A beautiful filter for SSH brute-force attacks for your admiration

A bad sysadmin changes the default SSH port.

A good sysadmin throttles the brute-force attackers by allowing one new connection in 20 seconds.

# requires xt_TARPIT
iptables -A INPUT -p tcp -m state --state NEW \
  --dport 22 -m recent --update --seconds 20 -j TARPIT
iptables -A INPUT -p tcp -m state --state NEW \
  --dport 22 -m recent --set -j ACCEPT

A brilliant sysadmin blocks intruders indefinitely

# requires hashlimit
iptables -A INPUT -p tcp -m tcp --dport 22 -m state \
  --state NEW -m hashlimit --hashlimit 1/hour \
  --hashlimit-burst 2 --hashlimit-mode srcip \
  --hashlimit-name SSH --hashlimit-htable-expire 60000 
  -j ACCEPT

iptables -A INPUT -p tcp -m tcp --dport 22 \
  --tcp-flags SYN,RST,ACK SYN -j DROP 

iptables -A INPUT -p tcp -m state \
  --state NEW -m tcp --dport 22 -j ACCEPT

In the last example, the 1st rule allows up to two connections per hour. After the limit of two connections per hour is reached, the second rule becomes active and the hashlimit module starts to countdown from 1 minute (60 000 milliseconds). If you connect within 1 minute, the hashlimit counter is reset to 60 000. If you connect after 1 minute, you drop to the 3rd rule and are allowed access.

Cherry-picked from Habr