Kernel debugging

what is KDB
http://www.ibm.com/developerworks/linux/library/l-kdbug/

Q Can we put assembly code in C ?
Ans Yes by asm(" ");

Q How to compile SO?
Ans -f PIC (Position independent code )
  http://tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

IPSec , IPsec and NAT

IPSec types its uses?
Problem with IPSec and NAT?
The IPSec Authentication Header (AH) . AH runs the entire IP packet, including invariant header fields such as source and destination IP address, through a message digest algorithm to produce a keyed hash. This hash is used by the recipient to authenticate the packet. If any field in the original IP packet is modified, authentication will fail and the recipient will discard the packet. AH is intended to prevent unauthorized modification, source spoofing, and man-in-the-middle attacks. But NAT, by definition, modifies IP packets. Therefore, AH + NAT simply cannot work.

When TCP or UDP are involved--as they are in transport mode ESP--there is a catch-22. Because NAT modifies the TCP packet, NAT must also recalculate the checksum used to verify integrity. If NAT updates the TCP checksum, ESP authentication will fail. If NAT does not update the checksum (for example, payload encrypted), TCP verification will fail.
If the transport endpoint is under your control, you might be able to turn off checksum verification. In other words, ESP can pass through NAT in tunnel mode, or in transport mode with TCP checksums dis- abled or ignored by the receiver.

Key Exchange mechanism? 
IPSEC transport mode and tunnel mode does IP header encrypted??
Diff in IKE1and IKE 2 

http://www.netbsd.org/docs/network/ipsec/

Good information

http://unixwiz.net/techtips/iguide-ipsec.html

http://www.kame.net/newsletter/20001119/

Commands
setkey -D : to see SAD entries d--dump
setkey -PD : Dumps all SPD entries, See Policy Database
setkey - F : flush SAD entries
setkey - FP : flush SPD Policy entries

# IPsec with IKE, with pre-shared secret
Racoon.conf
path pre_shared_key "/usr/local/v6/etc/psk.txt" ;

What is Diameter ? how it is better than Radius

Diameter is Advanced protocol than Radius
*  Its More Reliable uses TCP or SCTP               
*  Larger address space for attribute-value pairs (AVPs) and identifiers is 4 Bytes (32 bits instead of 8 bits)

Radius
*  uses UDP
*  identifiers is 1 Byte only 256 Attributes supported

---Remote Authentication Dial In User Service (RADIUS)
RADIUS server checks that the information is correct using authentication schemes like PAP, CHAP or EAP
NAS (Network Access Server (ASNGW) ) sends Access request to AAA

RADIUS -UDP ports
1812 for RADIUS Authentication and
1813 for RADIUS Accounting

dig utility for dns lookup

The dig command includes some timing stats and the actual query that will be performed.

ns lookup is also there :
but dig (Domain Information Groper)gives more Packet level information.
OPCODE etc ..

host command is also useful
Ref:
http://uw713doc.sco.com/en/NET_tcpip/dnsC.nslook.html

How to make sure process are running on both processor in SMP??

DNS -uses TCP or UDP Explain

port no 53 (both TCP and UDP)
DNS : DNS is used to resolve ip query this should be FAST enough so it uses UDP.
But there are some requirements where Reliability is of more concern in DNS like ZONE TRANSFER
also it is used when the response data size exceeds 512 bytes.
UDP DNS messages are limited to 512 bytes , longer messages are truncated and a special bit (TC) in the header is set to indicate that this has occurred. If a message being truncated causes a problem for its recipient, the query must be repeated using TCP, as described below.

ZONE Transfer (QTYPE: AXFR) : what happens there??
Whole data is replicated from server DNS to client DNS .
* Before sending the AXFR request, the AXFR client usually sends a preliminary SOA request to decide whether it wants to see the AXFR results. This SOA request may be sent through UDP or through TCP.
* TCP port 53 is simultaneously used by normal (non-AXFR) DNS clients requesting data that did not fit through UDP. A non-AXFR DNS client tries all queries through UDP first; however, if a UDP DNS server sets the ``TC'' bit in its response, the DNS client tries the query again through TCP.

The end of a zone transfer is marked by duplicating the SOA RR that started the zone.


REf:
http://www.tcpipguide.com/free/t_DNSMessageHeaderandQuestionSectionFormat.htm
http://technet.microsoft.com/en-us/library/cc781340(WS.10).aspx
http://cr.yp.to/djbdns/axfr-notes.html
http://www.faqs.org/rfcs/rfc1035.html