1. Ethernet Network Systems Security
Mort Anvari

2. Ethernet Most widely used LAN technology Low cost and high flexibility
Versions of different speed: 10Mbps, 100Mbps, Gigabit
Use globally unique media access control (MAC) address (hardware address) for every interface card
9/28/2004

3. Use of Hardware Address
Need an address to send a message to receiver on same Ethernet
IP address is not usable because network layer does not listen to wire
Use hardware address to identify receiver’s interface
Need to resolve receiver’s hardware address from receiver’s IP address
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4. Address Resolution Protocol
Protocol maps each IP address to corresponding hardware address in subnetwork
For computer i to get hardware address of computer j, i broadcasts a rqst message with IP address of j to the subnetwork
rqst(ipa.j) i
default
router
Internet
switch r j
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5. Address Resolution
If j sees a rqst message from i with its IP address, j sends a rply message with its IP address and hardware address to i
rply(ipa.j,hda.j) i
default
router
Internet
switch r j
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6. Functions of ARP Three functions of ARP Resolving IP addresses
Supporting dynamic assignment of addresses
Detecting destination failures
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7. ARP Spoofing Attack
To stop traffic from i to j, an adversary sends to i a spoofed rply message with IP address of j and a non-existent hardware address i
default
router
Internet
switch r j A
rply(ipa.j,hda.x)
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8. Another ARP Spoofing Attack
To stop traffic from i to default router r, an adversary sends to i a spoofed rply message with IP address of r and its own hardware address i
default
router
Internet
switch r j A
rply(ipa.r,hda.A)
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9. Countering ARP Spoofing Attacks
Proposed solutions include ARPWATCH and static ARP caches
ARPWATCH monitors transmission of rqst and rply messages over Ethernet and check them against a database of (IP addr, hardware addr) pairings
Static ARP cache stores permanent (IP addr, hardware addr) pairings of trusted hosts to avoid sending rqst and rply messages over Ethernet
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10. Insufficiencies of Proposed Solutions
ARPWATCH does not support dynamic assignment of IP addresses
Static ARP caches does not support dynamic assignment of IP addresses and detection of destination failures
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11. Need for Secure Address Resolution
When a computer receives a message m, it needs to determine whether m was indeed sent by claimed source, or was inserted, modified, or replayed by an adversary
Use secure address resolution protocol between each computer and a secure server
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12. Architecture of Secure Address Resolution Protocol
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13. Adversary
The adversary can perform three types of actions to disrupt communication between server s and any computer h[i] on the Ethernet
Message loss
Message modification
Message replay
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14. Secure Address Resolution Protocol
Use three mechanisms to counter adversary actions
timeouts to counter message loss
shared secrets to counter message modification
nonces to counter message replay
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15. Invite-Accept Protocol
Periodically, server s sends out an invt message to every computer on Ethernet
Every up computer is required to send back an acpt message including its IP address and hardware address
s updates its address database according to received acpt messages
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16. Invite-Accept Protocol
s  h[0..n-1]: invt(nc, md)
where md=MD(nc;scr[0])||MD(nc;scr[1])||…||MD(nc;scr[n-1])
h[i]  s: acpt(nc, ipa[i], hda[i], d)
where d=MD(nc;ipa[i];hda[i];scr[i])
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17. Request-Reply Protocol
When a computer needs to resolve a destination’s hardware address, it sends a rqst message to server s
If destination’s hardware address is still valid, s sends back a rply message with address information
If destination’s hardware address is not valid anymore, s sends back a rply message with no address information
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18. Request-Reply Protocol
h[i]  s: rqst(nc, ipa[j], d)
where d=MD(nc;ipa[j];scr[i])
If found,
s  h[i]: rply(nc, ipa[j], hda[j], d)
where d=MD(nc;ipa[j];hda[j];scr[i])
If not found,
s  h[i]: rply(nc, ipa[j], 0, d)
where d=MD(nc;ipa[j];0;scr[i])
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19. Extensions Four extensions of secure address resolution protocol
Insecure address resolution
Backup server
System diagnosis
Address resolution across multiple Ethernets
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20. Next Class IPsec Authentication Header (AH)
Encapsulation Security Payload (ESP)
key management
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