Small Changes to Increase Security on Ubuntu Servers

Here is somethings I’ve done to help increase security on my Ubuntu boxes.  The goal when securing a linux system you need to prevent, detect, and react.  These small changes will help in that goal.

Be careful with these changes, as you can lock yourself out of the server.
Also, Ubuntu can use admin(<10.x) or adm(>12.x) is the admin group!!

  1. Increase SSH security by reducing grace time, not allowing root to login (Ubuntu has no root user, but incase you are compromised and a root account is added), and only allow groups you want to login the box.  I run a shell for friends, so in order to allow them to login, I create a “ssh” group and put them into that group.
    Open /etc/ssh/sshd_config
    LoginGraceTime 20
    PermitRootLogin no
    AllowGroups adm ssh
  2. “su” program available to non-admin users
    sudo chown root:adm /bin/su
    sudo chmod 4750 /bin/su
  3. Install more apparmor profiles, read up on apparmor and make sure to think about it when troubleshooting issues.  Sometimes when you don’t use default file paths, apparmor will not allow an application to read/write to locations not whitelisted.
    sudo apt-get install apparmor-profiles
  4. Install denyhosts, this will block bots trying to brutforce you.
    sudo apt-get install denyhosts
  5. Here is an example of my changes to denyhosts
    Edit /etc/denyhosts.conf (diff -U3 denyhosts.conf.orig denyhosts.conf)
    --- denyhosts.conf.orig 2009-07-21 09:54:25.000000000 -0500
    +++ denyhosts.conf      2009-07-21 10:00:59.000000000 -0500
    @@ -57,13 +57,15 @@
    #            'y' = years
    #
    # never purge:
    -PURGE_DENY =
    +#PURGE_DENY =
    #
    # purge entries older than 1 week
    #PURGE_DENY = 1w
    #
    # purge entries older than 5 days
    #PURGE_DENY = 5d
    +# purge entries older than 4 weeks
    +PURGE_DENY = 4w
    #######################################################################
    #######################################################################
    @@ -90,9 +92,9 @@
    # eg.   sshd: 127.0.0.1  # will block sshd logins from 127.0.0.1
    #
    # To block all services for the offending host:
    -#BLOCK_SERVICE = ALL
    +BLOCK_SERVICE = ALL
    # To block only sshd:
    -BLOCK_SERVICE  = sshd
    +#BLOCK_SERVICE  = sshd
    # To only record the offending host and nothing else (if using
    # an auxilary file to list the hosts).  Refer to:
    # http://denyhosts.sourceforge.net/faq.html#aux
    @@ -218,7 +220,7 @@
    # Multiple email addresses can be delimited by a comma, eg:
    # ADMIN_EMAIL = foo@bar.com, bar@foo.com, etc@foobar.com
    #
    -ADMIN_EMAIL = root@localhost
    +#ADMIN_EMAIL = root@localhost
    #
    #######################################################################@@ -285,7 +287,7 @@
    #
    #SYSLOG_REPORT=NO
    #
    -#SYSLOG_REPORT=YES
    +SYSLOG_REPORT=YES
    #
    ######################################################################
  6. In order to whitelist a host from getting into denyhosts, list the ips in this file: /var/lib/denyhosts/allowed-hosts
  7. Make sure changes have been applied:
    sudo /etc/init.d/denyhosts restart
  8. Install performance monitor SAR
    sudo apt-get install sysstat
    Edit /etc/default/sysstat
    Set: ENABLE="true"
    sudo /etc/init.d/sysstat start
  9. Install logwatch and monitor the emails it sends you (root).  This will give you a good overview of your system if you don’t have a syslog server.
    sudo apt-get install logwatch
  10. Install Root Kit Hunter, this a cron job that will check your system for root kits.  It keeps track of your binaries and in case their MD5 changes.
    sudo apt-get install rkhunter
  11. Edit this file /etc/rkhunter.conf and add these changes to the very bottom, these may not work for you but they have been some false positives I needed to whitelist.
    MAIL-ON-WARNING=root@localhost
    ENABLE_TESTS="all"
    DISABLE_TESTS="suspscan hidden_procs deleted_files packet_cap_apps apps"ALLOWHIDDENDIR=/etc/.java
    ALLOWHIDDENDIR=/dev/.static
    ALLOWHIDDENDIR=/dev/.udev
    ALLOWHIDDENDIR=/dev/.initramfs
    ALLOWHIDDENFILE=/dev/.blkid.tab
    ALLOWHIDDENFILE=/dev/.blkid.tab.old
    SCRIPTWHITELIST=/usr/local/bin/lwp-request
  12. After installing rkhunter, you will get emails indicating if there is anything odd happening on your box.  Most of the time it’s from updates, so if you run apt-get upgrade or apt-get dist-upgrade, you need to run this command to update rkhunter:
    sudo rkhunter --propupd
  13. Shared Memory, edit /etc/fstab and add:
    tmpfs           /dev/shm        tmpfs   defaults,noexec,nosuid  0  0
  14. sudo mount -o remount /dev/shm

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