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We're on a 50 mb/s Comcast connection and our connection right now is coming in under 1.5 mb/s. Our roommate left for a week with BitTorrent running (Azureus client, we think). Our latency is approaching 300 ms. His door is locked up tight, and both his machine and the router for the house are located inside.

I've even flipped the power breaker in the house and that barely works for 2 minutes. His laptop keeps on running, and once the cable modem and router come back up and the machine reconnects, the torrents resume in earnest.

I've been running nmap and identified his IP on our LAN. Is there anything I can do over the LAN to make his torrents start to fail or slow down?

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closed as off topic by jscott, sysadmin1138 Feb 23 '11 at 15:38

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Do you have access to the router? If so, either pull the cable to his room (if the connection is physical) or change the wireless access key (if it's a wireless connection). –  Shadur Feb 6 '12 at 9:38
For some network i use netcut will kick user from WIFI lan :P –  Userpassword Jan 15 at 15:56

17 Answers 17

Pick his lock, or hire a locksmith Walk into his room and shut down his PC.

Otherwise, as Vartec says, you can try to block his MAC address on the router. There is a good probability that he has not changed its router admin password. If that's the case, try this:

  1. Check the Router/Gateway address that appear in your own computers network settings. ( usually or, if wifi, )
  2. go to the address in a web browser (eg: )
  3. when asked for a password, look up the default for his brand of router from here http://www.pcwintech.com/default-router-modem-passwords
  4. in the router config pages, find the list of recently/currently connected clients - it generally lists by MAC address
  5. add his MAC address to the list of banned devices
  6. Alternately, you could block all ports higher than 1023. This won't kick him off the network, but BT usually uses a higher port.

If he's changed the router admin pass, the best I would know how to do is "ping" (pings all devices on your subnet) to figure our which IP address he's using. From there you would actually need hack his computer - I wouldn't know how to help you with that.

EDIT: As listed by other posters here, you might be able to inject network traffic to kill his torrents. See the Comcast scandal over their injecting RST packets. I have no exprience with this. You also might be able to set up a second (ie: rogue) DHCP server to offer his computer a bad IP. My hunch is that it would be quicker, and more effective, to gain physical access to his room

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Couic [ michel.arboi.free.fr/couic.html ] is designed to automatically send TCP RST & ICMP Unreachable. And for being able to "sniff" packets on a switched network you can consult the "Sniffing FAQ" of Robert Graham [ newdata.box.sk/2001/jan/sniffing-faq.htm - The original URL seems broken ] –  Steve Schnepp May 19 '09 at 13:19
If it's one of the SerComm based routers, it may have a root backdoor access which you could use to change the wifi key github.com/elvanderb/TCP-32764 –  deed02392 Jan 7 at 13:08

This assumes your roomate is gone and you arn't able to contact him since talking with him is the best option. Keep the power off for three or four hours so his laptop battary dies or at least goes to sleep, then when you turn the power back on his laptop shouldn't start.

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And it is legal too. –  Tometzky Jun 8 '09 at 6:40
And he'd never know, "Oh the power was out for like 4 hours last night" –  SpaceManSpiff Jul 10 '09 at 11:15
with the added bonus of "having" to eat everything in the fridge ! –  Sirex Feb 23 '11 at 15:55

His Azureus HTTP server plug-in might be running, if you know it you could try HTTP'ing to his IP address with :6886 at the end - if it comes up with a GUI interface you should be able to reconfigure his download speeds or pause it altogether. There are other similar ports available that other SFers might be able to add via comments that you could try.

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Do you have your housemates permission to disable his bittorrent? I am guessing no because otherwise he would have told you the router password and you could simply block the laptop. It seems then this question is asking that we help you commit what could be a crime or at very least is probably bad housemate ettiquette. The correct answer to your question, though I doubt you will give me points for it, is to ask your housemate whether he is comfortable with you taking action and can you have the password.

Assuming you have permission to do this and cannot access the router then I would say the best way is using arp-spoofing to steal his IP and then issue TCP resets to all the connections (I think this will happen anyway when you steal his IP since the OS knows nothing of the connections). This requires that you are faster with the arp replies of cause than his laptop, and to you can cause that by DOS'ing the laptop.

I mean you basically have three options: hack the laptop, hack the router, hack the network. The fourth and one you seem to be overlooking is hack the door - he has a locked room sure but can't that lock be easily picked or isn't there another way into the room? We have locked doors for privacy but they are easily broken into.

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If they share the lease and internet connection, I doubt this is illegal (not that I'm a lawyer) but agreed it's not very neighborly. I suggested getting a locksmith to answer the question on hand, but in reality, if I couldn't get hold of the flatmate, I'd just use an internet cafe till he returns. It's not like a water pipe broke over his room :-) –  username May 17 '09 at 17:44
though if a water pipe did break over his room - that would take care of the torrent problem too. "awesome! we're standing knee deep in water, but just look at how fast these youtube videos are streaming!" –  username May 18 '09 at 2:05
I've done ARP poisoning to a roommate's computer before, and it does work quite well. –  Joseph Jun 1 '09 at 13:33
It wasn't very neighborly of the roommate to not set Azureus to cap its bandwidth usage and swamp the network. –  Shadur Feb 6 '12 at 9:36

Block his MAC in the router.

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the router is locked up with the laptop. –  antony.trupe Mar 11 '11 at 19:41

ARP Cache Poisoning (a longer description)

dsniff contains a good tool (arpspoof) to do this. IronGeek has a good tutorial on using it..

It's commonly used to route all traffic going between machine B and the router through your local machine, so you can sniff all traffic (it's "legitimate" uses are rather limited)..

The above tutorial describes this but you can use it to redirect traffic from your roommates machine to some non-existant MAC address (basically dropping all traffic)

arpspoof -t # being your roommate's IP

(the counter to this is a tool like arpwatch which simply keeps track of all MAC addresses -> IP, and alerts you when one changes)

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There's nothing inherently wrong about sniffing another machine's traffic. If, for example, you're trying to analyse a machine that's infected, and don't want to run the packet sniffer on that machine (as it may not give real information), ARP poisoning is a good option. –  Dentrasi Jun 11 '10 at 20:19
I use ettercap sudo ettercap -T --iface en1 -T -Q -M arp /ip-address/ //. NB. You can safely assume that it's illegal to intercept other peoples traffic, which is what this does, only use on traffic that is 'yours' –  mrmagooey Dec 18 '12 at 5:10

You could configure a computer with the same IP address as the computer you wan to block and ethernet will make the rest.

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I wonder which one wins... –  Ape-inago Jun 1 '09 at 1:43
+1 for ease of implementation for a semi-novice.... Wonder if modern hardware rejects the 'new' mac address as dupip... Once had a box enter the network with an IP of about 10+ years ago.... stopped everthing on that subnet! I presume you dont actually want to STOP the network however (though transmitting data with his IP while you 'reboot' the router via remote house power-cycling might make YOU the one with the 'original' mac address. Who pays the internet bill? –  ericslaw Jun 19 '09 at 23:05
-1 for inaccuracy: 1) "Ethernet" has nothing to do with IP Addressing and IP Address Conflicts--It is usually used to refer to the physical connectors/cabling, not Layer 3 of the OSI Model. 2) In an DHCP IP Addressing conflict, the "network interface on both on them gets disabled, causing each system to lose its connectivity until the conflict has been resolve" Now, this is not always the case, but for this particular scenario, this is more than likely what would happen –  Josh Brower Jun 23 '09 at 21:12
On our reasonably modern network at work, someone managed to duplicate a VM with a static IP and run both copies on different machines. They became alternately inaccessable for around 10 secs at a time. –  Colin Pickard Jul 10 '09 at 11:55

Figure out which breaker is his room. Flip it before you go to bed. When you wake up, turn the power back on. The cable modem and router should start back up but his laptop will likely not reconnect to the torrent clients. You would lose your network, but only while you sleep.

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Any legal and moral quarrels aside, if you're on the same local network segment, an ARP poisoning denial of service attack might do the trick - simply associate the laptop's IP with a non-existing MAC address for the router (or the other way around) and it would be shut out.

Depending on the door and/or window design they might be easier to just open up in some way though. Or get someone to do it for you.

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Umm, the roommate is away, the router etc are in his room. It is a locked door. You flip the power. It is pitch black. You are likely to be eaten by a grue.

Seriously, this is not

a. A social engineering problem


b. A network engineering problem

It is a simple, "I need to get into his room and unplug his laptop" engineering problem. If you want to take the brute force approach simply either pop the hinges on his door (assuming that they are on the outside) or just kick it in/pry it open.

I know that sounds heavy handed but there are times when it boils down to the simplest choice. If you have access to the router admin then yes, disable his machine in there. If the poster had the knowledge to be able to poison ARP, then they probably wouldn't have been asking this question. (No offense intended)

There were some useful suggestion re: flipping the power/breaker for the house/his room and letting the battery run down, but as for re-routing the coax cable to somewhere else and bringing up another cable modem/router. Come on people this problem isn't rocket science.

My opinion is that if your roommate is inconsiderate enough that he leaves his torrent client running unrestricted and unthrottled while he isn't there, then someone has to have words with him about fair usage of a shared resource or split the bill differently (this is a bad road to go down unless everyone agrees to it, I think that heavy users of the network should pay more and people that don't use it should pay a nominal amount (couple of dollars a month, literally $2$3 this means that if they do start to use it there has to be another agreement between everyone as to proportional usage).

Of course, all of my points are based on the assumption that it is a shared internet bill. If he's the guy paying for the bandwidth then he can use it whatever way he wants. If this is the case then suck it up and be glad he doesn't just switch it off when he's not there.

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Ok here is how I would approach this challenge.

I am assuming that he is using a static IP on the lan and running a microsoft OS on the lan.

  1. Power off the hub that the computer is connected to.
  2. With a non-microsoft OS bring up a machine with the same IP address.
  3. Re-connect the roommates machine to the network, It should detect a duplicate IP and then shut down.

The trick is that Microsoft OS's shut themselves down if they detect that their IP is in use, so you just need to make sure that his shuts down and not yours.

More details are here ... http://support.microsoft.com/kb/120599

If he is using DHCP then his machine will probably just grab another IP and then use UPNP to punch the correct hole in the firewall and you are back to square one again.

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You could try running up a copy of Untanlge's "Re-router" sytem that lets you use a virtual machine as the local network router.

In technical terms, what it does is spoof mac addresses (arp poisoning) so that the local machines use it as the primary gateway instead and then you can simply block his computer from accessing the internet for the time being.


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How devious do you want to be? You can start issuing fake TCP-IP packets destined to his MAC address. You can unplug his machine. :)

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They can't get in the room :) –  Jason Coco May 10 '09 at 1:28
Any recommendations on Mac OS software to do so? –  bob May 10 '09 at 1:36
@bob Try ettercap.sourceforge.net –  Joseph May 17 '09 at 13:29

You could set up a spare computer for a pfSense firewall. I set up one where both the WAN and LAN adapters were connected to the same network switch, and any new DHCP requests were intercepted by the firewall, until it was disconnected. This would require somehow getting his computer to reacquire its DHCP lease. After that just disable bittorent.

Personally I would try to jimmy his door open, or pick his lock.

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If you have access to the router you could turn off DHCP and reconfigure the router to serve up a different block of IP addresses. That should invalidate his machine's network connection.

Though if your router blocks mac addresses, that is probably easier.

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  • Set up QoS on your home router and throttle his bandwidth.

  • Install a firewall between him and the router, and set up iptables to limit the number of active connections to/from his IP.

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