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By default, both SSH server and client rely on TCP-based keep alive messages to detect broken sessions, which can be spoofed rather easily in order to keep a broken session opened (and vice versa). Since we rely on SSH-based keep alive messages, which are not vulnerable to this kind of tampering, there is no need to double-check connections via TCP keep alive as well. This patch thereof disables using TCP keep alive for both SSH client and server scenario. For usability reasons, a timeout of 5 minutes (10 seconds * 30 keep alive messages = 300 seconds) will be used for both client and server configuration, as 60 seconds were found to be too short for unstable connectivity scenarios. Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
37 lines
1.5 KiB
Plaintext
37 lines
1.5 KiB
Plaintext
# OpenSSH client configuration file for IPFire
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#
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# The full documentation is available at: https://man.openbsd.org/ssh_config
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#
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# Set some basic hardening options for all connections
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Host *
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# Disable undocumented roaming feature as it is known to be vulnerable
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UseRoaming no
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# Only use secure crypto algorithms
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KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
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Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
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MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
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# Always visualise server host keys (helps to identify key based MITM attacks)
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VisualHostKey yes
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# Use SSHFP (might work on some up-to-date networks) to look up host keys
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VerifyHostKeyDNS yes
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# Send SSH-based keep alive messages to connected server to avoid broken connections
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ServerAliveInterval 10
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ServerAliveCountMax 30
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# Disable TCP keep alive messages since they can be spoofed and we have SSH-based
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# keep alive messages enabled; there is no need to do things twice here
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TCPKeepAlive no
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# Ensure only allowed authentication methods are used
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PreferredAuthentications publickey,keyboard-interactive,password
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# Prevent information leak by hashing ~/.ssh/known_hosts
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HashKnownHosts yes
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# EOF
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