Securing your Linux server is important to protect your data, intellectual property, from the hands of crackers (hackers).
This Linux based firewall is controlled by the program called iptables to handles filtering for IPv4, and ip6tables handles filtering for IPv6.
Here, i have mentioned some iptables based rules which can filter high degree of illegitimate traffic.
# Modify this file accordingly for your specific requirement. # 1. Delete all existing rules iptables -F # 2. Set default chain policies iptables -P INPUT DROP iptables -P FORWARD DROP iptables -P OUTPUT DROP # 3. Block a specific ip-address #BLOCK_THIS_IP="x.x.x.x" #iptables -A INPUT -s "$BLOCK_THIS_IP" -j DROP # 4. Allow ALL incoming SSH #iptables -A INPUT -i eth0 -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT #iptables -A OUTPUT -o eth0 -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT # 5. Allow incoming SSH only from a sepcific network #iptables -A INPUT -i eth0 -p tcp -s 192.168.200.0/24 --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT #iptables -A OUTPUT -o eth0 -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT # 6. Allow incoming HTTP #iptables -A INPUT -i eth0 -p tcp --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT #iptables -A OUTPUT -o eth0 -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT # Allow incoming HTTPS #iptables -A INPUT -i eth0 -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT #iptables -A OUTPUT -o eth0 -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPT # 7. MultiPorts (Allow incoming SSH, HTTP, and HTTPS) iptables -A INPUT -i eth0 -p tcp -m multiport --dports 22,80,443 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp -m multiport --sports 22,80,443 -m state --state ESTABLISHED -j ACCEPT # 8. Allow outgoing SSH iptables -A OUTPUT -o eth0 -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT # 9. Allow outgoing SSH only to a specific network #iptables -A OUTPUT -o eth0 -p tcp -d 192.168.101.0/24 --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT #iptables -A INPUT -i eth0 -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT # 10. Allow outgoing HTTPS iptables -A OUTPUT -o eth0 -p tcp --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --sport 443 -m state --state ESTABLISHED -j ACCEPT # 12. Ping from inside to outside iptables -A OUTPUT -p icmp --icmp-type echo-request -j ACCEPT iptables -A INPUT -p icmp --icmp-type echo-reply -j ACCEPT # 13. Ping from outside to inside iptables -A INPUT -p icmp --icmp-type echo-request -j ACCEPT iptables -A OUTPUT -p icmp --icmp-type echo-reply -j ACCEPT # 14. Allow loopback access iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT # 15. Allow packets from internal network to reach external network. # if eth1 is connected to external network (internet) # if eth0 is connected to internal network (192.168.1.x) iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT # 16. Allow outbound DNS #iptables -A OUTPUT -p udp -o eth0 --dport 53 -j ACCEPT #iptables -A INPUT -p udp -i eth0 --sport 53 -j ACCEPT # 18. Allow rsync from a specific network iptables -A INPUT -i eth0 -p tcp -s 192.168.101.0/24 --dport 873 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 873 -m state --state ESTABLISHED -j ACCEPT # 19. Allow MySQL connection only from a specific network iptables -A INPUT -i eth0 -p tcp -s 192.168.200.0/24 --dport 3306 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 3306 -m state --state ESTABLISHED -j ACCEPT # 20. Allow Sendmail or Postfix iptables -A INPUT -i eth0 -p tcp --dport 25 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 25 -m state --state ESTABLISHED -j ACCEPT # 21. Allow IMAP and IMAPS iptables -A INPUT -i eth0 -p tcp --dport 143 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 143 -m state --state ESTABLISHED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 993 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 993 -m state --state ESTABLISHED -j ACCEPT # 22. Allow POP3 and POP3S iptables -A INPUT -i eth0 -p tcp --dport 110 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 110 -m state --state ESTABLISHED -j ACCEPT iptables -A INPUT -i eth0 -p tcp --dport 995 -m state --state NEW,ESTABLISHED -j ACCEPT iptables -A OUTPUT -o eth0 -p tcp --sport 995 -m state --state ESTABLISHED -j ACCEPT # 23. Prevent DoS attack iptables -A INPUT -p tcp --dport 80 -m limit --limit 25/minute --limit-burst 100 -j ACCEPT # 24. Port forwarding 80 to 8080 iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080 # 25. Log dropped packets iptables -N LOGGING iptables -A INPUT -j LOGGING iptables -A LOGGING -m limit --limit 2/min -j LOG --log-prefix "IPTables Packet Dropped: " --log-level 7 iptables -A LOGGING -j DROP
We can then add a few simple firewall rules to block the most common attacks, to protect our VPS from script-kiddies. We can’t really count on iptables alone to protect us from a full-scale DDOS or similar, but we can at least put off the usual network scanning bots that will eventually find our VPS and start looking for security holes to exploit.
First, we start with blocking null packets.
We told the firewall to take all incoming packets with tcp flags NONE and just DROP them. Null packets are, simply said, recon packets. The attack patterns use these to try and see how we configured the VPS and find out weaknesses
Fragmented Packet:
Packets with incoming fragments drop them. This attack result into Linux server panic such data loss.
Force SYN Packet Check:
Make sure NEW incoming tcp connections are SYN packets; otherwise we need to drop them,SYN-Flood-Attacks means that the attackers open a new connection, but do not state what they want (ie. SYN, ACK, whatever). They just want to take up our servers’ resources. We won’t accept such packages.
XMAS Packet:
Incoming malformed XMAS packets should be drop, XMAS packets is also a recon packet.
I also recommend you install and use sniffer such as tcpdupm and ngrep to test your firewall settings.
Conclusion:
This post only list basic rules for new Linux users. You can create and build more complex rules. This requires good understanding of TCP/IP, Linux kernel tuning via sysctl.conf, and good knowledge of your own setup. Stay tuned for next topics:
– Stateful packet inspection.
– Using connection tracking helpers.
– Network address translation.
– Layer 2 filtering.
– Firewall testing tools.
– Dealing with VPNs, DNS, Web, Proxy, and other protocols.
Thanks for reading….& stay tune for more info.
