使用endlessh来保护你的ssh服务

endlessh是一个简单的的 C语言 程序，整个源代码只有一个C文件.

git clone https://github.com/skeeto/endlessh
cd endlessh
make

这会生成一个可执行的 endlessh 程序. 我们可以把它移动到 /sbin 目录下

sudo mv endlessh /sbin

通过 -h 选项可以输出 endlessh 的使用方法

[root@VM_0_8_centos endlessh]# endlessh -h
Usage: endlessh [-vh] [-d MS] [-f CONFIG] [-l LEN] [-m LIMIT] [-p PORT]
  -d INT    Message millisecond delay [10000]
  -f        Set and load config file [/etc/endlessh/config]
  -h        Print this help message and exit
  -l INT    Maximum banner line length (3-255) [32]
  -m INT    Maximum number of clients [4096]
  -p INT    Listening port [2222]
  -v        Print diagnostics to standard output (repeatable)

配置文件

# The port on which to listen for new SSH connections.
Port 2222

# The endless banner is sent one line at a time. This is the delay
# in milliseconds between individual lines.
Delay 10000

# The length of each line is randomized. This controls the maximum
# length of each line. Shorter lines may keep clients on for longer if
# they give up after a certain number of bytes.
MaxLineLength 32

# Maximum number of connections to accept at a time. Connections beyond
# this are not immediately rejected, but will wait in the queue.
MaxClients 4096

# Set the detail level for the log.
#   0 = Quiet
#   1 = Standard, useful log messages
#   2 = Very noisy debugging information
LogLevel 0

kill -s SIGHUP $(pidof endlessh)

SSH登陆分三个阶段:

1. 协议协商阶段
2. 密钥和算法协商阶段

3. 认证阶段

   

而在 协议协商阶段 中，客户端向服务端建立链接后，服务端应该向客户端发送一个报文，该在报文中包括格式为“协议版本号 次协议版本号 软件版本号”的版本标识字符串。

而 endlessh 往客户端发送的是一段不断随机生成的字符串,相当于这个报文没有穷尽，也根本无法从这个字符串中解析出有意义的版本标识.

从而使ssh客户端卡在这一步，无法进行到下一步。