15-441 Computer Networking

1
15-441 Computer Networking

• Project II question/answer session
• October 18, 2001

2
Handin

• Send me email listing group members.
• Exact format described in project handout.
• Execute cklog cs.cmu.edu.
• Wait for confirmation email from me.
• Test read/write access.
• Hand in the following files
• Source files
• Makefile
• Project report.
• Make sure handin is correct!!

3
Progress Report

• Who has IP layer running?
• NAT/Firewall?
• DHCP?

4
IP Header ip_output()

• TCP and UDP layer already allocate space for the
  IP header in the pbuf.
• Breaks layering.
• Convenient.
• Makes TCP/UDP checksumming easier.
• p_data member of pbuf handed over to ip_output()
  points to allocated IP header.
• Do not modify p_len, p_data,

5
IP Header ip_input()

• TCP and UDP need access to the IP header.
• Do not modify p_len, p_data, when handing packet
  over to upper layer.

6
Freeing pbufs()

• ip_input()
• Always takes over responsibility for freeing a
  packet when called from link layer.
• TCP and UDP receive function will free packet
  unless port is unreachable.
• ICMP receive function always frees packets.
• ICMP send function takes over responsibility for
  freeing packet.
• Note icmp_send() calls ip_output().

7
Freeing pbufs()

• ip_output()
• Always takes over responsibility for freeing a
  packet when called from transport layer.
• Needs to free packet if error condition is
  detected and packet is not handed over to link
  layer.
• Link layer takes over responsibility when calling
  if_start().

8
Setsockopt()

• Need mechanism to set firewalling/NAT rules in
  kernel from user space.
• Setsockopt(int s, int level, int optname, const
  void optval, int optlen)
• First argument is routing socket.
• Used for configuring routing table (and other).
• Socket(AF_ROUTE, SOCK_RAW, 0)
• Second and third argument determine kernel
  function to be called.
• IPPROTO_IP/IP_FW_SET ? fw_setsockopt()
• IPPROTO_IP/IP_NAT_SET ? nat_setsockopt()

9
Setsockopt()

• Setsockopt(int s, int level, int optname, const
  void optval, int optlen)
• Fourth argument is pointer to arbitrary data
  structure.
• Fifth argument denotes length of this data
  structure.
• Setsockopt()will copy data structure from user to
  kernel space, call requested function, and pass
  it pointer to copied data structure and its
  length.

10
Setsockopt()

• How can we exploit Setsockopt() to configure
  NAT/firewalling rules in kernel?
• Define data structures containing mapping or
  filtering rule
• struct nat_rule
• struct filter_rule
• Pass pointer to it (and its length) to
  Setsockopt().
• Setsockopt() will copy data structure to kernel
  space.
• In fw_setsockopt()/nat_setsockopt(), cast const
  void pointer back to pointer to your data
  structure.
• Parse new rule and add it to set of rules kept in
  kernel.

11
Miscellaneous

• Initialize routing table with a default gateway
  before looking up route.
• rttest utility.
• Keep robustness of your code in mind, e.g.,
• byte ordering,
• checking for error/boundary conditions.
• You can ignore next packet in pbuf packet queue
  (p_nextpkt).
• Flag passed to ip_output() can be set to
  IP_FORWARDING (not to IP header flags!!).
• Should be set when calling ip_output() from
  ip_forwarding().

12
Questions?

• Read FAQ and bboard before sending email to TAs.