Archive for the 'Fedora' Category

Booting Fedora 24 cloud image with KVM

Fedora 24 is on the way, here’s how you can play with the cloud image on your local machine.

Download the image:
wget https://alt.fedoraproject.org/pub/alt/stage/24_RC-1.2/CloudImages/x86_64/images/Fedora-Cloud-Base-24-1.2.x86_64.qcow2

Make a new local backing image (so that we don’t write to our downloaded image) called my-disk.qcow2:
qemu-img create -f qcow2 -b Fedora-Cloud-Base-24-1.2.x86_64.qcow2 my-disk.qcow2

The cloud image uses cloud-init to configure itself on boot which sets things like hostname, usernames, passwords and ssh keys, etc. You can also run specific commands at two stages of the boot process (see bootcmd and runcmd below) and output messages (see final_message below) which is useful for scripted testing.

Create a file called meta-data with the following content:
instance-id: FedoraCloud00
local-hostname: fedoracloud-00

Next, create a file called user-data with the following content:
#cloud-config
password: password
chpasswd: { expire: False }
ssh_pwauth: True
 
bootcmd:
 - [ sh, -c, echo "=========bootcmd=========" ]
 
runcmd:
 - [ sh, -c, echo "=========runcmd=========" ]
 
# add any ssh public keys
ssh_authorized_keys:
  - ssh-rsa AAA...example...SDvZ user1@domain.com
 
# This is for pexpect so that it knows when to log in and begin tests
final_message: "SYSTEM READY TO LOG IN"

Cloud init mounts a CD-ROM on boot, so create an ISO image out of those files:
genisoimage -output my-seed.iso -volid cidata -joliet -rock user-data meta-data

If you want to SSH in you will need a bridge of some kind. If you’re already running libvirtd then you should have a virbr0 network device (used in the example below) to provide a local network for your cloud instance. If you don’t have a bridge set up, you can still boot it without network support (leave off the -netdev and -device lines below).

Now we are ready to boot this!
qemu-kvm -name fedora-cloud \
-m 1024 \
-hda my-disk.qcow2 \
-cdrom my-seed.iso \
-netdev bridge,br=virbr0,id=net0 \
-device virtio-net-pci,netdev=net0 \
-display sdl

You should see a window pop up and Fedora loading and cloud-init configuring the instance. At the login prompt you should be able to log in with the username fedora and password that you set in user-data.

How to find out which process is listening on a port

Say that you notice UDP port 323 is open (perhaps via netstat -lun) and you’ve no idea what that is!

With lsof it’s easy to find out which process is guilty:


[15:27 chris ~]$ sudo lsof -i :323
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
chronyd 1044 chrony 1u IPv4 19197 0t0 UDP localhost:323
chronyd 1044 chrony 2u IPv6 19198 0t0 UDP localhost:323

In this case, it’s chrony, the modern time keeping daemon.

As Jonh pointed out in the comments, you can also use netstat with the -p flag.

For example, show all processes listening (-l) on both TCP (-t) and UDP (-u) by port number (-n) showing the process (-p), while I grep for port 323 to show what’s running:

[19:08 chris ~]$ sudo netstat -lutnp |grep 323
udp 0 0 127.0.0.1:323 0.0.0.0:* 1030/chronyd
udp6 0 0 ::1:323 :::* 1030/chronyd

Signal Return Orientated Programming attacks

When a process is interrupted, the kernel suspends it and stores its state in a sigframe which is placed on the stack. The kernel then calls the appropriate signal handler code and after a sigreturn system call, reads the sigframe off the stack, restores state and resumes the process. However, by crafting a fake sigframe, we can trick the kernel into executing something else.

My friend Rashmica, an intern at OzLabs, has written an interesting blog post about this for some work she’s doing with the POWER architecture in Linux.

TRIM on LVM on LUKS on SSD, revisited

A few years ago I wrote about enabling trim on an SSD that was running with LVM on top of LUKS. Since then things have changed slightly, a few times.

With Fedora 24 you no longer need to edit the /etc/crypttab file and rebuild your initramfs. Now systemd supports a kernel boot argument rd.luks.options=discard which is the only thing you should need to do to enable trim on your LUKS device.

Edit /etc/default/grub and add the rd.luks.options=discard argument to the end of GRUB_CMDLINE_LINUX, e.g.:
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_DEFAULT=saved
GRUB_DISABLE_SUBMENU=true
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="rd.luks.uuid=luks-de023401-ccec-4455-832bf-e5ac477743dc rd.luks.uuid=luks-a6d344739a-ad221-4345-6608-e45f16a8645e rhgb quiet rd.luks.options=discard"
GRUB_DISABLE_RECOVERY="true"

Next, rebuild your grub config file:
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

If you’re using LVM, the setting is the same as the previous post. Edit the /etc/lvm/lvm.conf file and enabled the issue_discards option:
issue_discards = 1

If using LVM you will need to rebuild your initramfs so that the updated lvm.conf is in there.
sudo dracut -f

Reboot and try fstrim:
sudo fstrim -v /

Now also thanks to systemd, you can just enable the fstrim timer (cron) to do this automatically:
sudo systemctl enable fstrim.timer

Running scripts before and after suspend with systemd

I’ve had this question a few times, so it’s probably a good candidate for my blog.

If you want to do something before you suspend, like unload a module or run some script, it’s quite easy with systemd. Similarly, you can easily do something when the system resumes (like reload the module).

The details are in the systemd-suspend man page:
man systemd-suspend.service

Simply put an executable script of any name under /usr/lib/systemd/system-sleep/ that checks whether the first argument is pre (for before the system suspends) or post (after the system wakes from suspend).

If it is pre, then do the thing you want to before suspend, if it’s post then do the thing you want to do after resume. Simple!

Here’s a useless example:
#!/bin/sh
if [ "${1}" == "pre" ]; then
  # Do the thing you want before suspend here, e.g.:
  echo "we are suspending at $(date)..." > /tmp/systemd_suspend_test
elif [ "${1}" == "post" ]; then
  # Do the thing you want after resume here, e.g.:
  echo "...and we are back from $(date)" >> /tmp/systemd_suspend_test
fi

Automatic power saving on a Linux laptop with PowerTOP and systemd

If you have a laptop and want to get more battery life, you may already know about a handy tool from Intel called PowerTOP.

PowerTOP not only monitors your system for interrupts but has a tunable section where you can enable various power saving tweaks. Toggling one such tweak in the PowerTOP interface will show you the specific Linux system command it ran in order to enable or disable it.

PowerTOP Tweaks

Furthermore, it takes an argument ––auto-tune which lets you enable all of the power saving measures it has detected.

The package on Fedora comes with a systemd service, so enabling power saving on boot is simple, just enable the service:
sudo systemctl enable powertop

I noticed, however, that putting some devices into low-power mode on my laptop has unwanted side effects. In my case, the audio system outputs white noise and the USB mouse and keyboard are too slow to wake up (which I find annoying when I want to quickly click on, or type something).

So I took note of the Linux commands that PowerTOP was running for the USB peripherals and the audio device when it disabled power saving. The plan is to use the power of powertop ––auto-tune but I’ll then turn power saving back off for those specific devices.

I created an executable script under /usr/local/sbin/powertop-fixups.sh to disable power saving on those devices:
#!/bin/sh
# Don't do powersave on intel sound, we get static noise
echo '0' > '/sys/module/snd_hda_intel/parameters/power_save'
 
# Don't suspend USB keyboard and mouse
# This takes time to wake up which is annoying
echo 'on' > '/sys/bus/usb/devices/1-2.1.1/power/control'
echo 'on' > '/sys/bus/usb/devices/1-2.2/power/control'

Now, I just needed to tell systemd to start my script on boot, which should require and start after the powertop service.

I created the following service file at /etc/systemd/system/powertop-fixups.service:
[Unit]
Description=PowerTOP fixups
Requires=powertop.service
After=powertop.service
 
[Service]
Type=oneshot
ExecStart=/usr/local/sbin/powertop-fixups.sh
 
[Install]
WantedBy=multi-user.target

Then all I had to do was activate and enable it! Note that I don’t need to enable powertop.service itself, systemd will take care of that for me.

sudo systemctl daemon-reload
sudo systemctl enable powertop-fixups

Now I get the benefit of most of the power savings from PowerTOP, without the settings that were annoying.

Configuring Postfix to forward emails via localhost to secure, authenticated GMail

It’s pretty easy to configure postfix on a local Linux box to forward emails via an external mail server. This way you can just send via localhost in your programs or any system daemons and the rest is automatically handled for you.

Here’s how to forward via GMail using authentication and encryption on Fedora (23 at the time of writing). You should consider enabling two-factor authentication on your gmail account, and generate a password specifically for postfix.

Install packages:
sudo dnf install cyrus-sasl-plain postfix mailx

Basic postfix configuration:
#Only listen on IPv4, not IPv6. Omit if you want IPv6.
sudo postconf inet_protocols=ipv4
 
#Relay all mail through to TLS enabled gmail
sudo postconf relayhost=[smtp.gmail.com]:587
 
#Use TLS encryption for sending email through gmail
sudo postconf smtp_use_tls=yes
 
#Enable authentication for gmail
sudo postconf smtp_sasl_auth_enable=yes
 
#Use the credentials in this file
sudo postconf smtp_sasl_password_maps=hash:/etc/postfix/sasl_passwd
 
#This file has the certificate to trust gmail encryption
sudo postconf smtp_tls_CAfile=/etc/ssl/certs/ca-bundle.crt
 
#Require authentication to send mail
sudo postconf smtp_sasl_security_options=noanonymous
sudo postconf smtp_sasl_tls_security_options=noanonymous

By default postfix listens on localhost, which is probably what you want. If you don’t for some reason, you could change the inet_interfaces parameter in the config file, but be warned that then anyone on your network (or potentially the Internet if it’s a public address) could send mail through your system. You may also want to consider using TLS on your postfix server.

By default, postfix sets myhostname to your fully-qualified domain name (check with hostname -f) but if you need to change this for some reason you can. For our instance it’s not really necessary because we’re forwarding email through a relay and not accepting locally.

Check that our configuration looks good:
sudo postconf -n
sudo postfix check

Create a password file using a text editor:
sudoedit /etc/postfix/sasl_passwd

The content should be in this form (the brackets are required, just replace your username@gmail.com address and password):
[smtp.gmail.com]:587 username@gmail.com:password

Hash the password for postfix:
sudo postmap /etc/postfix/sasl_passwd

Tail the postfix log:
sudo journalctl -f -u postfix.service &

Start the service (you should see it start up in the log):
sudo systemctl start postfix

Send a test email, replace username@gmail.com with your real email address:
echo "This is a test." | mail -s "test message" username@gmail.com

You should see the email go through the journalctl log and be forwarded, something like:
Feb 29 04:32:51 hostname postfix/smtp[4115]: 87BE620221: to=, relay=smtp.gmail.com[209.85.146.108]:587, delay=1.9, delays=0.04/0.06/0.55/1.3, dsn=2.0.0, status=sent (250 2.0.0 OK 1456720371 m32sm102235580ksj.52 - gsmtp)

Permanently setting SELinux context on files

I’m sure there are lots of howtos on the Internet for this, but…

Say you are running a web server like nginx and your log files are in a non-standard location, you may have problems starting the service because SELinux is blocking nginx from reading or writing to the files.

You can set the context of these files so that nginx will be happy:
[user@server ~]$ sudo chcon -Rv --type=httpd_log_t /srv/mydomain.com/logs/

That’s only temporary however, and the original context will be restored if you run restorecon or relabel your filesystem.

So you can do this permanently using the semanage command, like so:

[user@server ~]$ sudo semanage fcontext -a -t httpd_log_t "/srv/mydomain.com/logs(/.*)?"

Now you can use the standard selinux command to restore the correct label and it will use the new one you set above.
[user@server ~]$ sudo restorecon -rv /srv/

Trusting a self-generated CA system-wide on Fedora

Say you’re using FreeIPA (or perhaps you’ve generated your own CA) and you want to have your machines trust it. Well in Fedora you can run the following command against the CA file:


# trust anchor rootCA.pem

Building a Mini-ITX NAS? Don’t buy a Silverstone DS380 case.

Edit: I made some changes which have dropped the temps to around 40 degrees at idle (haven’t tested at load yet). The case has potential, but I still think it’s slightly too cramped and the airflow is not good enough.

Here’s what I changed:

  • Rearranged the drives to leave a gap between each one, which basically limits the unit to 4 drives instead of 8
  • Inverted the PSU as per suggestion from Dan, so that it helps to draw air through the case. The default for the PSU is to draw air from outside and bypass the case.
  • Plugged the rear and side fans directly into the PSU molex connector, rather than through mainboard and rear of hard drive chassis

So I’m building a NAS (running Fedora Server) and thought that the Silverstone DS380 case looked great. It has 8 hot-swappable SATA bays, claims decent cooling with filters, neat form factor.

ds380-34

It requires an SFX PSU, but there are some that have enough juice on the 12v rail (although avoid the SilverStone SX500-LG, it’s slightly too long) so that it’s not a major problem (although I would prefer standard ATX).

So I got one to run low-power i3, C226 chipset mainboard and five HGST 3TB NAS drives. Unfortunately the cooling through the drives is pretty much non-existent. The two fans on the side draw air in but blow onto the hotswap chassis and nothing really draws air through it.

As a result, many of the drives run around 65 degrees Celsius at idle (tested overnight) which is already outside of the drives’ recommended temperature range of 0-60 degrees.

I’ve replaced the case with my second choice Fractal Design NODE 304 and the drives at idle all sit at around 35 degrees.

node

It has two smaller fans at the front to bring air directly over the drives and a larger one at the rear, with a manual L/M/H speed controller for all three on the rear of the case. As a bonus, it uses a standard ATX power supply and has plenty of room for it.

The only downside I’ve found so far is the lack of hot-swap, but my NAS isn’t mission-critical so that’s not a deal breaker for me.

Your mileage might vary, but I won’t buy the DS380 for a NAS again, unless it’s going to run full of SSDs or something (or I heavily mod the case). It’s OK for a small machine though without a bunch of disks (shame!) and that’s what I’ve re-purposed it for now.

-c