Re: [PATCH 1/2 v3] add hardened SSH server configuration

[Michael Tremer <michael.tremer@ipfire.org>]

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On Sun, 2018-09-09 at 18:47 +0200, Peter Müller wrote:
> In order to harden OpenSSH server in IPFire, using the upstream default
> configuration
> and edit it via sed commands in LFS file is error-prone and does not scale.
> 
> Thereof we ship a custom and more secure OpenSSH server configuration which
> is copied into the image during build time.
> 
> The third version of this patch corrects default value where no root login was
> possible (default is "prohibit-password") on default installations. It also
> enables password-based authentication by default so rewriting this during
> startup becomes obsolete.
> 
> Fixes #11750
> Fixes #11751
> Partially fixes #11538
> 
> Signed-off-by: Peter Müller <peter.mueller(a)link38.eu>
> Cc: Marcel Lorenz <marcel.lorenz(a)ipfire.org>
> Cc: Michael Tremer <michael.tremer(a)ipfire.org>
> ---
>  config/ssh/sshd_config | 82
> ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 82 insertions(+)
>  create mode 100644 config/ssh/sshd_config
> 
> diff --git a/config/ssh/sshd_config b/config/ssh/sshd_config
> new file mode 100644
> index 000000000..8b8e042c4
> --- /dev/null
> +++ b/config/ssh/sshd_config
> @@ -0,0 +1,82 @@
> +# ultra-secure OpenSSH server configuration
> +
> +# only allow version 2 of SSH protocol
> +Protocol 2
> +
> +# listen on port 22 by default
> +Port 22
> +
> +# listen on these interfaces and protocols
> +AddressFamily any
> +ListenAddress 0.0.0.0
> +ListenAddress ::
> +
> +# limit authentication thresholds
> +LoginGraceTime 30s
> +MaxAuthTries 3
> +
> +# limit maximum instanctes to prevent DoS
> +MaxStartups 5
> +
> +# ensure proper logging
> +SyslogFacility AUTH
> +LogLevel INFO
> +
> +# enforce permission checks before a login is accepted
> +# (prevents damage because of hacked systems with world-writeable
> +# home directories or similar)
> +StrictModes yes
> +
> +# only allow safe crypto algorithms (may break some _very_ outdated clients)
> +# see also: https://stribika.github.io/2015/01/04/secure-secure-shell.html
> +KexAlgorithms curve25519-sha256(a)libssh.org,diffie-hellman-group-exchange-
> sha256
> +Ciphers chacha20-poly1305(a)openssh.com,aes256-gcm(a)openssh.com,
> aes128-gcm(a)openssh.com
> +MACs hmac-sha2-512-etm(a)openssh.com,hmac-sha2-256-etm(a)openssh.com,
> umac-128-etm(a)openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128(a)openssh.com
> +
> +# enable data compression after successful login only
> +Compression delayed
> +
> +# only allow cryptographically safe SSH host keys (adjust paths if needed)
> +HostKey /etc/ssh/ssh_host_ed25519_key
> +HostKey /etc/ssh/ssh_host_ecdsa_key
> +HostKey /etc/ssh/ssh_host_rsa_key
> +
> +# only allow login via public key by default
> +PubkeyAuthentication yes
> +PasswordAuthentication yes

We cannot enable PasswordAuthentication in the image by default.

> +ChallengeResponseAuthentication no
> +PermitEmptyPasswords no
> +
> +# permit root login as there is no other user in IPFire 2.x
> +PermitRootLogin yes
> +
> +# specify preferred authentication methods (public keys come first)
> +AuthenticationMethods publickey,password
> +
> +# ignore user ~/.rhost* files
> +IgnoreRhosts yes
> +
> +# ignore user known hosts file
> +IgnoreUserKnownHosts yes
> +
> +# ignore user environments
> +PermitUserEnvironment no
> +
> +# do not allow any kind of forwarding (provides only low security)
> +# some of them might need to be re-enabled if SSH server is a jump platform
> +X11Forwarding no
> +AllowTcpForwarding no
> +AllowAgentForwarding no
> +PermitTunnel no
> +GatewayPorts no
> +PermitOpen none
> +
> +# detect broken sessions by sending keep-alive messages to
> +# clients (both via TCP and SSH)
> +TCPKeepAlive yes
> +ClientAliveInterval 10
> +
> +# close unresponsive SSH sessions which fail to answer keep-alive
> +ClientAliveCountMax 6
> +
> +# EOF