keys 文件

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### create ssh folder on apalis iMX8 device ###

apalis-imx8-07308034:~$ mkdir /home/testuser/.ssh

### add public key to apalis iMX8 authorized_keys file from Host PC remotely ###

$ ssh testuser@10.20.1.168 "tee -a /home/testuser/.ssh/authorized_keys" < /home/simon/local/tmp/ssh-test/id_rsa.pub

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./ 参考如下 patch 修改 Apalis iMX8 dropbear 启动配置来使 public key 验证生效

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--- a/lib/systemd/system/dropbear@.service

+++ b/lib/systemd/system/dropbear@.service

@@ -4,9 +4,9 @@

 After=syslog.target dropbearkey.service

 [Service]

-Environment="DROPBEAR_RSAKEY_DIR=/etc/dropbear"

+Environment="DROPBEAR_RSAKEY_DIR=/home/testuser/.ssh/"

 EnvironmentFile=-/etc/default/dropbear

-ExecStart=-/usr/sbin/dropbear -i -r ${DROPBEAR_RSAKEY_DIR}/dropbear_rsa_host_key $DROPBEAR

_EXTRA_ARGS

+ExecStart=-/usr/sbin/dropbear -i $DROPBEAR_EXTRA_ARGS

 ExecReload=/bin/kill -HUP $MAINPID

 StandardInput=socket

 KillMode=process

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./ 重启 Apalis iMX8 使配置生效后，再次尝试远程 SSH 登录，可以实现无需密码而是采用 public key 验证登录

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$ ssh -i /home/simon/local/tmp/ssh-test/id_rsa testuser@10.20.1.168

mkdir: cannot create directory '/run/user/1000': Permission denied

chmod: cannot access '/run/user/1000': No such file or directory

apalis-imx8-07308034:~$

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e). 更多关于 dropbear 工具命令参数说明可以参考如下

https://manpages.ubuntu.com/manpages/bionic/man8/dropbear.8.html

4). 总结

本文基于嵌入式 Linux 简单演示了 轻量化 SSH 工具软件 Dropbear 的增强安全性配置供参考。