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This article i based on a debian 9 vm

sshd basic

  • make sure ssh root is cert only
    • PermitRootLogin prohibit-password
  • AllowTcpForwarding no
  • X11Forwarding no
  • Only allow certain group to ssh
    • AllowGroups sshusers

harden SSH access

install fail2ban to make lock out brute force attacks

firewall to restrict traffic to and from bastion

iptables --flush

iptables --policy INPUT DROP
iptables --policy OUTPUT DROP

iptables --append OUTPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables --append INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

iptables --append INPUT -p tcp --dport 22 -j ACCEPT

iptables --append OUTPUT -d -p tcp --dport 22 -j DROP
iptables --append OUTPUT -p tcp --dport 22 -j ACCEPT
iptables --append OUTPUT -d -p udp --dport 53 -j ACCEPT
iptables --append OUTPUT -d -p tcp --dport 53 -j ACCEPT
iptables --append OUTPUT -d -p udp --dport 67:68 -j ACCEPT
iptables --append OUTPUT -p icmp -j ACCEPT

# depends on DNS so these must be after DNS rule
iptables --append OUTPUT -d -m comment --comment "" -j ACCEPT
iptables --append OUTPUT -d -m comment --comment "" -j ACCEPT

remember to persist with eg iptables-persistent

Harden OS


Make sure users can't see other users processes:

  • proc /proc proc defaults,hidepid=2 0 0

home dirs

make sure all homedirs are chmod 750

set DIR_MODE=0750 in /etc/adduser.conf


  • harden passwords
    • apt install libpam-pwquality
    • add password requisite try_first_pass minlen=12 retry=3 to /etc/pam.d/common-passwd
  • geoip - restrict ssh access based on geoip
  • limit su access
    • groupadd --system wheel
    • edit /etc/pam.d/su and uncomment the following line
    • #auth required

Consider adding pam_tally2 to auto-lock a user for a given period. Read more


Session Recording

Consider using