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Archives For 30 November 1999

Adding a hardware clock (RTC) to the Raspberry Pi

8 May 2013 — 20 Comments

The Raspberry Pi does not have a hardware clock on board. Instead, to keep track of time during reboots and downtime the ‘fake-hwclock’ package is used. It contains scripts to save the kernel’s current clock periodically (including at shutdown) and restore it at boot so that the system clock keeps at least close to realtime. Combined with NTP this is a simple, cheap and fairly accurate setup for most use-cases.

For a project at work I wanted to test a Raspberry Pi with a Real Time Clock (RTC) connected through the GPIO pins. This hardware clock needs to be very precise. I found the RasClock, by Afterthought Software, to be precise enough (about 3ppm or ~95 seconds deviation per year) while still affordable (about 15 euro).

Hardware installation

The hardware installation is simple. Just insert the battery and place the RasClock on the last 6 GPIO ports, at the end of the board. That’s it.

This is how the RasClock GPIO module looks like

This is how the RasClock GPIO module looks like

This is the RasClock with a battery backup installed

This is the RasClock with a battery backup installed

This is the RasClock installed on the Raspberry Pi, side view

This is the RasClock installed on the Raspberry Pi, side view

This is the RasClock installed on the Raspberry Pi, top view

This is the RasClock installed on the Raspberry Pi, top view

This is the RasClock installed on my two boxed Raspberry Pi's

This is the RasClock installed on two boxed Raspberry Pi’s

Looks pretty good!

Software installation

I recommend upgrading to the latest Raspbian version. At the time of writing Raspbian runs on kernel 3.6.11. By far the easiest way to upgrade firmware, is to use rpi-update.

Unfortunately the ‘rtc-pcf2127a’ module needed to operate the RasClock, is not in the default Linux kernel provided by Raspbian. Afterthought Software provides both a binary packages and source code for their forked raspberrypi/linux repository. They added an I2C driver for NXP/Philips PCF2127A device from Eckelmann AG.

To install the binary package:

wget http://afterthoughtsoftware.com/files/linux-image-3.6.11-atsw-rtc_1.0_armhf.deb
dpkg -i linux-image-3.6.11-atsw-rtc_1.0_armhf.deb
cp /boot/vmlinuz-3.6.11-atsw-rtc+ /boot/kernel.img

The binary package is nice and quick for testing. Installing from source allows you to only build the needed kernel module, instead of a complete kernel replacement.

In the comments of the Afterthought Software site, someone posted these steps. Thanks for sharing your work!

sudo apt-get -y install build-essential gcc make cmake i2c-tools
mkdir devel
cd devel
wget https://github.com/raspberrypi/linux/archive/rpi-3.6.y.tar.gz
wget https://github.com/afterthoughtsoftware/linux/commit/fd5ff2d88f470ed70ff58393eee61110b181816a.patch
tar vxzf rpi-3.6.y.tar.gz
cd linux-rpi-3.6.y/
SUW=`pwd`
patch -p1 < ../fd5ff2d88f470ed70ff58393eee61110b181816a.patch
zcat /proc/config.gz > .config
sed -i 's/# CONFIG_RTC_DRV_PCF2127A is not set/CONFIG_RTC_DRV_PCF2127A=m/g' .config
echo m | make oldconfig
wget https://github.com/raspberrypi/firmware/raw/master/extra/Module.symvers
make modules_prepare
cd drivers/rtc
make -C $SUW M=`pwd`
sudo cp rtc-pcf2127a.ko /lib/modules/3.6.11+/kernel/drivers/rtc/
sudo depmod

It doesn’t matter if you install the binary package or compile the kernel module on your own. The steps below are the same either way.

Make sure to load the needed modules:

cat >> /etc/modules <<EOL
i2c-bcm2708
rtc-pcf2127a
EOL

Make sure the i2c-tools package is installed:

apt-get install i2c-tools

I talked about the ‘fake-hwclock’ package. Now that we have a hardware clock, we should remove this package and it’s crons.

apt-get remove fake-hwclock
rm /etc/cron.hourly/fake-hwclock
update-rc.d -f fake-hwclock remove
rm /etc/init.d/fake-hwclock

Enable the ‘hwclock.sh’ script (part of util-linux), instead:

update-rc.d hwclock.sh enable

It’s now time to boot the new kernel. When you reboot, the RasClock will be available as /dev/rtc0.

dmesg

The kernel ring buffer should list something like this:

[ 32.737903] rtc-pcf2127a 1-0051: chip found
[ 32.739712] rtc-pcf2127a 1-0051: rtc core: registered rtc-pcf2127a as rtc0
[ 32.739775] i2c i2c-1: new_device: Instantiated device pcf2127a at 0x51

Because this is a new device, we need to set the time in the hardware clock. Do this by copying the system time to the RasClock.

hwclock --systohc --utc

Using the ‘hwclock’ command, you can read the RasClock:

hwclock --show --utc

Output:

Wed 08 May 2013 21:59:49 CEST -0.862324 seconds

Adding the ‘–debug’ flag adds some more interesting output, especially if you want to know exactly what goes on. Example output:

hwclock from util-linux 2.20.1
Using /dev interface to clock.
Assuming hardware clock is kept in UTC time.
Waiting for clock tick...
/dev/rtc0 does not have interrupt functions. Waiting in loop for time from /dev/rtc0 to change
...got clock tick
Time read from Hardware Clock: 2013/05/08 19:56:20
Hw clock time : 2013/05/08 19:56:20 = 1368042980 seconds since 1969
Wed 08 May 2013 21:56:20 CEST -0.752934 seconds

You can also access the hardware clock through the /sys pseudo filesystem like this:

cat /sys/class/rtc/rtc0/date
cat /sys/class/rtc/rtc0/name

The first command returns the date, the second command the name of the chip which is ‘rtc-pcf2127a’.

Using the clock

In my current setup, the RasClock keeps time during reboot and when the Raspberry Pi is turned off. My goal is to build a NTP server that is capable of keeping time also when no NTP peers are available to prevent time drifts. I assume the hardware clock that I have now available is more accurate than the clocksource that drives the system time. I need to experiment to see how the two clocks compare. I’ll come back on the subject in a follow-up post.

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One-liner to remove an incorrect entry in the SSH known_hosts file

4 May 2013 — 1 Comment

When a SSH-key changed, this warning is displayed:

ssh -l username 172.16.12.34

Warning:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the ECDSA key sent by the remote host is
aa:bb:cc:a2:b6:87:bd:43:f9:ff:<wbr />02:8e:a6:b8:29:42.
Please contact your system administrator.
Add correct host key in /home/remi/.ssh/known_hosts to get rid of this message.
Offending ECDSA key in /home/remi/.ssh/known_hosts:6

In case you know the host identification has changed, you can safely discard this warning.

You could run ‘vim ~/.ssh/known_hosts‘ enter ‘6G‘ to go to the 6th line, ‘dd‘ to delete that line and finally ‘:wq‘ to save the file. But, wouldn’t a one-liner be handy?

Try:

ssh-keygen -R 172.24.111.132

Output:

/home/remi/.ssh/known_hosts updated.
Original contents retained as /home/remi/.ssh/known_hosts.

I’ve tested this on Ubuntu, Debian, SUSE and Red Hat Enterprise Linux.

In case you get this error:

fopen: No such file or directory

There isn’t a ‘known_hosts’ file in ‘~/.ssh/’. You can use the -f flag to specify the right file.

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Forwarding a local MySQL socket to a remote MySQL server

3 May 2013 — 5 Comments

At times it can be very handy to be able to use a local MySQL socket, while the real MySQL server is in fact a remote one. I once used this technique to split a dozen LAMP servers into dedicated (clustered) web servers and MySQL servers. Without having all clients to update scripts, that is.

Today I ran into another use-case: while migrating an old 32bit MySQL Windows server to a modern Linux VM. We wanted to get some statistics from it. The famous ‘mysqltuner.pl‘ script is designed to run from localhost. Well, let’s fake MySQL is on localhost then 😉

For these tricks to work you need a tool called ‘socat‘, which can be installed like this:

apt-get install socat

This is how to create a socket in ‘/var/lib/mysql/mysql.sock’, owned by both user and group ‘mysql’ and forward the connections to ‘mysql-server’ on port 3306:

socat UNIX-LISTEN:/var/lib/mysql/mysql.sock,fork,\
reuseaddr,unlink-early,user=mysql,group=mysql,mode=777 \
TCP:mysql-server:3306 &

You can now connect:

mysql -u user -p

No need to supply a hostname, MySQL connects to the local socket by default and socat takes care of forwarding the connection to the real MySQL server.

It is important to note this works as long as socat is running. You could use a tool called runit to keep socat running at all times. In another post I’ll explain more about runit.

This works with other protocols as well.

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HOWTO discover the ip address of a Raspberry Pi

3 May 2013 — 2 Comments
When you boot your Raspberry Pi with Raspbian, it will get its ip address from a DHCP server. Most tutorials I’ve seen, ask you to login to a ‘router’ (that serves as DHCP server as well) to find the assigned ip address. When you haven’t access to the DHCP server, how do you find the Pi’s ip address?
You should know the MAC address of a Raspberry Pi always has a ‘b8:27:eb’ prefix. Using a Linux utility called ‘arp-scan‘ you can discover MAC addresses and their corresponding ip addresses. Arp-scan sends ARP packets to hosts on the local network and displays any responses that are received.
First, install arp-scan:
apt-get install arp-scan
Then locate all Raspberry Pi’s on the local network using this command:
arp-scan --interface=eth0 --localnet | grep b8:27:eb
You will need to be root, or arp-scan must be SUID root, in order to run arp-scan, because the functions that it uses to read and write  packets require root privilege.
Result:
172.16.12.34 b8:27:eb:00:11:ab (Unknown)
Using the ip address, it is now possible to SSH into your Raspberry Pi:
ssh -l pi 172.16.12.34
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Automatically adding and editing devices in Zenoss Core 4 using the API

26 April 2013 — 4 Comments

Today I figured out how to automatically add new devices (in my case those are mostly virtual machines) to the Zenoss monitoring system. This used to be done by hand, but no more 🙂

To add a new device (for example a Linux server called server001), simply call:

curl -u apiUser:apiPass \
'http://zenoss-server:8080/zport/dmd/DeviceLoader? \
deviceName=server001&devicePath=/Server/Linux&\
loadDevice:method=1'

It’s wise to create a dedicated Zenoss user just for these API calls, but you may use any account that has sufficient permissions to perform the action you’re calling.

As an alternative you can also use the ‘zenbatchload‘ command. Although you can only add new devices, not edit existing ones.The RESTful API does have the possibility to edit an existing device.

Let’s set some properties to the server we’ve just added:

curl -u apiUser:apiPass \
'http://zenoss-server:8080/zport/dmd/Devices/Server/\
Linux/devices/server001/manage_editDevice?serialNumber=1234&\
tag=tagname&productionState=1000&\
groupPaths=Group1&groupPaths=Group2&priority=3&\
comments=Api%20Test%20Comment&systemPaths=/System\
&rackSlot=Virtual'

It took me some time to figure out all available attributes, although once I found some of them I was able to Google the full list with an explaination.

Attributes:

  • deviceName – the name or IP of the device. If its a name it must resolve in DNS
  • devicePath – the device class where the first “/” starts at “/Devices” like “/Server/Linux” the default is “/Discovered”
  • tag – the tag of the device@
  • serialNumber – the serial number of the device
  • zSnmpCommunity – snmp community to use during auto-discovery if none is given the list zSnmpCommunities will be used
  • zSnmpPort – snmp port to use default is 161
  • zSnmpVer – snmp version to use default v1 other valid values are v2
  • rackSlot – the rack slot of the device.
  • productionState – production state of the device default is 1000 (Production)
  • comments – any comments about the device
  • hwManufacturer – hardware manufacturer this must exist in the database before the device is added
  • hwProductName – hardware product this must exist in the manufacturer object specified
  • osManufacturer – OS manufacturer this must exist in the database before the device is added
  • osProductName – OS product this must exist in the manufacturer object specified
  • locationPath – path to the location of this device like “/Building/Floor” must exist before device is added
  • groupPaths – list of groups for this device multiple groups can be specified by repeating the attribute in the url
  • systemPaths – list of systems for this device multiple groups can be specified by repeating the attribute in the url
  • statusMonitors – list of status monitors (zenping) for this device default is “localhost”
  • performanceMonitor – performance monitor to use default is “localhost”
  • discoverProto – discovery protocol default is “snmp” other possible value is “none”

Finally, in case you wish to delete a device, that can be done as well:

curl -u apiUser:apiPass \
'http://zenoss-server:8080/zport/dmd/Devices/Server/Linux/\
devices/server001/deleteDevice'

Personally I prefer not to delete devices. I rather set the ‘productionState’ to ‘-1’ (Decommissioned) to keep the history in Zenoss.

These simple API calls make it possible to automatically add a new server to the monitoring, or sync information from another source. But you can use the API for gathering all sorts of data as well. For example the load-average:

curl -u apiUser:apiPass \
'http://zenoss-server:8080/zport/dmd/Devices/Server/Linux/\
devices/server001/getRRDValue?dsname=laLoadInt5_laLoadInt5'

Result:

2.0

If you want to start playing with it, have a look at the Zenoss API documentation.

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