Andrew's blog

At the start of July, the LCA2019 team announced that the Call for Proposals for linux.conf.au 2019 were open! This Call for Proposals will close on July 30. If you want to submit a proposal, you don't have much time!

linux.conf.au is one of the best-known community driven Free and Open Source Software conferences in the world. In 2019 we welcome you to join us in Christchurch, New Zealand on Monday 21 January through to Friday 25 January.

For full details including those not covered by this announcement visit https://linux.conf.au/call-for-papers/, and the full announcement is here.

IMPORTANT DATES

• Call for Proposals Opens: 2 July 2018
• Call for Proposals Closes: 30 July 2018 (no extensions)
• Notifications from the programme committee: early-September 2018
• Conference Opens: 21st January 2019

I found to make all this work I had to piece together a bunch of information from different locations. This fills in some of the blanks from the official Raspberry Pi documentation. See here, here, and here.

Image

Download the latest raspbian image from https://www.raspberrypi.org/downloads/raspbian/ and unzip it. I used the lite version as I'll install only what I need later.

To extract the files from the image we need to jump through some hoops. Inside the image are two partitions, we need data from each one.

 # Make it easier to re-use these instructions by using a variable
 IMG=2017-04-10-raspbian-jessie-lite.img
 fdisk -l $IMG

You should see some output like:

 Disk 2017-04-10-raspbian-jessie-lite.img: 1.2 GiB, 1297862656 bytes, 2534888 sectors
 Units: sectors of 1 * 512 = 512 bytes
 Sector size (logical/physical): 512 bytes / 512 bytes
 I/O size (minimum/optimal): 512 bytes / 512 bytes
 Disklabel type: dos
 Disk identifier: 0x84fa8189
 
 Device                               Boot Start     End Sectors  Size Id Type
 2017-04-10-raspbian-jessie-lite.img1       8192   92159   83968   41M  c W95 FAT32 (LBA)
 2017-04-10-raspbian-jessie-lite.img2      92160 2534887 2442728  1.2G 83 Linux

You need to be able to mount both the boot and the root partitions. Do this by tracking the offset of each one and multiplying it by the sector size, which is given on the line saying "Sector size" (typically 512 bytes), for example with the 2017-04-01 image, boot has an offset of 8192, so I mount it like this (it is VFAT):

 mount -v -o offset=$((8192 * 512)) -t vfat $IMG /mnt
 # I then copy the data off:
 mkdir -p /data/diskless/raspbian-lite-base-boot/
 rsync -xa /mnt/ /data/diskless/raspbian-lite-base-boot/
 # unmount the partition now:
 umount /mnt

Then we do the same for the root partition:

 mount -v -o offset=$((92160 * 512)) -t ext4 $IMG /mnt
 # copy the data off:
 mkdir -p /data/diskless/raspbian-lite-base-root/
 rsync -xa /mnt/ /data/diskless/raspbian-lite-base-root/
 # umount the partition now:
 umount /mnt

DHCP

When I first set this up, I used OpenWRT on my router, and I had to patch /etc/init/dnsmasq to support setting DHCP option 43. As of the writting of this article, a similar patch has been merged, but isn't in a release yet, and, well, there may never be another release of OpenWRT. I'm now running LEDE, and the the good news is it already has the patch merged (hurrah!). If you're still on OpenWRT, then here's the patch you'll need:

https://git.lede-project.org/?p=source.git;a=commit;h=9412fc294995ae2543fabf84d2ce39a80bfb3bd6

This lets you put the following in /etc/config/dnsmasq, this says that any device that uses DHCP and has a MAC issued by the Raspberry PI Foundation, should have option 66 (boot server) and option 43 set as specified. Set the IP address on option 66 to the device that should be used for tftp on your network, if it's the same device that provides DHCP then it isn't required. I had to set the boot server, as my other network boot devices are using a different server (with an older tftpd-hpa, I explain the problem further down).

 config mac 'rasperrypi'
         option mac 'b8:27:eb:*:*:*'
         option networkid 'rasperrypi'
         list dhcp_option '66,10.1.0.253'
         list dhcp_option '43,Raspberry Pi Boot'

tftp

Initially I used a version of tftpd that was too old and didn't support how the RPi tried to discover if it should use the serial number based naming scheme. The version of tftpd-hpa Debian Jessie works just fine. To find out the serial number you'll probably need to increase the logging of tftpd-hpa, do so by editing /etc/default/tftpd-hpa and adding "-v" to the TFTP_OPTIONS option. It can also be useful to watch tcpdump to see the requests and responses, for example (10.1.0.203 is the IP of the RPi I'm working with):

  tcpdump -n -i eth0 host 10.1.0.203 and dst port 69

This was able to tell me the serial number of my RPi, so I made a directory in my tftpboot directory with the same serial number and copied all the boot files into there. I then found that I had to remove the init= portion from the cmdline.txt file I'm using. To ease debugging I also removed quiet. So, my current cmdline.txt contains (newlines entered for clarity, but the file has it all on one line):

idwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/nfs
nfsroot=10.1.0.253:/data/diskless/raspbian-lite-base-root,vers=3,rsize=1462,wsize=1462
ip=dhcp elevator=deadline rootwait hostname=rpi.etc.gen.nz

NFS root

You'll need to export the directories you created via NFS. My exports file has these lines:

/data/diskless/raspbian-lite-base-root	10.1.0.0/24(rw,no_root_squash,sync,no_subtree_check)
/data/diskless/raspbian-lite-base-boot	10.1.0.0/24(rw,no_root_squash,sync,no_subtree_check)

And you'll also want to make sure you're mounting those correctly during boot, so I have in /data/diskless/raspbian-lite-base-root/etc/fstab the following lines:

10.1.0.253:/data/diskless/raspbian-lite-base-root   /       nfs   rw,vers=3       0   0
10.1.0.253:/data/diskless/raspbian-lite-base-boot   /boot   nfs   vers=3,nolock   0   2

Network Booting

Now you can hopefully boot. Unless you into this bug, as I did. Where the RPi will sometimes fail to boot. Turns out the fix, which is mentioned on the bug report, is to put bootcode.bin (and only bootcode.bin) onto an SD card. That'll then load the fixed bootcode, and which will then boot reliably.

Something I've been wanting to do with our Asterisk PBX at Catalyst for a while is to allow having callers that hit VoiceMail to be forwarded the callee's cellphone if allowed. As part of an Asterisk migration we're currently carrying out I finally decided to investigate what is involved. One of the nice things about the VoiceMail application in Asterisk is that callers can hit 0 for the operator, or * for some other purpose. I decided to use * for this purpose.

I'm going to assume a working knowledge of Asterisk dial plans, and I'm not going to try and explain how it works. Sorry.

When a caller hits * the VoiceMail application exits and looks for a rule that matches a. Now, the simple approach looks like this within our macro for handling standard extensions:

[macro-stdexten]
...
exten => a,1,Goto(pstn,027xxx,1)
...

(Where I have a context called pstn for placing calls out to the PSTN).

This'll work, but anyone who hits * will be forwarded to my cellphone. Not what I want. Instead we need to get the dialled extension into a place where we can perform extension matching on it. So instead we'll have this (the extension is passed into macro-stdexten as the first variable - ARG1):

[macro-stdexten]
...
exten => a,1,Goto(vmfwd,${ARG1},1)
...

Then we can create a new context called vmfwd with extension matching (my extension is 7231):

[vmfwd]
exten => 7231,1,Goto(pstn,027xxx,1)

I actually have a bit more in there to do some logging and set the caller ID to something our SIP provider will accept, but you get the gist of it. All I need to do is to arrange for a rule per extension that is allowed to have their VoiceMail callers be forwarded to voicemail. Fortunately I have that part automated.

The only catch is for extensions that aren't allowed to be forwarded to a cellphone. If someone calling their VoiceMail hits * their call will be hung up and I get nasty log messages about no rule for them. How do we handle them? Well, we send them back to VoiceMail. In the vmfwd context we add a rule like this:

exten => _XXXX,1,VoiceMail(${EXTEN}@sip,${voicemail_option})
  same => n,Hangup

So any extension that isn't otherwise matched hits this rule. We use ${voicemail_option} so that we can use the same mode as was used previously.

Easy! Naturally this approach won't work for other people trying to do this, but given I couldn't find write ups on how to do this, I thought it be might be useful to others.

Here's my macro-stdexten and vmfwd in full:

[macro-stdexten]
exten => s,1,Progress()
exten => s,n,Dial(${ARG2},20)
exten => s,n,Goto(s-${DIALSTATUS},1)
exten => s-NOANSWER,1,Answer
exten => s-NOANSWER,n,Wait(1)
exten => s-NOANSWER,n,Set(voicemail_option=u)
exten => s-NOANSWER,n,Voicemail(${ARG1}@sip,u)
exten => s-NOANSWER,n,Hangup
exten => s-BUSY,1,Answer
exten => s-BUSY,n,Wait(1)
exten => s-BUSY,n,Set(voicemail_option=b)
exten => s-BUSY,n,Voicemail(${ARG1}@sip,b)
exten => s-BUSY,n,Hangup
exten => _s-.,1,Goto(s-NOANSWER,1)
exten => a,1,Goto(vmfwd,${ARG1},1)
exten => o,1,Macro(operator)

[vmfwd]

exten => _XXXX,1,VoiceMail(${EXTEN}@sip,${voicemail_option})
  same => n,Hangup

#include extensions-vmfwd-auto.conf

And I then build extensions-vmfwd-auto.conf from a script that is used to generate configuration files for defining accounts, other dial plan rule entries and phone provisioning files.

With thanks to John Kiniston for the suggestion about the wildcard entry in vmfwd.