Securing Debian Manual
Footnotes

1

At a given time it was superseded by the "Linux Security Knowledge Base". This documentation is also provided in Debian through the lskb package. Now it's back as the Lasg again.

2

A very good example of this kind of attacks using /tmp is detailed in The mysteriously persistently exploitable program (contest) and The mysteriously persistently exploitable program explained (notice that the incident is Debian-related). It is basicly an attack in which a local user stashes away a vulnerable setuid application by making a hard link to it, effectively avoiding any updates (or removal) of the binary itself made by the system administrator. Dpkg was recently fixed to prevent this (see 225692) but other setuid binaries (not controlled by the package manager) are at risk if partitions are not setup correctly.

3

Since Debian GNU/Linux 4.0, codename etch

4

The footprint in Debian 3.0 and earlier releases wasn't as tight, since some inetd services were enabled by default. Also standard installations of Debian 2.2 installed the NFS server as well as the telnet server.

5

This is desirable if you are setting up a development chroot, for example.

6

For example, in Debian woody it is around 400-500 Mbs, try this:

       $ size=0
       $ for i in `grep -A 1 -B 1 "^Section: base" /var/lib/dpkg/available |
       grep -A 2 "^Priority: required" |grep "^Installed-Size" |cut -d : -f 2
       `; do size=$(($size+$i)); done
       $ echo $size
       47762

7

Many intrusions are made just to get access to resources to do illegitimate activity (denial of service attacks, spam, rogue ftp servers, dns pollution...) rather than to obtain confidential data from the compromised system.

8

You can make (on another system) a dummy package with equivs.

9

In etch and later releases

10

Even though the libraries have been removed from the filesystem the inodes will not be cleared up until no program has an open file descriptor pointing to them.

11

Depending on your lsof version you might need to use $8 instead of $9

12

This happened, for example, in the upgrade from libc6 2.2.x to 2.3.x due to NSS authentication issues, see http://lists.debian.org/debian-glibc/2003/debian-glibc-200303/msg00276.html.

13

Unless you have installed a kernel metapackage like linux-image-2.6-686 which will always pull in the latest kernel minor revision for a kernel release and a given architecture.

14

A sample script called testnet is available in the Remotely rebooting Debian GNU/Linux machines article. A more elaborate network connectivity testing script is available in the Testing network connectivity article.

15

Setting up a serial console is beyond the scope of this document, for more information read the Serial HOWTO and the Remote Serial Console HOWTO.

16

The /etc/securetty is a configuration file that belongs to the login package.

17

Or ttyvX in GNU/FreeBSD, and ttyE0 in GNU/KNetBSD.

18

Or comX in GNU/Hurd, cuaaX in GNU/FreeBSD, and ttyXX in GNU/KNetBSD.

19

The default configuration in woody includes 12 local tty and vc consoles, as well as the console device but does not allow remote logins. In sarge the default configuration provides 64 consoles for tty and vc consoles. You can safely remove this if you are not using that many consoles.

20

Look for the getty calls.

21

Some of this includes the package manager dpkg since the installation (post,pre) and removal (post,pre) scripts are at /var/lib/dpkg/ and Smartlist

22

This dependency is not fixed, however, in the Debian 3.0 package. Please see Bug #112965.

23

libpam-chroot has not been yet thoroughly tested, it does work for login but it might not be easy to set up the environment for other programs

24

Setting HISTSIZE to a very large number can cause issues under some shells since the history is kept in memory for every user session. You might be safer if you set this to a high-enough value and backup user's history files (if you need all of the user's history for some reason)

25

Without the append-only flag users would be able to empty the contents of the history file running > .bash_history

26

Ttys are spawned for local logins and remote logins through ssh and telnet

27

As defined in /etc/adduser.conf (USERGROUPS=yes). You can change this behaviour if you set this value to no, although it is not recommended

28

Chpasswd cannot handle MD5 password generation so it needs to be given the password in encrypted form before using it, with the -e option.

29

On older Debian releases you might need to do this:

       $ apt-cache showpkg libwrap0 | egrep '^[[:space:]]' | sort -u | \
             sed 's/,libwrap0$//;s/^[[:space:]]\+//'

30

be sure to use uppercase here since spawn will not work

31

there's a very good article on it written by Lance Spitzner

32

Notice that this patch conflicts with patches already included in Debian's 2.4 kernel source package. You will need to use the stock vanilla kernel. You can do this with the following steps:

     # apt-get install kernel-source-2.4.22 kernel-patch-debian-2.4.22
     # tar xjf /usr/src/kernel-source-2.4.22.tar.bz2
     # cd kernel-source-2.4.22
     # /usr/src/kernel-patches/all/2.4.22/unpatch/debian

For more information see #194225, #199519, #206458, #203759, #204424, #210762, #211213, and the discussion at debian-devel

33

So common, in fact, that they have been the basis of 20% of the reported security vulnerabilities every year, as determined by statistics from ICAT's vulnerability database

34

In previous releases, checksecurity was integrated into cron and the file was /etc/cron.daily/standard

35

In Debian the kernel-source-version packages copy the sources to /usr/src/kernel-source-version.tar.bz2, just substitute version to whatever kernel version sources you have installed

36

To reproduce this (example provided by Felix von Leitner on the Bugtraq mailing list):

        host a (eth0 connected to eth0 of host b):
          ifconfig eth0 10.0.0.1
          ifconfig eth1 23.0.0.1
          tcpserver -RHl localhost 23.0.0.1 8000 echo fnord
     
        host b:
          ifconfig eth0 10.0.0.2
          route add 23.0.0.1 gw 10.0.0.1
          telnet 23.0.0.1 8000

It seems, however, not to work with services bound to 127.0.0.1, you might need to write the tests using raw sockets.

37

The fact that this behavior can be changed through routing was described by Matthew G. Marsh in the Bugtraq thread:

     eth0 = 1.1.1.1/24
     eth1 = 2.2.2.2/24
     
     ip rule add from 1.1.1.1/32 dev lo table 1 prio 15000
     ip rule add from 2.2.2.2/32 dev lo table 2 prio 16000
     
     ip route add default dev eth0 table 1
     ip route add default dev eth1 table 2

38

There are some patches available for this behavior as described in Bugtraq's thread at http://www.linuxvirtualserver.org/~julian/#hidden and http://www.fefe.de/linux-eth-forwarding.diff.

39

An attacker might have many problems pulling the access through after configuring the IP-address binding if he is not on the same broadcast domain (same network) as the attacked host. If the attack goes through a router it might be quite difficult for the answers to return somewhere.

40

Gdm will not append -nolisten tcp if it finds a -query or -indirect on the command line since the query wouldn't work.

41

To retrieve the list of mailer daemons available in Debian try:

     $ apt-cache search mail-transport-agent

The list will not include qmail, which is distributed only as source code in the qmail-src package.

42

A list of servers/daemons which support these protocols in Debian can be retrieved with:

     $ apt-cache search pop3-server
     $ apt-cache search imap-server

43

Note that depending on your bind version you might not have the -g option, most notably if you are using bind9 in sarge (9.2.4 version).

44

This setup has not been tested for new release of Bind yet.

45

Unless you use the instdir option when calling dpkg but then the chroot jail might be a little more complex.

46

It does try to run them under minimum priviledge which includes running daemons with their own users instead of having them run as root.

47

Available since the kernel version 2.4 (which was the default kernel in Debian 3.0). Previous kernel versions (2.2, available in even older Debian releases) used ipchains. The main difference between ipchains and iptables is that the latter is based on stateful packet inspection which provides for more secure (and easier to build) filtering configurations. Older (and now unsupported) Debian distributions using the 2.0 kernel series needed the appropriate kernel patch.

48

Unlike personal firewalls in other operating systems, Debian GNU/Linux does not (yet) provide firewall generation interfaces that can make rules limiting them per process or user. However, the iptables code can be configured to do this (see the owner module in the iptables(8) manpage).

49

Translations are available in up to ten different languages.

50

The full capability questionnaire is available at CVE

51

Some operating systems have already been plagued with automatic-updates problems such as the Mac OS X Software Update vulnerabity.

FIXME: probably the Internet Explorer vulnerability handling certificate chains has an impact on security updates on Microsoft Windows.

52

Older releases, such as Debian 3.1 sarge can use this feature by using backported versions of this package management tool

53

Until an automatic mechanism is developed.

54

Technically speaking, this is an ASCII-armored detached gpg signature.

55

Or has poisoned your DNS, or is spoofing the server, or has replaced the file in the mirror you are using, etc.

56

"ziyi" is the name of the tool used for signing on the Debian servers, the name is based on the name of a Chinese actress.

57

Not all apt repository keys are signed at all by another key. Maybe the person setting up the repository doesn't have another key, or maybe they don't feel comfortable signing such a role key with their main key. For information on setting up a key for a repository see Release check of non Debian sources, Section 7.5.4.

58

Either because you are using the stable, sarge, release or an older release or because you don't want to use the latest apt version, although we would really appreciate testing of it.

59

Some of them are provided when installing the harden-remoteaudit package.

60

If you use this last package and are running an official Debian, the database will not be updated with security updates. You should either use clamav-freshclam, clamav-getfiles to generate new clamav-data packages or update from the maintainers location:

       deb http://people.debian.org/~zugschlus/clamav-data/ /
       deb-src http://people.debian.org/~zugschlus/clamav-data/ /

61

Actually, there is an installer package for the F-prot antivirus, which is non-free but gratis for home users, called f-prot-installer. This installer, however, just downloads F-prot's software and installs it in the system.

62

For more examples of how to configure gnupg check /usr/share/doc/mutt/examples/gpg.rc.

63

Some relevant threads discussing these drawbacks include http://lists.debian.org/debian-mentors/2004/10/msg00338.html and http://lists.debian.org/debian-devel/2004/05/msg01156.html

64

This might eventually be introduced as a dh_adduser in debhelper. See #81967, #291177 and #118787.

65

You can even provide a SELinux policy for it

66

You may also want to use the --quiet (-q) option to reduce the output of apt-get, which will stop the generation of any output if no packages are installed.

67

Note that some packages might not use debconf and updates will stall due to packages asking for user input during configuration.

68

This is a common issue since many users want to maintain a stable system while updating some packages to unstable to gain the latest functionality. This need arises due to some projects evolving faster than the time between Debian's stable releases.

69

An easy way to do this is using a Live CD, such as Knoppix Std which includes both the file integrity tools and the integrity database for your system.

70

There are over 28 capabilities including: CAP_BSET, CAP_CHOWN, CAP_FOWNER, CAP_FSETID, CAP_FS_MASK, CAP_FULL_SET, CAP_INIT_EFF_SET, CAP_INIT_INH_SET, CAP_IPC_LOCK, CAP_IPC_OWNER, CAP_KILL, CAP_LEASE, CAP_LINUX_IMMUTABLE, CAP_MKNOD, CAP_NET_ADMIN, CAP_NET_BIND_SERVICE, CAP_NET_RAW, CAP_SETGID, CAP_SETPCAP, CAP_SETUID, CAP_SYS_ADMIN, CAP_SYS_BOOT, CAP_SYS_CHROOT, CAP_SYS_MODULE, CAP_SYS_NICE, CAP_SYS_PACCT, CAP_SYS_PTRACE, CAP_SYS_RAWIO, CAP_SYS_RESOURCE, CAP_SYS_TIME, and CAP_SYS_TTY_CONFIG. All of them can be de-activated to harden your kernel.

71

You don't need to install lcap to do this, but it's easier than setting /proc/sys/kernel/cap-bound by hand.

72

You will typically use a bridge firewall so that the firewall itself is not detectable, see Setting up a bridge firewall, Appendix D.

73

If you are adventurous, you can login to the system and save information on all running processes (you'll get a lot from /proc/nnn/). It is possible to get the whole executable code from memory, even if the attacker has deleted the executable files from disk. Then pull the power cord.

74

In fact, this is the tool used to build the CD-ROMs for the Gibraltar project (a firewall on a live CD-ROM based on the Debian distribution).

75

This is a list of some CERTs, for a full list look at the FIRST Member Team information (FIRST is the Forum of Incident Response and Security Teams): AusCERT (Australia), UNAM-CERT (Mexico) CERT-Funet (Finland), DFN-CERT (Germany), RUS-CERT (Germany), CERT-IT (Italy), JPCERT/CC (Japan), UNINETT CERT (Norway), HR-CERT (Croatia) CERT Polskay (Poland), RU-CERT (Russia), SI-CERT (Slovenia) IRIS-CERT (Spain), SWITCH-CERT (Switzerland), TWCERT/CC (Taiwan), and CERT/CC (US).

76

Be very careful if using chroots, since if the binary uses a kernel-level exploit to increase its privileges it might still be able to infect your system

77

For example, based on some data, it might seem that Windows NT is more secure than Linux, which is a questionable assertion. After all, Linux distributions usually provide many more applications compared to Microsoft's Windows NT. This counting vulnerabilities issues are better described in Why Open Source Software / Free Software (OSS/FS)? Look at the Numbers! by David A. Wheeler

78

Without diminishing the fact that some distributions, such as Red Hat or Mandrake, are also taking into account security in their standard installations by having the user select security profiles, or using wizards to help with configuration of personal firewalls.

79

Note that this is 'security by obscurity', and will probably not be worth the effort in the long term.

80

Be careful, as this will traverse your whole system. If you have a lot of disk and partitions you might want to reduce it in scope.

81

There has been a declassification decision, voted in GR-2005-002, that might make some posts available in the future, however.

82

Typically the needed packages will be installed through the dependencies

83

It can also be downloaded from http://www.cert.org/kb/acid/, http://acidlab.sourceforge.net or http://www.andrew.cmu.edu/~rdanyliw/snort/.

84


Since version 9.2.1-5. That is, since Debian release sarge.

85

Such as knockd. Alternatively, you can open a different console and have the system ask for confirmation that there is somebody on the other side, and reset the firewall chain if no confirmation is given. The following test script could be of use:

     #!/bin/bash
     
     while true; do
         read -n 1 -p "Are you there? " -t 30 ayt
         if [ -z "$ayt" ] ; then
             break
         fi
     done
     
     # Reset the firewall chain, user is not available
     echo
     echo "Resetting firewall chain!"
     iptables -F
     iptables -P INPUT ACCEPT
     iptables -P FORWARD ACCEPT
     iptables -P OUTPUT ACCEPT
     exit 1

Of course, you should disable any backdoors before getting the system into production.

86

You can use the debug option to have it send the progress of the module to the authpriv.notice facility

87

You can create a very limited bash environment with the following python definition for makejail, just create the directory /var/chroots/users/foo and a file with the following contents and call it bash.py:

     chroot="/var/chroots/users/foo"
     cleanJailFirst=1
     testCommandsInsideJail=["bash ls"]

And then run makejail bash.py to create the user environment at /var/chroots/users/foo. To test the environment run:

     # chroot /var/chroots/users/foo/ ls
     bin  dev  etc  lib  proc  sbin  usr

88

In some occasions you might need the /dev/ptmx and /dev/pty* devices and the /dev/pts/ subdirectory. Running MAKEDEV in the /dev directory of the chrooted environment should be sufficient to create them if they do not exist. If you are using kernels (version 2.6) which dynamically create device files you will need to create the /dev/pts/ files yourself and grant them the proper privileges.

89

If you are using a kernel that implements Mandatory Access Control (RSBAC/SElinux) you can avoid changing this configuration just by granting the sshd user privileges to make the chroot() system call.

90

Notice that there are no SETUID files. This makes it more difficult for remote users to escape the chroot environment. However, it also prevents users from changing their passwords, since the passwd program cannot modify the files /etc/passwd or /etc/shadow.


Securing Debian Manual

Version: 3.13, Sun, 09 Oct 2011 13:15:05 +0000

Javier Fernández-Sanguino Peña jfs@debian.org
Authors, Section 1.1