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112 lines
4.8 KiB
Plaintext
112 lines
4.8 KiB
Plaintext
# Hardened SSH bastion config -- modify wisely!
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# Based on https://wiki.mozilla.org/Security/Guidelines/OpenSSH
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# With modifications where applicable/needed
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# hardened params follow. every non-needed feature is disabled by default,
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# following the principle of least rights and least features (more enabled
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# features mean a more important attack surface).
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# === FEATURES ===
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# disable non-needed sshd features
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Tunnel no
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ForwardAgent no
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ForwardX11 no
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GatewayPorts no
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ControlMaster no
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# === CRYPTOGRAPHY ===
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# enforce the use of ssh version 2 protocol, version 1 is disabled.
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# all sshd_config options regarding protocol 1 are therefore omitted.
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Protocol 2
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# list of allowed ciphers.
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# chacha20-poly1305 is a modern cipher, considered very secure
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# aes is still the standard, we prefer gcm cipher mode, but also
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# allow ctr cipher mode for compatibility (ctr is considered secure)
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# we deny arcfour(rc4), 3des, blowfish and cast
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# for older remote servers (or esoteric hardware), we might need to add: aes256-cbc,aes192-cbc,aes128-cbc
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# known gotchas:
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# - BSD (https://lists.freebsd.org/pipermail/freebsd-bugs/2013-June/053005.html) needs aes256-gcm@openssh.com,aes128-gcm@openssh.com DISABLED
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# - Old Cisco IOS (such as v12.2) only supports aes128-cbc,3des-cbc,aes192-cbc,aes256-cbc
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# - Ancient Debians (Sarge) and RedHats (7) only support aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,aes192-cbc,aes256-cbc,rijndael-cbc@lysator.liu.se,aes128-ctr,aes192-ctr,aes256-ctr
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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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# list of allowed message authentication code algorithms.
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# etm (encrypt-then-mac) are considered the more secure, we
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# prefer umac (has been proven secure) then sha2.
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# for older remote servers, fallback to the non-etm version of
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# the algorithms. we deny md5 entirely.
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# for older remote servers (or esoteric hardware), we might need to add: hmac-sha1
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# Known gotchas:
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# - Old Cisco IOS (such as v12.2) only supports hmac-sha1,hmac-sha1-96,hmac-md5,hmac-md5-96
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# - Ancient Debians (Sarge) and RedHats (7) only support hmac-md5,hmac-sha1,hmac-ripemd160,hmac-ripemd160@openssh.com,hmac-sha1-96,hmac-md5-96
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MACs umac-128-etm@openssh.com,umac-64-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128@openssh.com,umac-64@openssh.com,hmac-sha2-512,hmac-sha2-256
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# List of allowed key exchange algorithms.
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# we prefer curve25519-sha256 which is considered the most modern/secure,
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# and still allow diffie hellman with group exchange using sha256 which is
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# the most secure dh-based kex.
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# we avoid algorithms based on the disputed NIST curves, and anything based
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# on sha1.
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# known gotchas:
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# - Windows needs diffie-hellman-group14-sha1 and also needs to NOT have diffie-hellman-group-exchange-sha1 present in the list AT ALL
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# - OmniOS 5.11 needs diffie-hellman-group1-sha1
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# - Old Cisco IOS (such as v12.2) only supports diffie-hellman-group1-sha1
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# - Ancient Debians (Sarge) and RedHats (7) only support diffie-hellman-group-exchange-sha1,diffie-hellman-group1-sha1
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KexAlgorithms curve25519-sha256@libssh.org,diffie-hellman-group-exchange-sha256
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# === AUTHENTICATION ===
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# we allow only public key authentication ...
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PubkeyAuthentication yes
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# ... not password nor keyboard-interactive
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# ... (set to yes if sshpass is to be used)
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PasswordAuthentication no
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# ChallengeResponseAuthentication=yes forces KbdInteractiveAuthentication=yes in the openssh code!
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ChallengeResponseAuthentication yes
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KbdInteractiveAuthentication yes
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# ... not host-based
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HostbasedAuthentication no
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# now we specify the auth methods order we want for manual ssh calls.
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# NOTE1: as per the ssh source code, an auth method omitted hereafter
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# will not be used, even if set to "yes" above.
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# NOTE2: the bastion code (namely, ttyrec), will always set the proper
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# value explicitly on command-line (pubkey OR sshpass), so the value
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# specified hereafter will be ignored. if you want to force-disable
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# a method, set it to "no" in the list above, as those will never be
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# overridden by the code.
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PreferredAuthentications publickey,keyboard-interactive
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# === LOGIN ###
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# disable escape character use
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EscapeChar none
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# detect if a hostkey changed due to DNS spoofing
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CheckHostIP yes
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# ignore ssh-agent, only use specified keys (-i)
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IdentitiesOnly yes
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# disable auto-lookup of ~/.ssh/id_rsa ~/.ssh/id_ecdsa etc.
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IdentityFile /dev/non/existent/file
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# carry those vars to the other side (includes LC_BASTION)
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SendEnv LANG LC_*
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# allow usage of SSHFP DNS records
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VerifyHostKeyDNS ask
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# yell if remote hostkey changed
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StrictHostKeyChecking ask
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# === SYSTEM ===
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# don't hash the users known_hosts files, in the context of a bastion, this adds no security
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HashKnownHosts no
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# send an ssh ping each 57 seconds to the client and disconnect after 5 no-replies
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ServerAliveInterval 57
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ServerAliveCountMax 5
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