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Multiply
Equals Discretion-Based Networking
Multiply CEO Peter Pezaris has moved beyond his past of contributing to Xemacs
and selling companies to CBS Sportsline to help build a social networking site;
we managed to avoid discussion of a certain other social media site in a recent
conversation.
Hackers
Told To Hack Off Ponytails
Open source developers are the grunge rockers of the new millennium, the originals
long dissolved in corporate formulaic art. Unlike their barefoot hippie predecessors
(who woke up one day in Birkenstocks as Volvo-driving yuppies), they kept their
ponytails as an affront to The Man, sharing their warez with the Kool-Aid drinking
masses.
Making
VoIP Perform as Advertised
The promise of lower cost has motivated many enterprises to move their voice traffic
to the enterprise IP wide area network (WAN). Many have discovered, however, that
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Microsoft
Open Port 25 To Hackers
For once, the correct definition of hackers applies here; the new Port 25 website
from Microsoft offers communications from the company's Open Source Software Lab.
BGP
Adjacency States
To pass the BSCI exam, earn your CCNP certification, and become an outstanding
networker, you´ve got to master the many details of BGP - and trust me,
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Dynamic Trunking Protocol (DTP)
When you´re studying to pass the BCMSN exam on the way to earning your CCNP certification, you´re going to add to your CCNA knowledgebase every step of the way.
Static VLANs
BCMSN exam success and earning your CCNP certification requires you to add to
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US
Backs Microsoft In Antitrust Fight
Microsoft has long complained that antitrust regulators with the European Union
have been holding back information related to the investigation, and US diplomats
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What New DSL Router?
I'm going to try to get someone's opinion on this for the last time before I try
getting some replies in a different forum. This is the 3rd time I've asked this
question. I am looking to buy a new Wireless Cable/DSL Router for a hotel which
has 10 workstations and 1 server. It needs to have the following facilities:
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04.10.06
Who Said That?
By A.P. Lawrence
My wife and I had run out for errands and had stopped at a D'Angelo sub shop.
We were just about to get out of the car when the phone rang. My wife rolled her
eyes but I took the call because, well, because that's what I do. Somebody has
a problem, I try to answer the phone.
This was a long time customer. He explained that he had someone that needed remote access to an application running on port 7000 on a SCO machine. He said it was working fine within his office, but that he couldn't get remote access working. We did the usual things: telnet localhost 7000 (yup, that connected), ssh out to another box and try telnet back on 7000 (which just hung). I asked him to double check his router, and he did, but still no luck.
"Oh wait a minute,", I said, "you said this was a SCO box? You mean that old test box you've had for years?"
This guy sells accounting software and some of his customers still run on SCO. Most of them use Linux now, but he keeps the SCO up and running for testing and support. But I was pretty sure that box had never been connected to the internet.
"You need a default route. Packets can get to it, but it doesn't know how to send them back."
This comes up fairly frequently. It also happens sometimes with remote TCP printers: if you haven't given them a gateway address, they can't communicate back to you. Packets need routes.
But why? That's what my caller wondered. Why doesn't it just figure out where to send the packets?
Well, for starters, the only machines any box can talk to are machines on its local physical lan. So obviously it needs to send any other packets through a router. Is it supposed to guess where the router is?
No, of course not. Either somebody tells it (route add) or it finds out through some protocol like router broadcasts. But if it doesn't have either of those advantages, it's blind and dumb.
But is it really? Let's look at some tcpdump output.
On my Mac, I first ssh'ed to a local (on my network) Linux box. In another Window on the Mac, I typed "sudo tcpdump -e host redhat1". Over on the Linux box, I simply tapped the space bar (to avoid a lot of packets). Here's the result:
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on en0, link-type EN10MB (Ethernet), capture size 96 bytes
07:55:59.946527 00:16:cb:8d:48:f7 (oui Unknown) > 00:c0:f0:6b:b4:18
(oui Unknown), ethertype IPv4 (0x0800), length 114: 192.168.4.21.50050
> redhat1.ssh: P 1320087778:1320087826(48) ack 1103513747 win 65535
07:55:59.947496 00:c0:f0:6b:b4:18 (oui Unknown) > 00:16:cb:8d:48:f7
(oui Unknown), ethertype IPv4 (0x0800), length 114: redhat1.ssh >
192.168.4.21.50050: P 1:49(48) ack 48 win 10336
07:55:59.947565 00:16:cb:8d:48:f7 (oui Unknown) > 00:c0:f0:6b:b4:18
(oui Unknown), ethertype IPv4 (0x0800), length 66: 192.168.4.21.50050
> redhat1.ssh: . ack 49 win 65535
Those hex digits following the date on each line should look familiar to you: those are MAC addresses. One of them is the address of the MacBook itself (00:16:cb:8d:48:f7) and the other is the RedHat box. Specifically, if I run "arp -a", I can see that, yes, that Mac address matches redhat1:
$ arp -a
redhat1 (192.168.4.237) at 0:c0:f0:6b:b4:18 on en0 [ethernet]
router (192.168.4.249) at 0:8:0:c0:fa:4 on en0 [ethernet]
$
Now what about if I go somewhere off-lan?
$ sudo tcpdump -e host website
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on en0, link-type EN10MB (Ethernet), capture size 96 bytes
07:57:51.020660 00:16:cb:8d:48:f7 (oui Unknown) > 00:08:00:c0:fa:04
(oui Unknown), ethertype IPv4 (0x0800), length 106: 192.168.4.21.50027
> website.ssh: P 3652278386:3652278426(40) ack 193102205 win 65535
07:57:51.057326 00:08:00:c0:fa:04 (oui Unknown) > 00:16:cb:8d:48:f7
(oui Unknown), ethertype IPv4 (0x0800), length 106: website.ssh >
192.168.4.21.50027: P 1:41(40) ack 40 win 16752
07:57:51.057409 00:16:cb:8d:48:f7 (oui Unknown) > 00:08:00:c0:fa:04
(oui Unknown), ethertype IPv4 (0x0800), length 66: 192.168.4.21.50027
> website.ssh: . ack 41 win 65535
If you look carefully at those, you see that the packets are identified as coming from my router ( 0:8:0:c0:fa:4). We'd expect that - that is the only place they could come from.
So, back to our port 7000 problem. If the SCO box knows where the packets come from, why doesn't it just use that knowledge to route them back?
Well, I suppose it could. Someone could write an IP stack that configured routes from level 2 header information when there was nothing better to go by. That's pretty much what a switch does for its ports. I wouldn't want that feature on a computer, though: I like to be able to tear down routing entirely when I want to.
So I guess the answer to "Why?" is just "Because that's the way it is. Add the
route, and my wife and I will go split a sub."
*Originally published at APLawrence.com
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com), home of FREE CCNA and CCNP tutorials and daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages. |
