add hardened SSH server configuration

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 fourth version of this patch disables password authentication by
default, since this is required by some cloud hosters in order to apply
the image. Further, this method is less secure than pubkey
authentication.

Non-AEAD ciphers have been re-added to provide compatibility to older
RHEL systems.

Fixes #11750
Fixes #11751
Partially fixes #11538

Signed-off-by: Peter Müller <peter.mueller@link38.eu>
Cc: Marcel Lorenz <marcel.lorenz@ipfire.org>
Cc: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

config/ssh/sshd_config

@@ -0,0 +1,81 @@
+# 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
+
+# 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+# only allow safe crypto algorithms (may break some _very_ outdated clients)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@libssh.org,diffie-hellman-group-exchange-sha256
+Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
+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
+
+# 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 no
+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