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解决SSH no matching host key type found 问题

· 3 min read
Alan

原文 Solution to openssh-8.8-p1 update: no matching host key type found. Their offer: ssh-rsa

解决办法​

如果最近升级到了 openssh 8.8-p1 版, 你会发现连接某些之前连接得好好的服务器突然无法连接: 

Unable to negotiate with x.x.x.x port 2222: no matching host key type found. Their offer: ssh-rsa

或者仓库无法clone:

> git clone git@github.com:user/repository.git

Cloning into 'xxxx'...
Unable to negotiate with x.x.x.x port 22: no matching host key type found. Their offer: ssh-rsa,ssh-dss
fatal: Could not read from remote repository.

Please make sure you have the correct access rights

解决办法是ssh命令指定算法:

ssh -o HostKeyAlgorithms=+ssh-rsa -o PubkeyAcceptedKeyTypes=+ssh-rsa  user@host -p 2222

上面比较麻烦, 可以修改ssh配置文件 ~/.ssh/config, 对于无法成功连接的host, 增加配置项:

~/.ssh/config
HostKeyAlgorithms +ssh-rsa
PubkeyAcceptedKeyTypes +ssh-rsa

完整配置如下:

~/.ssh/config
Host ssh.alanwei.com
  HostName ssh.alanwei.com
  User alan
  Port 22
  HostKeyAlgorithms +ssh-rsa
  PubkeyAcceptedKeyTypes +ssh-rsa
info

完整配置里的 HostName, User, Port 可以省略

或者对所有Host指定:

~/.ssh/config
Host *
  ServerAliveInterval 10
  HostKeyAlgorithms +ssh-rsa
  PubkeyAcceptedKeyTypes +ssh-rsa

为什么会有这个错误​

根据 OpenSSH Release Notes:

info

Future deprecation notice It is now possible[1] to perform chosen-prefix attacks against the SHA-1 algorithm for less than USD$50K.

In the SSH protocol, the “ssh-rsa” signature scheme uses the SHA-1 hash algorithm in conjunction with the RSA public key algorithm. OpenSSH will disable this signature scheme by default in the near future.

Note that the deactivation of “ssh-rsa” signatures does not necessarily require cessation of use for RSA keys. In the SSH protocol, keys may be capable of signing using multiple algorithms. In particular, “ssh-rsa” keys are capable of signing using “rsa-sha2-256” (RSA/SHA256), “rsa-sha2-512” (RSA/SHA512) and “ssh-rsa” (RSA/SHA1). Only the last of these is being turned off by default.

也就是说 8.8p1 版的 openssh 的 ssh 客户端默认禁用了 ssh-rsa 算法, 但是对方服务器只支持 ssh-rsa, 当你不能自己升级远程服务器的 openssh 版本或修改配置让它使用更安全的算法时, 在本地 ssh 针对这些旧的ssh server重新启用 ssh-rsa 也是一种权宜之法.