解决 openssh-8.8-p1 升级: no matching host key type found. Their offer: ssh-rsa

来源 https://blogs.qingtianxueri.com/jie-jue-openssh-8-8-p1-sheng-ji-no-matching-host-key-type-found-their-offer-ssh-rsa/

配置文件: /etc/ssh/sshd_config 

# This file is automatically generated at startup
KexAlgorithms [email protected],diffie-hellman-group-exchange-sha256
Port 22
Protocol 2
HostKey /etc/ssh/ssh_host_rsa_key
HostKey /etc/ssh/ssh_host_ed25519_key
Compression delayed
ClientAliveInterval 30
PermitRootLogin yes
# Login via Key or Password
ChallengeResponseAuthentication yes
PasswordAuthentication yes
PubkeyAuthentication yes
UseDNS no
LoginGraceTime 30s
VersionAddendum none
AllowAgentForwarding no
X11Forwarding no
Ciphers [email protected],[email protected],[email protected],aes256-ctr,aes192-ctr,aes128-ctr
MACs [email protected],[email protected],[email protected],[email protected],hmac-sha2-512,hmac-sha2-256,hmac-ripemd160,[email protected]
# override default of no subsystems
Subsystem       sftp    /usr/libexec/sftp-server

如果你最近升级到了 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

解决办法

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

当然, 每次连接敲这么一长串也不太好.

编辑用户 ssh 配置 ~/.ssh/config, 对于无法成功连接的host, 增加配置项:

HostKeyAlgorithms         +ssh-rsa
PubkeyAcceptedKeyTypes    +ssh-rsa

完整的配置可能看起来像这样:

Host myhost
Hostname 	1.1.1.1
User user001
IdentityFile     ~/.ssh/id_rsa
# fixup for openssh 8.8
HostKeyAlgorithms +ssh-rsa
PubkeyAcceptedKeyTypes +ssh-rsa

或者，像我一样的懒人:

Host *
     ServerAliveInterval 10
     HostKeyAlgorithms +ssh-rsa
     PubkeyAcceptedKeyTypes +ssh-rsa

为什么会有这个错误

根据 OpenSSH Release Notes

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.

Refs

http://www.openssh.com/releasenotes.html

https://www.openssh.com/legacy.html

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