Semantics-Preserving Simplification of Real-World Firewall Rule Sets

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1 Semantics-Preserving Simplification of Real-World Firewall Rule Sets Formal Methods 2015 Cornelius Diekmann * Lars Hupel Georg Carle * * Chair for Network Architectures and Services Chair for Logic and Verification Munich, Germany With contributions by Lars Noschinski, Julius Michaelis *, Andreas Korsten *, Manuel Eberl, Lukas Schwaighofer *, and Fabian Immler. FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets 1

2 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

3 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

4 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

5 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

6 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

7 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

8 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

9 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

10 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

11 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

12 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

13 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

14 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

15 Introduction to Firewalls Chain INPUT (policy ACCEPT) DOS_PROTECT all / /0 ACCEPT all / /0 state RELATED,ESTABLISHED DROP tcp / /0 tcp dpt:22 DROP tcp / /0 multiport dports 21,873,5005,... DROP udp / /0 multiport dports 123,111,2049,... ACCEPT all / /0 DROP all / /0 Chain DOS_PROTECT (1 references) RETURN icmp / /0 icmptype 8 limit: avg 1/sec... DROP icmp / /0 icmptype 8 RETURN tcp / /0 tcp flags:0x17/0x04 limit:... DROP tcp / /0 tcp flags:0x17/0x04... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 2

16 Introduction to Firewalls Firewalls are usually managed manually FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 3

17 Introduction to Firewalls Firewalls are usually managed manually... which is extremely error-prone FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 3

18 Introduction to Firewalls Firewalls are usually managed manually... which is extremely error-prone There are tools to analyze rulesets and discover errors Margrave ITVal FIREMAN Firewall Builder Firewall Policy Advisor ConfigChecker... FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 3

19 Example: IPSpace Partition Ruleset from the introduction... treats all packets equally... except for the last two rules FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 4

20 Example: IPSpace Partition Ruleset from the introduction... treats all packets equally... except for the last two rules Expected output /16 is accepted Everything else is dropped ITVal output There is 1 class: The Universe FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 4

21 Problems in Firewall Analysis Tools This talk is not about ITVal Many tools have similar problems 1 Complex Chain model Calling to and returning from user-defined chains May lead to errors in tools FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 5

22 Problems in Firewall Analysis Tools This talk is not about ITVal Many tools have similar problems 2 Vast amount of primitive matches Check man iptables Now check man iptables-extensions Now check if you have custom extensions running Now think about future features Supporting everything is infeasible Certain features cannot be supported by some tool s algorithm FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 5

23 Summary Problem Tools cannot understand complex real-word rulesets Our Solution Semantics-preserving simplification λ β A α FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Introduction to Firewalls 6

24 Agenda 1 Semantics 2 Simplification 3 Evaluation FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Agenda 7

25 Agenda 1 Semantics 2 Simplification 3 Evaluation FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Semantics 8

26 Syntax Rule: ( mexpr, action ) Example: ( icmp icmptype 8 limit : avg1/sec..., Return ) Ruleset: rule list Firewall state:!, %,? Primitive matcher: γ Primitive Packet Bool FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Semantics 9

27 Syntax Rule: ( mexpr, action ) Example: ( icmp icmptype 8 limit : avg1/sec..., Return ) Ruleset: rule list Firewall state:!, %,? Primitive matcher: γ Semantics: Primitive Packet Bool γ, p rs, s t FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Semantics 9

28 Syntax Rule: ( mexpr, action ) Example: ( icmp icmptype 8 limit : avg1/sec..., Return ) Ruleset: rule list Firewall state:!, %,? Primitive matcher: γ Primitive Packet Bool Semantics: γ }{{} primitive matcher, p }{{} packet rs }{{} ruleset, s }{{} start state t }{{} final state FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Semantics 9

29 Determinism If γ, p rs, s t and γ, p rs, s t then t = t FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Semantics 10

30 Agenda 1 Semantics 2 Simplification 3 Evaluation FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 11

31 Rewriting simple actions Remove Log actions Unfolding custom chains Eliminates Call/Return Linux kernel only accepts acyclic call graphs unfolding terminates FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 12

32 Rewriting simple actions Unfolding custom chains Example Chain INPUT X a Chain X Return Accept b c Result [( a ( b) c, Accept )] FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 13

33 Simplification Summary Actions left: Accept, Drop Semantics are preserved γ, p simplify rs, t t iff γ, p rs, t t FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 14

34 Simplification Summary Actions left: Accept, Drop Semantics are preserved γ, p simplify rs, t t iff γ, p rs, t t Remaining problems 1 Unknown primitives matches 2 Complex nested match-expressions after unfolding unsupported by iptables FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 14

35 Unknown primitives Lifting to ternary logic Kleene s 3-valued logic Primitive matcher may now return unknown Default decision strategy: in-doubt-allow or in-doubt-deny γ, p rs, s allow t γ, p rs, s deny t FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 15

36 Unknown primitives Let m u be an unknown match. in-doubt-allow more permissive ruleset (m u, Accept) (True, Accept) (m u, Drop) (False, Drop) Example ( icmp icmptype 8 limit : avg1/sec..., Drop ) ( icmp icmptype 8 False, Drop ) FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 16

37 Closure Property { p γ, p rs,? deny! } { p γ, p rs,?! } { p γ, p rs,? allow! } We continue with one of the approximations FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 17

38 Normalization Impossible: # iptables (tcp udp) -j ACCEPT Impossible: # iptables (src ip tcp) -j ACCEPT FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 18

39 Normalization Problem iptables supports only negation-normal form with the connective Solution normalize: rule rule list where all rules share the same action Example (exclude ip from accessing an HTTP server) [( src ip (tcp port 80), Accept )] [( src ip ( tcp port 80), Accept )] [( src ip tcp, Accept ), ( src ip port 80, Accept )] FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Simplification 19

40 Agenda 1 Semantics 2 Simplification 3 Evaluation FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Evaluation 20

41 Evaluation Ruleset 1 Shorewall firewall on a home router; 500 rules. Unfolding: firewall does not unconditionally drop packets from private IP ranges FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Evaluation 21

42 Evaluation Ruleset 1 Shorewall firewall on a home router; 500 rules. Unfolding: firewall does not unconditionally drop packets from private IP ranges Ruleset 2 Small firewall script found online (networking.ringofsaturn.com) Most rules are dead; contrary to documented behavior Author probably confused: -I (insert at top) and -A (append at tail) FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Evaluation 21

43 Evaluation Ruleset 1 Shorewall firewall on a home router; 500 rules. Unfolding: firewall does not unconditionally drop packets from private IP ranges Ruleset 2 Small firewall script found online (networking.ringofsaturn.com) Most rules are dead; contrary to documented behavior Author probably confused: -I (insert at top) and -A (append at tail) Ruleset 3 & 4 & 5 Main firewall of our lab Snapshot 2013: 2800 rules Firewall Builder: import errors ITVal: erroneous results After simplification: success Upper closure: 1000 rules Lower closure: 500 rules Snapshot 2014: 4000 rules Snapshot 2015: almost 5000 rules FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Evaluation 21

44 Fakulta t fu r Informatik Technische Universita t Mu nchen Future Work FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Future Work 22

45 Q & A FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Q & A 23

46 Backup Slides FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 24

47 Specifying Primitive Matchers in Ternary Logic Very easy: Specify what you know/want, the rest in unknown FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 23

48 Semantics (1) SKIP γ, p [], t t ACCEPT DROP REJECT match m p γ, p [(m, Accept)],?! match m p γ, p [(m, Drop)],? % match m p γ, p [(m, Reject)],? % NOMATCH match m p γ, p [(m, a)],?? DECISION t? FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 24

49 Semantics (2) γ, p rs 1,? t γ, p rs2, t t SEQ γ, p rs 1 ::: rs 2,? t LOG EMPTY match m p γ, p [(m, Log)],?? match m p γ, p [(m, Empty)],?? FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 25

50 Semantics (3) Background ruleset Γ : chain name rule list CALLRESULT match m p γ, p Γ c,? t γ, p [(m, Call c)],? t CALLRETURN match m p Γ c = rs 1 ::: (m, Return) :: rs 2 match m p γ, p rs 1,?? γ, p [(m, Call c)],?? FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 26

51 Ruleset 3 (excerpt, 22 of 2800 rules displayed) 1 Chain FORWARD ( policy ACCEPT ) 2 target prot opt source destination 3 LOG_DROP all / /0 4 ACCEPT tcp /0 multiport sports 389,636 5 ACCEPT tcp /0 multiport sports 389,636 6 ACCEPT udp /0 udp spt :88 7 ACCEPT udp /0 udp spt :88 8 ACCEPT tcp / /0 tcp spt : ACCEPT tcp / /27 tcp dpt : ACCEPT tcp / /27 tcp dpt : ACCEPT udp /0 12 ACCEPT udp / multiport dports 4569,5000: ACCEPT all /0 14 ACCEPT all / ACCEPT all /0 16 ACCEPT all / tcp / /23 state NEW tcp dpt :22 flags : 0x17/0x02 recent : SET name : ratessh side : source 18 tcp / /23 state NEW tcp dpt :22 flags : 0x17/0x02 recent : SET name : ratessh side : source 19 mac_ 96 all / /0 20 LOG_DROP all --! / / Chain LOG_DROP (21 references ) 23 target prot opt source destination 24 LOG all / /0 limit : avg 100/ min burst 5 LOG flags 0 level 4 prefix "[ IPT_DROP ]:" 25 DROP all / / Chain mac_ 96 (1 references ) 28 target prot opt source destination 29 RETURN all /0 MAC XX:XX:XX:XX:XX:XX 30 DROP all /0 FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 27

52 Ruleset 3 Upper Closure (excerpt) 1 Chain FORWARD ( policy ACCEPT ) 2 3 DROP all / /0 4 ACCEPT tcp / /0 5 ACCEPT tcp / /0 6 ACCEPT udp / /0 7 ACCEPT udp / /0 8 ACCEPT tcp / /0 9 ACCEPT tcp / /27 10 ACCEPT tcp / /27 11 ACCEPT udp / /0 12 ACCEPT udp / /32 13 ACCEPT all / /0 14 ACCEPT all / /32 15 ACCEPT all / /0 16 ACCEPT all / /32 17 DROP all! / /0 FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 28

53 Ruleset 3 Lower Closure (excerpt) 1 Chain FORWARD ( policy ACCEPT ) 2 3 DROP all / /0 4 ACCEPT udp / /0 5 ACCEPT all / /0 6 ACCEPT all / /32 7 ACCEPT all / /0 8 ACCEPT all / /32 9 DROP all / /0 10 DROP all / / (unfolded DROPs from chain mac 96) 12 DROP all! / /0 FM15, Semantics-Preserving Simplification of Real-World Firewall Rule Sets: Backup Slides 29

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