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  • Network throughput issue (ARP-related)

    - by Joel Coel
    The small college where I work is having some very strange network issues. I'm looking for any advice or ideas here. We were fine over the summer, but the trouble began few days after students returned to campus in force for the fall term. Symptoms The main symptom is that internet access will work, but it's very slow... often to the point of timeouts. As an example, a typical result from Speedtest.net will return .4Mbps download, but allow 3 to 8 Mbps upload speed. Lesser symptoms may include severely limited performance transferring data to and from our file server, or even in some cases the inability to log in to the computer (cannot reach the domain controller). The issue crosses multiple vlans, and has effected devices on nearly every vlan we operate. The issue does not impact all machines on the network. An unaffected machine will typically see at least 11Mbps download from speedtest.net, and perhaps much more depending on larger campus traffic patterns at the time. There is one variation on the larger issue. We have one vlan where users were unable to log into nearly all of the machines at all. IT staff would log in using a local administrator account (or in some cases cached credentials), and from there a release/renew or pinging the gateway would allow the machine to work... for a while. Complicating this issue is that this vlan covers our computer labs, which use software called Deep Freeze to completely reset the hard drives after a reboot. It could just the same issue manifesting differently because of stale data on machines that have not permanently altered low-level info for weeks. We were able to solve this, however, by creating a new vlan and moving the labs over to the new vlan wholesale. Instigations Eventually we noticed that the effected machines all had recent dhcp leases. We can predict when a machine will become "slow" by watching when a dhcp lease comes up for renewal. We played with setting the lease time very short for a test vlan, but all that did was remove our ability to predict when the machine would become slow. Machines with static IPs have pretty much always worked normally. Manually releasing/renewing an address will never cause a machine to become slow. In fact, in some cases this process has fixed a machine in that state. Most of the time, though, it doesn't help. We also noticed that mobile machines like laptops are likely to become slow when they cross to new vlans. Wireless on campus is divided up into "zones", where each zone maps to a small set of buildings. Moving to a new building can place you in a zone, thereby causing you to get a new address. A machine resuming from sleep mode is also very likely to be slow. Mitigations Sometimes, but not always, clearing the arp cache on an effected machine will allow it to work normally again. As already mentioned, releasing/renewing a local machine's IP address can fix that machine, but it's not guaranteed. Pinging the default gateway can also sometimes help with a slow machine. What seems to help most to mitigate the issue is clearing the arp cache on our core layer-3 switch. This switch is used for our dhcp system as the default gateway on all vlans, and it handles inter-vlan routing. The model is a 3Com 4900SX. To try to mitigate the issue, we have the cache timeout set on the switch all the way down to the lowest possible time, but it hasn't helped. I also put together a script that runs every few minutes to automatically connect to the switch and reset the cache. Unfortunately, this does not always work, and can even cause some machines to end up in the slow state for a short time (though these seem to correct themselves after a few minutes). We currently have a scheduled job that runs every 10 minutes to force the core switch to clear it's ARP cache, but this is far from perfect or desirable. Reproduction We now have a test machine that we can force into the slow state at will. It is connected to a switch with ports set up for each of our vlans. We make the machine slow by connecting to different vlans, and after a new connection or two it will be slow. It's also worth noting in this section that this has happened before at the start of prior terms, but in the past the problem has gone away on it's own after a few days. It solved itself before we had a chance to do much diagnostic work... hence why we've allowed it to drag so long into the term this time 'round; the expectation was this would be a short-lived situation. Other Factors It's worth mentioning that we have had about half a dozen switches just outright fail over the last year. These are mainly 2003/2004-era 3Coms (mostly 4200's) that were all put in at about the same time. They should still be covered under warranty, buy HP has made getting service somewhat difficult. Mostly in power supplies that have failed, but in a couple cases we have used a power supply from a switch with a failed mainboard to bring a switch with a failed power supply back to life. We do have UPS devices on all but three of four switches now, but that was not the case when I started two and a half years ago. Severe budget constraints (we were on the Dept. of Ed's financially challenged institutions list a couple years back) have forced me to look to the likes of Netgear and TrendNet for replacements, but so far these low-end models seem to be holding their own. It's also worth mentioning that the big change on our network this summer was migrating from a single cross-campus wireless SSID to the zoned approach mentioned earlier. I don't think this is the source of the issue, as like I've said: we've seen this before. However, it's possible this is exacerbating the issue, and may be much of the reason it's been so hard to isolate. Diagnosis At first it seemed clear to us, given the timing and persistent nature of the problem, that the source of the issue was an infected (or malicious) student machine doing ARP cache poisoning. However, repeated attempts to isolate the source have failed. Those attempts include numerous wireshark packet traces, and even taking entire buildings offline for brief periods. We have not been able even to find a smoking gun bad ARP entry. My current best guess is an overloaded or failing core switch, but I'm not sure on how to test for this, and the cost of replacing it blindly is steep. Again, any ideas appreciated.

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  • Multicore system. How are the cores "sorted"

    - by asdrubael
    If I have a system with let's say two Quadcore CPUs and HyperThreading enabled, how do I know how the phsyical and virtual cores are sorted? For example if I force a process to run on "CPU 0" how to know which Core this really is? I remember a complex drawing about this, but didn't find anything useful.

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  • Can a password change in Active Directory be reverted?

    - by Jordan Milne
    I often need to log in with a user's domain account to make sure that I've set up their profile correctly, but there's no easy way to do this other than to call them and give them a temp pass and force them to change it (bad, since they often forget it,) or ask for their password (worse.) Is there any way to change a user's password, then set it back to the original later? I tried to see if there was any way to get the NTLM hash so I could set it back later, but there doesn't seem to be a facility for that built into AD.

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  • Can a password change in Active Directory be reverted?

    - by Jordan Milne
    I often need to log in with a user's domain account to make sure that I've set up their profile correctly, but there's no easy way to do this other than to call them and give them a temp pass and force them to change it (bad, since they often forget it,) or ask for their password (worse.) Is there any way to change a user's password, then set it back to the original later? I tried to see if there was any way to get the NTLM hash so I could set it back later, but there doesn't seem to be a facility for that built into AD.

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  • Group Policy is not being applied from Server 2003 to win7 client

    - by John Hoge
    Hi, I'm experimenting with Group Policy settings. My DC is running Server 2003, and the client I am using for this test is running Win7. I've restarted the client a few times, and tried running gpupdate/force for good measure. This machine is in it's own OU with a group policy applied to change one setting, Computer Configuration/Administrative Templates/Network/Offline Files. When I run MMC and look at Local Computer Policy on the client this setting shows up as "not configured". Thanks, John

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  • PuTTY - Server Unexpectedly Closed Network Connection

    - by Austin
    I have two servers that I have been able to connect fine as s1.website.com and s2.website.com I connected to s1.website.com earlier today without any issues, however, when I connected to s2.website.com it gave me the "Server Unexpectedly Closed Network Connection" I have researched this elsewhere and someone concluded it to a "brute force attempt" However, I know this is not the case. If anyone else has had this issue please let me know. Thanks

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  • How do I keep windows XP from automatically changing the screen resolution?

    - by roamn
    I have an Asus EEEBox (EB202, intel GMA 950) hooked up to my 1080p TV (DVI-HDMI cable). I use it to watch standard definition movies and TV shows. I prefer to run at 1280x720 so that I can see things more easily, but every time I turn off the TV, then back on again, the resolution defaults to 1080p (1920x1080). How can I force a specific resolution? If that's not possible, is there a way to use a batch script to switch to the desired resolution faster?

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  • How to move windows that open up offscreen?

    - by Matthew Scharley
    I have a dual monitor setup, and I recently played around with the positioning settings, and some of my single window applications do the favour of preserving where they were last closed, and opening in the same position later. Unfortuanately, that now places them out of the viewable area of my screens! Is there some way to force a particular window into the viewable area? If it matters at all, this is on Windows XP 32b.

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  • Remote Server: Please wait for the System Event Notification Service

    - by Jeff Handley
    I was rebooting a remote server (Windows Server 2008 R2 Standard) over remote desktop and the session now shows the blue screen during the shutdown sequence, and the message "Please wait for the System Event Notification Service..." It seems that everything is still running on the server (for instance, http://jeffhandley.com is still responding), but I need to get the machine to finish the reboot sequence. How can I force the machine past this point? It's been stuck there for about 30 minutes.

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  • How to get hibernate working on Windows 8?

    - by A T
    I have been using Windows 8 x64 Developers Preview as my primary OS on my laptop. Unfortunately I have been unable to enable hibernate (tried through registry and through powercfg.cpl). The closest I have got is that I was able to enable hibernate on pressing of power button, but that just turns of screen and keeps everything else running, I have to force shutdown to do anything. Do you know how I can get hibernate to work on my Dell Studio 17?

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  • Sendmail: Use SMART_HOST for local mail

    - by bradlis7
    We are running an outbound SPAM filter, and we'd like to be able to have the mail sent through that filter, even when it's to a local user. Is there a way to force sendmail to use the SMART_HOST, even when it is going to be delivered locally?

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  • Printing without breaking accross pages.

    - by jedberg
    I have a page with a bunch of HTML tables that I need to print. Each table should fit on one page, but both Firefox and Safari want to break them over multiple pages and put more than one on a page. Is there any way to force it to not break the tables across pages? I can use any Mac program to print. Thanks.

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  • how to set php SERVER_PORT var to 80, behind varnish?

    - by Daniel
    how to force php to read SERVER_PORT as 80, when apache listens on 8080 and varnish listens on 80 ?? if my apache vhost is set to 8080, SERVER_PORT will always be 8080, this is troubling me a little since in many parts of the application some links are calculated with SERVER_NAME and SERVER_PORT together, .. so what I need is that php "believes" that SERVER_PORT is 80, so all links will pass trough varnish regards Daniel

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  • An device with an unknown MAC address is connected to my router

    - by Yar
    There is a computer that is not mine that is accessible on my network. I can even access its filesystem via AFP. What I want to know is how the computer could get on my network. My network is secured like this: Does that mean that they've used password cracking tools? The pass is not easy to guess but not hard to figure out via brute-force hacking, I guess. If I am being hacked, should I switch to WPA?

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  • Learn to use both sides of the keyboard

    - by brewerja
    I'd like to force myself to use the correct (right side or left side) Shift, Ctrl, and Alt keys depending on what letter I'm typing. For instance I'd like to use the right Shift key when typing 'A' and the left Shift key when typing 'P'. I find myself using only the left side a lot and I'm looking for a way to set the mappings on my machine so that it only responds to correct pairings. I'm running Fedora, but any Linux distro support would be an acceptable answer.

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  • An unknown Mac is connected to my router?

    - by Yar
    There is a computer that is not mine that is accessible on my network. I can even access its filesystem via AFP. What I want to know is how the computer could get on my network. My network is secured like this: Does that mean that they've used password cracking tools? The pass is not easy to guess but not hard to figure out via brute-force hacking, I guess. If I am being hacked, should I switch to WPA?

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  • Does Google bot(and/or search engines) index a forwarded page? [migrated]

    - by user2889419
    Let say I have foo.bar domain, and I force the user to use the https over http. The question is as browsers just accept and load the forwarded/new page(when the request for http://foo.bar - https://foo.bar), does the google bot(or other search engines) accept the forwarded page and index the new page and just ignore the old page? in other word, does search engines accept https beside the http? thanks in advance.

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  • Stop windows resuming from hibernation to install automatic updates

    - by swref
    I always hibernate my work (Windows 7, x64) laptop when leaving for the night. Sometimes when I come in again in the morning, the laptop is powered up and there's a message telling me that new updates have been installed. (I've changed the registry so automatic updates don't force a reboot.) I'm happy to have Windows apply updates automatically, but I don't want it to resume from hibernation to do so. How can I stop this behaviour so I'm not wasting electricity?

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  • Mac on My Router?

    - by Yar
    There is a computer that is not mine that is accessible on my network. I can even access its filesystem via AFP. What I want to know is how the computer could get on my network. My network is secured like this: Does that mean that they've used password cracking tools? The pass is not easy to guess but not hard to figure out via brute-force hacking, I guess. If I am being hacked, should I switch to WPA?

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  • Recovered folders from Camera show as JPEG files and can can't be viewed.

    - by user642111
    I have recovered a CCTV camera hard disk after a crash and have managed to get most of the data using EasyRecovery Pro. The problem is now that all the data that I have recovered appear like File09.JPG with and image icon in windows XP, but the files can't be viewed in any JPEG viewer software. I suspect that the .JPG files are indeed folders, but I can't force windows XP the change the file type. Very Odd. Any help is appreciated. Thanks, Hoo

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