解决 ssh 找不到对应主机密钥类型

解决办法

如果最近升级到了 openssh 8.8 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 

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，增加以下配置项:

HostKeyAlgorithms +ssh-rsa PubkeyAcceptedKeyTypes +ssh-rsa 

完整的配置如下：

Host jump     Port 2222     HostName x.x.x.x     User ***     IdentityFile ~/.ssh/id_rsa     UseKeychain yes     AddKeysToAgent yes     PreferredAuthentications publickey     HostKeyAlgorithms +ssh-rsa     PubkeyAcceptedKeyTypes +ssh-rsa 

问题原因

根据 OpenSSH 8.8 Release Notes 信息：

This release disables RSA signatures using the SHA-1 hash algorithm by default. This change has been made as the SHA-1 hash algorithm is cryptographically broken, and it is possible to create chosen-prefix hash collisions for <USD$50K [1]  For most users, this change should be invisible and there is no need to replace ssh-rsa keys. OpenSSH has supported RFC8332 RSA/SHA-256/512 signatures since release 7.2 and existing ssh-rsa keys will automatically use the stronger algorithm where possible.  Incompatibility is more likely when connecting to older SSH implementations that have not been upgraded or have not closely tracked improvements in the SSH protocol. For these cases, it may be necessary to selectively re-enable RSA/SHA1 to allow connection and/or user authentication via the HostkeyAlgorithms and PubkeyAcceptedAlgorithms options. For example, the following stanza in ~/.ssh/config will enable RSA/SHA1 for host and user authentication for a single destination host:      Host old-host         HostkeyAlgorithms +ssh-rsa         PubkeyAcceptedAlgorithms +ssh-rsa  We recommend enabling RSA/SHA1 only as a stopgap measure until legacy implementations can be upgraded or reconfigured with another key type (such as ECDSA or Ed25519). 

从这里可以知道，从 openssh 8.8 版本开始默认禁用了 ssh-rsa 算法，对于大部分情况，这次的更改是无感知的，当服务器仅支持 ssh-rsa 算法时才会出现不兼容的情况。

本地启用 RSA/SHA1 支持仅仅只是一种权宜之计，官方建议是升级密钥算法或使用另一种密钥算法（例如 ECDSA 或 Ed25519 算法）。