This is one of my favorite history of Science quotes….
“In 1697, Isaac Newton received and solved Jean Bernoulli’s brachistochrone problem. The swiss mathematician Bernouilli had challenged his colleagues to solve it within six months. Newton not only solved the problem before going to bed that same night, but in doing so, invented a new branch of mathematics called the calculus of variations. He had resolved the issue of specifying the curve connecting two points displayed from each other laterally, along which a body, acted upon only by gravity, would fall in the shortest time. Newton, age 55, sent the solution to be published, at his request, anonymously. But the brilliant originality of the work betrayed his identity, for when Bernoulli saw the solution he commented, ‘We recognize the lion by his claw.’”
February 25th, 2009
jesse
i don’t want to work. I want to play in the fort all day!
Charlotte loves her new building blocks.
… and so does Obama is his own special way.
One annoyance about OSX is that is leaves behind a .DS_Store file on network share drives (Samba, etc.) This file contains folder-specific display preferences for the way those files and folders should be viewed.
How to get rid of them?
Apple has as article How to prevent .DS_Store file creation over network connections on exactly how to do this but to summarize, from a terminal, type the following:
% defaults write com.apple.desktopservices DSDontWriteNetworkStores true
Press return and reboot your mac.
Since this command turns off this behavior on a per user basis, it has to be repeated for every account on the computer that access a remote file share.
February 22nd, 2008
jesse
To determine an endpoint’s operating system one must first understand how to measure differences in a network protocol stack. Start by studying the TCP state machine, understanding how TCP connections are made, where implementations may vary, and creating a series of tests to expose those variances.
To initiate and manage TCP connections endpoints (hosts) exchange packets with certain flags, or control bits, set. There are eight flags available; to identify a host operating system we are interested with the following three:
- ACK – Indicates that the acknowledgment field is significant
- RST – Resets the connection
- SYN – Synchronizes sequence numbers
Transmission Control Protocol is defined in RFC 793 and uses a 3-way handshake protocol to initialize new connections between hosts.
The packet sequence is:
- Caller sends SYN
- Recipient responds with SYN, ACK
- Caller sends ACK
If the connection cannot be made the recipient will typically respond with a RST packet to abort the connection instead of the SYN + ACK to establish it. A successful 3-way handshake is illustrated in the host packet exchange shown below. The process of establishing a connection can be traced on the TCP state diagram below right as (CLOSED) > (SYN_SENT) > (ESTABLISHED) while a failed connection would be (CLOSED > (SYN_SENT) > (CLOSED).
To detect a TCP connection and determine between caller and recipient one must capture only two packets with the SYN (Step 1) and SYN+ACK (Step 2) or RST bits set. To do this note the structure of a TCP packet header and build a packet filter expression.
A TCP header typically holds 20 octets of data. The first line of the packet header contains 32 bits or 4 octets numbered 0 – 3, the second line octets 4 – 7, etc. The TCP control bits are contained in octet 13. In a TCP datagram with the SYN bit set octet 13 resembles the TCP Flags detail illustrated above. By filtering on TCP flags one may capture all packets necessary to profile a host operating system.
References