Archive for the ‘linux’ Category

NSA-proof SSH

Tuesday, January 6th, 2015

ssh-pictureOne of the biggest takeaways from 31C3 and the most recent Snowden-leaked NSA documents is that a lot of SSH stuff is .. broken.

I’m not surprised, but then again I never am when it comes to this paranoia stuff. However, I do run a ton of SSH in production and know a lot of people that do. Are we all fucked? Well, almost, but not really.

Unfortunately most of what Stribika writes about the “Secure Secure Shell” doesn’t work for old production versions of SSH. The cliff notes for us real-world people, who will realistically be running SSH 5.9p1 for years is hidden in the repo.

Edit your /etc/ssh/sshd_config:

Ciphers aes256-ctr,aes192-ctr,aes128-ctr
MACs hmac-sha2-512,hmac-sha2-256,hmac-ripemd160
KexAlgorithms diffie-hellman-group-exchange-sha256

Basically the nice and forward secure aes-*-gcm chacha20-poly1305 ciphers, the curve25519-sha256 Kex algorithm and Encrypt-Then-MAC message authentication modes are not available to those of us stuck in the early 2000s. That’s right, provably NSA-proof stuff not supported. Upgrading at this point makes sense.

Still, we can harden SSH, so go into /etc/ssh/moduli and delete all the moduli that have 5th column < 2048, and disable ECDSA host keys:

cd /etc/ssh
mkdir -p broken
mv moduli ssh_host_dsa_key* ssh_host_ecdsa_key* ssh_host_key* broken
awk '{ if ($5 > 2048){ print } }' broken/moduli > moduli
# create broken links to force SSH not to regenerate broken keys
ln -s ssh_host_ecdsa_key ssh_host_ecdsa_key
ln -s ssh_host_dsa_key ssh_host_dsa_key
ln -s ssh_host_key ssh_host_key

Your clients, which hopefully have more recent versions of SSH, could have the following settings in /etc/ssh/ssh_config or .ssh/config:

Host all-old-servers


Note: Sadly, the -ctr ciphers do not provide forward security and hmac-ripemd160 isn’t the strongest MAC. But if you disable these, there are plenty of places you won’t be able to connect to. Upgrade your servers to get rid of these poor auth methods!

Handily, I have made a little script to do all this and more, which you can find in my Gone distribution.

There, done.

sshh obama

Updated Jan 6th to highlight the problems of not upgrading SSH.
Updated Jan 22nd to note CTR mode isn’t any worse.
Go learn about COMSEC if you didn’t get trolled by the title.

sound sound

Monday, December 8th, 2014


Recently I been doing some video editing.. less editing than tweaking my system tho.
If you want your jack output to speak with Kdenlive, a most excellent video editing suite,
and output audio in a nice way without choppyness and popping, which I promise you is not nice,
you’ll want to pipe it through pulseaudio because the alsa to jack stuff doesn’t do well with phonom, at least not on this convoluted setup.

Remember, to get that setup to work, ALSA pipes to jack with the pcm.jack { type jack .. thing, and remove the alsa to pulseaudio stupidity at /usr/share/alsa/alsa.conf.d/50-pulseaudio.conf

So, once that’s in place, it won’t play even though Pulse found your Jack because your clients are defaulting out on some ALSA device… this is when you change /etc/pulse/client.conf and set default-sink = jack_out.

danger at the docks

Friday, November 14th, 2014 is taking the world by storm, but a day at the docks is not without its perils. Here I hope to inspire you to try out docker by showing you how to avoid its pitfalls.

In the days of yore

As the FreeBSD jailers and Solaris zoners will attest to, containerizing your services is a great boon, saving space and resources and providing easy management akin to chroots and potential security benefits, without the overheads of full-blown virtual machines.

FreeBSD Jail Linux has had containers for the longest time, in the ancient form of User Mode Linux, which actually ran a kernel in userland, and more recently OpenVZ, which was more like jails.

The former didn’t lend itself to production deployments and the latter never made it into the linux mainline, coming at a time when people were more interested in virtualization than containment. In recent years, a kernel facility named Cgroups has made LinuX Containers (LXC) possible, which as afforded the management, if not security, of bsd jails.

what can be gained

The biggest potential benefit from containers is that CPU, memory and disk resources are 100% shared at native speeds, so no libraries and no data need ever be duplicated on disk nor in memory.

In FreeBSD jails, this was achieved by providing most of the system read-only like /usr, /lib and /bin, and sharing it amongst jails. This worked quite well, but was surprisingly tricky to update.

You can do similar stuff with LXC, just as long as you understand that if it breaks, you get to keep all the pieces. This gives you full control, and means that I for one have LXC instances in production with uptimes of 1200 days and counting.


Taking the approach of single-container-single-responsibility further, you could instead of deploying whole system containers create image filesystems that contained only the bare necessities. For instance, your python application would have apart from its code,just the python runtime, libc and other dependant libraries, and naught much else.

Inspired by the “leaner is better” philosophy backed by the experience of running LXC in production, we built this minimal deployment framework complete with a tool to magically find all the required libraries.
leaner is better
Awesomely small images come from this approach, where the “contact surface” of the application has shrank to nothing but the app itself. It was far from perfect, serving to make the images awesomely less debuggable and managable, and never made it into production proper.

layer upon layer is two steps further

In comes Docker, and its concept of filesystem image layers based on AUFS. The approach isn’t novel itself, having been used by live-CD distributions for the longest time, but it’s the first that provides tools to manage the layers effortlessly for containers. So you can now have 100 servers with 100 application layers, and all your Ruby applications share one runtime layer and your Python applications share another, and they all run on the same base image of Ubuntu, and they do all that transparently, without you having to consciously think about which bit goes where.

layersDocker takes another step further, borrowing heavily from distributed social source control ala github, allowing you to clone, build, push, pull, commit, share and remix images as easy as that.

This is the type of thing that blows disk-image-based virtualization straight out of the water.

Perils and rough starts

The Docker docs are well written and will get you spawning containers and dockerizing applications in no time at all. What they will not tell you is how to run containers in production for all values of production.

In particular, the following things require special attention:

  • changing ips
  • service discovery
  • dns issues
  • cache clash

.. and that is precisely what we will talk about next time.
see you later

systemd crash course, with remote logging

Saturday, September 20th, 2014

live a better life

The world is taking systemd by storm and there is no looking back now.

Still, there are some elements that you would expect to be there that are missing. One of them is remote logging!

Another thing missing is a decent crash course [*]. This is frustrating because making init scripts and checking logs is the staple diet of any old sysadmin.

Read on to readjust gently but quickly.
she wants it

Systemd crash course

Find “unit” – that’s the new name for “init script name” to us oldtimers:

systemctl list-units --type=service
# this one is way more verbose
systemctl list-units

Start, stop, restart, reload, status:

systemctl start sshd
systemctl stop sshd
systemctl restart sshd
systemctl reload sshd
# status, gives some log output too
systemctl status sshd

Check ALL the logs, follow the logs, get a log for a service:

journalctl -l
journalctl -f
journalctl -u sshd

Install a systemd service:
(This is what a systemd service description looks like)

    cat > ossec.service << EOF
Description=OSSEC Host-based Intrusion Detection System

ExecStart=/var/ossec/bin/ossec-control start
ExecStop=/var/ossec/bin/ossec-control stop


# now copy that file into the magic place, /etc/init.d in the old days
install -Dm0644 ossec.service /usr/lib/systemd/system/ossec.service

# now make systemd pick up the changes
systemctl daemon-reload

Enable or disable a service:

systemctl enable ossec
systemctl disable ossec

systemd components

Remote logging

OK so you now know your way around this beast.
Now you want remote logging.

According to the Arch wiki [#], systemd doesn’t actually do remote logging (yet. what else doesn’t it do?) but it will helpfully spew its logs onto the socket /run/systemd/journal/syslog if you knock twice, gently.

To convince systemd to write to this socket, go to /etc/systemd/journald.conf and set


then issue a journald restart

systemctl restart systemd-journald

You can install syslog-ng and it should pick up the logs. Test it now by making a log entry with

logger -t WARN zool

and check /var/log/syslog.log

If you have a distro running systemd, then hopefully syslog-ng will be recent enough to be aware enough of systemd that things should just work at this point.

If it don’t, syslog-ng.conf’s source src { system(); }; isn’t picking up the socket file. Fix this by adding the socket explicitly by changing the source in /etc/syslog-ng/syslog-ng.conf like so:

source src {

if you are working with a laptop or desktop then the console_all on tty12 is handy too:

log { source(src); destination(console_all); };

the systemd monster

[*] IMHO Fedora’s cheatsheet on systemd is a little too cluttered
[#] Arch has a decent intro to systemd

No sockpuppets were harmed in the making of this blog entry. Any and all images are © whomever made them, and I love you for not sueing me.

Grsecurity on the desktop

Friday, August 8th, 2014

In my last post I presented Grsecurity kernel packages for Debian Wheezy. Now would be a good time to review the hows and whys of Grsecurity, so you can decide if it is something you need.

Today we will quickly look at Grsecurity’s viability and impact on a typical desktop or laptop.

Why do I need Grsecurity on the desktop?

  • Often run unsecure code? Limit its impact on the system.
  • Employ chroots and containers? Enforce stricter containment.
  • Connect to hostile networks? Reduce and mitigate impact of exploitation attacks.
  • Allow others to use your system? Increase monitoring and control over your machine.

Or perhaps you choose to run Grsecurity on your laptop simply for the sheer paranoia-factor and to impress friends.

How does Grsecurity behave on the desktop?

In addition to the invisible yet significant hardening efforts againt kernel exploitation, there are some changes that an experienced user will notice immediately:

  • you need root to dmesg
  • grsec reports denied resource oversteps in dmesg
  • top only shows this user’s running processes
  • mappings and other sensitive information is only available to the process owner in /proc

The very few programs that depend on a specific kernel version, or that read /sys/kcore or write directly to kernel constructs will not work under Grsecurity.

And of course, there is the feeling of solidity and the sight of the reported kernel version:

~# uname -a
Linux amaeth 3.2.60-grsec-lied #1 SMP Wed Aug 6 17:40:27 CEST 2014 x86_64 GNU/Linux

kernel security for debian

Wednesday, August 6th, 2014

TL;DR – links to Grsecurity-enabled up-to-date debian wheezy-kernel packages at the bottom of this post.

Kernel security is becoming more important nowadays, especially since the Linux kernel has grown so big and the platform is rife with misbehaving programs.

Some enthusiasts have illusions that the Linux kernel is somehow magically very secure, but those in the know will tell you there are quicker ways to gain root than sudo.
you did what?
Grsecurity is by far the best patchset for increasing the security of a system, whether it be a virtual LAMP box, a login shell server or a desktop machine. I have been tracking it and using it for years and consider it superior to the SELinux and AppArmor approaches.

It has only one major drawback: most distributions of Linux do not provide tested and up-to-date grsec-patched kernel packages, making Grsec-level security features nearly unobtainium for the common mortal.

I have been rolling my own kernel patches since the millenium and so I put in the work to put Grsecurity back into Debian.

So far I have built and tested kernels for Debian 7.5 and 7.6 Stable codenamed Wheezy. This is the standard, debian-patched kernel with added Grsecurity.

I have built separate packages which are suitable for servers, virtualized servers and desktops, and these have been tested on run-of-the-mill LAMP boxen as well as custom and well-trafficed shell servers, and of course my trusty desktops and laptops.

Download and Install

You can download and install the grsec desktop. server and virtual server debian packages today!

Note, to avoid running out of space in /boot, change MODULES=most to MODULES=dep in /etc/initramfs-tools/initramfs.conf

Install the lied apt repository

sudo -i
wget -O- | apt-key add -
wget -O /etc/apt/sources.list.d/lied.list
apt-get update

Grsecurity Desktop/Laptop HOWTO

apt-get install linux-image-3.2.60-grsec-lied

Grsecurity Server HOWTO

dpkg -i linux-image-3.2.54-grsec-nose-server_3.2.54-grsec-nose-server-1_amd64.deb

Grsecurity Virtual Server HOWTO

dpkg -i linux-image-3.2.54-grsec-nose-virtual_3.2.54-grsec-nose-virtual-1_amd64.deb

Furthermore I commit to also merging the patchsets and making available Grsecurity packages for Debian 8/Jessie and providing it all in a debian repo. I will then make this available in a repo so that people can easily add it to their setup.
I also commit to keeping these packages up to date on all the platforms I care about.

Quick Howto build your own
To build your own Grsec-enabled debian kernel packages, execute the following commands:

gpg grsec-201408032014-debian-3.2.60-1.patch.gpg
apt-get source linux
cd linux-3.2.60
patch -p1 < ../grsec-201408032014-debian-3.2.60-1.patch
wget -O .config
make deb-pkg LOCALVERSION=-myversion

You can replace “grsec-config-server” with “grsec-config-desktop” or “-virtual” if you need one of the other configurations.

3g wwan pain revisited with gobi

Monday, June 30th, 2014

Hi all,
after a long hiatus I’ve finally found a something annoying enough to share with you: namely, my 3g modem.
I have spoken at lengths about 3g on linux before.

I have a Thinkapd X201 laptop and it has a Qualcomm Gobi 2000 3g modem. This modem does some fancy mode switching, but not in the regular way by getting some control bytes. Therefore, usb-modeswitch can’t help you.

Instead, the modem needs firmware loaded to switch from usb id 05c6:9204 to usb id 05c6:9205.
On linux, the firmare loading is achieved with gobi-loader.

All this is nicely documented at thinkwiki, unfortunately it doesn’t make it one bit easier for the regular joe.

The trouble is, the firmware is not redistributable, so the whole thing is quite tricky!

  1. download 7xwc48ww.exe from the Thinkpad support site,
  2. unpack the drivers with wine or cabextract. I used wine:
    cp 7xwc48ww.exe ~/.wine/drive_c
    wine 7xwc48ww.exe

    Make sure you run the driver installation after extraction, otherwise execute setup again: wine ~/.wine/drive_c/DRIVERS/WWANQL/setup.exe

  3. copy the firmware:
    cd ~/.wine/drive_c/Program Files/QUALCOMM/Images/Lenovo
    sudo mkdir /lib/firmware/gobi
    sudo cp 6/UQCN.mbn UMTS/* /lib/firmware/gobi/

    This was the tricky part, unpacking and selecting the correct firmware out of the 12 different sets of files provided in that directory.

  4. reload the driver: modprobe -r qcserial; modprobe qcserial
  5. dmesg should now show you have three USB serial devices /dev/ttyUSB0 (control interface), /dev/ttyUSB1 (the actual modem), and /dev/ttyUSB2 (the GPS, which you need windows to enable once).
    usb 2-1.4: Product: Qualcomm Gobi 2000
    usb 2-1.4: Manufacturer: Qualcomm Incorporated
    qcserial 2-1.4:1.1: Qualcomm USB modem converter detected
    usb 2-1.4: Qualcomm USB modem converter now attached to ttyUSB0
    qcserial 2-1.4:1.2: Qualcomm USB modem converter detected
    usb 2-1.4: Qualcomm USB modem converter now attached to ttyUSB1
    qcserial 2-1.4:1.3: Qualcomm USB modem converter detected
    usb 2-1.4: Qualcomm USB modem converter now attached to ttyUSB2
  6. If you have gotten this far, your 3g modem is basically working and you can set up wvdial as in my previoius post pointing at the /dev/ttyUSB1 modem.

    Note however you still need to enable the modem with echo enable > /proc/acpi/ibm/wan

bup quick reference

Thursday, April 25th, 2013

Git is nice and flexible. I wish my backups were that flexible. Thankfully, my wishes have been answered, as bup was created.
I used to lookup the 28c3 bup slides for a quick reference, until I realized I was always looking for just one page of the slides. Best docs are short docs.

# Install
sudo apt-get install python2.6-dev python-fuse python-pyxattr python-pylibacl
git clone
cd bup && make && make test && sudo make install
# index zz's home directory
bup index -ux /home/zz
# backup to default BUP_DIR and label the backup 'laptop'
bup save -n laptop /home/zz
# backup to remote myserver, naming the backup 'laptop'
bup save -r myserver -n laptop /home/zz
# index /home/zz on myserver
bup on myserver index -ux /home/zz
# backup myserver:/home/zz, naming the backup 'server'
bup on myserver save -n server /home/zz
# check the latest laptop backup
bup ls laptop/latest/home/zz

It’s hard to migrate from tivoli, rsnapshot, tarsnap and friends when you don’t know how. So here we go, without further ado, all you needed to know about bup but never daret to ask, ie

Some reasons to use bup:

  • global deduplication
    • rsnapshot: 4.97G = 2.18G with bup
    • rsnapshot: 12.6G = 4.6G with bup
  • save transmission time
  • backups are oneliners
  • anytime snapshots
  • uid,gid,permissions,acl,selinux
  • par2 anti-bitrot and corruption resistance
  • runs on dd-wrt

This is awesome, but there are two caveats. One is I am unaware of Enterprise&tm; shops using bup yet, the other is a common question: no, bup doesn’t encrypt data.

You can either encrypt or deduplicate. Choose. If you want the other, you probably want duplicity or tarsnap.

prads-0.3.2: ya skipped that one

Monday, November 5th, 2012

Ever since HACK.LU (where we spoke about VSF), Ebf0 and I have had quite some activity on PRADS, wonder why?

We really enjoyed the design of POM-NG, we find this little program quite inspiring and will keep in touch with GMsoft.

This might be the right time to announce PRADS-me! at, a service to actively fingerprint your own self. Real useful even just for an IP check, geolocation or to see what types of fingerprints you are matching at any given time.

Some of you might recall that PRADS was the subject of a Masters thesis in 2011: “Investigating Passive Operating System Detection” by Petter Bjerke Falch from UiO. Well, it’s happened again.

Jostein Haukeli at the University of Oslo Department of Informatics has written a paper on “False positive reduction through IDS network awareness”. We are excited about the prospect that our work is being used in data correlation, and we would like to see more event correlation stuff done in a scalable context.

Last year PRADS was a featured ip6-ready tool at the ISC.
Furthermore, in July this year PRADS was included in OSSIM, the Open Source SIEM

In other news, PRADS is about to be baked into the next release of the Security Onion network monitoring linux distro. Version 12.04 beta already comes with PRADS included (replacing old-timers sancp and pads) but it did require some bug-squashing from our end. You know what that means? 0.3.2-rc1 was tagged in the tree recently. That’s right: a new PRADS release is coming up Real Soon Now.

the paranoid console viewer

Tuesday, June 19th, 2012

Hi all,
I know it’s been a while since my last post.
There is lots to talk about, but let’s start afresh with something short and sweet.

Let me paint a picture for you:
There is something broken in the most holy and secure part of your network. You need to call support to have them look at it. The support rep wants console access, but you can’t give them axx to your holiest cream pie.
They offer to take over your desktop with a java rootkit app like TeamViewer, GoToMeeting or WebEx.
You decline. You need to stay in control, but show them what they need to see, that and only that.

Let me be clear on the problem statement:
Read-only shell access to the most secure host, which is not available over the wire, viewed by multiple parties at the same time.

Here’s how to do that with SSH, screen(1) and some foo,
with ssh->chroot->rbash->readonly multiuser screen->reverse ssh->openvpn:

You will need a linux server in an “unsafe” zone which is exposed to your support rep on the internet or thru VPN.

  1. Create the user to be contained on your unsafe box, with the restricted bash shell:
    unsafe# export user=rep; adduser $user; chage -s /usr/bin/rbash $user
  2. (Bonus:) chroot/contain the user within sshd_config
  3. Setup multiuser screen on the unsafe box. There are lots of guides for it, but the short and sweet of it is: unsafe# chmod +s `which screen`; chmod 755 /var/run/screen Indeed, this increases the attack surface, and therefore we call this box the unsafe one.
  4. ssh from secure zone to unsafe server:
    secure# ssh -R 2222:localhost:22 screen
  5. Run screen from YOUR account and do :addacl $user :chacl $user -w "#" :chacl $user -x "?" Replace $user with whatever from step 1. Then, still in your screen: :multiuser on
  6. Win! Now you can reverse ssh back to the secure zone and let $user on the unsafe box read the terminal without being able to access anything but what you show her.
  7. Bonus: Add `screen -r $youraccount` in $user/.profile and $user will drop straight into locked screen, and remember that multiuser screen is read-write-execute for all accounts that are addacl’d
    so you might want to chacl before enabling the $user account login.

    And there you have it, superparanoid reverese secure-unsecure remote shell viewer.