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Lab 5: NAT

Lab 5: NAT. CS144 Review Session 7 November 13 th , 2009 Roger Liao. Announcements. Lab 5 is out Due Thursday, December 3 rd Layered on top of Lab 3 (sr) Pass a command flag (-nat) to turn on NAT behavior Lab 3 grade = max(lab 3 grade, lab 5 grade). Overview. Basic NAT functionality

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Lab 5: NAT

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  1. Lab 5: NAT CS144 Review Session 7 November 13th, 2009 Roger Liao

  2. Announcements • Lab 5 is out • Due Thursday, December 3rd • Layered on top of Lab 3 (sr) • Pass a command flag (-nat) to turn on NAT behavior • Lab 3 grade = max(lab 3 grade, lab 5 grade)

  3. Overview • Basic NAT functionality • ICMP Requirements • TCP Requirements • General NAT processing logic • Suggestions

  4. NAT • Network Address Translation • Translates private IP addresses to facilitate Internet communication • 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16 • Single device with single IP address • Hides details of internal network • But interferes with many applications

  5. “Reverse” NAT • myth (you) is behind NAT • Distinguish internal (eth0) and external (eth1) by interface name • Translate packets from myth (VNS firewall) so that it appears the NAT sent them

  6. ICMP echo src IP: 171.64.15.11 dst IP: 171.67.236.21 myth eth0: 171.67.236.16 ICMP echo src IP: 171.67.236.20 dst IP: 171.67.236.21 NAT eth1: 171.67.236.20 ICMP echo src IP: 171.67.236.20 dst IP: 171.67.236.21 App Server App Server 171.67.236.21 171.67.236.23

  7. ICMP echo reply src IP: 171.67.236.21 dst IP: 171.64.15.11 myth eth0: 171.67.236.16 ICMP echo reply src IP: 171.67.236.21 dst IP: 171.64.15.11 NAT eth1: 171.67.236.20 ICMP echo reply src IP: 171.67.236.21 dst IP: 171.67.236.20 App Server App Server 171.67.236.21 171.67.236.23

  8. ICMP Requirements • Support echo requests/replies • Echo requests are external host independent • Using the same query identifier to two different hosts will preserve mapping • If A sends an ICMP request with id q1q1’ to B and another request with id q1q2’ to C, then q1’==q2’. • Do not timeout ICMP query mappings for at least 60 seconds

  9. TCP Requirements 1. Endpoint-Independent Mapping behavior for TCP • Same translation (X1:x1)(X1’:x1’) for packets destined to any external host • UNSAF: Unilateral Self-Address Fixing mechanism 2. Support all valid sequences of TCP packets - TCP implementations should work 3. Endpoint-Independent Filtering behavior for TCP - Like Endpoint-Independent Mapping, just for accepting inbound packets from external hosts

  10. TCP Requirements 4. Don’t respond to inbound SYN for at least 6 seconds. Drop if outbound SYN received, send Port Unreachable otherwise - Used for supporting simultaneous open - Compromise to have this support and signal error for invalid SYN 5. Abandon idle TCP connections after 2 hours 4 minutes - Rationale: Default keep-alive of 2 hours and transitory period (open/close) of 4 minutes - Can drop or send RST packets for non-SYN pkts with no mapping

  11. TCP Requirements 6. No port assignment behavior of port overloading for TCP - Disallow different internal endpoints from using the same mapping - This means for (X1:x1)(X1’:x1’) and (X2:x2)(X2’:x2’), (X1’:x1’) != (X2’:x2’) 7. Support hairpinning for TCP of type “External source IP address and port” - Rewrite source IP and port when receiving packet from internal host with a mapping

  12. src IP, port - X:x dst IP, port – Y’:y’ src IP, port – X’:x’ dst IP, port – Y:y Hairpinning Mapping Y:yY’:y’ X:xX’:x’ eth1: 171.67.236.20 NAT eth0: 171.67.236.16 myth myth X:x Y:y

  13. General Logic • Check whether packet is inbound or outbound • Determine if it is ICMP or TCP • If outbound, add a globally unique mapping • If inbound, check for existing mapping. • If none, discard (unless TCP SYN or hairpinning)

  14. General Logic • Rewrite IP src/dst • Don’t forget to recompute checksum • Rewrite ICMP identifier/TCP port • Recompute checksum again • TCP checksum covers pseudoheader and payload • Reuse router logic to determine how to forward packet • Don’t worry about UDP

  15. Threads • Spawn a thread to handle timing out NAT entries • Similar to ARP cache • Synchronize access to shared data • NAT mappings • Locks • Create thread in sr_router.c • Takes a pointer to a C routine. This is where you implement timeout logic. • Can rely on main program exit to terminate thread

  16. Data Structures • Need to store NAT mappings • Linked list is fine, O(n) traversal • Keep a time field to remember when a mapping was last used • Need to remember used ICMP identifiers and used port numbers • Separate structures for identifier and port number

  17. Implementation Suggestions • Implement NAT code in separate files (e.g. sr_nat.h, sr_nat.c) • Don’t forget to update the Makefile • Handle command line flags in sr_main.c • http://www.gnu.org/software/hello/manual/libc/Getopt.html#Getopt • Create necessary NAT data structures in sr_instance (sr_router.h) • Initialize in sr_router.c

  18. Other Suggestions • Work on ICMP first and then TCP • Note that ARP is unchanged • Save logfile (-l logfile to ./sr) and examine packet flow in Wireshark/tcpdump • Start early – report VNS issues to staff list and VNS admin (dgu@cs.stanford.edu)

  19. Upcoming Updates • Reference binary for comparison • Will be released next week, accessible from /usr/class/cs144/bin • New topology for testing • Most likely will be nested NATs • Web server will likely be updated to show observed IP address/port on home page

  20. Questions? 32 + 16 > 128

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