Linux Restricted Shells: rssh and scponly

Restricted shells like rssh and scponly give sysadmin the possibility to limit the operations that Linux user can do, for example you can create user that will be allowed to copy files via scp but won’t be permitted to login into system’s command line. This is quite important security feature that should be considered by every sysadmin to prevent unauthorized activity by users for example over SSH.

If you have some online storage that is used for uploading backup data over scp or rsync/ssh from remote hosts then it is highly recommended to use restricted shells for those incoming connections and make sure that even if the attacker has got username/password (or key) then he (or she!) won’t be able to break into your system.

scponly is extremely simple restricted shell, user account that has scponly binary as its shell won’t be able to do anything except transfer data from remote host via scp protocol or via rsync/scp. rssh provides little bit more features: you can limit users to use selected protocols like scp, sftp, rsync, cvs or rdist either in chroot environment or not.

Installation

I prefer using yum or aptitude to install such kind of software like rssh or scponly so the fastest way is to try one of below commands depending on your needs:

apt-get install rssh
apt-get install scponly
yum install rssh
yum install scponly

If there are problems to find desired restricted shell in your Linux distro’s repository then you should download sources and do some ./configure, make and make install. Here are the links: latest rssh .tar.gz, latest scponly .tgz.

Configuration

scponly doesn’t need any configuration and works out of the box so you just should set it as a shell for user account. Here are some examples.

Create new user account with scponly as shell:

useradd -s /usr/sbin/scponly user1

Modify user account to set rssh as a shell:

usermod -s /usr/sbin/rssh user2

Where /usr/sbin/scponly is binary executable of scponly.

rssh comes with text configuration file usually stored in /etc/rssh.conf. You can either setup per-user settings there or configure global restrictions for all accounts which are using rssh. Default rssh.conf file is well commented so there shouldn’t be any problems to configure rssh as you needs. At the same time, here are some examples.

If you wish to restrict all users to scp and rsync only then you should uncomment lines in rssh.conf like below:

allowscp
#allowsftp
#allowcvs
#allowrdist
allowrsync

Now coming to per-user examples. User peter is allowed to use scp protocol only, the following line in rssh.conf will do that:

user=sbk:022:00001:

User ann is allowed to scp, rsync only:

user=sbk:022:10001:

As you can see enabled protocols in per-user setup are specified as 11000 (scp, sftp), 11111 (scp, sftp, cvs, rdist, rsync) or 00000 (no protocols enabled). 022 in above examples specifies umask.

Testing

Let’s assume you’ve created user1 and enabled only scp and rsync using rssh. An attempt to access the server via SSH under user1 account will end with the following output:

artiomix$ ssh user1@1.2.3.4
user1@1.2.3.4's password: 
 
This account is restricted by rssh.
Allowed commands: scp rsync
 
If you believe this is in error, please contact your system administrator.
 
Connection to 1.2.3.4 closed.

At the same time scp transfers will work without problems:

artiomix$ scp -P 23451 /etc/test.file user1@1.2.3.4:/tmp
user1@1.2.3.4's password:
test.file                             100%  983     1.0KB/s   00:00

Further Reading

rssh support chroot environments for scp, rsync and other transfer protocols. It means that you can restrict users not only by command they can use but also by filesystems they reach. For example, user1 can be chrooted to /chroot_user1 so it can’t be used to copy something from /etc or /var/www directories of the server. Here is nice manual about chroot in rssh.

Failover and Load Balancing using HAProxy

HAProxy sample topology

HAProxy is open source proxy that can be used to enable high availability and load balancing for web applications. It was designed especially for high load projects so it is very fast and predictable, HAProxy is based on single-process model.

In this post I’ll describe sample setup of HAProxy: users’ requests are load balanced between two web servers Web1 and Web1, if one of them goes down then all the request are processed by alive server, once dead servers recovers load balancing enables again. See topology to the right.

Installation

HAProxy is included into repositories for major Linux distributions, so if you’re using Centos, Redhat or Fedora type the following command:

yum install haproxy

If you’re Ubuntu, Debian or Linux Mint user use this one instead:

apt-get install haproxy

Configuration

As soon as HAProxy is installed it’s time to edit its configuration file, usually it’s placed in /etc/haproxy/haproxy.cfg. Official documentation for HAProxy 1.4 (stable) is here.

Here is configuration file to implement setup shown at the diagram and described above:

global
        user daemon
        group daemon
        daemon
        log 127.0.0.1 daemon
 
listen http
        bind 1.2.3.4:80
        mode http
        option tcplog
 
        log global
        option dontlognull
 
        balance roundrobin
        clitimeout 60000
        srvtimeout 60000
        contimeout 5000
        retries 3
        server web1 web1.example.com:80 check
        server web2 web2.example.com:80 check
        cookie web1 insert nocache
        cookie web2 insert nocache

Let’s stop on most important parts of this configuration file. Section global specifies user and group which will be used to run haproxy process (daemon in our example). Line daemon tells HAProxy to run in background, log 127.0.0.1 daemon specifies syslog facility for sending logs from HAProxy.

Section listen http contains line bind 1.2.3.4:80 that specifies IP address and port that will be used to accept users’ requests (they will be load balanced between Web1 and Web2). Line mode http means that HAProxy will filter all requests different from HTTP and will do load balancing over HTTP protocol.

Line balance roundrobin specifies load balancing algorithm according to which each web server (Web1 and Web2) will be used in turns according to their weights. In our example weights for both servers are the same so load balancing is fair.

Lines server web1 … and server web2 … specify web servers for load balancing and failover, in our case they are load balanced according to round robin algorithm and have the same priority/weight.

The last two lines in configuration files are optional, they makes it possible to preserve cookies, it means for example that if you logged in to web application hosted at Web1 and then HAProxy forwarded your next request to Web2 you will still have logged in session opened as cookies with session id from Web1 will be sent to you from Web2 as well.

Top 5 Password Managers for Linux [Guest Post]

In this post you will find set of password managers for Linux which provides secure storage for your passwords for sensitive data. If you still keep the passwords in plain text then you must consider one of available password managers so this article is for you.

KeePassX

KeePassX has been a very popular and famous password manager for Linux for a very long time and still trusted by pretty big number of users. When user launches the KeePassX password manager first it requires to set up of a master password to add an extra layer of security to password storage. As an option you can use a file with encryption key instead of the password. This key file can be used along with the master password to provide stronger security. KeePassX application is rather simple so you can easily create one or more databases which will have a master password and will contain all the login credentials stored encrypted. This manager is considered to be one of the most secure managers. If you’re Ubuntu user just type in terminal the following command:

sudo apt-get install keepassx

GPassword Manager

Gpassword Manager (GPM) is also one of the most secure and highly rated password managers which have more friendly and easy to use interface that KeePassX. This utility has many features that make it to be a good choice for most of the high level computer users. This password manager allows to set and add favorites into system-tray that is one of the unique features of this application. GPM utility uses the crypto++ method for encryption which can be used in Windows and Linux hence it enables the same database to be used on different platforms without the need to convert anything.

My Passwords

My Passwords is a simple and easy to use utility that allows you to store all your login credentials in an encrypted manner within a file. The most exciting feature of this utility are its speed and no requirement of an installation. Encryption algorithm that is used there is AES. Storage in Derby Database format along with AES encryption gives the user the power to create secure and fast password repository. The interface for this utility is fairly simple.

Fiagaro’s Password Manager 2

Fiagaro’s Password Manager 2 is another powerful tool with strong encryption methods that makes it one of the most secure utility for managing passwords in Linux. Fiagaro’s Password Manager 2 uses the AES-256 encryption of the database files which hold all your login credentials (it uses master password that should be set up once you started the program first).

Gringotts

Gringotts is rather old project: its application for Linux/Unix provides the user the possibility to store his or her notes in secure storage encrypted by symmetrical ciphers. Gringotts has a set of eight different algorithms that can be used to encrypt the desired data. This utility also provides different methods for hashing as well as compression. The interface of Gringotts is not as simple as of other password Managers but still easy to use and most effective for old school bearded Unix users.

About the author: Kelly Marsh is a blogger by profession. She loves writing on technology and luxury. Beside this she is fond of technology. Recently an article on Maruti Ritz attracted her attention. These days she is busy in writing an article on johnnie walker blue.

Grub Fallback: Boot good kernel if new one crashes

It’s hard to believe but I didn’t know about Grub fallback feature. So every time when I needed to reboot remote server into a new kernel I had to test it on local server to make sure it won’t panic on remote unit. And if kernel panic still happened I had to ask somebody who has physical access to the server to reboot the hardware choose proper kernel in Grub. It’s all boring and not healthful – it’s much better to use Grub’s native fallback feature.

Grub is default boot loader in most Linux distributions today, at least major distros like Centos/Fedora/RedHat, Debian/Ubuntu/Mint, Arch use Grub. This makes it possible to use Grub fallback feature just out of the box. Here is example scenario.

There is remote server hosted in New Zealand and you (sitting in Denmark) have access to it over the network only (no console server). In this case you cannot afford that the new kernel makes server unreachable, e.g. if new kernel crash during boot it won’t load network interface drivers so your Linux box won’t appear online until somebody reboots it into workable kernel. Thankfully Grub can be configured to try loading new kernel once and if it fails Grub will load another kernel according to configuration. You can see my example grub.conf below:

default=saved
timeout=5
splashimage=(hd0,1)/boot/grub/splash.xpm.gz
hiddenmenu
fallback 0 1
title Fedora OpenVZ (2.6.32-042stab053.5)
        root (hd0,1)
        kernel /boot/vmlinuz-2.6.32-042stab053.5 ro root=UUID=6fbdddf9-307c-49eb-83f5-ca1a4a63f584 rd_MD_UUID=1b9dc11a:d5a084b5:83f6d993:3366bbe4 rd_NO_LUKS rd_NO_LVM rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYTABLE=sv-latin1 rhgb quiet crashkernel=auto
        initrd /boot/initramfs-2.6.32-042stab053.5.img
        savedefault fallback
title Fedora (2.6.35.12-88.fc14.i686)
        root (hd0,1)
        kernel /boot/vmlinuz-2.6.35.12-88.fc14.i686 ro root=UUID=6fbdddf9-307c-49eb-83f5-ca1a4a63f584 rd_MD_UUID=1b9dc11a:d5a084b5:83f6d993:3366bbe4 rd_NO_LUKS rd_NO_LVM rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYTABLE=sv-latin1 rhgb quiet
        initrd /boot/initramfs-2.6.35.12-88.fc14.i686.img
        savedefault fallback

According to this configuration Grub will try to load ‘Fedora OpenVZ’ kernel once and if it fails system will be loaded into good ‘Fedora’ kernel. If ‘Fedora OpenVZ’ loads well you’ll be able to reach the server over the network after reboot. Notice lines ‘default=saved’ and ‘savedefault fallback’ which are mandatory to make fallback feature working.

Alternative way

I’ve heard that official Grub fallback feature may work incorrectly on RHEL5 (and Centos 5) so there is elegant workaround (found here):

1. Add param ‘panic=5′ to your new kernel line so it looks like below:

title Fedora OpenVZ (2.6.32-042stab053.5)
        root (hd0,1)
        kernel /boot/vmlinuz-2.6.32-042stab053.5 ro root=UUID=6fbdddf9-307c-49eb-83f5-ca1a4a63f584 rd_MD_UUID=1b9dc11a:d5a084b5:83f6d993:3366bbe4 rd_NO_LUKS rd_NO_LVM rd_NO_DM LANG=en_US.UTF-8 SYSFONT=latarcyrheb-sun16 KEYTABLE=sv-latin1 rhgb quiet crashkernel=auto panic=5
        initrd /boot/initramfs-2.6.32-042stab053.5.img

This param will make crashed kernel to reboot itself in 5 seconds.

2. Point default Grub param to good kernel, e.g. ‘default=0′.

3. Type in the following commands (good kernel appears in grub.conf first and new kernel is second one):

# grub
 
grub> savedefault --default=1 --once
savedefault --default=1 --once
grub> quit

This will make Grub to boot into new kernel once and if it fails it will load good kernel. Now you can reboot the server and make sure it will 100% appear online in a few minutes. I usually prefer native Grub fallback feature but if you see it doesn’t work for you it makes sense to try above mentioned workaround.

Why Mosh is better than SSH?

Mosh screenshot

Mosh (stands for Mobile Shell) is replacement of SSH for remote connections to Unix/Linux systems. It brings a few noticeable advantages over well known SSH connections. In brief, it’s faster and more responsive, especially on long delay and/or unreliable links.

Key benefits of Mosh

  • Stays connected if your IP is changed. Roaming feature of Mosh allows you to move between Internet connections and keep Mosh session online. For example, if your wifi connection changes IP you don’t need to reconnect.
  • Keeps session after loosing connection. For example, if you lost Internet connection for some time, or your laptop went offline due to exhausted battery – you’ll be able to pick up previously opened Mosh session easily.
  • No root rights needed to use Mosh. Unlike SSH Mosh server is not a daemon that needs to listen on specific port to accept incoming connections from clients. Mosh server and client are executables that could be run by ordinary user.
  • The same credentials for remote login. Mosh uses SSH for authorization so in order to open connection you need the same credentials as before.
  • Responsive Ctrl+C combination. Unlike SSH Mosh doesn’t fill up network buffers so even if you accidentally requested to output 100 MB file you’ll be able to hit Ctrl+C and stop it immediately.
  • Better for slow or lagged links. Have you ever tried to use SSH on satellite link where average RTT is 600 ms or more? Wish Mosh you don’t need to wait until server replies to see your typing. It works in CLI and such programs as vi or emacs so on it makes it possible to do the job slow connections more comfortably.

Well, there are some disadvantages too:

  • No IPv6 support.
  • UTF-8 only.

Mosh is available for all major Linux distributions, FreeBSD and Mac OS X systems:

Ubuntu (12.04 LTS) or Debian (testing/unstable):

sudo apt-get install mosh

Gentoo:

emerge net-misc/mosh

Arch Linux:

packer -S mobile-shell-git

FreeBSD:

portmaster net/mosh

Mac OS X:

<a  class="colorbox" href="https://github.com/downloads/keithw/mosh/mosh-1.1.3-2.pkg">mosh-1.1.3-2.pkg</a>

Sources: mosh-1.1.3.tar.gz

Project’s website

P.S. It’s better that combination of SSH and GNU Screen.

Add physical NIC to XenServer

If you add new physical network interface to the hardware that runs XenServer it won’t appear in XenCenter by default.

In order to attach it to VMs or change its settings you’ll need to type in a few commands to XenServer’s CLI.

1. Connect XenServer via SSH using root rights:

ssh root@192.168.10.1 -v

2. Make sure that new NIC is attached to hardware and detected by Linux, in below command’s output you can see there are three Ethernet controllers (the last one was just attached to hardware):

[root@localhost ~]# lspci  | grep -i ethernet
10:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168B PCI Express Gigabit Ethernet controller (rev 01)
1e:00.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5723 Gigabit Ethernet PCIe (rev 10)
30:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 10)

As you can see this NIC isn’t shown in XenCenter and below command doesn’t show its UID among detected interfaces:

root@localhost ~]# xe pif-list
uuid ( RO)                  : 095abcc1-4d64-7925-200f-a91d558ec872
                device ( RO): eth1
    currently-attached ( RO): true
                  VLAN ( RO): -1
          network-uuid ( RO): 9da74476-ffcb-6824-25ad-62d46f34e252
 
uuid ( RO)                  : 555844b2-4061-47e0-52ef-01e42f182eef
                device ( RO): eth0
    currently-attached ( RO): true
                  VLAN ( RO): -1
          network-uuid ( RO): 90a0e347-9246-7ac9-c939-30983602c14e

As well as no new eth2 in ifconfig’s output

[root@localhost ~]# ifconfig     
eth0      Link encap:Ethernet  HWaddr 68:B5:99:E3:1C:56  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:1953 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2475 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:201110 (196.3 KiB)  TX bytes:1929408 (1.8 MiB)
          Interrupt:19 
 
eth1      Link encap:Ethernet  HWaddr 00:30:4F:33:43:6E  
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:110 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:14435 (14.0 KiB)  TX bytes:0 (0.0 b)
          Interrupt:17 Base address:0xe000
[root@localhost ~]# ifconfig eth2
ifconfig: interface eth2 does not exist

3. Solution is pretty easy – you just need to find out UUID of XenServer host to which you’d like to attach new NIC. You can do it by the following commands:

[root@localhost ~]# xe host-list 
uuid ( RO)                : c5ab0df3-440a-4164-b1a4-6febf1ff0052
          name-label ( RW): XenServer HP Proliant ML 110
    name-description ( RW): Default install of XenServer

and

[root@localhost ~]# xe pif-scan host-uuid=c5ab0df3-440a-4164-b1a4-6febf1ff0052

That’s it, from now you’ll see new NIC in XenCenter.

[root@localhost ~]# xe pif-list
uuid ( RO)                  : 095abcc1-4d64-7925-200f-a91d558ec872
                device ( RO): eth1
    currently-attached ( RO): true
                  VLAN ( RO): -1
          network-uuid ( RO): 9da74476-ffcb-6824-25ad-62d46f34e252
 
uuid ( RO)                  : 555844b2-4061-47e0-52ef-01e42f182eef
                device ( RO): eth0
    currently-attached ( RO): true
                  VLAN ( RO): -1
          network-uuid ( RO): 90a0e347-9246-7ac9-c939-30983602c14e
 
uuid ( RO)                  : 7f3b59d7-1508-835a-b268-4476bbac33d5
                device ( RO): eth2
    currently-attached ( RO): false
                  VLAN ( RO): -1
          network-uuid ( RO): 9584917b-e49a-f075-f1e0-8ba2c4a4bf02