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Bài giảng Mạng máy tính nâng cao - Chương 13: Firewall

Bài giảng Mạng máy tính nâng cao - Chương 13: Firewall bao gồm những nội dung về Firewalls (Stateless packet filtering, Stateful packet filtering, Application Gateways); Intrusion Detection Systems (IDS), Denial of Service Attacks.

Chủ đề:

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Firewalls & IDS Outline Firewalls & IDS Outline

(cid:1) Firewalls

◦ Stateless packet filtering ◦ Stateful packet filtering (cid:2) Access Control Lists ◦ Application Gateways

(cid:1) Intrusion Detection Systems (IDS)

◦ Denial of Service Attacks

Firewalls Firewalls

Firewall

isolates organization’s internal net from larger Internet, allowing some packets to pass, blocking others.

public Internet

administered network

firewall

Why Firewalls? Why Firewalls?

prevent denial of service (DoS) attacks: • SYN flooding: attacker establishes many bogus TCP connections, no resources left for “real” connections. prevent illegal modification/access of internal data. • e.g., attacker replaces CIA’s homepage with something

else.

allow only authorized access to inside network (set of allow only authorized access to inside network (set of

authenticated users/hosts)

three types of firewalls: stateless packet filters 1. stateful packet filters 2. 3. application gateways

Stateless Packet Filtering Stateless Packet Filtering

Should arriving packet be allowed in? Departing packet let out?

(cid:1) internal network connected to Internet via router

firewall.

(cid:1) router filters packet-by-packet, decision to

forward/drop packet based on: ◦ source IP address, destination IP address ◦ TCP/UDP source and destination port numbers ◦ ICMP message type ◦ TCP SYN and ACK bits.

Stateless Packet Filtering: Example Stateless Packet Filtering: Example

Example 1: Block incoming and outgoing datagrams with IP

protocol field = 17 and with either source or dest port = 23.

(cid:1) all incoming, outgoing UDP flows and telnet

connections are blocked. connections are blocked.

Example 2: Block inbound TCP segments with ACK=0. (cid:1) prevents external clients from making TCP connections with internal clients, but allows internal clients to connect to outside.

Stateless Packet Filtering: Stateless Packet Filtering: More Examples More Examples

Firewall Setting

Policy

No outside Web access.

Drop all outgoing packets to any IP address, port 80

Drop all incoming TCP SYN packets to any IP except 130.207.244.203, port 80 80

No incoming TCP connections, except those for institution’s public Web server only. public Web server only.

Drop all incoming UDP packets - except DNS and router broadcasts.

Prevent Web-radios from eating up the available bandwidth.

Prevent your network from being used for a smurf DoS attack.

Drop all ICMP packets going to a “broadcast” address (eg 130.207.255.255).

Drop all outgoing ICMP TTL expired traffic

Prevent your network from being tracerouted.

Access Control Lists Access Control Lists

(cid:1) ACL: table of rules, applied top to bottom to incoming packets:

(action, condition) pairs.

protocol

action

source port

dest port

flag bit

source address

dest address

any

allow

222.22/16

TCP

> 1023

outside of 222.22/16 222.22/16

allow

222.22/16

TCP

> 1023

ACK

outside of 222.22/16

allow

222.22/16

UDP

> 1023

---

outside of 222.22/16

allow

222.22/16

UDP

> 1023

----

outside of 222.22/16

deny

all

Packet Filtering Stateful Packet Filtering Stateful

(cid:1) stateless packet filter: heavy handed tool

◦ admits packets that “make no sense,” e.g., dest port = 80, ACK bit

set, even though no TCP connection established:

action protocol source address dest address source port dest port flag bit

•

allow 222.22/16 TCP TCP 80 80 > 1023 > 1023 ACK ACK outside of 222.22/16 222.22/16

stateful packet filter: track status of every TCP connection. o track connection setup (SYN), teardown (FIN): to determine

whether incoming, outgoing packets “makes sense”. o timeout inactive connections at firewall: no longer admit

packets.

Advanced Computer

Networks Firewalls and IDS 9

Packet Filtering Stateful Packet Filtering Stateful

ACL augmented to indicate need to check connection state table

before admitting packet.

proto action check conxion dest address source address source port dest port flag bit

any allow 222.22/16 TCP > 1023 80 outside of 222.22/16 222.22/16

222.22/16 allow TCP 80 > 1023 ACK outside of 222.22/16

allow 222.22/16 UDP > 1023 53 --- outside of 222.22/16

222.22/16 allow UDP > 1023 ---- 53 outside of 222.22/16

deny all all all all all all

Application Gateways Application Gateways

(cid:1) Filters packets on application data as well as on

IP/TCP/UDP fields.

Example: Allow select internal users to telnet outside.

gateway-to-remote gateway-to-remote host telnet session

router and filter

application gateway

host-to-gateway telnet session

Limitations of Firewalls and Gateways Limitations of Firewalls and Gateways

(cid:1) Filters often use all or nothing policy for UDP.

(cid:1) IP Spoofing: router can’t know if data “really” comes from claimed source.

(cid:1) Tradeoff: degree of communication with outside world, level of security. security.

(cid:1) If multiple app’s. need special treatment, special treatment, each has own app. gateway.

(cid:1) Many highly protected sites still suffer from attacks.

(cid:1) Client software must know how to contact gateway. ◦ e.g., must set IP address of proxy in Web browser.

Intrusion Detection Systems (IDS) Intrusion Detection Systems (IDS)

(cid:1) Packet filtering:

◦ operates on TCP/IP headers only. ◦ no correlation check among sessions.

IDS: Intrusion Detection System (cid:1) Deep packet inspection: look at packet contents (cid:1) Deep packet inspection: look at packet contents (e.g., check character strings in packet against database of known virus, attack strings).

(cid:1) Examine correlation among multiple packets:

(cid:2) port scanning (cid:2) network mapping (cid:2) DoS attack