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Anleitung zum Absichern von Debian
Kapitel 7 - Paket signierung unter Debian


Dieses Kapitel könnte auch mit "Wie man sein Debian GNU/Linux System sicher upgraded/updated" überschrieben werden, und es verdient hauptsächlich deshalb ein eigenes Kapitel, weil es in kein anderes passt.

Derzeit (Stand Dezember 2001) stellt Debian keine signierten Pakete für die Distribution zur Verfügung und der Woody Release (3.0) wird diese Fähigkeiten nicht integrieren. Es gibt eine Lösung für signierte Pakete, die - hoffentlich - in der nächsten release (sarge) integriert wird.


7.1 Der vorgeschlagene Plan zur Prüfung von Paket Signierungen

Der derzeitige (unimplementierte) Plan zur Prüfung von Paket Signaturen mit apt ist:

Durch diese Kette von md5-Summen ist apt in der Lage, zu verifizieren, dass ein Paket aus einer bestimmten Release stammt. Dies ist zwar unflexibler als jedes Paket einzeln zu signieren, kann aber auch mit den unten aufgeführten Plänen kombiniert werden.

Die Signierung von Paketen wurde innerhalb des Debian Projektes ausführlich diskutiert. Mehr Informationen hierzu finden Sie unter http://www.debian.org/News/weekly/2001/8/ und http://www.debian.org/News/weekly/2000/11/.


7.2 Alternative Einzel-Paket-Signierungs Schemata

Dieses zusätzliche Schemata jedes paket einzeln zu signieren erlaubt es, Pakete zu prüfen, selbst wenn sie nicht mehr in irgendeiner Packages-Datei erwähnt werden, und auch Pakete von Dritten, für die es keine Packages-Datei gibt, können unter Debian installiert werden.

Dieses Paket-Signierungs Schemata kann durch debsig-verify und debsigs implementiert werden. Diese beiden Pakete können in der .deb-Datei selbst eingebettete Signaturen erstellen und prüfen. Debian hat bereits jetzt die Möglichkeiten dies zu tun, aber das Regelwerk und die Werkzeuge hierfür werden erst nach dem Woody Release eingeführt (um den Release-Zyklus nicht zu verlangsamen).

Beachten Sie: Derzeit wird enthält /etc/dpkg/dpkg.cfg derzeit standardmässig die Option "no-debsig".


7.3 Prüfen veröffentlichter Pakaete

Für den Fall, dass Sie nun zusätzliche Sicherheits Prüfungen einführen wollen, können Sie das folgende Skript von Anthony Towns benutzen. Dieses Skript kann automatisch neue Sicherheits Checks durchführen, damit ein Nutzer sicher gehen kann, dass die Software, die er/sie herunterlädt, die gleiche ist, wie die, die von Debian bereitgestellt wird. 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.

Dieser Beispiel-Code, umbenannt nach apt-release-check, sollte auf die folgende Art benutzt werden: used in the following way:

     # apt-get update
     # apt-release-check
     (... Ergebnisse ...)
     # apt-get dist-upgrade

Zuerst müssen Sie jedoch:

     #!/bin/bash
     # This script is copyright (c) 2001, Anthony Towns
     #
     # 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
     # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     # 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
     
     >OK
     >MISSING
     >NOCHECK
     >BAD
     
     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"
                             return
                     fi
                     echo "$FILE" >>MISSING
                     echo "MISSING $Y"
                     return
             fi
             if [ "$Y" = "" ]; then
                     echo "$FILE" >>NOCHECK
                     echo "NOCHECK"
                     return
             fi
             X="`md5sum < /var/lib/apt/lists/$FILE` `wc -c < /var/lib/apt/lists/$FILE`"
             X="`echo "$X" | sed 's/^ *//;s/  */ /g'`"
             if [ "$X" != "$Y" ]; then
                     echo "$FILE" >>BAD
                     echo "BAD"
                     return
             fi
             echo "$FILE" >>OK
             echo "OK"
     }
     
     echo
     echo "Checking sources in /etc/apt/sources.list:"
     echo "~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
     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
     echo
     
     cat /etc/apt/sources.list | 
       sed 's/^ *//' | grep '^[^#]' |
       while read ty url dist comps; do
             if [ "${url%%:*}" = "http" -o "${url%%:*}" = "ftp" ]; then
                     baseurl="${url#*://}"
             else
                     continue
             fi
             echo "Source: ${ty} ${url} ${dist} ${comps}"
             
             rm -f Release Release.gpg
             wget -q -O Release "${url}/dists/${dist}/Release"
     
             if ! grep -q '^' Release; then
                     echo "  * NO TOP-LEVEL Release FILE"
             else
                     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"
                     fi
     
                     wget -q -O Release.gpg "${url}/dists/${dist}/Release.gpg"
                     sigline="`gpgv --status-fd 3 Release.gpg Release 3>&1 >/dev/null 2>&1 | sed -n "s/^\[GNUPG:\] GOODSIG [0-9A-Fa-f]* //p"`"
                     if [ "$sigline" ]; then
                             echo "  o Signed by: $sigline"
                     else
                             echo "  * NO VALID SIGNATURE"
                             >Release
                     fi
             fi
             okaycomps=""
             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"
                             else
                                     echo "  * PROBLEMS WITH $comp ($X, $Y)"
                             fi
                     elif [ "$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"
                             else
                                     echo "  * PROBLEMS WITH component $comp ($X, $Y)"
                             fi
                     fi
             done
             [ "$okaycomps" = "" ] || echo "  o Okay:$okaycomps"
             echo
       done
     
     echo "Results"
     echo "~~~~~~~"
     echo
     
     allokay=true
     
     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
         allokay=false
         (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
         echo
         sed 's/^/    /' < UNVALIDATED
         echo
     fi
     
     if grep -q ^ BAD; then
         allokay=false
         (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
             done
         fi) | fmt
         echo
         sed 's/^/    /' < BAD
         echo
     fi
     
     if grep -q ^ MISSING; then
         allokay=false
         (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
         echo
         sed 's/^/    /' < MISSING
         echo
     fi
     
     if grep -q ^ NOCHECK; then
         allokay=false
         (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
             done
         fi) | fmt
         echo
         sed 's/^/    /' < NOCHECK
         echo
     fi
     
     if $allokay; then
         echo 'Everything seems okay!'
         echo
     fi
     
     rm -rf /tmp/apt-release-check

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Anleitung zum Absichern von Debian

2.5 (beta) 20 septiembre 2003Sat, 17 Aug 2002 12:23:36 +0200

Javier Fernández-Sanguino Peña jfs@computer.org