Finish assignment 4

3 files changed, 35 insertions, 0 deletions

diff --git a/netsec-assignment4-S4498062/exercise4/exercise4a b/netsec-assignment4-S4498062/exercise4/exercise4a
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+This is in REQ-4. Error messages should only be in an existing session
+(otherwise we can send error messages `gratuitiously', like in gratuitious ARP,
+and thus influence a NAT from outside). So, we SHOULD drop these packets.
+
+If such an error message does belong to an existing session, NAT should simply
+translate the address and forward the message to the internal host, so that it
+receives the message correctly.
diff --git a/netsec-assignment4-S4498062/exercise4/exercise4b b/netsec-assignment4-S4498062/exercise4/exercise4b
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+This is in REQ-5. Error messages should only be in an existing session, so these
+packets SHOULD be dropped. However, there does not seem to be any risk with for-
+warding those packets (which is why it is no MUST).
+
+For messages belonging to an existing session, the source address should be
+translated and the packet should be forwarded to the external realm.
diff --git a/netsec-assignment4-S4498062/exercise4/exercise4c b/netsec-assignment4-S4498062/exercise4/exercise4c
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+Query
+    MUST be permitted unless explicitly overridden by local policy (REQ-1)
+    Session MUST be remembered for at least 60s (REQ-2)
+
+Error
+    MUST be traversed unless IP/ICMP checksum validation fails (REQ-3)
+    Packets SHOULD only be allowed to travel between realms when belonging to an
+        existing session (REQ-4, REQ-5)
+    NAT sessions MUST NOT not be refreshed.
+
+Non-QueryError
+    MAY be dropped or appropriately handled (REQ-11)
+
+DoS
+    The NAT device helps prevent DoS attacks with lots and lots of ICMP error
+    messages by blocking them if they are not linked to an existing session. If
+    the device would not do that, we could do something like DNS amplification.
+
+Destroying sessions
+    An attacker may attempt to send bogus error messages into the NAT network
+    in order to destroy the current sessions. To prevent this, the NAT device
+    won't delete or refresh a NAT session based on an error message.