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Securing Debian Manual
Chapter 7 - Debian Security Infrastructure

7.1 The Debian Security Team

Debian has a Security Team, that handles security in the stable distribution. Handling security means they keep track of vulnerabilities that arise in software (watching forums such as Bugtraq, or vuln-dev) and determine if the stable distribution is affected by it.

Also, the Debian Security Team is the contact point for problems that are coordinated by upstream developers or organizations such as CERT which might affect multiple vendors. That is, when problems are not Debian-specific. The contact point of the Security Team is team@security.debian.org which only the members of the security team read.

Sensitive information should be sent to the first address and, in some cases, should be encrypted with the Debian Security Contact key (as found in the Debian keyring).

Once a probable problem is received by the Security Team it will investigate if the stable distribution is affected and if it is, a fix is made for the source code base. This fix will sometimes include backporting the patch made upstream (which usually is some versions ahead of the one distributed by Debian). After testing of the fix is done, new packages are prepared and published in the http://security.debian.org site so they can be retrieved through apt (see Execute a security update, Section 4.2). At the same time a Debian Security Advisory (DSA) is published on the web site and sent to public mailing lists including debian-security-announce and Bugtraq.

Some other frequently asked questions on the Debian Security Team can be found at Questions regarding the Debian security team, Section 12.3.

7.2 Debian Security Advisories

Debian Security Advisories (DSAs) are made whenever a security vulnerability is discovered that affects a Debian package. These advisories, signed by one of the Security Team members, include information of the versions affected as well as the location of the updates. This information is:

DSAs are published both on Debian's frontpage and in the Debian security pages. Usually this does not happen until the website is rebuilt (every four hours) so they might not be present immediately. The preferred channel is the debian-security-announce mailing list.

Interested users can, however (and this is done in some Debian-related portals) use the RDF channel to download automatically the DSAs to their desktop. Some applications, such as Evolution (an email client and personal information assistant) and Multiticker (a GNOME applet), can be used to retrieve the advisories automatically. The RDF channel is available at http://www.debian.org/security/dsa.rdf.

DSAs published on the website might be updated after being sent to the public-mailing lists. A common update is adding cross references to security vulnerability databases. Also, translations[49] of DSAs are not sent to the security mailing lists but are directly included in the website.

7.2.1 Vulnerability cross references

Debian provides a fully crossreferenced table including all the references available for all the advisories published since 1998. This table is provided to complement the reference map available at CVE.

You will notice that this table provides references to security databases such as Bugtraq, CERT/CC Advisories and US-CERT Vulnerability Notes Database as well as CVE names (see below). These references are provided for convenience use, but only CVE references are periodically reviewed and included.

Advantages of adding cross references to these vulnerability databases are:

7.2.2 CVE compatibility

Debian Security Advisories were declared CVE-Compatible[50] in February 24, 2004.

Debian developers understand the need to provide accurate and up to date information of the security status of the Debian distribution, allowing users to manage the risk associated with new security vulnerabilities. CVE enables us to provide standardized references that allow users to develop a CVE-enabled security management process.

The Common Vulnerabilities and Exposures (CVE) project is maintained by the MITRE Corporation and provides a list of standardized names for vulnerabilities and security exposures.

Debian believes that providing users with additional information related to security issues that affect the Debian distribution is extremely important. The inclusion of CVE names in advisories help users associate generic vulnerabilities with specific Debian updates, which reduces the time spent handling vulnerabilities that affect our users. Also, it eases the management of security in an environment where CVE-enabled security tools -such as network or host intrusion detection systems, or vulnerability assessment tools- are already deployed regardless of whether or not they are based on the Debian distribution.

Debian provides CVE names for all DSAs released since September 1998. All of the advisories can be retrieved on the Debian web site, and announcements related to new vulnerabilities include CVE names if available at the time of their release. Advisories associated with a given CVE name can be searched directly through the Debian Security Tracker (see below).

In some cases you might not find a given CVE name in published advisories, for example because:

7.3 Security Tracker

The central database of what the Debian security teams know about vulnerabilities is the Debian Security Tracker. It cross references packages, vulnerable and fixed versions for different suites, CVE names, Debian bug numbers, DSA's and miscellaneous notes. It can be searched, e.g. by CVE name to see which Debian packages are affected or fixed, or by package to show unresolved security issues. The only information missing from the tracker is confidential information that the security team received under embargo.

The package debsecan uses the information in the tracker to report to the administrator of a system which of the installed packages are vulnerable, and for which updates are available to fix security issues.

7.4 Debian Security Build Infrastructure

Since Debian is currently supported in a large number of architectures, administrators sometimes wonder if a given architecture might take more time to receive security updates than another. As a matter of fact, except for rare circumstances, updates are available to all architectures at the same time.

Packages in the security archive are autobuilt, just like the regular archive. However, security updates are a little more different than normal uploads sent by package maintainers since, in some cases, before being published they need to wait until they can be tested further, an advisory written, or need to wait for a week or more to avoid publicizing the flaw until all vendors have had a reasonable chance to fix it.

Thus, the security upload archive works with the following procedure:

This procedure, previously done by hand, was tested and put through during the freezing stage of Debian 3.0 woody (July 2002). Thanks to this infrastructure the Security Team was able to have updated packages ready for the apache and OpenSSH issues for all the supported (almost twenty) architectures in less than a day.

7.4.1 Developer's guide to security updates

Debian developers that need to coordinate with the security team on fixing in issue in their packages, can refer to the Developer's Reference section Handling security-related bugs.

7.5 Package signing in Debian

This section could also be titled "how to upgrade/update safely your Debian GNU/Linux system" and it deserves its own section basically because it is an important part of the Security Infrastructure. Package signing is an important issue since it avoids tampering of packages distributed in mirrors and of downloads with man-in-the-middle attacks. Automatic software update is an important feature but it's also important to remove security threats that could help the distribution of trojans and the compromise of systems during updates[51].

Debian does not provide signed packages but provides a mechanism available since Debian 4.0 (codename etch) to check for downloaded package's integrity[52]. For more information, see Secure apt, Section 7.5.2.

This issue is better described in the Strong Distribution HOWTO by V. Alex Brennen.

7.5.1 The current scheme for package signature checks

The current scheme for package signature checking using apt is:

By following the chain of MD5 sums apt is capable of verifying that a package originates from a a specific release. This is less flexible than signing each package one by one, but can be combined with that scheme too (see below).

This scheme is fully implemented in apt 0.6 and is available since the Debian 4.0 release. For more information see Secure apt, Section 7.5.2. Packages that provide a front-end to apt need to be modified to adapt to this new feature; this is the case of aptitude which was modified to adapt to this scheme. Front-ends currently known to work properly with this feature include aptitude and synaptic.

Package signing has been discussed in Debian for quite some time, for more information you can read: http://www.debian.org/News/weekly/2001/8/ and http://www.debian.org/News/weekly/2000/11/.

7.5.2 Secure apt

The apt 0.6 release, available since Debian 4.0 etch and later releases, includes apt-secure (also known as secure apt) which is a tool that will allow a system administrator to test the integrity of the packages downloaded through the above scheme. This release includes the tool apt-key for adding new keys to apt's keyring, which by default includes only the current Debian archive signing key.

These changes are based on the patch for apt (available in Bug #203741) which provides this implementation.

Secure apt works by checking the distribution through the Release file, as discussed in Per distribution release check, Section 7.5.3. Typically, this process will be transparent to the administrator although you will need to intervene every year[53] to add the new archive key when it is rotated, for more information on the steps an administrator needs to take a look at Safely adding a key, Section

This feature is still under development, if you believe you find bugs in it, please, make first sure you are using the latest version (as this package might change quite a bit before it is finally released) and, if running the latest version, submit a bug against the apt package.

You can find more information at the wiki pages and the official documentation: Migration to APT 0.6 and APT Signature Checking.

7.5.3 Per distribution release check

This section describes how the distribution release check mechanism works, it was written by Joey Hess and is also available at the Debian Wiki. Basic concepts

Here are a few basic concepts that you'll need to understand for the rest of this section.

A checksum is a method of taking a file and boiling it down to a reasonably short number that uniquely identifies the content of the file. This is a lot harder to do well than it might seem, and the most commonly used type of checksum, the MD5 sum, is in the process of being broken.

Public key cryptography is based on pairs of keys, a public key and a private key. The public key is given out to the world; the private key must be kept a secret. Anyone possessing the public key can encrypt a message so that it can only be read by someone possessing the private key. It's also possible to use a private key to sign a file, not encrypt it. If a private key is used to sign a file, then anyone who has the public key can check that the file was signed by that key. No one who doesn't have the private key can forge such a signature.

These keys are quite long numbers (1024 to 2048 digits or longer), and to make them easier to work with they have a key id, which is a shorter, 8 or 16 digit number that can be used to refer to them.

gpg is the tool used in secure apt to sign files and check their signatures.

apt-key is a program that is used to manage a keyring of gpg keys for secure apt. The keyring is kept in the file /etc/apt/trusted.gpg (not to be confused with the related but not very interesting /etc/apt/trustdb.gpg). apt-key can be used to show the keys in the keyring, and to add or remove a key. Release checksums

A Debian archive contains a Release file, which is updated each time any of the packages in the archive change. Among other things, the Release file contains some MD5 sums of other files in the archive. An excerpt of an example Release file:

      6b05b392f792ba5a436d590c129de21f            3453 Packages
      1356479a23edda7a69f24eb8d6f4a14b            1131 Packages.gz
      2a5167881adc9ad1a8864f281b1eb959            1715 Sources
      88de3533bf6e054d1799f8e49b6aed8b             658 Sources.gz

The Release files also include SHA-1 checksums, which will be useful once MD5 sums become fully broken, however apt doesn't use them yet.

Now if we look inside a Packages file, we'll find more MD5 sums, one for each package listed in it. For example:

         Package: uqm
         Priority: optional
         Filename: unstable/uqm_0.4.0-1_i386.deb
         Size: 580558
         MD5sum: 864ec6157c1eea88acfef44d0f34d219

These two checksums can be used to verify that you have downloaded a correct copy of the Packages file, with a md5sum that matches the one in the Release file. And when it downloads an individual package, it can also check its md5sum against the content of the Packages file. If apt fails at either of these steps, it will abort.

None of this is new in secure apt, but it does provide the foundation. Notice that so far there is one file that apt doesn't have a way to check: The Release file. Secure apt is all about making apt verify the Release file before it does anything else with it, and plugging this hole, so that there is a chain of verification from the package that you are going to install all the way back to the provider of the package. Verification of the Release file

To verify the Release file, a gpg signature is added for the Release file. This is put in a file named Release.gpg that is shipped alongside the Release file. It looks something like this [54] , although only gpg actually looks at its contents normally:

     -----BEGIN PGP SIGNATURE-----
     Version: GnuPG v1.4.1 (GNU/Linux)
     -----END PGP SIGNATURE----- Check of Release.gpg by apt

Secure apt always downloads Release.gpg files when it's downloading Release files, and if it cannot download the Release.gpg, or if the signature is bad, it will complain, and will make note that the Packages files that the Release file points to, and all the packages listed therein, are from an untrusted source. Here's how it looks during an apt-get update:

     W: GPG error: http://ftp.us.debian.org testing Release: The following signatures
      couldn't be verified because the public key is not available: NO_PUBKEY 010908312D230C5F

Note that the second half of the long number is the key id of the key that apt doesn't know about, in this case that's 2D230C5F.

If you ignore that warning and try to install a package later, apt will warn again:

     WARNING: The following packages cannot be authenticated!
       libglib-perl libgtk2-perl
     Install these packages without verification [y/N]?

If you say Y here you have no way to know if the file you're getting is the package you're supposed to install, or if it's something else entirely that somebody that can intercept the communication against the server[55] has arranged for you, containing a nasty suprise.

Note that you can disable these checks by running apt with --allow-unauthenticated.

It's also worth noting that newer versions of the Debian installer use the same signed Release file mechanism during their debootstrap of the Debian base system, before apt is available, and that the installer even uses this system to verify pieces of itself that it downloads from the net. Also, Debian does not currently sign the Release files on its CDs; apt can be configured to always trust packages from CDs so this is not a large problem. How to tell apt what to trust

So the security of the whole system depends on there being a Release.gpg file, which signs a Release file, and of apt checking that signature using gpg. To check the signature, it has to know the public key of the person who signed the file. These keys are kept in apt's own keyring (/etc/apt/trusted.gpg), and managing the keys is where secure apt comes in.

By default, Debian systems come preconfigured with the Debian archive key in the keyring.

     # apt-key list
     pub   1024D/4F368D5D 2005-01-31 [expires: 2006-01-31]
     uid                  Debian Archive Automatic Signing Key (2005) <ftpmaster@debian.org>

Here 4F368D5D is the key id, and notice that this key was only valid for a one year period. Debian rotates these keys as a last line of defense against some sort of security breach breaking a key.

That will make apt trust the official Debian archive, but if you add some other apt repository to /etc/apt/sources.list, you'll also have to give apt its key if you want apt to trust it. Once you have the key and have verified it, it's a simple matter of running apt-key add file to add it. Getting the key and verifying it are the trickier parts. Finding the key for a repository

The debian-archive-keyring package is used to distribute keys to apt. Upgrades to this package can add (or remove) gpg keys for the main Debian archive.

For other archives, there is not yet a standard location where you can find the key for a given apt repository. There's a rough standard of putting the key up on the web page for the repository or as a file in the repository itself, but no real standard, so you might have to hunt for it.

The Debian archive signing key is available at http://ftp-master.debian.org/ziyi_key_2006.asc (replace 2006 with current year).[56]

gpg itself has a standard way to distribute keys, using a keyserver that gpg can download a key from and add it to its keyring. For example:

     $ gpg --keyserver pgpkeys.mit.edu --recv-key 2D230C5F
     gpg: requesting key 2D230C5F from hkp server pgpkeys.mit.edu
     gpg: key 2D230C5F: public key "Debian Archive Automatic Signing Key (2006) <ftpm
     aster@debian.org>" imported
     gpg: Total number processed: 1
     gpg:               imported: 1

You can then export that key from your own keyring and feed it to apt-key:

     $ gpg -a --export 2D230C5F | sudo apt-key add -
     gpg: no ultimately trusted keys found

The "gpg: no ultimately trusted keys found" warning means that gpg was not configured to ultimately trust a specific key. Trust settings are part of OpenPGPs Web-of-Trust which does not apply here. So there is no problem with this warning. In typical setups the user's own key is ultimately trusted. Safely adding a key

By adding a key to apt's keyring, you're telling apt to trust everything signed by the key, and this lets you know for sure that apt won't install anything not signed by the person who possesses the private key. But if you're sufficiently paranoid, you can see that this just pushes things up a level, now instead of having to worry if a package, or a Release file is valid, you can worry about whether you've actually gotten the right key. Is the http://ftp-master.debian.org/ziyi_key_2006.asc file mentioned above really Debian's archive signing key, or has it been modified (or this document lies).

It's good to be paranoid in security, but verifying things from here is harder. gpg has the concept of a chain of trust, which can start at someone you're sure of, who signs someone's key, who signs some other key, etc., until you get to the archive key. If you're sufficiently paranoid you'll want to check that your archive key is signed by a key that you can trust, with a trust chain that goes back to someone you know personally. If you want to do this, visit a Debian conference or perhaps a local LUG for a key signing [57].

If you can't afford this level of paranoia, do whatever feels appropriate to you when adding a new apt source and a new key. Maybe you'll want to mail the person providing the key and verify it, or maybe you're willing to take your chances with downloading it and assuming you got the real thing. The important thing is that by reducing the problem to what archive keys to trust, secure apt lets you be as careful and secure as it suits you to be. Verifying key integrity

You can verify the fingerprint as well as the signatures on the key. Retrieving the fingerprint can be done for multiple sources, you can check The Debian System Book, talk to Debian Developers on IRC, read the mailing list where the key change will be announced or any other additional means to verify the fingerprint. For example you can do this:

     $ GET http://ftp-master.debian.org/ziyi_key_2006.asc | gpg --import
     gpg: key 2D230C5F: public key "Debian Archive Automatic Signing Key (2006)
       <ftpmaster&debian.org>" imported
     gpg: Total number processed: 1
     gpg:               imported: 1
     $ gpg --check-sigs --fingerprint 2D230C5F
     pub   1024D/2D230C5F 2006-01-03 [expires: 2007-02-07]
           Key fingerprint = 0847 50FC 01A6 D388 A643  D869 0109 0831 2D23 0C5F
     uid   Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.org>
     sig!3        2D230C5F 2006-01-03  Debian Archive Automatic Signing Key
                                       (2006) <ftpmaster@debian.org>
     sig!         2A4E3EAA 2006-01-03  Anthony Towns <aj@azure.humbug.org.au>
     sig!         4F368D5D 2006-01-03  Debian Archive Automatic Signing Key
                                       (2005) <ftpmaster@debian.org>
     sig!         29982E5A 2006-01-04  Steve Langasek <vorlon@dodds.net>
     sig!         FD6645AB 2006-01-04  Ryan Murray <rmurray@cyberhqz.com>
     sig!         AB2A91F5 2006-01-04  James Troup <james@nocrew.org>

and then check the trust path from your key (or a key you trust) to at least one of the keys used to sign the archive key. If you are sufficiently paranoid you will tell apt to trust the key only if you find an acceptable path:

     $ gpg --export -a 2D230C5F | sudo apt-key add -

Note that the key is signed with the previous archive key, so theoretically you can just build on your previous trust. Debian archive key yearly rotation

As mentioned above, the Debian archive signing key is changed each year, in January. Since secure apt is young, we don't have a great deal of experience with changing the key and there are still rough spots.

In January 2006, a new key for 2006 was made and the Release file began to be signed by it, but to try to avoid breaking systems that had the old 2005 key, the Release file was signed by that as well. The intent was that apt would accept one signature or the other depending on the key it had, but apt turned out to be buggy and refused to trust the file unless it had both keys and was able to check both signatures. This was fixed in apt version There was also confusion about how the key was distributed to users who already had systems using secure apt; initially it was uploaded to the web site with no announcement and no real way to verify it and users were forced to download it by hand.

In January 2006, a new key for 2006 was made and the Release file began to be signed by it, but to try to avoid breaking systems that had the old 2005 key, the Release file was signed by that as well. In order to prevent confusion on the best distribution mechanism for users who already have systems using secure apt, the debian-archive-keyring package was introduced, which manages apt keyring updates. Known release checking problems

One not so obvious problem is that if your clock is very far off, secure apt will not work. If it's set to a date in the past, such as 1999, apt will fail with an unhelpful message such as this:

     W: GPG error: http://archive.progeny.com sid Release: Unknown error executing gpg

Although apt-key list will make the problem plain:

     gpg: key 2D230C5F was created 192324901 seconds in the future (time warp or clock problem)
     gpg: key 2D230C5F was created 192324901 seconds in the future (time warp or clock problem)
     pub   1024D/2D230C5F 2006-01-03
     uid                  Debian Archive Automatic Signing Key (2006) <ftpmaster@debian.org>

If it's set to a date too far in the future, apt will treat the keys as expired.

Another problem you may encouter if using testing or unstable is that if you have not run apt-get update lately and apt-get install a package, apt might complain that it cannot be authenticated (why does it do this?). apt-get update will fix this. Manual per distribution release check

In case you want to add now the additional security checks and don't want or cannot run the latest apt version[58] you can use the script below, provided by Anthony Towns. This script can automatically do some new security checks to allow the user to be sure that the software s/he's downloading matches the software Debian's distributing. This stops Debian developers from hacking into someone's system without the accountability provided by uploading to the main archive, or mirrors mirroring something almost, but not quite like Debian, or mirrors providing out of date copies of unstable with known security problems.

This sample code, renamed as apt-check-sigs, should be used in the following way:

     # apt-get update
     # apt-check-sigs
     # apt-get dist-upgrade

First you need to:

This is the example code for apt-check-sigs, the latest version can be retrieved from http://people.debian.org/~ajt/apt-check-sigs. This code is currently in beta, for more information read http://lists.debian.org/debian-devel/2002/debian-devel-200207/msg00421.html.

     # Copyright (c) 2001 Anthony Towns <ajt@debian.org>
     # This program is free software; you can redistribute it and/or modify
     # it under the terms of the GNU General Public License as published by
     # the Free Software Foundation; either version 2 of the License, or
     # (at your option) any later version.
     # This program is distributed in the hope that it will be useful,
     # but WITHOUT ANY WARRANTY; without even the implied warranty of
     # GNU General Public License for more details.
     rm -rf /tmp/apt-release-check
     mkdir /tmp/apt-release-check || exit 1
     cd /tmp/apt-release-check
     arch=`dpkg --print-installation-architecture`
     am_root () {
             [ `id -u` -eq 0 ]
     get_md5sumsize () {
             cat "$1" | awk '/^MD5Sum:/,/^SHA1:/' | 
               MYARG="$2" perl -ne '@f = split /\s+/; if ($f[3] eq $ENV{"MYARG"}) {
     print "$f[1] $f[2]\n"; exit(0); }'
     checkit () {
             local FILE="$1"
             local LOOKUP="$2"
             Y="`get_md5sumsize Release "$LOOKUP"`"
             Y="`echo "$Y" | sed 's/^ *//;s/  */ /g'`"
             if [ ! -e "/var/lib/apt/lists/$FILE" ]; then
                     if [ "$Y" = "" ]; then
                             # No file, but not needed anyway
                             echo "OK"
                     echo "$FILE" >>MISSING
                     echo "MISSING $Y"
             if [ "$Y" = "" ]; then
                     echo "$FILE" >>NOCHECK
                     echo "NOCHECK"
             X="`md5sum < /var/lib/apt/lists/$FILE | cut -d\  -f1` `wc -c < /var/lib
             X="`echo "$X" | sed 's/^ *//;s/  */ /g'`"
             if [ "$X" != "$Y" ]; then
                     echo "$FILE" >>BAD
                     echo "BAD"
             echo "$FILE" >>OK
             echo "OK"
     echo "Checking sources in /etc/apt/sources.list:"
     echo "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
     (echo "You should take care to ensure that the distributions you're downloading
     echo "are the ones you think you are downloading, and that they are as up to"
     echo "date as you would expect (testing and unstable should be no more than"
     echo "two or three days out of date, stable-updates no more than a few weeks"
     echo "or a month)."
     ) | fmt
     cat /etc/apt/sources.list | 
       sed 's/^ *//' | grep '^[^#]' |
       while read ty url dist comps; do
             if [ "${url%%:*}" = "http" -o "${url%%:*}" = "ftp" ]; then
             echo "Source: ${ty} ${url} ${dist} ${comps}"
             rm -f Release Release.gpg
             lynx -reload -dump "${url}/dists/${dist}/Release" >/dev/null 2>&1
             wget -q -O Release "${url}/dists/${dist}/Release"
             if ! grep -q '^' Release; then
                     echo "  * NO TOP-LEVEL Release FILE"
                     origline=`sed -n 's/^Origin: *//p' Release | head -1`
                     lablline=`sed -n 's/^Label: *//p' Release | head -1`
                     suitline=`sed -n 's/^Suite: *//p' Release | head -1`
                     codeline=`sed -n 's/^Codename: *//p' Release | head -1`
                     dateline=`grep "^Date:" Release | head -1`
                     dscrline=`grep "^Description:" Release | head -1`
                     echo "  o Origin: $origline/$lablline"
                     echo "  o Suite: $suitline/$codeline"
                     echo "  o $dateline"
                     echo "  o $dscrline"
                     if [ "${dist%%/*}" != "$suitline" -a "${dist%%/*}" != "$codeline" ]; then
                             echo "  * WARNING: asked for $dist, got $suitline/$codeline"
                     lynx -reload -dump "${url}/dists/${dist}/Release.gpg" >/dev/null 2>&1
                     wget -q -O Release.gpg "${url}/dists/${dist}/Release.gpg"
                     gpgv --status-fd 3 Release.gpg Release 3>&1 >/dev/null 2>&1 | sed -n "s/^\[GNUPG:\] //p" | (okay=0; err=""; while read gpgcode rest; do
                             if [ "$gpgcode" = "GOODSIG" ]; then
                                 if [ "$err" != "" ]; then
                                     echo "  * Signed by ${err# } key: ${rest#* }"
                                     echo "  o Signed by: ${rest#* }"
                             elif [ "$gpgcode" = "BADSIG" ]; then
                                 echo "  * BAD SIGNATURE BY: ${rest#* }"
                             elif [ "$gpgcode" = "ERRSIG" ]; then
                                 echo "  * COULDN'T CHECK SIGNATURE BY KEYID: ${rest %% *}"
                             elif [ "$gpgcode" = "SIGREVOKED" ]; then
                                 err="$err REVOKED"
                             elif [ "$gpgcode" = "SIGEXPIRED" ]; then
                                 err="$err EXPIRED"
                         if [ "$okay" != 1 ]; then
                             echo "  * NO VALID SIGNATURE"
             for comp in $comps; do
                     if [ "$ty" = "deb" ]; then
                             X=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/binary-${arch}/Release" | sed 's,//*,_,g'`" "${comp}/binary-${arch}/Release")
                             Y=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/binary-${arch}/Packages" | sed 's,//*,_,g'`" "${comp}/binary-${arch}/Packages")
                             if [ "$X $Y" = "OK OK" ]; then
                                     okaycomps="$okaycomps $comp"
                                     echo "  * PROBLEMS WITH $comp ($X, $Y)"
                     elif [ "$ty" = "deb-src" ]; then
                             X=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/source/Release" | sed 's,//*,_,g'`" "${comp}/source/Release")
                             Y=$(checkit "`echo "${baseurl}/dists/${dist}/${comp}/source/Sources" | sed 's,//*,_,g'`" "${comp}/source/Sources")
                             if [ "$X $Y" = "OK OK" ]; then
                                     okaycomps="$okaycomps $comp"
                                     echo "  * PROBLEMS WITH component $comp ($X, $Y)"
             [ "$okaycomps" = "" ] || echo "  o Okay:$okaycomps"
     echo "Results"
     echo "~~~~~~~"
     cd /tmp/apt-release-check
     diff <(cat BAD MISSING NOCHECK OK | sort) <(cd /var/lib/apt/lists && find . -type f -maxdepth 1 | sed 's,^\./,,g' | grep '_' | sort) | sed -n 's/^> //p' >UNVALIDATED
     cd /tmp/apt-release-check
     if grep -q ^ UNVALIDATED; then
         (echo "The following files in /var/lib/apt/lists have not been validated."
         echo "This could turn out to be a harmless indication that this script"
         echo "is buggy or out of date, or it could let trojaned packages get onto"
         echo "your system."
         ) | fmt
         sed 's/^/    /' < UNVALIDATED
     if grep -q ^ BAD; then
         (echo "The contents of the following files in /var/lib/apt/lists does not"
         echo "match what was expected. This may mean these sources are out of date,"
         echo "that the archive is having problems, or that someone is actively"
         echo "using your mirror to distribute trojans."
         if am_root; then 
             echo "The files have been renamed to have the extension .FAILED and"
             echo "will be ignored by apt."
             cat BAD | while read a; do
                 mv /var/lib/apt/lists/$a /var/lib/apt/lists/${a}.FAILED
         fi) | fmt
         sed 's/^/    /' < BAD
     if grep -q ^ MISSING; then
         (echo "The following files from /var/lib/apt/lists were missing. This"
         echo "may cause you to miss out on updates to some vulnerable packages."
         ) | fmt
         sed 's/^/    /' < MISSING
     if grep -q ^ NOCHECK; then
         (echo "The contents of the following files in /var/lib/apt/lists could not"
         echo "be validated due to the lack of a signed Release file, or the lack"
         echo "of an appropriate entry in a signed Release file. This probably"
         echo "means that the maintainers of these sources are slack, but may mean"
         echo "these sources are being actively used to distribute trojans."
         if am_root; then 
             echo "The files have been renamed to have the extension .FAILED and"
             echo "will be ignored by apt."
             cat NOCHECK | while read a; do
                 mv /var/lib/apt/lists/$a /var/lib/apt/lists/${a}.FAILED
         fi) | fmt
         sed 's/^/    /' < NOCHECK
     if $allokay; then
         echo 'Everything seems okay!'
     rm -rf /tmp/apt-release-check

You might need to apply the following patch for sid since md5sum adds an '-' after the sum when the input is stdin:

     @@ -37,7 +37,7 @@
             local LOOKUP="$2"
             Y="`get_md5sumsize Release "$LOOKUP"`"
     -       Y="`echo "$Y" | sed 's/^ *//;s/  */ /g'`"
     +       Y="`echo "$Y" | sed 's/-//;s/^ *//;s/  */ /g'`"
             if [ ! -e "/var/lib/apt/lists/$FILE" ]; then
                     if [ "$Y" = "" ]; then
     @@ -55,7 +55,7 @@
             X="`md5sum < /var/lib/apt/lists/$FILE` `wc -c < /var/lib/apt/lists/$FILE`"
     -       X="`echo "$X" | sed 's/^ *//;s/  */ /g'`"
     +       X="`echo "$X" | sed 's/-//;s/^ *//;s/  */ /g'`"
             if [ "$X" != "$Y" ]; then
                     echo "$FILE" >>BAD
                     echo "BAD"

7.5.4 Release check of non Debian sources

Notice that, when using the latest apt version (with secure apt) no extra effort should be required on your part unless you use non-Debian sources, in which case an extra confirmation step will be required by apt-get. This is avoided by providing Release and Release.gpg files in the non-Debian sources. The Release file can be generated with apt-ftparchive (available in apt-utils 0.5.0 and later), the Release.gpg is just a detached signature. To generate both follow this simple procedure:

     $ rm -f dists/unstable/Release
     $ apt-ftparchive release dists/unstable > dists/unstable/Release
     $ gpg --sign -ba -o dists/unstable/Release.gpg dists/unstable/Release

7.5.5 Alternative per-package signing scheme

The additional scheme of signing each and every packages allows packages to be checked when they are no longer referenced by an existing Packages file, and also third-party packages where no Packages ever existed for them can be also used in Debian but will not be default scheme.

This package signing scheme can be implemented using debsig-verify and debsigs. These two packages can sign and verify embedded signatures in the .deb itself. Debian already has the capability to do this now, but there is no feature plan to implement the policy or other tools since the archive signing scheme is prefered. These tools are available for users and archive administrators that would rather use this scheme instead.

Latest dpkg versions (since 1.9.21) incorporate a patch that provides this functionality as soon as debsig-verify is installed.

NOTE: Currently /etc/dpkg/dpkg.cfg ships with "no-debsig" as per default.

NOTE2: Signatures from developers are currently stripped when they enter off the package archive since the currently preferred method is release checks as described previously.

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Securing Debian Manual

Version: 3.13, Sun, 08 Apr 2012 02:48:09 +0000

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