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How many I/O paths would a virtual machine -- like an EC2 instance or a Linode instance -- have available? I'm interested in learning more about this because I don't know an appropriate value for PostgreSQL's effective_io_concurrency setting. Thanks!
You don't know what's behind the mountpoints of your virtual machines. So the best thing you can and, actually, should do: test the performance of the disks alone and then testing it with different values of effective_io_concurrency.
I would took each /dev/* device that is used in your mountpoints as a separate IO path, at least this can be a good start.
I also think, that this configuration parameter has more value on the dedicated physical servers, rather then virtual ones (provided externally of course).
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I am using RHEL 6.3 (2.6.x kernel). I have tcp based client server applications. I noticed that when i run them on the same host, the throughput is approx the same, irrespective whether server binds to loopback or local IP assigned to NIC.
What is the reason behind it? My understanding is that loopback is software based routing, where as when local IP assigned to NIC is involved, the hardware is involved in the data path. Is that true?
The hardware does not get involved.
As soon as the routing function knows that the destination address is local, the packet is switched to ingress path. Which is incidentally why sniffers can't capture such packets, because that hook happens to be after the point of this decision.
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I want to test some keyloggers in order to see how they work.
If my host OS is clean, and I install the keyloggers on a guest OS running in VirtualBox, will the keylogger be able to log the keystrokes and screenshots in the HOST OS?
I think I know the answer to this question but I just want to make sure.
Unless your VMs are broken, key/screen-loggers will be fully contained in the VMs and will gain no access to the host OS keyboard/screen data that's outside of the VM.
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Is there any way another EC2 user can network sniff the traffic coming from or going to one of my EC2 instances, even if both our virtual instance are on the same physical host? And because this one is the most sensitive one to me, I will ask specifically, could another EC2 used find a way to sniff the traffic between my EC2 instance and my RDS database?
I'm not an expert on EC2, but if Amazon sets up NAT'ed DomU's for separate instances (and I'm sure they do), you should be safe.
With NAT, every instance only receives packets that were intended for that host. So no packet sniffing possible (at least in theory).
I should add that there are some interesting (as in "look we did it and wrote a paper about it") sniffing attacks based on L1 cache sniffing.
link to paper, if you're interested.
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Is there a way to enable the netbios name in a Linux without using Samba?
What I want is that I would like the hostname (or whatever) to appear when other computers (MS Windows) is scanning the device through ethernet.
This is achieved by netbios. Samba has an application called nmbd that will do the trick. But Samba will take up more that 0.5MB in my compressed image. Which is bursting my partitions. And I don't want to spend that much flash and ram on such a trivial function anyway. (I'm using an embedded linux device btw.)
Does anyone know another way? An alternative to nmbd perhaps?
I'm not aware of any free software alternatives to nmbd. The nmbd from older versions of Samba might be smaller in size. Alternatively, you might be able to use the nmbd from Samba-TNG .
Or if you're really feeling adventurous, you might be able to write your own (if you just care about the name appearing). See http://ubiqx.org/cifs/NetBIOS.html.
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I have recently started using iptables, and I executed iptables -F without knowledge of what it might do. And suddenly I have lost connection to the node. I can't even ping the node. Any help would be highly appreciated!
Thanks!
You will need physical access to the computer and either restart the firewall script or simply reboot the server (but that's the "rude" way of fixing this).
If this computer is hosted at colocation company you need to either contact their support and ask them to reboot the machine (do not give them your password) or sometimes they have some sort of remote rebooting mechanism. Look through the FAQ of the colocation provider.