=Paper=
{{Paper
|id=Vol-2058/paper-16
|storemode=property
|title=Firewall Management With FireWall Synthesizer
|pdfUrl=https://ceur-ws.org/Vol-2058/paper-16.pdf
|volume=Vol-2058
|authors=Chiara Bodei,Pierpaolo Degano,Riccardo Focardi,Letterio Galletta,Mauro Tempesta,Lorenzo Veronese
|dblpUrl=https://dblp.org/rec/conf/itasec/BodeiDFGTV18
}}
==Firewall Management With FireWall Synthesizer==
https://ceur-ws.org/Vol-2058/paper-16.pdf
Firewall Management With FireWall Synthesizer
Chiara Bodei1 , Pierpaolo Degano1 , Riccardo Focardi2 ,
Letterio Galletta1 , Mauro Tempesta2 , and Lorenzo Veronese2
1
Dipartimento di Informatica, Università di Pisa, Italy
{chiara,degano,galletta}@di.unipi.it
2
DAIS, Università Ca’ Foscari Venezia, Italy
{focardi,tempesta}@unive.it, 852058@stud.unive.it
Abstract
Firewalls are notoriously hard to configure and maintain. Policies are written in low-
level, system-specific languages where rules are inspected and enforced along non-trivial
control flow paths. Moreover, firewalls are tightly related to Network Address Translation
(NAT) since filters need to be specified taking into account the possible translations of
packet addresses, further complicating the task of network administrators. To simplify
this job, we propose FireWall Synthesizer (FWS), a tool that decompiles real firewall
configurations from different systems into an abstract specification. This representation
highlights the meaning of a configuration, i.e., the allowed connections with possible address
translations. We show the usage of FWS in analyzing and maintaining a configuration on
a simple (yet realistic) scenario and we discuss how the tool scales on real-world policies.
1 Introduction
Firewalls are fundamental mechanisms for the protection of computer networks. The effec-
tiveness of a firewall system crucially depends on the correctness of its configuration, since an
oversight in the policy may have dramatic effects on the security or the functionality of the
entire network.
Firewall management is a complex task also for skilled system administrators. A typical
configuration consists of a large number of rules and it is often hard to understand their effect
on the overall firewall behavior. Moreover, from time to time, configurations must be adjusted
to reflect the updates of the desired security policies. Since rules interact with each other,
maintenance must be carefully carried out to avoid introducing unintended behaviors and subtle
bugs that could expose the network to external threats.
Typically, policies are written in low-level configuration languages that are specific to the
firewall system in use and support non-trivial control flow constructs, such as jumps and gotos.
Network administrators must also keep in mind how packets are processed by the network
stack of the operating system when writing their policy. Further complications come from
Network Address Translation (NAT), the mechanism for translating addresses and performing
port redirection, which modifies the packets while they traverse the firewall.
To simplify the work of system administrators, we have studied the problem of decompiling
real configurations into abstract specifications that represent the set of the permitted connec-
tions. The abstract version of a firewall policy exposes the meaning of the configuration while
discarding all its low-level details, so that it is easier to verify whether the intended security
policy is correctly implemented. In addition, by comparing two abstract specifications, an ad-
ministrator can spot the differences between the corresponding real configurations and check
whether the updates have the desired effect on the firewall behavior.
Our tool FireWall Synthesizer (FWS) supports the above activities. It decompiles a real
configuration into a human-readable form and analyzes it checking if it meets a given security
Firewall Management With FWS Bodei et al.
54.230.203.47
LAN
10.0.0.0/8
DMZ
54.230.203.0/24
eth0 eth1
10.0.0.1 54.230.203.1
ext
23.1.8.15
Internet
Figure 1: Network setup of our case study.
goal. Differently from the other tools and techniques proposed in the literature (see e.g., [1, 3]),
the novelty of FWS is that it does not target a specific firewall system. Indeed, FWS analyzes
configurations for the most used firewall tools in Linux and Unix [5, 2, 4] and provides partial
support for Cisco IOS routers. Furthermore, FWS can be easily extended to support new
firewall systems by providing the corresponding front-ends. At [6] you can download FWS and
the examples shown below. A full research paper describing the tool internals, the intermediate
language, the algorithm and the underlying theory is presently submitted for presentation at
an international conference.
2 FWS in Action
This section illustrates how FWS is used to analyze and manage real firewall configurations. For
lack of space, we only consider the case of iptables but our approach can be directly applied
to other systems. Besides synthesizing abstract specifications of the firewall policies, possibly
projected using the query language provided by FWS, we show how the analyses implemented
in the tool can be exploited while performing maintenance operations. These analyses include:
• Reachability: is a certain host reachable from another one, possibly through NAT?
• Policy implication and equivalence: are the packets accepted by one configuration at
least/exactly those accepted by a different configuration?
• Policy difference: what are the packets accepted by one configuration and denied by
another one?
• Related rules: which rules affect the destiny of the packets satisfying a given query?
Case study
We consider the simple (yet realistic) network setup shown in Figure 1:
• the LAN network is assigned the set of private IP addresses 10.0.0.0/8;
• servers and production machines, including the HTTPS web server with address 54.230.203.47
are placed in the demilitarized zone (54.230.203.0/24).
The firewall has three interfaces: eth0 connected to the LAN with IP 10.0.0.1, eth1 connected
to the DMZ with IP 54.230.203.1 and ext connected to the Internet with public IP 23.1.8.15.
We want to enforce the following policy on the traffic passing through the firewall:
2
Firewall Management With FWS Bodei et al.
1 * nat
2 # ACCEPT policy in nat chains
3 : PREROUTING ACCEPT [0:0]
4 : INPUT ACCEPT [0:0]
5 : OUTPUT ACCEPT [0:0]
6 : POSTROUTING ACCEPT [0:0]
7
8 # R3 - Apply SNAT on connections from the LAN towards the Internet
9 -A POSTROUTING -s 10.0.0.0/8 -o ext -j MASQUERADE
10
11 COMMIT
12
13 * filter
14 # DROP policy in filtering chains
15 : INPUT DROP [0:0]
16 : FORWARD DROP [0:0]
17 : OUTPUT DROP [0:0]
18
19 # Allow established packets
20 -A FORWARD -m state -- state ESTABLISHED -j ACCEPT
21 # R2 - LAN hosts can connect to DMZ
22 -A FORWARD -s 10.0.0.0/8 -d 5 4. 2 30 .2 03 . 0/ 24 -j ACCEPT
23 # R3 - LAN hosts can connect to the Internet over HTTP / HTTPS
24 -A FORWARD -s 10.0.0.0/8 -o ext -p tcp -- dport 80 -j ACCEPT
25 -A FORWARD -s 10.0.0.0/8 -o ext -p tcp -- dport 443 -j ACCEPT
26 # R1 - Any host can connect to the HTTPS server in the DMZ
27 -A FORWARD -d 54.230.203.47 -p tcp -- dport 443 -j ACCEPT
28
29 COMMIT
Figure 2: Firewall configuration in iptables.
R1 Hosts from the Internet can connect to the HTTPS server in the DMZ;
R2 LAN hosts can freely connect to any host in the DMZ;
R3 LAN hosts can connect to the Internet over HTTP and HTTPS (with source NAT).
Analysis of an iptables configuration
Figure 2 shows a commented iptables configuration (in the iptables-save format) that
implements the above security policy. In the rules we use both IP addresses and network
interfaces: the mapping between interfaces and addresses is provided to FWS with a separate
file. Now we use FWS for verifying that the configuration meets the requirements above. For
instance, we can check R1 and R2 simultaneously by asking FWS the query
dstIp == 54.230.203.0/24 && state == NEW
where dstIp is the destination address of the packet entering the firewall and state is used to
distinguish between new and established connections. The operator == constrains a variable to
be equal to a value or inside a certain interval, while && is the logical and. In particular, the
query checks whether new connections are allowed towards hosts in the subnet 54.230.203.0/24.
The output of the tool is in Table 1a, where * denotes any value. The first line of the table
states that any host can communicate over HTTPS (port 443) with the server 54.230.203.47
(R1); whereas the second line says that any machine in the LAN can freely communicate with
the hosts in the DMZ (R2).
3
Firewall Management With FWS Bodei et al.
Src IP Src Port Dst IP Dst Port Protocol State
* * 54.230.203.47 443 tcp NEW
10.0.0.0/8 * 54.230.203.0/24 * * NEW
(a) Requirements R1 and R2
Src IP Src Port SNAT IP Dst IP Dst Port Protocol State
10.0.0.0/8 * 23.1.8.15 * \ { 80 tcp NEW
10.0.0.0/8 443
54.230.203.0/24
127.0.0.0/8
}
(b) Requirement R3
Table 1: Analysis of the iptables policy.
We now check requirement R3. To do that, we ask which packets from the LAN can reach
hosts that are not in the DMZ:
srcIp == 10.0.0.0/8 && not(dstIp == 54.230.203.0/24) && state == NEW
The answer to the query is in Table 1b: Internet hosts can be reached over HTTP and HTTPS
(ports 80 and 443) and the source IP address of outgoing packets is rewritten to 23.1.8.15, the
external address of the firewall. In the fourth column we use \ (set difference) on addresses to
denote all addresses except those of the LAN, DMZ and loopback interface (the Internet).
Maintaining existing configurations
Suppose to add a new computer with IP address 10.13.3.7 to the LAN. Differently from other
machines, we allow access to the hosts on the Internet only over HTTPS. The other requirements
on traffic should be preserved. To this purpose, we can add the following rule to the FORWARD
chain, which drops connections to port 80 from host 10.13.3.7:
-A FORWARD -s 10.13.3.7 -p tcp --dport 80 -j DROP
However, we must be careful about where to place this rule in order to fulfill the desired
requirement and avoid to unintentionally block legal traffic.
If we place the new rule at the end of the FORWARD chain, the policy equivalence analysis
implemented in FWS reports that the new policy is equivalent to the previous version. Then,
we use the related rules analysis to understand which rules are relevant for processing new
HTTP packets. The output of this analysis includes the filtering rules at lines 22 and 24 of the
FORWARD chain (see Figure 2):
-A FORWARD -s 10.0.0.0/8 -d 54.230.203.0/24 -j ACCEPT
-A FORWARD -s 10.0.0.0/8 -o ext -p tcp --dport 80 -j ACCEPT
If we add the new rule before the line 21, FWS reports that the policy is not equivalent to
the previous one. In order to check the impact of our changes we run the policy difference
analysis projected over the HTTP traffic using the query protocol == tcp && dstPort ==
80. The output is in Table 2. In the first column, + (resp. -) stands for lines that appear
(resp. disappear) in the synthesis after the updates. Now the host 10.13.3.7 cannot connect to
the Internet over HTTP, as desired (lower part of Table 2). However, our update also prevents
4
Firewall Management With FWS Bodei et al.
+/- Src IP Src Port Dst IP Dst Port Protocol State
+ 10.0.0.0/8 \ {10.31.3.37} * 54.230.203.0/24 80 tcp NEW
- 10.0.0.0/8 * 54.230.203.0/24 80 tcp NEW
+/- Src IP Src Port SNAT IP Dst IP Dst Port Protocol State
+ 10.0.0.0/8 \ { * 23.1.8.15 * \ { 80 tcp NEW
10.31.3.37 10.0.0.0/8
} 54.230.203.0/24
127.0.0.0/8
}
- 10.0.0.0/8 * 23.1.8.15 * \ { 80 tcp NEW
10.0.0.0/8
54.230.203.0/24
127.0.0.0/8
}
Table 2: Policy differences after the wrong update.
communications over HTTP with machines on the DMZ, thus violating the requirement R2
(upper part of Table 2).
The correct placement of the new rule is between the lines 22 and 23, i.e. the rule for the
requirement R2 and those for the requirement R3. In this way we allow HTTP traffic from
10.13.3.7 only to the DMZ. If we repeat the analysis, we see that now the only difference is just
in the HTTP traffic towards the Internet, as desired.
3 Conclusions
We have shown how our tool FWS can be used to verify the compliance of a firewall policy with
the desired functionality and security requirements of a network in a simple yet realistic case
study. Moreover, we have discussed how the various analyses implemented by the tool can to
simplify maintenance operations.
In order to evaluate the effectiveness of FWS on real-world scenarios we have tested on
several iptables policies collected by the authors of [1]. We have measured the time needed
by FWS to synthesize policies of size ranging from 15 to 263 rules and most of the analyses
terminate in less than 3 seconds. The 3 most complex cases of size 77, 90 and 263 rules terminate
in 28, 25 and 355 seconds, respectively. We stress that the reported times are those required
to perform a complete synthesis of a policy, whereas in a typical usage the user will run the
analyses on a specific set of hosts or services using the query language of FWS.
References
[1] Cornelius Diekmann, Julius Michaelis, Maximilian P. L. Haslbeck, and Georg Carle. Verified ipta-
bles Firewall Analysis. In Proceedings of the 15th IFIP Networking Conference, Vienna, Austria,
May 17-19, 2016, pages 252–260, 2016.
[2] The IPFW Firewall. https://www.freebsd.org/doc/handbook/firewalls-ipfw.html.
[3] Robert M. Marmorstein. Formal Analysis of Firewall Policies. PhD thesis, College of William and
Mary, Williamsburg, VA, May 2008.
[4] Packet Filter (PF). https://www.openbsd.org/faq/pf/.
[5] The Netfilter Project. https://www.netfilter.org/.
[6] FireWall Synthesizer (FWS): Tool and Examples. https://github.com/secgroup/fws.
5
Firewall Management With FWS Bodei et al.
Appendix: Using FWS
In this section we describe how using our tool to replicate the results of Section 2. FWS can
be downloaded from [6], and installed following the instruction inside README.MD file.
Once installed, the command fws is available on the command line. The syntax is
fws FRONTEND ACTIONS OPTIONS
where FRONTEND specifies which the firewall system we want to decompile: the supported fire-
walls are cisco, ipfw, iptables, pf. The second argument ACTION defines the operation to
perform on the configuration. Currently, fws implements the following:
synthesis Synthesize a specification
implication Check for policy implication
equivalence Check for policy equivalence
diff Synthesize differences between two firewalls
convert Convert a configuration to the generic language
query Display the rules that affect the selected packets
The thirds argument allows the user to provide fws with input file, the query to execute, the
format of the output (e.g., ASCII, LATEX), etc. The options we are using in the following are:
-i Interfaces file
-f Main configuration file
-s Configuration to compare with the main one
-q Query to check on the main configuration
Additionally, some useful shorthands are available to project the analysis on specific subsets of
flows that packets can traverse:
--loopback Flows with local source and local destination
--input Flows with remote source and local destination
--output Flows with local source and remote destination
--forward Flows with remote source and remote destination
--nat Flows with NATs
--filter Flows without NATs
In the rest of this section we assume that the current directory contains the files of the
directory examples/itasec18 of the archive at [6]. To decompile the iptables configuration
of Figure 1 we invoke fws as follows
$ fws iptables synthesis -i interfaces -f iptables . txt
where the file iptables.txt contains the configuration and the file interface the information
about each network interface. More precisely, it includes a line for each network interface; each
line contains the name of the interface, the address range of the sub-network it communicates
with and its IP address. For example, the interface file for our case study contains the following
lo 127.0.0.0/8 127.0.0.1
eth0 10.0.0.0/8 10.0.0.1
eth1 54.230.203.0/24 54.230.20
ext 0.0.0.0/0 23.1.8.15
For example, the interface eth0 communicates with hosts in the sub-network 10.0.0.0/8 and its
IP address is 10.0.0.1.
The content of Tables 1a and 1b are obtained by the following invocations of fws:
6
Firewall Management With FWS Bodei et al.
+/- Src IP Src Port SNAT IP Dst IP Dst Port Protocol State
+ 10.0.0.0/8 \ { * 23.1.8.15 * \ { 80 tcp NEW
10.31.3.37 10.0.0.0/8
} 54.230.203.0/24
127.0.0.0/8
}
- 10.0.0.0/8 * 23.1.8.15 * \ { 80 tcp NEW
10.0.0.0/8
54.230.203.0/24
127.0.0.0/8
}
Table 3: Policy differences after the correct update.
$ fws iptables synthesis -f iptables . txt -i interfaces -- forward \
-q dstIp == 5 4. 2 30 .2 03 . 0/ 24 && state == NEW
$ fws iptables synthesis -f iptables . txt -i interfaces -- forward \
-q srcIp == 10.0.0.0/8 && not ( dstIp == 5 4 . 2 30 . 2 0 3 . 0 / 2 4 ) && state == NEW
To check that the policy obtained by adding the rule to drop connections to port 80 from
host 10.13.3.7 at the end of the FORWARD chain is equivalent to the original one, we can run the
command
$ fws iptables equivalence -i interfaces -f iptables . txt \
-s i p t a b l e s _ w r o n g _ u p d a t e _ 1 . txt
To get the rules that are relevant in processing HTTP packets of new connections we launch
fws as follows:
$ fws iptables query -i interfaces -f iptables . txt \
-q " protocol == tcp && dstPort == 80 && state == NEW "
The output of the tool is the following:
-t nat -A POSTROUTING -s 10.0.0.0/8 -o ext -j MASQUERADE
-t filter -A FORWARD -s 10.0.0.0/8 -d 5 4. 23 0. 2 03 .0 /2 4 -j ACCEPT
-t filter -A FORWARD -s 10.0.0.0/8 -o ext -p tcp -- dport 80 -j ACCEPT
Finally, we can compute the difference between the original configuration in Figure 1 and
the new version concerning the connections to port 80 running:
$ fws iptables diff -i interfaces -f iptables . txt \
-s i p t a b l e s _ c o r r e c t _ u p d a t e . txt \
-q " protocol == tcp && dstPort == 80 " -- forward
The output of the tool, reported in Table 3, shows that our modification affects only connections
to the Internet, as expected.
7