Firewall. IPTables and its use in a realistic scenario. José Bateira ei10133 Pedro Cunha ei05064 Pedro Grilo ei09137 FEUP MIEIC SSIN

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1 Firewall IPTables and its use in a realistic scenario FEUP MIEIC SSIN José Bateira ei10133 Pedro Cunha ei05064 Pedro Grilo ei09137

2 Topics 1- Firewall How they work? Why use them? NAT and Firewall Requirement analysis Servers and services Ports.

3 Topics (cont) 2 - Iptables Background History What is iptables? Basic concepts and commands

4 Topics (cont) 3 - A case study Topology of the network The Requirements analysis.

5 1 - Firewall Security system that control in/out network traffic by analysing data packets. Based on rules, the Firewall lets the packet get in or not. Basically, it filters incoming and outgoing traffic.

6 1.1 - How they work There are 4 types of firewalls.. 1. Circuit-filter 2. Application Gateway

7 1.1 - How they work (2) 3. Packet-filtering 4. Stateful Inspection

8 1.2 - Why use them? Pros Cons 1. Allows for definition of NATs. 2. Can block malicious connections to system/network. 3. Allows separation between accessible hosts and private hosts. 1. Can be hard to configure all the rules. Conclusion: Taking the time to configure it can GREATLY improve security. Don t use it at your own risk

9 1.3 - NAT and Firewall WAN Network Address Translation (NAT) is a mechanism that allows for internal hosts of a network communicate with the outside networks using a single, common ip address The Firewall standing at the edge can support this by masquerading outgoing connections and translating incoming connections It is used to protect the internal network from direct access from the outside, since internal IPs are not public Web Server

10 1.4 - Requirements Analysis Why? What? Who? When? Where?

11 Servers and Services Identification of what servers/hosts are running in our network and in which subnetwork. For each server/host, identify if it s running a service (and which one), if it s allowed to access a service (and where) and who is allowed to access the service it s providing.

12 Ports For each service running in our network, identify which port is it running on. Normal ports: HTTPS: We could make them run on different ports though. If so, which ports and we have to consider this in the rules specification (port-forwarding).

13 2.1 - Iptables: Background History Linux kernels have had packet filtering since the 1.1 series. ipfw - incorporated into the kernel 1.1. ipfwadm and ipchains - userspace tool used in kernel 2.0 and 2.2. Iptables - the fourth-generation tool used since kernel 2.4

14 ipchains Input Routing Forward Output Receive Process Send Process iptables Routing Forward input Output Receive Process Send Process

15 2.2 - What is iptables? In Linux, the packet filter framework is divided in 2 parts: Netfilter - implemented in kernel space. Iptables - the user space module. When we talk about Iptables, commonly its means booths parts.

16 2.2 - What is iptables? Iptables in an application that allow administrators manage the netfilter configuration. Main characteristic: Filtering considering/regardless of the state of the package. Support for nat, addresses and ports. Plugins.

17 2.3 - Basic concepts. Rules - What the firewall must do. Chain - Group of rules that are store. The rules are executed in order.every chain have a standard rule. If any rule isn't applied, the standard rule will be used. Tables - iptables organizes its flow in tables, each with a set of predefined chains.

18 2.3 - Basic concepts. There are 4 kind of tables: Filter Table - for doing the actual packet filtering. Default table INPUT OUTPUT FOWARD NAT Table - rewrite packet source and/or destination. PREROUTING OUTPUT POSTROUTING

19 2.3 - Basic concepts. MANGLE Table - allow packet change like header and content. PREROUTING INPUT OUTPUT FOWARD POSTROUTING RAW Table - for avoiding connection tracking.

20 NETWORK PREROUTING MANGLE NAT (DNAT) local IP? YES NO INPUT MANGLE FILTER FORWARD MANGLE FILTER LOCAL PROCESS OUTPUT MANGLE NAT (DNAT) FILTER POSTROUTING MANGLE NAT (SNAT/MASQUERADE) NETWORK

21 2.3 - Basic concepts - commands Save and restore We can use a script file and/or use iptables commands. # sudo iptables-save > FileWithRules # sudo iptables-restore < FileWithRules We can also save counters with -C plan

22 2.3 - Basic concepts - commands # iptables <table> <Add/Insert/Delete> <CHAIN> <PKT_MATCHING_CRITERIA> <ACTION>

23 2.3 - Basic concepts - commands <table> -t filter ( DEFAULT) -t nat -t mangle <Add/Insert/Delete> -A (ADD AT THE BOTTOM OF THE CHAIN) -I (PUT IN THE BEGINNING OF THE CHAIN) -D ( DELETE RULE)

24 2.3 - Basic concepts - commands <CHAIN> PREROUTING INPUT FORWARD OUTPUT POSTROUTING USER_DEFINED_CHAIN Exemples iptables -t filter -A INPUT -p tcp --dport 22 -j ACCEPT iptables -D INPUT -p tcp --dport 22 -j ACCEPT

25 2.3 - Basic concepts - commands <PKT_MATCHING_CRITERIA> OSI MODEL LAYER 2 -i INTERFACE -o INTERFACE -mac-source [!] <xx-xx-xx-xx-xx-xx> OSI MODEL LAYER 3 -s or --src SRC_IP -d or --destination DST_IP

26 2.3 - Basic concepts - commands OSI MODEL LAYER 4 -p (udp tcp icmp) --icmp-type [!] <icmp_type> /***UDP AND TCP*****/ --source-port or --sport PORT --destination-port or --dport PORT /*****TCP ONLY*******/S --tcp-flags (SYN ACK FIN RST URG PSH ALL NONE) --syn --tcp-option [!] <tcp_option#>

27 2.3 - Basic concepts - commands <ACTION> -j ACCEPT -j DROP DROP PACKET -j REJECT DROP AND NOTIFY -j USER_DEFINED_CHAIN START A CHAIN -j RETURN LEAVE THE CHAIN -j LOG CREATE LOGS

28 2.3 - Basic concepts - commands NAT table specific -j SNAT REWRITE SOURCE IP -j MASQUERADE WHEN SNAT IS DYNAMIC -j DNAT REWRITE DESTINATION SOURCE -j REDIRECT MANGLE table specific -j ROUTE ADD A ROUTE -j TOS SET IP HEADER TYPE OF SERVICE -j TTL TIME TO LIVE

29 3 - Case study To aid with the understanding of IPTables, let s apply it to a fictional but realistic scenario. We will decide what rules apply in which firewall, which traffic goes in, out, to where and from where.

30 3.1 - Topology Internet DMZ Web DNS VPN / eth eth0 eth1 Protected Servers SMTP Admin Server File Server External Firewall eth0 eth / Internal Firewall eth2 Users /24 user 1 user 2 user 3 user 4 user 5

31 DMZ Web DNS VPN Requirements Analysis / For each server, we start by defining his incoming and outgoing connections that are allowed. Web Server ( ) Service Protocol Port Source Address HTTP TCP 80 All iptables -A INPUT -p tcp --dport 80 -j ACCEPT HTTPS TCP 443 All iptables -A INPUT -p tcp --dport 443 -j ACCEPT SSH TCP 22 Admin Server iptables -A INPUT -p tcp -s dport 22 -j ACCEPT

32 DMZ Web DNS VPN Requirements Analysis(2) / DNS Server ( ) Service Protocol Port Source Address DNS UDP 53 All iptables -A INPUT -p udp --dport domain -j ACCEPT SSH TCP 22 Admin Server iptables -A INPUT -p tcp -s dport ssh -j ACCEPT VPN Server ( ) Service Protocol Port Source Address OpenVPN/PPTP TCP 1194,1723 All iptables -A INPUT -p tcp -m multiport --destination-ports 1194,1723 -j ACCEPT SSH TCP 22 Admin Server iptables -A INPUT -p tcp -s dport ssh -j ACCEPT

33 Protected Servers SMTP Admin Server File Server Requirements Analysis(3) / Mail Server ( ) Service Protocol Port Source Address SMTP,POP,POPS, IMAP, IMAPS TCP 25,110,995,143,993 VPN Server, User Network iptables -A INPUT -p tcp -s m multiport --destination-ports smtp, pop,pops,imap,imaps -j ACCEPT iptables -A INPUT -p tcp -s /24 -m multiport --destination-ports smtp, pop,pops,imap,imaps -j ACCEPT VPN User x SSH TCP 22 Admin Server iptables -A INPUT -p tcp -s dport ssh -j ACCEPT

34 Protected Servers SMTP Admin Server File Server Requirements Analysis(4) / Admin Server( ) Service Protocol Port Source Address SSH TCP 22 VPN Server, User Network iptables -A INPUT -p tcp -s dport ssh -j ACCEPT iptables -A INPUT -p tcp -s /24 --dport ssh -j ACCEPT VPN User x

35 Protected Servers SMTP Admin Server File Server Requirements Analysis(5) / FTP Server ( ) Service Protocol Port Source Address FTP, SFTP TCP 22,21 VPN Server, User Network iptables -A INPUT -p tcp -s /24 -m multiport --destination-ports ftp,ssh -j ACCEPT iptables -A INPUT -p tcp -s m multiport --destination-ports ftp,ssh -j ACCEPT User x VPN SSH TCP 22 Admin Server iptables -A INPUT -p tcp -s dport ssh -j ACCEPT

36 3.2 - Requirements Analysis(6) Users /24 user 1 user 2 user 3 user 4 user 5 User Hosts ( /24) Service Protocol Port Source Address SSH TCP 22 VPN Server, Admin Server iptables -A INPUT -p tcp -s dport ssh -j ACCEPT iptables -A INPUT -p tcp -s dport ssh -j ACCEPT VPN Admin

37 3.2 - Requirements Analysis(7) All iptables -P INPUT DROP iptables -P OUTPUT ACCEPT iptables -P FORWARD DROP iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT By default, all other packets are dropped. But all connections that are made because of allowed connections are allowed (for instance, the transfer of a file using FTP).

38 3.2 - Requirements Analysis(8) We then move on to the firewalls themselves. Starting with the internal firewall, we decide the traffic that is allowed in, out, to where and where from, protocols and ports included. Internal Firewall Servers Services Ports File Server FTP,SFTP 21,22 Origin Network/Server User s Network, VPN Server, AdminServer iptables -A FORWARD -p tcp -d s /24 -m multiport --destination-port ssh,ftp -j ACCEPT iptables -A FORWARD -p tcp -d s m multiport --destination-port ssh,ftp -j ACCEPT iptables -A FORWARD -p tcp -d s dport ssh -j ACCEPT

39 3.2 - Requirements Analysis(9) Internal Firewall Servers Services Ports Admin Server SSH 22 Origin Network/Server User s Network, VPN Server iptables -A FORWARD -p tcp -d s /24 --dport ssh -j ACCEPT iptables -A FORWARD -p tcp -d s dport ssh -j ACCEPT

40 3.2 - Requirements Analysis(10) Internal Firewall Servers Services Ports User s Computers SSH 22 Origin Network/Server User s Network, Admin Server, VPN Server iptables -A FORWARD -p tcp -s /24 -d /24 --dport ssh -j ACCEPT iptables -A FORWARD -p tcp -s d /24 --dport ssh -j ACCEPT iptables -A FORWARD -p tcp -s d /24 --dport ssh -j ACCEPT

41 3.2 - Requirements Analysis(11) Internal Firewall Servers Services Ports Origin Network/Server Mail Server SMTP,POP3,POP3S,IMAP, IMAPS 25,110,995,143,993 User s Network, Admin Server, VPN Server iptables -A FORWARD -p tcp -s /24 -d m multiport --destination-port smtp,pop,pops, imap,imaps -j ACCEPT iptables -A FORWARD -p tcp -s d m multiport --destination-port smtp,pop,pops, imap,imaps -j ACCEPT iptables -A FORWARD -p tcp -s d dport ssh -j ACCEPT iptables -A FORWARD -i eth1 -s /24 -j ACCEPT iptables -A FORWARD -i eth2 -s /24 -j ACCEPT iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT iptables -P FORWARD DROP

42 3.2 - Requirements Analysis(12) On the external firewall, we implement NAT protocols, stop access to the internal network and allow access to the DMZ. External Firewall Accepted INPUT traffic iptables -A INPUT -p udp -s sport domain -j ACCEPT iptables -A INPUT -i eth2 -p tcp -m multiport --destination-port http,https,domain, 1194,1723 -j ACCEPT

43 3.2 - Requirements Analysis(13) External Firewall Servers Services Ports Origin Network/Server DNS DNS 53 No Restrictions iptables -A FORWARD -p udp -d dport domain -j ACCEPT iptables -A FORWARD -p tcp -d dport domain -j ACCEPT Web HTTP,HTTPS 80, 443 No Restrictions iptables -A FORWARD -p tcp -d m multiport --destination-port http, https -j ACCEPT VPN Server OpenVPN/PPTP 1194,1723 No Restrictions iptables -A FORWARD -p tcp -d m multiport --destination-port 1194,1723 -j ACCEPT iptables -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

44 3.2 - Requirements Analysis(14) External Firewall NAT Mechanism Connection s Origin Protocols SNAT Users Network No Restrictions iptables -t NAT -A POSTROUTING -o eth2 -j MASQUERADE DNAT From outside (Internet) HTTP, DNS, OpenVPN, PPTP iptables -t NAT -A PREROUTING -i eth2 -p tcp -m multiport --destination-port http,https -j DNAT --to iptables -t NAT -A PREROUTING -i eth2 -p tcp --dport domain -j DNAT --to iptables -t NAT -A PREROUTING -i eth2 -p udp -m multiport --destination-port domain -j DNAT --to iptables -t NAT -A PREROUTING -i eth2 -p tcp -m multiport --destination-port 1194,1723 -j DNAT --to

45 3.2 - Requirements Analysis(15) iptables -P INPUT DROP iptables -P OUTPUT ACCEPT iptables -P FORWARD DROP External Firewall Default Policies

46 Thank you for your attention! Firewall IPTables and its use in a realistic scenario FEUP MIEIC SSIN Authors: José Bateira ei10133 Pedro Cunha ei05064 Pedro Grilo ei09137

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