Limit ssh connections

firewall-sshd-limit.sh

#!/bin/sh

# requires hashlimit
iptables -I INPUT -p tcp -m tcp --dport 22 -m state --state NEW \
  -m hashlimit --hashlimit 1/min \
  --hashlimit-burst 5 --hashlimit-mode srcip \
  --hashlimit-name SSH --hashlimit-htable-expire 120000 \
  -j ACCEPT

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

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

# The 1st rule allows up to 5 connections per min. After the limit of 5 
# connections per min is reached, the second rule becomes active and 
# the hashlimit module starts to countdown from 2 minutes (2 x 60 000 milliseconds). 
# If you connect within 1 minute, the hashlimit counter is reset to 120000. 
# If you connect after 1 minute, you drop to the 3rd rule and are allowed access.