Sshbastion

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 outgoing traffic from bastion

#!/bin/bash
iptables --flush OUTPUT
iptables --append OUTPUT -d security.debian.org -j ACCEPT
iptables --append OUTPUT -d ftp.dk.debian.org -j ACCEPT
iptables --append OUTPUT -d 192.168.12.1 -p tcp --dport 22 -j DROP
iptables --append OUTPUT -m state --state NEW -p tcp --dport 22 -j ACCEPT
iptables --append OUTPUT -m state -d 192.168.12.1 -p udp --dport 53 -j ACCEPT
iptables --append OUTPUT -m state -d 192.168.12.1 -p tcp --dport 53 -j ACCEPT
iptables --append OUTPUT -m -d 192.168.12.1 -p udp --dport 67:68 -j ACCEPT
iptables --append OUTOUT -p icmp -j ACCEPT
iptables --append OUTPUT -m state --state NEW -j REJECT

remember to persist with eg iptables-persistent

Harden OS

proc

Make sure users can't see other users processes:

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

PAM

• harden passwords
  • apt install libpam-pwquality
  • add password requisite pam_pwquality.so minlen=12 retry=3 to /etc/pam.d/common-passwd
• geoip - restrict ssh access based on geoip
  • apt install geoip-database-extra libpam-geoip
  • see https://github.com/vetinari/pam_geoip/blob/master/geoip.conf and modify /etc/security/geoip.conf
  • add account required pam_geoip.so geoip_db=/usr/share/GeoIP/GeoIPCity.dat to /etc/pam.d/sshd
• limit su access
  • groupadd --system wheel
  • edit /etc/pam.d/su and uncomment the following line
  • #auth required pam_wheel.so

home dirs

make sure all homedirs are chmod 750

set DIR_MODE=0750 in /etc/adduser.conf

selinux

• https://wiki.debian.org/SELinux/Setup
• test that the setup is ok and monitor with /var/log/audit
• when everything is ok, set selinux to enforce mode and check with sestatus

Session Recording

• https://aws.amazon.com/blogs/security/how-to-record-ssh-sessions-established-through-a-bastion-host/
  • if using this script use
  • chattr +a /var/log/bastion/
  • add umask 0027 to /usr/bin/bastion/shell

Consider using http://scribery.github.io/tlog/