# Hardened SSHD bastion config -- modify wisely!
# Based on https://wiki.mozilla.org/Security/Guidelines/OpenSSH
# With additional restrictions where applicable

# -lo and -rt users only have local console login
DenyUsers *-rt
DenyUsers *-lo

# hardened params follow. every non-needed feature is disabled by default,
# following the principle of least rights and least features (more enabled
# features mean a more important attack surface).

# === FEATURES ===

# disable non-needed sshd features
AllowAgentForwarding no
AllowTcpForwarding no
X11Forwarding no
PermitTunnel no
PermitUserEnvironment no
GatewayPorts no

# === INFORMATION DISCLOSURE ===

# don't yell to the world that we're running debian,
# this disables the debian string version on the server hello message
DebianBanner no

# however, display a legal notice for each connection
Banner /etc/ssh/banner

# don't print the bastion MOTD on connection
PrintMotd no

# === CRYPTOGRAPHY ===

# enforce the use of ssh version 2 protocol, version 1 is disabled.
# all sshd_config options regarding protocol 1 are therefore omitted.
Protocol 2

# only use hostkeys with RSA
HostKey /etc/ssh/ssh_host_rsa_key

# list of allowed ciphers.
# aes is a trusted standard, only allow it's ctr mode (cbc is not considered secure)
# we deny arcfour(rc4), 3des, blowfish and cast
Ciphers aes256-ctr,aes192-ctr,aes128-ctr

# list of allowed message authentication code algorithms.
# we prefer umac (has been proven secure) then sha2.
# we deny md5 and sha1
MACs umac-64@openssh.com,hmac-sha2-512,hmac-sha2-256

# List of allowed key exchange algorithms.
# we allow diffie hellman with group exchange using sha256 which is
# the most secure dh-based kex.
# we avoid algorithms based on the disputed NIST curves, and anything based
# on sha1.
KexAlgorithms diffie-hellman-group-exchange-sha256

# === AUTHENTICATION ===

# we allow only public key authentication ...
PubkeyAuthentication yes
# ... not password
PasswordAuthentication no
# ... keyboard interactive (needed for MFA through PAM)
KbdInteractiveAuthentication yes
# ... not kerberos
KerberosAuthentication no
# ... challenge-response (needed for MFA through PAM)
ChallengeResponseAuthentication yes
# ... not host-based
HostbasedAuthentication no
# ... and not gssapi auth.
GSSAPIAuthentication no
GSSAPIKeyExchange no

# just in case, we also explicitly deny empty passwords
PermitEmptyPasswords no

# this needs to be set at "yes" to allow PAM keyboard-interactive authentication,
# which is not a security issue because the AuthenticationMethods below force the use of
# either publickey or publickey+keyboard-interactive, hence password-only login is never
# possible, for root or any other account for that matter
PermitRootLogin yes

# === LOGIN ===

# disconnect after 30 seconds if user didn't log in successfully
LoginGraceTime 30

# not more than 1 session per network connection (connection sharing with ssh client's master/shared mode)
MaxSessions 1

# maximum concurrent unauth connections to the sshd daemon
MaxStartups 50:30:500

# accept LANG and LC_* vars (also includes LC_BASTION)
AcceptEnv LANG LC_*

# === SYSTEM ===

# Use kernel sandbox mechanisms where possible in unprivilegied processes (seccomp)
UsePrivilegeSeparation sandbox

# sshd log level at verbose in auth facility for auditing purposes
LogLevel VERBOSE
SyslogFacility AUTH

# check sanity of user HOME dir before allowing user to login
StrictModes yes

# never use dns (slows down connections)
UseDNS no

# use PAM facility
UsePAM yes

# Debian 7 doesn't support the AuthenticationMethods, hence we can't benefit
# from the bastion-nopam group (accountModify --pam-auth-bypass will not work)
#Match Group bastion-nopam
#	AuthenticationMethods publickey
#Match All
#	AuthenticationMethods publickey,keyboard-interactive:pam