ssh_configでの落とし穴 ref: https://qiita.com/tsuyoshi_cho/items/90038d14da3a71a84aba

file0.txt

# Github
Host github.com
  # Alt/Use for Over HTTPS at 443
  # Hostname ssh.github.com
  # Port 443
  Hostname github.com

# gist(use ssh)
Host gist.github.com
  # Over HTTPS
  # Hostname ssh.github.com
  # Port 443
  Hostname gist.github.com

# altssh server direct setting
Host ssh.github.com
  Hostname ssh.github.com
  Port 443

# 2nd or later account
# Example
Host example.github.com
  Hostname github.com
  # key define : first match check
  IdentityFile ~/.ssh/id_rsa.github.example

# github common setting
Match originalhost github.com,gist.github.com,ssh.github.com
  IdentityFile ~/.ssh/id_ecdsa.github
  IdentityFile ~/.ssh/id_ed25519.github
  # fallback
  IdentityFile ~/.ssh/id_rsa.github

# github global setting
Host *github.com
  User git
  TCPKeepAlive yes
  IdentitiesOnly yes
  Compression yes
  # Note:key
  # https://api.github.com/users/tsuyoshicho/keys

file1.txt

$ ssh -G [email protected]
user git
hostname github.com
 <中略>
identityfile ~/.ssh/id_rsa.github.example
identityfile ~/.ssh/id_ecdsa.github
identityfile ~/.ssh/id_ed25519.github
identityfile ~/.ssh/id_rsa.github
 <後略>

file2.txt

# operation server
Host op.example.com
  User tsuyoshicho
  TCPKeepAlive yes
  IdentitiesOnly no
  PubkeyAuthentication no
  ProxyCommand none

# server instance
Host dev1.example.com
  Hostname [2001:db8::1]
  User tsuyoshicho
  TCPKeepAlive yes
  # key define : first match check
  IdentityFile ~/.ssh/id_rsa.example
  IdentitiesOnly yes
  ProxyCommand none