Yearly Archives: 2011

Integrating BIND with AD-integrated Microsoft DNS

I recently set up BIND9 to run secondary zones for an ActiveDirectory-integrated DNS server (the reason being that I hated effectively losing internet access when I rebooted my W2k8R2 server). While that was really easy (add the Linux server to the nameservers tab in DNS Admin, allow zone transfers and notifications, add slave zones in the named.conf), I thought that it shouldn’t be too difficult to also automatically replicate AD-integrated Conditional Forwarders.

While they are easily found in the DC=DomainDnsZones and DC=ForestDnsZones branch inside the AD, it turns out that the server information is stored in dnsproperty attributes containing binary data. However, Microsoft actually provides a specification for their DNS data structures, which is certainly very commendable. But as it turns out, it appears to have been written by someone who had no clue about Endianness or how many bits are in a byte (*).
The essence is: everything is Big Endian, except for IP addresses (the spec claims they are Network Byte Order, but in reality they are Little Endian), and every occurence of “1 byte” in section dnsProperty should be replaced with “4 byte”.

So after taking about two hours for something that I expected would only take a couple minutes to hack together, I ended up with 400 lines of code that generate a file you can include in your named.conf that will look something like this:
zone "" {
type forward;
forward first;
forwarders {; 2001:470:20:0:0:0:0:2; };

zone “” {
type forward;
forward first;
forwarders {; 2001:470:20:0:0:0:0:2; };
(For those curious, this sample configuration would point and at Hurricane Electric’s DNS server so that you get AAAA records, a.k.a. Google over IPv6)

After this worked, I decided to also pull my slave zone definitions through the same mechanism. It only took me a minute to do that.
zone "" {
type slave;
file "";
masters {; };
allow-notify {; };

So here we are: BIND9 as a fully-blown sync partner for AD-integrated DNS zones. To add a zone or conditional forwarder to BIND, add it to AD, set it to replicate to all DNS/domain controllers in this domain or forest, add the BIND server to the nameservers tab and allow zone transfers and notifications, and wait for the cron job to kick in.

I ended up having to write this script in PHP because Python’s LDAP module appears to have a broken SASL implementation, and you need SASL to use Kerberos for an LDAP connection.

The PHP script takes two parameters (1. the AD server’s address or the AD DNS domain name; 2. the AD base DN) and requires a valid Kerberos ticket.
The shell script (which you will most likely want to run from a cron job), which shares much of its code with my script from ISC DHCPd: Dynamic DNS updates against secure Microsoft DNS, needs to be configured with your realm, domain, base DN, user name (principal) and path to a keytab for that user (instruction on how to generate the keytab using ktutil are in the script’s comments).

(*) After doing all this, I figured that people from projects like Samba that write open source software to re-implement or interface with Microsoft products are doing an absolutely amazing job. They most likely aren’t getting any better specs than the one I found on MS DNS (if they get specs at all), and yet still somehow create almost perfect software that is a lot more complex than the simple stuff I did here.

UPDATE 2011-10-30: Apparently, AD refuses all requests from Linux clients that come in via IPv6. To force IPv4, line 7 of the PHP script needs to be changed to $conn = ldap_connect(gethostbyname($adserver), 389);, which is also fixed in the downloadable script.

Mount ext3 VMDK in VMWare Fusion using VMDKMounter

VMWare Fusion 3 comes with a tool called It allowed you to simply double-click NTFS or FAT32 VMDKs and they would be mounted on your desktop.

VMWare Fusion 4 dropped this tool, but you can download version 3.1.3 and extract /Library/Application Support/VMware Fusion/ from the package using Pacifist (just make sure that has the sticky bit set and is owned by root:wheel after you extract it).

Next, install OSXFUSE (the successor to MacFUSE) and fuse-ext2 if you don’t already have them installed.

VMDKMounter attempts to mount EXT2 using /System/Library/Filesystems/ext2.fs/Contents/Resources/mount_ext2, so we need to create two symlinks:

cd /System/Library/Filesystems
sudo ln -s fuse-ext2.fs ext2.fs
cd ext2.fs/Contents/Resources
sudo ln -s ../../mount_fuse-ext2 mount_ext2

Now we’re all set, you can simply open a VMDK by double-clicking it, or you can right-click a VMWare VM and open it with and automatically have all its VMDKs mounted.

If you are receiving an NTFS-3G error message when mounting a non-NTFS VMDK: that’s perfectly normal, so you can just click OK. The error message is due to VMDKMounter simply trying a bunch of file system mounters until it finds one that doesn’t fail. As far as I can tell, it tries (in that order) ntfs, msdos, ntfs-3g, hfs, ext2, ext3.

How-To: Converting Xen Linux VMs to VMWare ESXi

I have a couple Linux VMs I created on Xen using xen-create-image (as such, they are using pygrub and have one virtual disk file per partition). Now I want to migrate those over to a VMWare ESXi box. To convert your raw Xen disk images to VMWare vmdk files, do this:

1. In VMWare Fusion or Workstation, do a basic install of Debian Squeeze onto a flat-file (not split into 2GB segments and preallocated) VMDK that is slightly larger than your virtual Xen disk with a separate VMDK for swap.
2. Downgrade it to Grub 1 using apt-get install grub-legacy, grub-install /dev/sda, update-grub (as Grub 2 is not compatible with /boot/grub/menu.lst files as generated by xen-create-image).
3. Shut down and make a copy of the VMDK.
4. Boot the VM back up and re-install Grub2 using apt-get install grub.
5. Edit /boot/grub/grub.cfg and replace root=UUID=xxxxxxxxxx in the linux lines with root=/dev/sda1
6. Shut down the VM and attach the VMDK you copied in step 3 as an additional disk (this will be the target disk for our conversion).
7. Boot it up and make sure that you’re getting a Grub2 screen (i.e. it is not booting from the copied VMDK).
8. Using mount, check that your root disk is sda1 (which usually should be the first disk, not the copied disk). Using ls /dev/sd*, make sure it sees the target disk as sdc.
9. dd if=/path/to/xen/vm/disk.img of=/dev/sdc1 bs=1048576
10. mount /dev/sdc1 /mnt; cd /mnt
11. nano etc/fstab: replace swap disk /dev/xvda1 with /dev/sdb1 and root disk /dev/xvda2 with /dev/sda1
12. nano etc/inittab: replace hvc0 with tty1
13. nano boot/grub/menu.lst: replace /dev/xvda2 with /dev/sda1
14. umount /mnt
15. Attach the new virtual disk to a VM and boot a rescue system. There, drop to a shell on /dev/sda1 and apt-get update, apt-get install grub
16. Reboot
17. Done!

Using Intel AMT’s VNC server

Newer Intel Chipsets with vPro/Intel AMT, such as the Q57, Q67 and C206 (as long as they’re paired with a Core i5/i7 or Xeon with integrated graphics), have a feature called Remote KVM.

To use it, press Ctrl-P at the BIOS splash screen to get to the MEBx menu, set a password (minimum 8 characters, mixed case, numbers and special characters are enforced), configure the network settings (they can even match the OS’s IP address), enable Remote KVM and disable User Opt-In.

Next, download the Intel AMT SDK, extract the ZIP and open .\Windows\Intel_AMT\Bin\KVM\KVMControlApplication.exe . There, you can enable KVM as seen in the following screenshot:

KVM Status can either be set to “redirection ports” (meaning it will only be accessible to VNC clients that specifically support Intel AMT, such as RealVNC Viewer Plus or Intel’s KVM Console, the former of which costs $100, the latter of which constantly overlays a RealVNC logo on the screen), to “default port” (meaning it will be accessible on TCP port 5900 to any VNC client), or to “all ports” (which is the combination of both).
If you enable VNC access, you will also need to set an RFB Password. As I found out the hard way (Intel actually has it hidden in their documentation as well), it gets truncated at 8 characters and at the same time has the same security requirements as the general AMT password.
If you disabled User Opt-In in the MEBx menu, you can disable it here as well.

So that’s it, now you can use almost any VNC client you like (RealVNC and Chicken of the VNC work fine, while Apple Remote Desktop appears to cause the VNC server to freeze) and control the machine just as if you were sitting in front of it.
Two things I noticed: On my machine, the BIOS splash screen was not visible during a KVM connection (not even on a directly-attached screen), so to get to the BIOS I needed to blindly hit the corresponding key. Also, it is not possible to enter the MEBx menu during a KVM connection (probably for some obscure security reasons): if you hit the corresponding key, it immediately exits and continues normal bot; if you establish a KVM connection while in MEBx, you get disconnected immediately.

After about half an hour of playing with Intel AMT, I have to say it’s really cool. If you’re buying/building a home server, you should definitely consider getting a mainboard with Intel AMT 6.0 or later: You get server-grade remote management capabilities for a very small premium, which are very useful if you ever lock yourself out while remotely connected to the server.

Xen 4.0 and Citrix WHQL PV drivers for Windows

Xen 4.0 is supposed to be able to use Citrix’s WHQL certified Windows paravirtualization drivers. Their advantage over the GPLPV drivers is that they are code-signed, meaning they run on 64-bit Windows without disabling some of Windows’ security features.

UPDATE 2011-10-17: Signed GPLPV drivers are now available. I have not yet tested them, but I assume the fix below is no longer necessary.

While the Citrix drivers included in XenServer 5.5 work (after making a single registry tweak), the more recent ones included in e.g. Xen Cloud Platform 1.0 do not work right away:

If you install the XCP drivers, make that registry tweak and reboot the DomU, you’ll notice messages like XENUTIL: WARNING: CloseFrontend: timed out in XenbusWaitForBackendStateChange: /local/domain/0/backend/console/[id]/0 in state INITIALISING; retry. in your /var/log/xen/qemu-dm-*.log and Windows just gets stuck during boot and keeps spinning forever. To get it back to work, you’ll need to
xenstore-rm /local/domain/0/backend/console/[id]
xenstore-rm /local/domain/0/backend/vfb/[id]

after starting the VM (thanks to Keith Coleman‘s mailing list post!).

To automatically run these commands upon DomU start, create a script named /usr/lib/xen/bin/qemu-dm-citrixpv with the following contents

xenstore-rm /local/domain/0/backend/console/$2
xenstore-rm /local/domain/0/backend/vfb/$2

sh -c "sleep 10; xenstore-rm /local/domain/0/backend/console/$2; xenstore-rm /local/domain/0/backend/vfb/$2" &

exec /usr/lib/xen/bin/qemu-dm $*
and chmod +x it.

Then, edit your DomU config file and modify the device_model line and point it to your new script:
device_model = '/usr/lib/xen/bin/qemu-dm-citrixpv'

Now your Windows Server 2008 R2 x64 HVM-DomU is all set!

Asterisk: Remotely retrieving voicemail by pressing *

Many howtos around the internet on how to remotely access your voicemail box involve a dedicated extension reachable from the outside or an IVR menu entry. But wouldn’t it be much nicer if you could just press the * DTMF key during the announcement? Turns out, this is quite simple:

exten => s,1,Dial(SIP/1234,20)
exten => s,n,Voicemail(1234,us)
exten => a,1,VoiceMailMain(1234)
exten => a,n,Hangup()

And it even works when you’re using macros (like I am):

exten => 5551234,1,Macro(incoming-plus-voicemail,SIP/1234,20,1234)
exten => 5551337,1,Macro(incoming-plus-voicemail,SIP/1337,20,1337)

[macro-incoming-plus-voicemail] ; SIP/xxx, wait time, voicemail
exten => s,1,Dial(${ARG1},${ARG2}
exten => s,n,Voicemail(${ARG3},us)
push * during the announcement to access your mailbox
exten => a,1,VoiceMailMain(${ARG3})
exten => a,n,Hangup()

Asterisk: Compile SRTP Module without recompiling Asterisk

I recently installed Asterisk 1.8.3 (the Asterisk team now provides pre-built Debian packages at
Unfortunately, that package came without the res_srtp SRTP module. (UPDATE: Starting in 1.8.4, it does come with it.) Because I didn’t feel like re-compiling the entire package, I just took the corresponding version of res_srtp.c from the SVN, added the following lines to the beginning of it:

#ifndef AST_MODULE
#define AST_MODULE "res_srtp"

and compiled and installed it using

gcc -shared res_srtp.c -o -lsrtp
sudo cp /usr/lib/asterisk/modules/

You’ll need to have libsrtp0-dev and asterisk-dev installed, otherwise the compile will fail.
Then, you can do sudo asterisk -r and load the module using module load res_srtp (or just restart Asterisk).

I’m still working on getting SRTP working flawlessly both incoming and outgoing and with stuff like transfers. Asterisk Secure Calling Specifics are a good starting point, but I’m also planning to write another post about this in the near future.

Asterisk: Change Callee-ID using CONNECTEDLINE

It’s easy to change your Caller ID (assuming your phone provider doesn’t filter it) in Asterisk using something like Set(CALLERID(name)=blah). This is often used to choose which number to use for an outgoing call if you have multiple on a single SIP or ISDN trunk.

But did you know it’s just as easy to change the Callee ID on an outgoing call, i.e. change what your phone displays during the call? This can be very useful to display on the phone which one of several possible outgoing lines (multiple SIP providers, ISDN, …) was used or at which point in an IVR menu you are at the moment. To do this, use Set(CONNECTEDLINE(name)=blah). Before getting started, set sendrpid = pai in your sip.conf.

To make things easier, I created two macros in my dialplan:

exten => s,1,Set(CONNECTEDLINE(name,i)=${ARG1})
exten => s,n,Set(CONNECTEDLINE(number,i)=${ARG2})
exten => s,n,Set(CONNECTEDLINE(pres)=allowed)

exten => s,1,Macro(connectedline-name-number,${ARG1}, ${MACRO_EXTEN})

Now I can do things like

exten => 101,1,Answer()
exten => 101,n,Macro(connectedline-name,Hello World)
exten => 101,n,Playback(hello-world)
exten => 101,n,Hangup()

in my dialplan (IVR example).

Or how about

exten => 100,1,Macro(connectedline-name,Mailbox)
exten => 100,n,VoiceMailMain(${CALLERID(num)},s)
exten => _XXX.,n,Macro(connectedline-name,VoIP 1)
exten => _XXX.,n,Dial(SIP/${EXTEN}@voipprovider)

(outgoing line example).

The Asterisk Wiki also has an entire page on Manipulating Party ID Information.

NFS Performance: rsize and wsize

NFS is supposed to be a very simple and fast network file protocol. However, when I tried to use it on my Xen box between a Debian Squeeze DomU and an NFS server running on the Debian Squeeze Dom0, I noticed that write performance was abysmal: any write more than a couple KB in size would not only slow down to a crawl, but also bog down the DomU, making it rather difficult to even cancel the write.

After some researching and testing, I tracked it down to the rsize and wsize mount options: they specify the size of the chunks sent at a single time. Apparently, they are set to 1M if you don’t specify anything else. In my case, wsize=131072 and rsize=262144 showed the highest write and read speeds respectively. However, wsize=131072 is not too far away from the cliff after which writing drops to a crawl, so I decided to back it down to 65536.

Patching DSDT in recent Linux kernels without recompiling

Up until a year or two ago, the Linux kernel let you replace the ACPI DSDT by adding a customized version to the initrd. However, more recent versions disable that by default. If you’re using Grub2 as your bootloader though, the alternative is simple: just add acpi /boot/dsdt.aml to your Grub config. If you prefer a more elegant solution, just add the attached 01_acpi.txt to /etc/grub.d, renamed it to 01_acpi and chmod +x it; then run update-grub2 to rebuild your Grub config. It originally came from and I removed the -e flag in the acpi line, which caused the new DSDT only to be visible to Grub, but not to the OS.

I believe the acpi command in Grub2 originally came from the Hackintosh community – messing around with DSDTs is a lot more common there because Mac OS X is rather picky.

For those of you who don’t know what I’m talking about: the DSDT describes certain hardware features your PC has, such as buttons, CPU power save modes, and lots of other things. Some mainboards have very poorly done BIOSes that have equally messed up DSDT tables. I won’t go into a lot of detail regarding how to fix those here though (it’s as simple as cat /proc/acpi/dsdt > dsdt.dat; iasl -d dsdt.dat; editing dsdt.dsl to your liking; iasl -tc dsdt.dsl (this last step will probably produce a number of errors that can be solved by googling for the error number and making the appropriate changes in dsdt.dsl)).

Another valuable hint for people messing around with DSDTs: the DSDT is not the only place that can contain this kind of information, the other place would be the SSDT and possibly additional SSDTs. You can find them in /sys/firmware/acpi/tables and decompile them just like the DSDT as described above. Instead of recompiling the SSDT by itself, you could probably also consider merging it into your custom DSDT at the appropriate places.