sshの.ssh/configでホスト名と鍵を指定してる際、違う鍵を使って接続するときの注意

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先日、ssh接続する際に.ssh/configで指定していた鍵ではない秘密鍵を使って接続しようとしてたところ、公開鍵を登録していない秘密鍵を指定しても接続されてしまうという事があった。
なんだこりゃと思って-vオプションを付与して見たところ、以下のような動きをしていた。

blacknon@BS-PUB-DEVELOP:~$ cat .ssh/config
Host 192.168.0.119
HostName 192.168.0.119
User root
Port 22
IdentityFile /home/blacknon/.ssh/id_rsa
blacknon@BS-PUB-DEVELOP:~$ 
blacknon@BS-PUB-DEVELOP:~$ ssh -v -i testkey root@192.168.0.119
OpenSSH_7.2p2 Ubuntu-4ubuntu2.1, OpenSSL 1.0.2g  1 Mar 2016
debug1: Reading configuration data /home/blacknon/.ssh/config
debug1: /home/blacknon/.ssh/config line 1: Applying options for 192.168.0.119
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: /etc/ssh/ssh_config line 19: Applying options for *
debug1: Connecting to 192.168.0.119 [192.168.0.119] port 22.
debug1: Connection established.
debug1: identity file testkey type 1
debug1: key_load_public: No such file or directory
debug1: identity file testkey-cert type -1
debug1: identity file /home/blacknon/.ssh/id_rsa type 1
debug1: key_load_public: No such file or directory
debug1: identity file /home/blacknon/.ssh/id_rsa-cert type -1
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_7.2p2 Ubuntu-4ubuntu2.1
debug1: Remote protocol version 2.0, remote software version OpenSSH_6.6.1
debug1: match: OpenSSH_6.6.1 pat OpenSSH_6.6.1* compat 0x04000000
debug1: Authenticating to 192.168.0.119:22 as 'root'
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: algorithm: curve25519-sha256@libssh.org
debug1: kex: host key algorithm: ecdsa-sha2-nistp256
debug1: kex: server->client cipher: chacha20-poly1305@openssh.com MAC:  compression: none
debug1: kex: client->server cipher: chacha20-poly1305@openssh.com MAC:  compression: none
debug1: expecting SSH2_MSG_KEX_ECDH_REPLY
debug1: Server host key: ecdsa-sha2-nistp256 SHA256:3pbP7iBBSKrGlSX/dES1WdvfOsRX7Zjyb9G8tXeeHi4
debug1: Host '192.168.0.119' is known and matches the ECDSA host key.
debug1: Found key in /home/blacknon/.ssh/known_hosts:2
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: rekey after 134217728 blocks
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Next authentication method: gssapi-keyex
debug1: No valid Key exchange context
debug1: Next authentication method: gssapi-with-mic
debug1: Unspecified GSS failure.  Minor code may provide more information
No Kerberos credentials available
debug1: Unspecified GSS failure.  Minor code may provide more information
No Kerberos credentials available
debug1: Unspecified GSS failure.  Minor code may provide more information
debug1: Unspecified GSS failure.  Minor code may provide more information
No Kerberos credentials available
debug1: Next authentication method: publickey
debug1: Offering RSA public key: testkey
debug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-mic,password
debug1: Offering RSA public key: /home/blacknon/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 279
debug1: Authentication succeeded (publickey).
Authenticated to 192.168.0.119 ([192.168.0.119]:22).
debug1: channel 0: new [client-session]
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: pledge: network
debug1: Sending environment.
debug1: Sending env LANG = ja_JP.UTF-8
Last login: Wed Jun 21 07:57:32 2017 from 192.168.0.171
[root@BS-PUB-CENT7-02 ~]#

どうやら、オプションで指定した秘密鍵で接続できなかった場合、そのまま.ssh/configに記載されている秘密鍵で接続されてしまうようだ。
(オプションで指定した鍵で接続できる場合は、そのままその鍵で接続される)

新しい鍵を登録したりする際のテスト時などにちょっと紛らわしい…。
違う鍵を使って接続する場合は、対象のホストを一時的にコメントアウトするなり、.ssh/configを一時的にリネームするといった対応をすると良さそう。

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