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Security in Computer Networks: Ipsec VPNs

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GC 2019-2020Virtual 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 VPNs 2
GC 2019-2020
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
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
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
GC 2019-2020IPsec 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
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
IP datagram with ESP
IPsec IPsec
IP datagram with ESP
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)
GC 2019-2020SA 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 ( and of the destination SA interface (
the type encryption used (e.g., AES)
the encryption key
the type of integrity check used (e.g., HMAC with MD5)
the authentication key
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.
GC 2019-2020Inside 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 IP header
GC 2019-2020Split 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
Split tunneling is a high performance solution with low bandwidth requirement
GC 2019-2020
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
GC 2019-2020Firewalls
A combination of hardware and software that isolates an organization’s internal network from the Internet, allowing some packets to pass, blocking others
Router filters packet-by-packet
• source IP address, destination IP address
• TCP/UDP source and destination port numbers
• ICMP message type
Stateless packet filtering
should be dropped?
TCP > 1023 80 any
allow outside of 222.22/16
UDP > 1023 53 ---
Access Control Lists
Firewall rules are implemented in routers with Access Control Lists (ACLs)
Each router interface has its own list
R u le
GC 2019-2020Stateful 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
action source address
222.22/16 TCP 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”
allow 222.22/16 outside of
222.22/16 UDP > 1023 53 ---
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
GC 2019-2020Application 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-gateway telnet session
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.
An application firewall is specific to each application and may be a bottleneck due to the additional processing for application-level protocols. 23
GC 2019-2020Intrusion 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)
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
internal network
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
GC 2019-2020
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
GC 2019-2020Trusted 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
Semitrusted IPsec topology
GC 2019-2020Semitrusted 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
The firewall functionality is integrated into the IPsec gateway
The corporate firewall is not used for VPN users
GC 2019-2020Network edge design
The redundancy has not been considered to better visualize the flows through the network
Nowadays obsolete
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
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
GC 2019-2020Three-tier web design (cont.)
GC 2019-2020Basic 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
Dedicated servers for e-mail antivirus (AV) processing
GC 2019-2020References
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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