Linux – How easy is it to migrate a Linux VM image from one VM env to another

linuxvirtualization

If I stick to one of the standard, well-supported VM disk images (like a raw image, or VDI, VMDK, …), are Linux VMs typically easy to move between VM environments? E.g., between (say) VirtualBox and KVM, or VMWare and Xen? I'm talking here of fully virtualized environments, not paravirtualization requiring support within the guest OS.

It seems to me that the kernels in most Linux distributions these days are configured to…keep an open mind and detect things at boot time, so you don't have the issue that you sometimes have moving a Windows VM from one virtualization system to another (I'm thinking particularly of HAL issues that Windows has, like ACPI vs. non-ACPI; I've also just had Windows VMs generally acting strangely when moved from VMWare to VirtualBox, for instance).

I'm looking for a general answer, but if it helps, specifically I'm mostly going to be doing this with Ubuntu 8.04 LTS and 10.04 LTS guests. But that could change.

Best Answer

Linux is able to load modules (drivers) for it's hardware platform on demand. As long as your kernel is able to find modules for you hardware, it does not even matter on what platform you start your VM, save for a couple of caveats:

for all hypervisor types, use generic hardware: use IDE disks (not SCSI, KVM sucks with SCSI disks) and use e1000 network adapters. For portability, stay away from KVM's vda disks or VMware vmxnet. This is not alway possible (VMware uses SCSI by default), but it'll help.
make sure you use the same MAC addresses for the migrated VM on the new platform as you did on the old one
use LVM for your disks: the bootloader will find your kernel and feed it your initrd; if you use LVM, you don't have to worry about disks going awol (worry less, anyway).
leave your network configuration at DHCP if possible
do not install VMwareTools and similar
generate an initrd and try to put as many of the drivers you'll need on the other platforms in it. This will save your behind when trying to go from KVM (IDE disks) to VMware (SCSI disks) and you already have support for VMware's LSI Logic SCSCI controller in your initrd.

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Linux – How to run a server on port 80 as a normal user on Linux

Short answer: you can't. Ports below 1024 can be opened only by root. As per comment - well, you can, using CAP_NET_BIND_SERVICE, but that approach, applied to java bin will make any java program to be run with this setting, which is undesirable, if not a security risk.

The long answer: you can redirect connections on port 80 to some other port you can open as normal user.

Run as root:

# iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080

As loopback devices (like localhost) do not use the prerouting rules, if you need to use localhost, etc., add this rule as well (thanks @Francesco):

# iptables -t nat -I OUTPUT -p tcp -d 127.0.0.1 --dport 80 -j REDIRECT --to-ports 8080

NOTE: The above solution is not well suited for multi-user systems, as any user can open port 8080 (or any other high port you decide to use), thus intercepting the traffic. (Credits to CesarB).

EDIT: as per comment question - to delete the above rule:

# iptables -t nat --line-numbers -n -L

This will output something like:

Chain PREROUTING (policy ACCEPT)
num  target     prot opt source               destination         
1    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:8080 redir ports 8088
2    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:80 redir ports 8080

The rule you are interested in is nr. 2, so to delete it:

# iptables -t nat -D PREROUTING 2

Best Answer

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