Transcript Ethernet

Ethernet
Network Systems Security
Mort Anvari
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
2
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
9/28/2004
3
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
i
rqst(ipa.j)
switch
default
router
r
Internet
j
9/28/2004
4
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
i
rply(ipa.j,hda.j)
switch
default
router
r
Internet
j
9/28/2004
5
Functions of ARP

Three functions of ARP



Resolving IP addresses
Supporting dynamic assignment of
addresses
Detecting destination failures
9/28/2004
6
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
switch
default
router
r
Internet
j
A
9/28/2004
rply(ipa.j,hda.x)
7
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
switch
default
router
r
Internet
j
A
9/28/2004
rply(ipa.r,hda.A)
8
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
9/28/2004
9
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
9/28/2004
10
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
9/28/2004
11
Architecture of
Secure Address Resolution Protocol
h[i]
s
Applications
Transport
Network
Applications
Transport
Network

Subnetwork
hn[i]
hr[i]
Interface
Subnetwork
invite-accept protocol
request-reply protocol
sn
write arrays
ipa, hda, valid
sr
Interface
Ethernet
9/28/2004
12
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
9/28/2004
13
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
9/28/2004
14
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
9/28/2004
15
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])
9/28/2004
16
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
9/28/2004
17
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])
9/28/2004
18
Extensions

Four extensions of secure address
resolution protocol




Insecure address resolution
Backup server
System diagnosis
Address resolution across multiple
Ethernets
9/28/2004
19
Next Class

IPsec



Authentication Header (AH)
Encapsulation Security Payload (ESP)
key management
9/28/2004
20