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Issue 5 (May 16, 2000)
___________________________________________________________________________
The gh0st.net project: http://www.gh0st.net/index.html
FireSt0rm homepage: http://www.firest0rm.org/index.html
URL of the day: http://www.cs.wisc.edu/condor/index.html
All content copyright © 2000 by the individual authors, All Rights Reserved
___________________________________________________________________________
- Editor's Comments
- URLs
- Readers' Questions
- Readers' Comments
- Contemporary Telenet I
- Ethernet - The Bottom Two Layers
- Music Reviews
- Future Issues
- Credits
***********************************************************************
*** Editor's Comments : Kynik
***********************************************************************
I'm glad to see that some readers are actually submitting questions and
comments. If you send us something, and it's coherent and topical, we'll
probably include it in the next issue. If we can't answer a question you
have, it's pretty likely that someone reading out there will be able to
point you in the right direction. I've also increased the number of songs
that are reviewed to 2, just because it looks more competitive that way,
and you're not bored with a single choice. We got very lucky on the last
issue, as we were posted on HNN on a Friday (which means we'd have
exposure for the whole weekend) and the L0pht (pronounced /loft/ dammit!)
guys didn't do any updates on it until Wednesday. Ok, so I'm easily
amused. We're still looking for interesting articles on damn near
everything, so if you'd like to help us out and get read by a thousand or
so people, this is how you can do it pretty easily. Also, if you submit
something to us, and you'd like to remain anonymous, just indicate this in
your email and we won't include your name or address or both. I'd also
like to give thanks to my co-editor ajax, who's been very helpful in
proofreading and reformatting awkward parts, as well as doing commentary.
[ /me blushes. Heh, I remember way back when an old associate first
showed me the l0pht homepage. He insisted on calling it "low-fat".
I dunno where that extra vowel came from. Did we mention being easily
amused? {ajax} ]
***********************************************************************
*** Random good URLs : Kynik
***********************************************************************
The Open Server Architecture Project: The Win32 solution for Apache
http://www.opensa.com/
Lance Spitzner's "Passive Fingerprinting" article
http://www.enteract.com/~lspitz/finger.html
A 'handmade' crypto challenge
http://www.jdueck.org/challenge.html
Keep an eye on security incidents, maybe report your own
http://www.sans.org/giac.htm
An interesting article about cyborgs
http://home.fuse.net/mllwyd/cyborgs.html
***********************************************************************
*** Readers' Questions
***********************************************************************
Lockdown <llh@student-net.co.uk> wrote:
I'd like to find out more about you ghost net project..is it a vpn, or what?
I've also got a few crap articles I could give ya.
Cheers,
Lee 'Lockdown' Hughes
[ Well, technically, it's not 'my' gh0st.net project. It's something I
stumbled into, and am starting to get more involved in. The gh0st.net
URL is posted at the top of every issue so far, and the most complete
information is there. If you have specific questions, you can send them
to phatal@gh0st.net - he's the guy running the show and cracking the
whip. And as for 'crap articles' - no thanks. The world has enough noise
already. Quality articles are acceptable though. {kynik} ]
[ And just to pre-answer some questions: gh0st.net is primarily about
security research. This is a pretty wide spec, and more than a few
things would be considered "in the scope". Among them might be code
auditing, VPN setups, capture-the-flag games... basically, if in doing
it, we learn a concept about security that we can apply, it counts.
Current projects in the pipeline include setting up various VPN
implementations (possibly using IPv6), the various open boxes (tulkas,
english) that are up for intrusion attempts, probably some others I'm
forgetting. Of course, we all have lives too (well, *I* don't, but
everyone else claims to), so progress may seem a little slow at times;
in fact, I partly wrote this whole description to save phatal from a
deluge of email about it - we'd all rather be doing than talking anyway.
Not An Official Gh0st Net Statement, but probably close. {ajax} ]
-----------------------------------------------------------------------
Jason Holt <jason@community.net> wrote:
Your just intonation article was *great*. It's by far the clearest
article I've ever read on the mathematics and actualities of music. I
wondered about it for years, and finally worked out the x*2^(n/12) formula
on my own - then this article filled in all the gaps.
One thing I'm still wondering, though. I've heard that baroque
instruments were tuned slightly differently than they are today.
Something about even tempering vs. some other kind of tempering. Any
idea why that was, or what the differences were?
Thanks for a great article.
[ Baroque instruments were tuned to just scales. That's why a lot of
pieces written before the piano and harpsichord were in one key; or,
some say, when they modulated, they still sounded like they were in the
old key, lending a different tone to the piece. Hope this answers your
question. {ajax} ]
***********************************************************************
*** Readers' Comments
***********************************************************************
NIBLE <n1bl3@yahoo.com> wrote:
The article on AI Security has good points in preventing some of the root
problems on system security. One alteration that we could make to improve
performance of inspection would be to assign one host as the trusted
inspecting station where all new patches for a domain/cluster would be
inspected and tested before installed on other internal systems. This
method will accomplish the following:
a) Isolate performance degradation of inspecting updates on one host
versus all participating hosts.
b) Detect possible malicious code prior to installing on all machines
thus allowing early isolation.
c) Provide a single point for maintaining new methods of inspection versus
updating all participating hosts.
Although the argument of "How much can you trust this one host?" can be
raised, there are some answers.
The primary assumption was that the distribution host was trusted.
Another approach would be to set up two hosts that both receive the
updates, verifying the updates between them for possible infection upon
transition, and after all checks have been completed designate one host
to be the primary distributor and the other the backup, thus resolving
redundancy as well. :-)
Regards
D' n1bl3 (nible)
[ Thanks for your input. Soon I'll be releasing AI security II--don't miss
it! A traffic monitor daemon using a backprop neural net is currently in
the works. I think it'll be interesting to see if I can train and
release a set-weight neural network that's effective in detecting
probing and intrusion attempts. I'm confident that article will raise
from eyebrows. {Blakboot} ]
***********************************************************************
*** Contemporary Telenet I : blakboot
***********************************************************************
Introduction
-------------
Security awareness and exploitation is a fast game on the Internet.
Staying on top, whether it be for intrusion or consultation, requires
onerous research; research that never ends. Before I came into this
scene, most of my experience came from esoteric networks, BBSing,
wardialing spoils, et cetera. Regardless, nothing has sharpened my
knowledge and awareness of computer systems more than this vast network
of hustle and bustle. If we could look back in in time, what wonders;
what system vulnerabilities would we laugh about? If we could step back
in time a bit, what things could we get into? What industries never
quite caught up with the future, and what would their ignorance allow us
to plunder?
Please excuse me, I have left out a lot of information for sake of time
(our favorite editor wants results), and file size. There will be an
article forthcoming that will cover much more on contemporary usage. This
is a primer.
[ Yeah, working under a pseudo-deadline sucks, eh? Turns out that we're
already over my target per-issue size even without this article, but
that's ok. I'm confident that this is quality. {kynik} ]
Enter Telenet
-------------
Telenet, commercially known as Sprintnet, but forever referred to as
otherwise, is an X.25 network. Dialups nationwide are still active, and
systems still lie sparsely about it. Herein I have provided a working scan
script, and some of the spoils from that.
- What systems can you find on Telenet?
This isn't a definitive list by far, but what I've been: VMS, Primenet,
assorted unix clones, Lantronix type deals, arbitrary systems/databases.
- How do you get on Telenet?
Anyone with basic telecommunications knowledge doesn't have to read
this. First, get a terminal emulator. These programs allow you to receive
relatively protocol-free data. It's nothing like your damned PPP/SLIP
connection; raw data (with the exception of emulation) is displayed from
the remote computer. I suggest Telemate, Telix; anything but
hyperterminal.
For the connection to be possible and coherent, set your baud rate to
1200bps (some dialups support 14.4) and data bits to 7. Most connections
to remote computers are 8 bits, although X.25 networks are an exception.
You should know that the possible combination of 8 bits is 256; it means
that on an 8 bit connection, we can take advantage of 256 characters.
Telenet can only send and receive data consisting of one of the 127
bytes, combinations of 7 bits.
[ Correct me if I'm wrong here, but won't most modern modems auto-set
their baud rate depending on how the dialup handshakes? {kynik} ]
[ We'd like to think so. Some old modems don't like to talk to newer ones
though. Backwards combatibility. And besides, it can't hurt. {ajax} ]
With that said, know that if you want to transfer binary files over
Telenet, you have to use the kermit protocol, because zmodem, ymodem,
xmodem, etc. are 8 bit protocols. Kermit is a slow bastard and time has
blessed us with its death in modern file transfers. My suggestion for
transfering files over a 7 bit connection is to use uuencoding (unix to
unix encoding). This will break down those extended ascii characters
into plaintext, and then all you have to do is uudecode on the remote
system.
Once you've configured your terminal program with the two
specifications above, it's time to connect to Telenet. The toll free
Telenet dialup is 1-800-546-2000.
[ When dialed from some area codes, you may receive a message saying "You
have entered a number that can not be reached within your calling area."
then a unique number code, in my case "47530" I don't exactly know what
the numbers there stand for, but it is interesting that it looks quite
like a zip code :-/ {Reverse Corruption} ]
Once you've connected, press enter two times; it will ask you for what
type of terminal to use. Just type in D1, vt100, whatever. From here,
you've a @ prompt. To get your local dialup, type "mail". It'll enter a
login procedure. Use the login/password: phones/phones; this will execute
a script which allows you to list all local dialups.
- Connecting to computers
This is easy, and the article shouldn't cover it, although I'm going
to get past it, and open up into more dynamic aspects of the network in
Contemporary Telenet II.
From the @ prompt, you can connect to systems hosted by sprintnet,
and other X.25 networks. To connect to a system on the current network,
just type the NUA (Network User Address); if you want to connect to a
computer on another network, you'll have to provide a DNIC. (Data Network
Idenification Code). An NUA consists of two things. An NPA (area code)
and an address, which can be any floating point number greater than 1
(there's a limit - that i do not know). Decimal places of an NUA usually
indicate something similar to ports in TCP/IP.
So, if I wanted to connect to a system in Tallahassee, FL. An example
session would be something like:
@ c 90423
904 23 CONNECTED
Username:
To disconnect from the system or interrupt a pending connection, press
@ followed by a carriage return; complete the disconnect by typing D from
your pad.
Now, if you wanted to connect to a system on Tymnet (another X.25
network), you would type an NUA something like:
@ c 0310690423
Where 03106 is your DNIC, 904 the area code, 23 the address. Easy pie.
Here's the NUA scanner script. It's for Telemate (IMO, one of the best
emulators), and you need TMS.EXE, the script compiler. I also highly
recommend this scripting language; I learned it in under 30min and it's
quite useful, taking the hassle out of communication routines.
The scanner works well on my dialup, though I suspect the different
nodes sometimes will act strangely; causing the scanner to get off beat.
That's just speculation though; I believe I'd gotten all the bugs out.
it's sensitive and will reconnect to telenet with the smallest signs of
what it suspects as a frozen node; and so, sometimes it disconnects
unnecessarily. Please excuse that. Otherwise, it's sleek and records
connections better than the old NUA Attacker program by Docter Dissector,
which was good, but somewhere along the line Telenet return messages may
have changed, causing NUAA to record unwanted connection attempts. If I
remember correctly, it would record network congestion (which you will
get frequently these days).
; NUA SCANNER v1.0 : TMscript
; Compiled & tested w/ Telemate v4.20
; Blakboot [FS] '00
; BUG:
; Only in applied scan mode, it doesn't increment the NUA
; when the pad freezes on a pending connection.
integer nua,dialtelenet,t1,t2,cw,npa,max,pending,float,c,aspm,odata,obaud
string telenet,past,present,tmp1,tmp2,filename,i
; ---- configuration ----
filename = "C:\TERMINAL\SCAN\N.TXT"; Full path
telenet = "1-800-546-2000" ; You can add any prefixes you want
npa = 305 ; Area code and
nua = 22 ; NUA to scan
max = 1000 ; NUA to stop at
cw = 10 ; Time in seconds to wait for connect
aspm = 0 ; Applied Scan Mode [1/0]
;-------------------------
procedure esc
inputch i
if success
if i="^["
print "^M^MTerminating scan."
close
put "@"
put "hang"
hangup
set baud,obaud
set data,odata
stop
endif
endif
endproc
query data,odata
query baud,obaud
set baud,1200
set data,7
put "ats11=40"
delay 5
clear text
print "Press escape at any time to terminate the scan."
print "Opening NUA log file: ",filename
append filename
if not success
print "Error opening ",filename,"^MTerminating script."
stop
endif
date tmp2
time past
strset tmp1,"-",1,79
write
write "Scan session started on ",tmp2,", ",past
if aspm
write "* Applied Scanning."
endif
write "NPA/NUA: ",npa,nua," - ", npa,max
write tmp1
print "Dialing Telenet..."
repeat
repeat
dialtelenet=0
put "atdt",telenet
time past
prob=0
while not connected
esc
time present
substr present,4,5,tmp1
substr past,4,5,tmp2
atoi tmp1,t1
atoi tmp2,t2
waitfor "busy","no carrier","voice",1
if found
prob=1
exit
endif
if (t1-t2)>= 2
prob=1
exit
endif
endwhile
if prob
print "^M^MRedialing..."
put "^M~~"
endif
until not prob
delay 20
put "^M^MD1"
delay 20
clear com
repeat
esc
itoa npa,tmp1
itoa nua,tmp2
concat tmp1,tmp2
clear com
if c
concat tmp1, "."
itoa float, tmp2
concat tmp1, tmp2
endif
put tmp1
waitfor " connected","not","dis","81","00","BB","D4",cw
if not found
clear com
put "@"
waitfor "telenet","@",5
if not found
dialtelenet=1
errmsg="Node froze."
exit
else
clear com
put "d"
waitfor "@",10
if not found
dialtelenet=1
errmsg="Node froze when trying to abort."
exit
endif
endif
else
clear com
switch found
case 1:
clear com
if c
write " ",
endif
write tmp1
close
append filename
delay 10
put "@"
put "d"
if aspm
if not c
float=0
cw=cw+10
c=1
endif
endif
waitfor "disconnected",5
case 5:
endswitch
if not found=1 ; if not connected
waitfor "@",5
endif
clear com
if not found ; found could = "@",
dialtelenet=1
t1=nua
if c
nua=nua+float
endif
print "PENDING: ",pending," NUA: ",nua," T1: ",t1
if pending=nua
nua=nua+1
else
pending=nua
endif
nua=t1
errmsg="Node froze when pending another connection"
exit
endif
clear com
endif
if c
if float=9
c=0
cw=cw-10
nua=nua+1
float=0
else
float=float+1
endif
else
nua=nua+1
endif
until nua>max
print errmsg
print "Reconnecting to Telenet..."
hangup
until not dialtelenet
; [SNIP--end of code]
Here are some scan results. No commenting 'cus I was lazy; this is
basically just some spoil I'm grabbing out of my archive. These are not
very old. Maybe a few months.
NPA/NUA: 30556 - 3051000
-------------------------------------------------------------------------------
30559
30559.1
30559.2
30559.3
30559.4
30559.5
30559.6
30559.7
30559.8
30559.9
NPA/NUA: 7160 - 7167000
-------------------------------------------------------------------------------
71623
71623.1
71623.2
71623.3
71623.4
71623.5
71623.6
71623.7
71623.8
71623.9
71625
71625.1
71625.2
71625.3
71625.4
71625.5
71625.6
71625.7
71625.8
71625.9
***********************************************************************
*** Ethernet - The Bottom Two Layers : bobtfish
***********************************************************************
There are lots and lots of articles about TCP/IP, how it works and how
to hack it, however there is very little information (for the poor hacker
who cannot afford text books) about actual ethernet itself, where it came
from and how it works. I hope to go some way to correct that in this
article.
Using the OSI (Open Systems Interconnection) networking reference model
ethernet takes the bottom two layers, the data link layer and the
physical layer. I intend to talk about both of these layers in detail
however first I will give a brief introduction to the ethernet system.
Introduction
------------
Ethernet was invented by Xerox, DEC and Intel. It grew from a system
researched ar Xerox PARC (Where such things as mice and GUIs came from)
where they built a 2.94Mbps system. (Mbps = Million bits per second) This
system was the son of a system called ALOHA constructed to allow radio
communication between the Hawaiian Islands.
[ The great thing was, this was rounded up to 3Mbps for marketing. Some
people objected to a roundoff error greater than the entire bandwidth
of ARPANET at the time... {ajax} ]
Ethernet is sometimes called IEEE 802.3 however this is wrong. IEEE
802.3 is *very* similar to actual ethernet except 802.3 describes a whole
slew of systems running from 1-10Mbps on various media (more than
ethernet) and a field in the packet header is differs between ethernet
and 802.3. Now, you're thinking, there is an 802.3, but what happened to
802.1 and 802.2? Well, 802.1 is an introduction to the 802 standards and
defines a set of primitives and 802.2 describes the upper part of the data
link layer (which we don't give a toss about right now). Additional info
ref #1
Ok, back to ethernet then. Ethernet is a CSMA/CD protocol, which stands
for Carrier Sense Multiple Access with Collision Detection. Don't worry,
I didn't understand it first time either so I'll run through it bit by
bit:
Carrier Sense - The system looks at the cable to see if anything is
transmitting before it does. (So that two machines are not
trying to send data down the same wire at the same time.)
Multiple Access - Multiple machines can access the same communication
channel to send data. Ergo there is only one set of
wires no matter how many machines you have.
Collision Detection - If a station is transmitting and two stations are
waiting then when the first station stops they will
both try to transmit at once, meaning the data will
be garbled. Collision detection means they detect
this and sort it out somehow. (More on this later)
Note that Ethernet does *not* guarantee reliable delivery of the data -
even if it is sent correctly without problems the receiving machine may be
so loaded that it does not have spare buffers to put the data in so it may
be erased.
Types of ethernet - The boring stuff.
-------------------------------------
Since Ethernet refers to the 'ether' ie the medium the signal passes
through we may as well start our discussion on cables.
Name Cable Max segment Nodes/seg Comments
-------------------------------------------------------------------------
10Base5 Thick coax 500m 100 Old - Not used
10Base2 Thin coax 200m 30 Cheap
10BaseT Twisted Pair 100m 1024 Standard
10BaseF Fibre 2000m 1024 Building<>Building
100BaseTX Twisted Pair 100m 1024 Fast
100BaseFX Fibre 2000m 1024 Expensive
I will deal with these in order in the table.
10base5 is the oldest (and obsolete in anywhere but the poorest
universities). It is called thick ethernet because it is yellow and
resembles a garden hose with markings every 2.5 meters. (The 802.3
standard suggests the cable should be yellow but does not require it ;) )
Connections are made using vampire taps in which a pin is forced 1/2 way
into the core which are then connected to a transceiver. This transceiver
invariably connects to the host computer using AUI which if you see it on
a hub or network card look like parallel ports (D shaped connector)
10base2 is known as thin ethernet and in contrast to 10base5 bends
easily. Connections are made using BNC type connectors to form T
junctions in the cable. Thin ethernet is MUCH cheaper and easier to
install than 10base5 but can only run 200 meters and can handle only 30
machines per segment. Both of these systems have a big problem: any bad
connection, wonky BNC connector or cable break will cause the entire
network to fall apart. The only reliable way to find these breaks is to
pull out each cable and T-piece and replace them one by one (which means
quite a long network downtime with 30 machines) or to use an expensive
machine called a 'time domain reflectometer' which injects a specially
shaped pulse into the cable and waits for it to echo back (the echo is
caused by the fault). This allows the fault to be pin-pointed. The phreaks
amongst you will know that a time domain reflectometer can also tell you
if someone is tapping your phone. Well, before it gets to the exchange
that is...
These types of problems prompted the development of 10baseT which uses
a different kind of wiring pattern with every machine going to a central
hub which receives and re-transmits the signals to every other connected
station meaning that a cable break will disable one machine, not the
whole network. A large hub for many stations costs a lot of money but it
means that adding or removing a station can be done without halting the
network.
Another option is 10BaseF which uses fibre optics. This is expensive
due to the cost of fibre and the connectors and terminators but has
excellent noise (and tempest) immunity and is the connection of choice
for low speed links between buildings.
[ There are sub-standards 10BaseFB, for inter-repeater links, and
10BaseFL, for links to workstations. As far as I can tell, this was
done simply to aggravate people. You may also run into an older
standard called Fiber Optic Inter-Repeater Link, or FOIRL. If so,
good luck to you. {ajax} ]
A quick note about repeaters - 10base5, 10base2 and 10baseT all have
quite small maximum segment lengths so to allow larger networks segments
can be connected with repeaters. These are a physical layer device which
take the signal, amplify it and send it on its way. As far as the
network is concerned there is no difference (other than electronic delay
introduced by the repeater) A network can contain as many segments and
repeaters as required as long as no two machines are > 2.5km apart and no
path between two machines has more than 4 repeaters. (Why these
restrictions are present will be discussed later.)
100baseTX is now quickly becoming the standard for new installations and
is almost the same as 10baseT technically. (coax cables were dropped due
to the overwhelming advantages of a hub-based design) Another good feature
for the network engineer is that the same wires are used for the same
thing meaning you don't need different cables. (However some poor-quality
cables that work at 10Mb/s will not work at 100Mb/s.
A coding scheme called 4B5B is used at 125MHz with 5 clock periods
transmitting 4 bits of data. 100baseFX uses two strands of multimode
fibre, one for each direction and has the same advantages discussed with
10baseF. This is all this paper will say about fast ethernet. Readers are
referred to ref #2 if interested.
Manchester encoding - The interesting stuff
-------------------------------------------
Ethernet does not use straight binary encoding with 0 volts for 0 and 5
volts for 1 as it would lead to ambiguities because stations would not be
able to tell the difference between an idle sender (0 volts) and a zero
bit (0 volts).
What is needed is a system that lets receivers tell the start, middle
and end of each bit with no reference to an external clock. A system
called manchester encoding is used where binary 1 is sent by having the
voltage high during the 1st half of the bit and low during the second. A
binary 0 is sent as a low during the first 1/2 of the bit and a high
during the second. This means every bit has a transition in the middle
making it easier for the receiver to synchronize with the sender. The
disadvantage of Manchester encoding is it requires twice as much
bandwidth as straight binary encoding because the pulses are 1/2 the
width. It is shown below:
Bit stream: 1 0 0 0 0 1 0 1 1 1 1
Binary : --________--__--------
Manchester: -__-_-_-_--__--_-_-_-_
[ Hey bobtfish - did Manchester encoding actually come from Manchester in
the UK, or was it arbitrarily named? {kynik} ]
The high signal in ethernet is +0.85V and the low signal is -0.85V. This
gives a DC value of 0V.
The MAC sublayer protocol - The really interesting bit.
-------------------------------------------------------
Bytes:
7 1 6 6 2 0-1500 0-46 4
|Preamble| |Destination| Source | | Data | Pad | Checksum |
| | | address |address | | | | |
Each frame starts with a preamble of 7 bytes, each containing the bit
pattern 10101010. This, when manchester encoded produces a 10MHz square
wave for 5.6usec to allow the receiver's clock to synchronize to the
transmitter's. Next comes a start of frame byte containing 10101011. The
source and destination addresses come next. The address containing all 1
bits is reserved for broadcast which is delivered to all stations on the
network. The minimum frame length is 64 bytes, from destination address to
checksum and so if the data is less than 46 bytes then the pad field is
used to pad the data to 64 bytes. This stops a station that is
transmitting a short frame from completing before the first bit has
reached the other end of the cable, where it may collide with another
frame. (Remember we can have 2.5km of cable and 4 repeaters in there -
quite a large delay).
If a station detects a collision (by sensing more power on the cable
than it is putting out) then is aborts its transmission and transmits
48bits of noise to warn all the other stations. It then waits a random
amount of time before sensing the cable to try and transmit again. If the
frame was too short then if a collision occurs the sender could conclude
that it was successful as the noise burst does not get back before it has
stopped transmitting.
As network speed increases the minimum frame length must go up or the
maximum cable length must come down. For a 1Gbps LAN the minimum frame
size would be 6400bytes with a 2.5Km maximum distance. This is called the
long fat pipe problem. (Which if you do any studies of high-speed
communication you will come across quite often)
The final field is called the checksum. It is a 32bit hash code of the
data using a cyclic redundancy check. If some of the data is wrong then
the checksum will almost certainly be wrong.
Binary Exponential Backoff (And other things with no amusing acronym)
---------------------------------------------------------------------
We now know how ethernet stops two machines transmitting at the same
time, however how does it arbitrate between them? Well since there is no
designated 'master' machine, (which is why receivers have to synchronize
their clock to the sender - there is no master clock), the two stations
must perform this arbitration between themselves.
After a collision stations divide time up into discrete slots of length
512 bit times, or 51.2usec.
After the first collision, each station waits either 0 or 1 slot times
before trying to transmit again. If they collide again each station picks
0, 1, 2 or 3 at random and waits that number of slot times. If a third
collision occurs then the next time the number of slots to wait is chosen
at random from 0 to (2^3)-1. This random time is increased exponentially
until ten collisions have happened, at this point the randomization is
stopped at a maximum of 1023 slots. After 16 collisions the controller
gives up, goes for a beer and reports failure to transmit.
[ Ethernet beer? Sounds like an IPO! ;) {kynik} ]
This is called binary exponential back off (and has with and without
beer options ;) ) and was chosen to dynamically adapt to the number of
stations trying to send. If the randomization interval was fixed at 1023
the chance of 2 stations colliding a second time would be greatly reduced
but the average delay would be 100s of slots. However if each station
always delayed 0 or 1 slots then if 100 stations were waiting to transmit
then they would collide until 99 picked 0 and 1 picked 1 or vice versa..
By having the random time grow exponentially the system gets the lowest
delay at low load but enables the collision to be resolved when lots of
stations want to transmit.
Switching
---------
As you add more stations to an Ethernet the traffic (naturally) goes up.
Eventually the system will saturate (And with lots of machines waiting to
send efficiency goes down the toilet). There are a number of ways to
resolve this. First one could increase the speed of the LAN. (ie rip out
all the 10BaseT cards and put 100BaseT cards in) however this is not
necessarily practical. Another route to go is to segment groups of
machines that communicate a lot onto different physical networks and use a
bridge or router to connect them. The way we will look at here is a
switch.
A switch is like a hub except that it has inside it a microprocessor and
a very fast internal bus. When a station sends a frame the switch checks
where it is destined for and copies it across its internal bus then
sends it out to the other station. If the internal bus is busy then the
switch buffers the packet in internal ram and then forwards it when the
bus is available. This means that (since the internal bus is many times
faster than the ethernet) you can theoretically get an aggregate bandwidth
of number of stations * speed of network. This is because each port forms
it's own collision domain. This also gives that added advantage that
full-duplex operation can be supported. (ie A station, if its ethernet
card supports it, can be both transmitting and recieving a frame at the
same time)
However, if all stations on the switch are trying to contend to send to
one particular station there can be problems. If one machine is a server
and the rest are clients, all of which are making requests (Using all the
10Mb/s bandwidth on their port) then you have an aggregate of 120Mb/s (on
a 12 port switch) which can never get through. How a switch handles this
situation is manufacturer dependent.
Nowadays a common item is a switch with one or two 100baseT ports and
10 or so 10baseT ports. This goes some way to solve the above problem as
few workstations need more than 10Mb/s however a server can easily use
100Mb/s to serve it's clients.. So with 10 clients at 10Mb/s and a server
at 100Mb/s then each client can get a full (and both ways) 10Mb/s of
throughput. And you can happily boot over the network and run all your X
applications on the remote machine at 10Mb/s. (Ok I wouldn't like to try
remote Quake 3... But hey...)
Another advantage of a switch (to a network administrator) and
disadvantage (to a black-hat) is that any machine connected to the switch
will only see traffic destined for that machine. That is, an ethernet
sniffer will catch no more than local users accounts and passwords.
[ Not entirely true. Most switches have a MAC (ethernet) address table
in internal memory, with possibly multiple MAC addresses associated
with a single port. Some switches will forget MAC addresses after a
period of inactivity. Occasionally, a host will have the MAC address
of another host in its ARP cache, but since the switch no longer knows
what port the destination host is on, it will be forced to broadcast
the packet. Oops. Of course, some switches are even smarter and have
their own MAC address, and can do ARP queries for machines they forget
about. Not a major problem, but don't trust switches to protect you
from sniffing; besides all this, some are just buggy. {ajax} ]
End notes
---------
I hope this article has given you a few clues about how Ethernet
actually works if you didn't know already, and even if you did it might
have told you some interesting history, but maybe not.
Just a couple of (well 4) quick notes:
1) When transmitting IP over ethernet there is a system called arp for
matching IP addresses and physical network addresses. (Look in
/proc/net/arp under linux I believe for the arp of the rest of your
network or ifconfig for that of your ethernet adaptor.)
2) Ethernet hardware addresses are meant to be unique - I know of
multiple instances of people having 2 cards with the same address.
3) Microsoft Office products embed your hardware address in documents.
This is a pretty unique identifier. (And how they got the dude who
wrote Melissa). Get vi now.
4) Microsoft Windows 95 (I believe but I'm not sure - its one of them) is
dumb. If you make an ethernet packet addressed to FFFFFFFFFFFF
(broadcast) but with the station's IP address then it will accept it
as arriving at the station's IP address, not by broadcast. (This would
work for any ethernet address but the hardware in the ethernet card
filters out packets not for the machine or broadcast.)
References
----------
#1 - Stallings, W - Local and Metropolitan Area Networks. 4th Ed.
Macmillan 1993
#2 - Johnson, H. W. - Fast Ethernet-Dawn of a new network, Prentice Hall
1996
***********************************************************************
*** Music Reviews : kynik, bobtfish, ajax, orbitz
***********************************************************************
We have two songs this issue from fairly different genres. The first is
"One Day" by the Pinkerton Thugs. They can be found online at:
http://www.pinkertonthugs.org/
BobtFish's review
-----------------
Originality - 2
Talent - 4.5
Production - 4
I like it - 3.5
What can I say about this song, it's a 1:56 long, speedy punk song about
hating the world with a dodgy sample at the start.
However whilst it isn't anything that ground breaking here it is a good
song, the riffs are solid and the drumming is interesting. The vocals
are good and appropriate and you can hear all the parts individually so
it's reasonably well mixed. The thing that I really liked about this song
is it's catchy, very catchy, makes you want to bounce out the house and
throw bricks through the neighbors' windows :) Which is what good punk is
all about.
I don't think I'd run out and buy an album by these guys, or even be able
to listen to an album all the way through, but as a single song I rather
enjoy it and would probably dance to it if I heard it in a club and was
stood up.
Kynik's Review
--------------
Originality - 2.5
Talent - 3
Production - 4.5
I Like It - 4
I'll admit, I'm a big punk fan, and this song makes me yearn for the days
of oldschool hardcore. Straightforward, to the point, and undoubtedly
punk. Unfortunately this particular song is a bit bland, even for punk.
The vocals are good, and it sounds like the singer (unlike many punk
bands) might actually have the ability to sing if he wanted to. I would
have brought the level of the bass guitar up a bit (being a bassist, I
want to be able to hear it) and the drums down just a touch. Nothing
new here, but good punk if you like punk.
ajax's Review
--------------
Originality - 1.5
Talent - 4
Production - 4.5
I Like It - 3
Here's the thing about punk rock: there's very little room for
creativity. Watch, I shall demonstrate. The verse and instrumental bits
consist of a I-V-vi-IV chord progression. The chorus runs "Your so-called
order amounts to inequality / One day, we'll make the bastards pay / Oi!".
The chorus goes iii-IV-V-I. It runs for one minute and fifty-six seconds.
See? You now know exactly what this song sounds like.
Maybe I'm disillusioned, but punk still hasn't changed the world, and
neither have punk rock kids. To its credit, the song sounds very well
produced, and the band sounds tight and doesn't drag for a second.
Punk's got its place, and every once in a while I enjoy it. Every once in
a while I like hip-hop, too. And while I'd certainly see these guys live
- I'd like to know what the rhythm guy is using for his distortion, nice
and crunchy - I can't see myself spending money for punk CDs. I like a
little creativity in my guitar rock, and most of this "revolutionary",
idealistic punk sounds like a broken record. I'm amazed the groove hasn't
worn through yet.
The other is "Preacher" by My Ruin. My Ruin's homepage is unsurprisingly
at:
http://www.myruin.com/
ajax's Review
--------------
Originality - 3.5
Talent - 4
Production - 3.5
I Like It - 4
Okay, so I'm biased in favor of female vocalists. Sue me. This song is
downright creepy, while still rocking. The rhythm guitar line is not
terribly original, but the lead makes up for it by being un-obvious, and
the bass counterpoints it well.
The singer's got a better-than-decent voice; it's a shame she hides it
behind that stereo chorus effect. Maybe if she turned the intensity down
a notch, the difference between the right and left is a little harsh. Of
course, I only listened to it in headphones, so this probably isn't a
problem. The drummer, on the other hand, sounds like he's kicking a
cereal box, and the snare drum sounds basically the same but with more
reverb. A shame, since it makes him sound terribly untalented, and the
treble on the drums takes sonic space away from the vocals.
Overall, though, turn up the bass and scare your neighbors.
Kynik's Review
--------------
Originality - 3.5
Talent - 3
Production - 3
I Like It - 4
This is a pretty decent song. I thought at first it was something new by
the Genitorturers, as the sounds are VERY close. (I'm actually not sure
who came first, My Ruin or Genitorturers) If you like one, you'll probably
like the other. I'm a fan of female-fronted rock bands (ask any of my
friends) and while I like this one, I'm not really impressed as much as I
have been before. I tend to go for more extreme vocal ranges, such as very
pure almost operatic singing to screaming or screeching. While the singer
does hit on both of those, it's not used to its fullest "wake up boy!"
potential. It's good guitar-driven industrial-ish music, and with a better
producer (the mix was weak at times) this song would have a bigger public
appeal.
Orbitz's review
-----------------
Originality - 4
Talent - 3.5
Production - 4
I Like It - 1
I did not care much for this song. Hence the 1.0 on 'I Like It'. I
thought the opening bass was pretty nice. I am not much into goth type
music. Talent got a 3.5 because I did not much care for the lyrics but I
liked the opening bass a lot so the song got points for that. Tune
sounded like it was mixed together pretty good. Originality is up
because I haven't heard much music like that. Overall I did not like this
song and found the lyrics to be weak.
Overall Rating, "One Day"
-------------------------
Originality - 2.00
Talent - 3.83
Production - 4.33
I Like It - 3.50
Total - 13.67/20.00 (68.35%)
Overall Rating, "Preacher"
-------------------------
Originality - 3.67
Talent - 3.50
Production - 3.50
I Like It - 3.00
Total - 13.67/20.00 (68.35%)
[ I swear I did not plan for that tie to happen. {kynik} ]
***********************************************************************
*** Future Issues
***********************************************************************
Contemporary Telenet II
***********************************************************************
*** Credits
***********************************************************************
Editor: Kynik <kynik@firest0rm.org>
Co-Editor: ajax <ajax@firest0rm.org>
Article Contributions: Blakboot <blakboot@firest0rm.org>
bobtfish <bobtfish@firest0rm.org>
Music Reviews: orbitz <orbitz@firest0rm.org>
Commentary: revcorrupt <revcrupt@firest0rm.org>
***********************************************************************
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