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GC2019-2020

Security in Computer Networks:Ipsec VPNs

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GC2019-2020Virtual Private Networks

� Private networks can be very costly: an institution needs to purchase, install and maintain its own physical network infrastructure

� Today many companies use Virtual Private Networks (VPN) to establish secure private network connections over any other network (such as the public Internet), allowing them to reduce their communications expenses� A VPN emulates a private network

� Company’s inter-office traffic is sent over the (public) shared network• logically separate from other traffic

• encrypted before entering the shared network

� Currently, the two most common techniques used for creating Internet-based VPNs are IPsec and SSL/TLS

� IPsec VPNs are generally grouped in two categories:� Site-to-Site VPNs

� Remote User VPNs2

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� Also known as gateway-to-gateway VPNs� Used to connect two networks together� Built around the idea that the benefits provided by IPsec are not

needed from host to host, only from one site to another� All communications are authenticated and encrypted before they

enter the VPN Service Provider’s network

Site-to-Site VPNs

IPsec process

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GC2019-2020Remote User VPNs

� Also called host-to gateway VPNs

� Usually used to link with a corporate office from remote locations

� Provide mobility and are economical

� One of the important points is to create a strong authentication

� Mobile users connect to the network using a VPN client software which encapsulates, encrypts, and signs the traffic before sending it through the Internet to the VPN gateway

IPsec process 4

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GC2019-2020

� IPsec (IP security) provides security at the network level

� Suite defined in more than a dozen RFCs

� Two main protocols:� Authentication Header (AH)

• does not provide confidentiality, only source authentication and data integrity

� Encapsulation Security Payload (ESP)• provides source authentication, data integrity and confidentiality

• more widely used than AH (therefore, we will focus exclusively on the ESP protocol)

� A network-layer logical connection, named Security Association(SA), is built is between two IPsec entities� An SA is simplex (unidirectional from source to destination)

� If both entities want to send secure datagrams to each other, then two SAs are needed, one in each direction

� IPSec operates in one of two different modes: transport mode or tunnel mode

IPsec

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GC2019-2020IPsec transport mode

� When transport mode is used, IPsec protects only the IP payload (a TCP segment, a UDP segment or an ICMP message), not the IP header

� Transport mode is used for end-to-end communications (for example, for communications between a client and a server).

� An SA is built between two hosts

IPsec-aware IPsec-aware

A B

IP Source = AIP Destination=B

Internet

SA

IP datagram with ESP

ESP MAC

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GC2019-2020IPsec – tunnel mode

� In tunnel mode, IPsec protects the entire IP packet

� Tunnel mode is typically used when an SA is built is between two IPsec gateway (site-to-site VPN) or between an IPsec gateway and a host (remote user VPN)

� Tunnel mode is preferred because it allows private addressing

IPsec IPsec

Tunnel mode Gateway-to-Gateway R1 R2 BAtunnel

IP datagram with ESP

Internet

(new) (original)

IP SRC = R1IP DST=R2

IP SRC = AIP DST=B

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GC2019-2020IPsec – tunnel mode (cont.)

IPsec IPsec

R1 R2 BAtunnelTunnel mode Host-to-Gateway

IP datagram with ESP

Internet

(new) (original)

IP SRC = AIP DST=R2

IP SRC = AIP DST=B

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� The endpoints maintain state information about the SA

� An IPsec entity holds SA state information for all of its SAs in Security Association Database (SAD)

� Along with SAD, the IPsec entity also maintains a Security Police Database (SPD)

� The SPD contains the rules which indicate what types of datagrams (as a function of source IP address, destination IP address, protocol type) are to be IPsec processed and which SA should be used

� For large VPNs, instead of manual configuration, an automatic mechanism can be used to create the SAs (the two IPsec entities exchange certificates, negotiate authentication and encryption algorithms, create session keys, … )

� IPsec IKE (Internet Key Exchange)

Security associations (SAs)

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GC2019-2020SA from R1 to R2

R1 stores for the SA:� an 32-bit SA identifier: Security Parameter Index (SPI)

� the IP addresses of the origin SA interface (200.168.1.100) and of the destination SA interface (193.68.2.23)

� the type encryption used (e.g., AES)

� the encryption key

� the type of integrity check used (e.g., HMAC with MD5)

� the authentication key

193.68.2.23200.168.1.100

172.16.1/24172.16.2/24

security association

Internetheadquartersbranch office

R1R2

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GC2019-2020IPsec datagram

new IPheader

ESPhdr

originalIP hdr

Original IPdatagram payload

ESPtrl

ESPMAC

encrypted

“enchilada” authenticated

paddingpad

lengthnext

headerSPI Seq#

193.68.2.23200.168.1.100

172.16.1/24172.16.2/24

security association

Internetheadquartersbranch office

R1R2

� Focus on tunnel mode with ESP

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GC2019-2020What happens?

R1 converts the original IP datagram to an IPsec datagram:

� appends to the back of the original datagram an “ESP trailer”;

� encrypts the result using the algorithm and the key specified by the SA;

� appends to the front of this encrypted quantity the “ESP header, creating the “enchilada”;

� creates the authentication MAC over the whole enchilada, using the algorithm and key specified by the SA;

� appends the MAC to the back of enchilada, forming the payload;

� creates a new IP header, with all the classic IPv4 header fields, which it appends before the payload.

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GC2019-2020Inside the enchilada:

� ESP trailer: � padding, for block ciphers� next header, for identifying the type of payload data

� ESP header: � SPI, so that the receiving entity knows what to do� Sequence number, to thwart replay attacks

� MAC in the ESP auth field is created with a shared secret key

new IPheader

ESPhdr

originalIP header

Original IPdatagram payload

ESPtrl

ESPMAC

encrypted

“enchilada” authenticated

paddingpad

lengthnext

headerSPISeq

#

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GC2019-2020ESP trailer: next header

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GC2019-2020Split tunneling

� Typically, an SPD contains a policy that encrypts information between entities of an organization

� Instead, any traffic destined for a network outside the organization (typically the Internet) is sent unencrypted

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� Split tunneling is a high performance solution with low bandwidth requirement

GC2019-2020

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Split tunneling (cont.)

� Without split tunneling, instead, all traffic is encrypted and sent to the central site

� The central site decrypts the traffic and sends Internet-bound traffic back out to the Internet through the central site’s Internet edge and security solution

� This clearly increases response times, increases the crypto load on the remote router or PC, and increases bandwidth requirements

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GC2019-2020

A review on firewalls

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GC2019-2020Firewalls

A combination of hardware and software that isolates an organization’s internal network from the Internet, allowing some packets to pass, blocking others

firewall

administerednetwork

publicInternet

firewalltrusted “good guys” untrusted “bad guys”

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GC2019-2020

� Internal network connected to Internet via a router firewall

� Router filters packet-by-packet

� Decisions to forward/drop packets are typically 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

Should the packet be allowed to pass or

should be dropped?

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GC2019-2020

actionsource

addressdest

addressprotocol

sourceport

destport

flagbit

allow 222.22/16outside of222.22/16

TCP > 1023 80any

allow outside of222.22/16

222.22/16TCP 80 > 1023 ACK

allow 222.22/16outside of222.22/16

UDP > 1023 53 ---

allow outside of222.22/16

222.22/16UDP 53 > 1023 ----

deny all all all all all all

Access Control Lists

� Firewall rules are implemented in routers with Access Control Lists (ACLs)

� Each router interface has its own list

Rule

s ar

e ap

plie

d fro

m t

op t

o d

ow

n

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GC2019-2020Stateful packet filtering

� A stateless packet filter makes decisions on each packet in isolation

� admits packets that “make no sense,” e.g., dest port = 80, ACK bit set, even though no TCP connection is established:

� Is vulnerable to IP Spoofing attacks

actionsourceaddress

destaddress

protocolsource

portdestport

flagbit

allow outside of222.22/16

222.22/16TCP 80 > 1023 ACK

� Stateful packet filters track all ongoing TCP connection in a connection table

� track connection setup (SYN), teardown (FIN): determine whether incoming, outgoing packets “make sense”

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GC2019-2020

actionsource

address

dest

addressproto

source

port

dest

port

flag

bitcheck

connection

allow 222.22/16outside of

222.22/16TCP > 1023 80

any

allow outside of

222.22/16

222.22/16TCP 80 > 1023 ACK X

allow 222.22/16outside of

222.22/16UDP > 1023 53 ---

allow outside of

222.22/16

222.22/16UDP 53 > 1023 ----

deny all all all all all all

Stateful packet filtering (cont.)

� ACL extended to indicate the need to check connection state table before admitting a packet

� Stateful packet filters don’t detect attacks at layers above the fourth one and are vulnerable to DoS attacks that saturate the state table 22

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GC2019-2020Application gateways (application firewalls)

� They filter packets on application-layer data as well as on IP/TCP/UDP fields.

� example: allow to select internal users to telnet outside.

host-to-gatewaytelnet session

gateway-to-remote host telnet session

applicationgateway router and filter

1. require all telnet users to telnet through a gateway.

2. for authorized users, the gateway sets up a telnet connection to the remote host. The gateway relays data between the two connections

3. R1 blocks all telnet connections not originating from gateway.

R1

� An application firewall is specific to each application and may be a bottleneck due to the additional processing for application-level protocols. 23

GC2019-2020Intrusion Detection Systems (IDSs)

� IDSs:� perform deep packet inspection: look at packet contents (e.g.,

check character strings in the packet against a database of known virus, attack strings)

� examine correlation among multiple packets

• port scanning

• DoS attack

• …

� generate alerts when they observe potentially malicious traffic

• IPS (Intrusion Prevention System): a device that filters out suspicious traffic

� Host-based IDS (HIDS)

� Network IDS (NIDS) 24

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GC2019-2020

Webserver FTP

server

DNSserver

Internet

DMZ (demilitarized zone)

firewall

IDS sensors

Intrusion detection systems

� Multiple Network IDSs (NIDS) are typically employed� each sensor sees only a fraction of the organization’s traffic

� different types of checking at different locations

� NIDs work in concert, sending alerts to a central IDS processor, which aggregates alerts and sends them to network administrators

internalnetwork

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GC2019-2020Distributed Firewalls

L2 switch

L3 switch

� To deploy firewalls throughout the enterprise network in key locations

� The distributed firewall strategy helps to protect against internal threats

� New network security perimeters: between switches and back-end servers, between different departments, where a wireless LAN meets the wired network

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GC2019-2020

Firewall and IPsec Gateway placement for IPsecVPNs

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GC2019-2020

� Site-to-site and remote user VPNs are combined onto the same gateway device

� Once authenticated, the IPsecVPN is trusted and granted connectivity on par with internal users

� after authentication and decryption, the traffic is passed directly to the internal network

� Additional security controls in firewalls and NIDS are not used

Trusted IPsec topology

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GC2019-2020Trusted IPsec topology (cont.)

� In this alternative topology the IPsec traffic is routed from a dedicated interface onto the firewall

� The firewall does not provide any added security, save one: to detect attacks against the IPsec gateway, you can audit the access control logging information

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� Traffic is routed into a firewall after decryption

Semitrusted IPsec topology

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GC2019-2020Semitrusted IPsec topology (cont.)

� In this alternative topology all traffic flows into and out of two interfaces on the same firewall

� To detect attacks against the IPsec gateway, you can audit the access control logging information

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GC2019-2020Semitrusted IPsec topology (cont.)

� The firewall functionality is integrated into the IPsec gateway

� The corporate firewall is not used for VPN users

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GC2019-2020Network edge design

� The redundancy has not been considered to better visualize the flows through the network

Nowadays obsolete

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DMZ

GC2019-2020

Some examples on application security design: web and e-mail

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DMZ

Three-tier web design

� The web server displays the content to the user, the application server processes user input and generates the content accessing, if needed, the database server

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� By separating the three functions, it is more difficult for the attacker to get at the applications and at the database, which is where real damage can result

GC2019-2020Three-tier web design (cont.)

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GC2019-2020Three-tier web design (cont.)

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GC2019-2020Basic two-tier e-mail design

� The antivirus (AV) software can be added on the mail servers directly

� Without antivirus filtering on the e-mail servers, signatures must be updated on each and every host, increasing the risk that damage can be done before you are able to stop it

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DMZ

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GC2019-2020Distributed two-tier e-mail design

� Dedicated servers for e-mail antivirus (AV) processing

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DMZ

GC2019-2020References

� J. Kurose e K.W. Ross, “Computer Networking. A Top-Down Approach”, sixth edition, Pearson Addison-Wesley

� S. Convery, “Network Security Architecture”, Cisco Press

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