How to run multiple guest OS in QEMU?

byMarcin Cylke
March 27, 2011
2 minute read

This weekend I’ve been fiddling with

QEMU. I’ve installed OpenBSD on a single image and wanted to have two instances of it communicating via network. Installing the system was easy, but the networking setup was quite a pain. See how I did that… To make QEMU instances communicate with each other I needed to plug them to a “network”. That’s why I’ve created a bridge to which Virtual Instances would connect to.

I’ve used the following script:

#!/bin/bash                                                                                                                                                 
# 1st, release all DHCP address and remove all IP address associated
# with the original eth0
#/sbin/dhcpcd -k
kill pidof dhclient
/sbin/ip addr flush eth0
# then take the interface down so we can rename it
/sbin/ip link set eth0 down
# now rename the original eth0 to reth0 (Real ETH0)
nameif reth0 00:24:81:43:61:5b
# OK, bring the same interface (with new name though) back up
/sbin/ip link set reth0 up
# 2nd let's create a bridge called eth0 so other programs think they are
# talking to the same old interface (actually they will talk to the
# bridge which is a clone of the original eth0 - with name MAC addr)
/usr/sbin/brctl addbr eth0
# then add both origianl eth0 and tap1 device to the bridge
/sbin/brctl addif eth0 tap1
/usr/sbin/brctl addif eth0 reth0
echo "showing bridge mac addresses"
/usr/sbin/brctl showmacs eth0
# 3rd, we need to bring the newly created bridge UP
/sbin/ip link set eth0 up
# 4th, renew the DHCP address if possible
#/sbin/dhcpcd -n
dhclient eth0
/sbin/ip addr show

Then I just needed to start Qemu with this command line:

sudo qemu openbsd-4.7.img  -net tap -net nic,macaddr=52:54:00:12:34:57,model=ne2k_pci

Since I’ve set up bridge for Qemu instances, I’ve plugged TAP interfaces into it. That’s why I’ve needed to specify this in my qemu exec line. I’ve also added macaddress setting since both my instances were getting the same one. And that’s all! It works like a charm. Now on to some harder things!

Marcin Cylke

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Private fields and methods are not private in groovy

byTomasz Kalkosiński
July 12, 2012

I used to code in Java before I met groovy. Like most of you, groovy attracted me with many enhancements. This was to my surprise to discover that method visibility in groovy is handled different than Java!

Consider this example:

class Person {
private String name
public  String surname

private Person() {}

private String signature() { "${name?.substring(0, 1)}. $surname" }

public String toString() { "I am $name $surname" }
}

How is this class interpreted with Java?

Person has private constructor that cannot be accessed
Field "name" is private and cannot be accessed
Method signature() is private and cannot be accessed

Let's see how groovy interpretes Person:

public static void main(String[] args) {
def person = new Person()   // constructor is private - compilation error in Java
println(person.toString())

    person.@name = 'Mike'       // access name field directly - compilation error in Java
println(person.toString())

    person.name = 'John'        // there is a setter generated by groovy
println(person.toString())

    person.@surname = 'Foo'     // access surname field directly
println(person.toString())

    person.surname = 'Bar'      // access auto-generated setter
println(person.toString())

println(person.signature()) // call private method - compilation error in Java
}

I was really astonished by its output:

I am null null
I am Mike null
I am John null
I am John Foo
I am John Bar
J. Bar

As you can see, groovy does not follow visibility directives at all! It treats them as non-existing. Code compiles and executes fine. It's contrary to Java. In Java this code has several errors, pointed out in comments.

I've searched a bit on this topic and it seems that this behaviour is known since version 1.1 and there is a bug report on that: http://jira.codehaus.org/browse/GROOVY-1875. It is not resolved even with groovy 2 release. As Tim Yates mentioned in this Stackoverflow question: "It's not clear if it is a bug or by design". Groovy treats visibility keywords as a hint for a programmer.

I need to keep that lesson in mind next time I want to make some field or method private!

DDevelopment & Design