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<article lang="fr">
<articleinfo>
<title>
Compressions des sessions TCP/IP avec des outils de type SSH
</title>
<subtitle>
Version française du <foreignphrase>Compressed TCP/IP-Sessions using
SSH-like tools</foreignphrase>
</subtitle>
<author>
<firstname>Sebastian</firstname>
<surname>Schreiber</surname>
<affiliation>
<address><email>Schreib CHEZ SySS POINT de</email></address>
</affiliation>
</author>
<othercredit role="traduction">
<firstname>Vincent</firstname>
<surname>Chartier</surname>
<affiliation>
<address><email>vchartier CHEZ kyxar POINT fr</email></address>
</affiliation>
<contrib>Adaptation française</contrib>
</othercredit>
<othercredit role="relecture">
<firstname>Claude</firstname>
<surname>Thomassin</surname>
<affiliation>
<address><email>cthomassin CHEZ waika9 POINT com</email></address>
</affiliation>
<contrib>Relecture de la version française</contrib>
</othercredit>
<othercredit role="publication">
<firstname>Jean-Philippe</firstname>
<surname>Guérard</surname>
<affiliation>
<address><email>jean TIRET philippe POINT guerard CHEZ tigreraye POINT org</email></address>
</affiliation>
<contrib>Préparation de la publication de la v.f.</contrib>
</othercredit>
<revhistory>
<revision>
<revnumber>v2000-02-02.fr.1.0</revnumber>
<date>2003-07-23</date>
<authorinitials>VC,CT,JPG</authorinitials>
</revision>
<revision>
<revnumber>v2000-02-02</revnumber>
<date>2000-02-02</date>
<authorinitials>SS</authorinitials>
</revision>
</revhistory>
<pubdate>2003-12-31</pubdate>
<releaseinfo>Version : 2000-02-02.fr.1.0</releaseinfo>
<abstract><para>
Ce guide pratique explique pas à pas comment compresser ses connexions
en utilisant SSH. Compresser les connexions permet, d'une part, de
limiter la bande passante utilisée et, d'autre part, d'accélérer les
connexions à faible débit (modem, GSM, RNIS).
</para></abstract>
</articleinfo>
<section>
<title>Introduction</title>
<para>
Dans le passé, nous avions l'habitude de compresser les fichiers
pour économiser de l'espace disque. Aujourd'hui, l'espace disque est
peu onéreux mais la bande passante est limitée. En compressant les flux
de données réseau, vous atteindrez deux objectifs :
</para>
<para>
<orderedlist>
<listitem><para>
Vous économiserez la bande passante et le volume transféré (ce
qui est important si vous devez payer le trafic ou si votre réseau est
chargé).
</para></listitem>
<listitem><para>
Vous accélérez les connexions à faible bande passante (Modem,
GSM, RNIS).
</para></listitem>
</orderedlist>
</para>
<para>
Ce guide va vous expliquer comment économiser la bande passante et le
temps de connexion en utilisant des outils comme SSH1, SSH2, OpenSSH ou
LSH.
</para>
</section>
<section>
<title>Compresser HTTP, FTP…
</title>
<para>
Mon bureau est connecté à internet par une ligne RNIS à 64 kbit/s.
Le taux de transfert maximum de cette ligne est donc d'environ
7 ko/s. Il est possible d'accélérer le débit de cette connexion en
compressant les données échangées : quand je télécharge des
fichiers, Netscape affiche un taux de transfert allant jusqu'à
40 ko/s (Il est possible de compresser les fichiers d'historique
par un facteur de 15). SSH est un outil principalement conçu pour la
mise en place de connexions sécurisées au travers de réseaux
non-sécurisés. De plus, SSH est capable de compresser les données
échangées et de faire suivre des ports (comme le font rinetd ou redir).
C'est donc l'outil approprié pour compresser n'importe quelle connexion
TCP simple. « Simple » signifie que seule une connexion TCP
est ouverte. Une connexion FTP ou la connexion entre les produits
Microsoft Outlook et Exchange ne sera pas simple puisque qu'elle
impliquera la mise en place de plusieurs connexions. SSH utilise
l'algorithme de compression Lempel-Ziv (LZ77) — donc vous
atteindrez les mêmes importants taux de compression que winzip et pkzip.
Pour compresser toutes les connexions HTTP provenant de mon intranet
vers internet, il me suffit d'exécuter une commande sur ma
machine :
</para>
<screen>
ssh -C -L8080:mandataire:80 -f identifiant@nom_d_hote "sleep 10000"
</screen>
<para>
<literal>nom_d_hote</literal> = la machine
hébergée par le fournisseur d'accès internet (FAI). Un accès via SSH à
cette machine est nécessaire.
</para>
<para>
<literal>identifiant</literal> = mon nom d'utilisateur sur
<literal>nom_d_hote</literal>.
</para>
<para>
<literal>mandataire</literal> = le serveur mandataire (<foreignphrase>proxy</foreignphrase>)
de mon FAI.
</para>
<para>
Le navigateur de mon PC est configuré pour utiliser
<literal>localhost:8080</literal> comme serveur mandataire (<literal>localhost</literal>
étant le nom d'hôte par défaut permettant de se connecter à la machine
locale). Mon portable se connecte lui aussi sur le port <literal>8080</literal>
de mon PC (en utilisant l'adresse IP de mon PC au lieu de
<literal>localhost</literal>). La connexion est compressée et redirigée vers le
véritable serveur mandataire via SSH. L'infrastructure ressemble à
celle-ci :
</para>
<screen>
64 kbit/s RNIS
Mon PC ----------------------------- Un PC
Client SSH compressé (Unix/Linux/Win-NT)
Port 8080 chez mon FAI
| Serveur SSH
| Port 22
| |
| |
| |
| Ethernet 10 mbit/s | 100 mbit/s
| non compressé | non compressé
| |
| |
Mon second PC Serveur mandataire (proxy)
avec Netscape,... de mon FAI
(portable) Port 80
</screen>
</section>
<section>
<title>Compression du courrier électronique
</title>
<section>
<title>Courrier entrant (POP3, IMAP4)
</title>
<para>
La plupart des internautes rapatrient leur courrier électronique à
partir de leur serveur de messagerie via le protocole POP3. Ce
protocole présente de nombreux inconvénients :
</para>
<para>
<orderedlist>
<listitem>
<para>
POP3 fait transiter le mot de passe en clair (des versions de
POP et d'IMAP utilisant SSL ont été développées et un mécanisme
d'authentification via un protocole de type défi-réponse est défini
dans les RFC 2095 et 2195).
</para>
</listitem>
<listitem>
<para>
POP3 est un protocole très bavard : le client doit tout
d'abord demander un message. Puis le serveur lui envoie le message.
Ensuite, le client demande la suppression du message transféré. Le
serveur lui confirme la suppression. Une fois cela fait, le serveur
est prêt à passer au message suivant. Cela nécessite donc 4
transactions par courrier électronique.
</para>
</listitem>
<listitem>
<para>
POP3 transfère les courriers électroniques sans aucune
compression, bien que les courriers électroniques soient hautement
compressible (d'un facteur de 3,5).
</para>
</listitem>
</orderedlist>
</para>
<para>
Vous pourriez compresser les connexions POP3 en redirigeant le port
110 de la machine locale (<literal>localhost:110</literal>) via une connexion
compressée vers le serveur POP3 de votre fournisseur d'accès. Après
cela, il suffirait de configurer votre client de messagerie pour qu'il
se connecte sur l'adresse <literal>localhost:110</literal> afin de télécharger le
courrier électronique. Ce qui permettrait de sécuriser et d'accélérer
la connexion — mais le temps de téléchargement reste inhérent à
la surcharge due au protocole POP3.
</para>
<para>
Il est intéressant de substituer au protocole POP3 un protocole plus
efficace. L'idée est de télécharger la boîte aux lettres toute entière
en une seule fois sans avoir à supporter la surcharge due au protocole.
De plus, il est intéressant de compresser cette connexion. L'outil
approprié offrant ces deux caractéristiques est SCP. Vous pouvez
télécharger votre fichier boîte aux lettres de la manière
suivante :
</para>
<screen>
scp -C identifiant@serveur-de-messagerie:/var/spool/mail/identifiant \
/tmp/nouveau_courrier
</screen>
<para>
Mais il y a un problème : que se passe-t-il si un nouveau
courrier arrive sur le serveur pendant que vous téléchargez votre boîte
aux lettres ? Le nouveau courrier serait perdu. On a plutôt
intérêt à utiliser les commandes suivantes :
</para>
<screen>
ssh identifiant@serveur-de-messagerie \
"mv /var/spool/mail/identifiant /tmp/identifiant_a_rapatrier"
scp -C identifiant@serveur-de-messagerie:/tmp/identifiant_a_rapatrier \
/tmp/nouveau_courrier
</screen>
<para>
Un déplacement de fichier (mv) est une opération élémentaire, donc
vous n'aurez pas de problème si vous recevez de nouveaux courriers
durant l'exécution des commandes. Cependant si les répertoires /tmp et
/var/spool/mail/ du serveur de messagerie ne sont pas sur le même
disque, vous pourriez avoir des problèmes. Une solution est de créer un
fichier de verrouillage sur le serveur avant d'exécuter la commande de
déplacement :
</para>
<screen>
touch /var/spool/mail/identifiant.lock
</screen>
<para>
Vous devrez le supprimer après coup. Une meilleure solution est de
déplacer le fichier identifiant dans le même répertoire :
</para>
<screen>
ssh identifiant@serveur-de-messagerie \
"mv /var/spool/mail/identifiant /var/spool/mail/identifiant_a_rapatrier"
</screen>
<para>
Après cela, vous pourrez utiliser formail à la place de procmail
pour filtrer /tmp/nouveau_courrier dans les répertoires
correspondants :
</para>
<screen>
formail -s procmail < /tmp/nouveau_courrier
</screen>
</section>
<section>
<title>Courrier sortant (SMTP)
</title>
<para>
Envoyez des courriers électroniques au travers de connexions SSH
compressées et chiffrées, permet de :
</para>
<para>
<itemizedlist>
<listitem>
<para>
Économiser du trafic réseau.
</para>
</listitem>
<listitem>
<para>
Sécuriser la connexion (cela n'a pas d'intérêt si le courrier
électronique transite par des réseaux non sécurisés par la suite).
</para>
</listitem>
<listitem>
<para>
Authentifier l'émetteur. Beaucoup de serveurs de messagerie
n'autorisent pas le transit des courriers pour éviter les abus. Si
vous envoyez un courrier au travers d'une connexion SSH, le serveur de
messagerie distant (par exemple Sendmail ou MS Exchange) pensera
être connecté localement, et acceptera donc de faire transiter le
message.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Si vous avez un accès SSH au serveur de messagerie, vous aurez
besoin de la commande suivante :
</para>
<screen>
ssh -C -L2525:serveur_de_messagerie:25 identifiant@serveur_de_messagerie
</screen>
<para>
Si vous n'avez pas d'accès SSH au serveur de messagerie, mais que
vous avez un accès SSH à un serveur autorisé à utiliser le serveur de
messagerie comme relais, la commande sera la suivante :
</para>
<screen>
ssh -C -L2525:serveur_de_messagerie:25 identifiant@autre_server
</screen>
<para>
Après cela, vous pourrez configurer votre client de messagerie local
(ou le serveur de messagerie, recherchez de la documentation sur les
relais de messagerie (<foreignphrase>smarthosts</foreignphrase>)) pour envoyer les messages
sortants vers le port 2525 de la machine locale (<literal>localhost</literal>).
</para>
</section>
</section>
<section>
<title>Notes sur les performances
</title>
<para>
Bien sûr, la compression et le chiffrement utilisent du temps
processeur. Il a été calculé qu'un vieux Pentium-133 peut chiffrer et
compresser environ 1 Go / heure — ce qui est assez
important. Si vous compilez SSH avec l'option « --with-none »
vous indiquerez à SSH de ne pas utiliser de chiffrement. Cela permet
d'augmenter un peu les performances. Voici une évaluation de
</para>
<para>
le compromis entre
différentes méthodes de téléchargement (pendant le test, un fichier non
compressé de 6 Mo est transféré à partir d'un Pentium à
133 MHz vers un portable équipé d'un Pentium 2 à 233 MHz
et ceci sur un segment Ethernet à 10 Mbit/s sans autre charge).
</para>
<screen>
+--------------------------+---------+---------+-----------+-----------+
| | FTP | chiffré | compressé | compressé |
| | | | | & chiffré |
+--------------------------+---------+---------+-----------+-----------+
+--------------------------+---------+---------+-----------+-----------+
| Temps écoulé (s) | 7.6 | 26 | 9 | 23 |
+--------------------------+---------+---------+-----------+-----------+
| Taux de transfert (ko/s) | 790 | 232 | 320 | 264 |
+--------------------------+---------+---------+-----------+-----------+
| Facteur compression | 1 | 1 | 3.8 | 3.8 |
+--------------------------+---------+---------+-----------+-----------+
</screen>
</section>
<section>
<title>Remerciements
</title>
<para>
Merci à Harald König
<email>koenig CHEZ tat POINT physik POINT uni TIRET tuebingen POINT de</email>,
qui a utilisé rcp pour télécharger les boîtes aux lettres complètes. La
dernière version de ce guide pratique en version originale est
disponible à l'adresse suivante :
<ulink url="http://www.tldp.org/HOWTO/Compressed-TCP.html"/>.
</para>
<para>
La dernière version française de ce document est toujours disponible à
l'adresse <ulink url="&howto;Compressed-TCP.html"/>.
</para>
</section>
<section>
<title>Droits d'utilisation</title>
<para>
Copyright © 2002 Sebastian Schreiber pour la version originale.
</para>
<para>
Copyright © 2003-2004 Vincent Chartier, Claude Thomassin et
Jean-Philippe Guérard pour la version française.
</para>
<para>
Vous est autorisé de copier, distribuer ou modifier la version originale
de ce document selon les termes de la licence de documentation libre GNU
(GFDL), version 1.2 ou ultérieure, telle que publiée par la Free
Software Foundation ; sans section inaltérable, ni texte de
première de couverture, ni texte de quatrième de couverture. Une copie
de cette licence est incluse dans la section intitulée « GNU Free
Documentation License ».
</para>
<para>
<foreignphrase>Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documentation License,
Version 1.2 or any later version published by the Free Software
Foundation; with no Invariant Sections, no Front-Cover Texts, and no
Back-Cover Texts. A copy of the license is included in the section
entitled "GNU Free Documentation License".</foreignphrase>
</para>
</section>
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Title, and add to it an item stating at least the title, year, new
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Page. If there is no section Entitled "History" in the Document,
create one stating the title, year, authors, and publisher of the
Document as given on its Title Page, then add an item describing the
Modified Version as stated in the previous sentence.
</simpara></listitem>
<listitem><simpara>Preserve the network location, if any, given in the
Document for public access to a Transparent copy of the Document, and
likewise the network locations given in the Document for previous
versions it was based on. These may be placed in the "History"
section. You may omit a network location for a work that was
published at least four years before the Document itself, or if the
original publisher of the version it refers to gives permission.
</simpara></listitem>
<listitem><simpara>For any section Entitled "Acknowledgements" or
"Dedications", Preserve the Title of the section, and preserve in the
section all the substance and tone of each of the contributor
acknowledgements and/or dedications given therein.
</simpara></listitem>
<listitem><simpara>Preserve all the Invariant Sections of the Document,
unaltered in their text and in their titles. Section numbers or the
equivalent are not considered part of the section titles.
</simpara></listitem>
<listitem><simpara>Delete any section Entitled "Endorsements".
Such a section may not be included in the Modified Version.
</simpara></listitem>
<listitem><simpara>Do not retitle any existing section to be Entitled
"Endorsements" or to conflict in title with any Invariant Section.
</simpara></listitem>
<listitem><simpara>Preserve any Warranty Disclaimers.
</simpara></listitem>
</orderedlist>
<para>If the Modified Version includes new front-matter sections or
appendices that qualify as Secondary Sections and contain no material
copied from the Document, you may at your option designate some or all
of these sections as invariant. To do this, add their titles to the
list of Invariant Sections in the Modified Version's license notice.
These titles must be distinct from any other section titles.</para>
<para>You may add a section Entitled "Endorsements", provided it
contains nothing but endorsements of your Modified Version by various
parties--for example, statements of peer review or that the text has
been approved by an organization as the authoritative definition of a
standard.</para>
<para>You may add a passage of up to five words as a Front-Cover Text,
and a passage of up to 25 words as a Back-Cover Text, to the end of the
list of Cover Texts in the Modified Version. Only one passage of
Front-Cover Text and one of Back-Cover Text may be added by (or through
arrangements made by) any one entity. If the Document already includes
a cover text for the same cover, previously added by you or by
arrangement made by the same entity you are acting on behalf of, you may
not add another; but you may replace the old one, on explicit permission
from the previous publisher that added the old one.</para>
<para>The author(s) and publisher(s) of the Document do not by this
License give permission to use their names for publicity for or to
assert or imply endorsement of any Modified Version.</para>
</section>
<section id="gfdl-5"><title>COMBINING DOCUMENTS</title>
<para>You may combine the Document with other documents released under
this License, under the terms defined in <link linkend="gfdl-4">section
4</link> above for modified versions, provided that you include in the
combination all of the Invariant Sections of all of the original
documents, unmodified, and list them all as Invariant Sections of your
combined work in its license notice, and that you preserve all their
Warranty Disclaimers.</para>
<para>The combined work need only contain one copy of this License, and
multiple identical Invariant Sections may be replaced with a single
copy. If there are multiple Invariant Sections with the same name but
different contents, make the title of each such section unique by adding
at the end of it, in parentheses, the name of the original author or
publisher of that section if known, or else a unique number. Make the
same adjustment to the section titles in the list of Invariant Sections
in the license notice of the combined work.</para>
<para>In the combination, you must combine any sections Entitled
"History" in the various original documents, forming one section
Entitled "History"; likewise combine any sections Entitled
"Acknowledgements", and any sections Entitled "Dedications". You must
delete all sections Entitled "Endorsements".</para>
</section>
<section id="gfdl-6"><title>COLLECTIONS OF DOCUMENTS</title>
<para>You may make a collection consisting of the Document and other
documents released under this License, and replace the individual copies
of this License in the various documents with a single copy that is
included in the collection, provided that you follow the rules of this
License for verbatim copying of each of the documents in all other
respects.</para>
<para>You may extract a single document from such a collection, and
distribute it individually under this License, provided you insert a
copy of this License into the extracted document, and follow this
License in all other respects regarding verbatim copying of that
document.</para>
</section>
<section id="gfdl-7"><title>AGGREGATION WITH INDEPENDENT WORKS</title>
<para>A compilation of the Document or its derivatives with other
separate and independent documents or works, in or on a volume of a
storage or distribution medium, is called an "aggregate" if the
copyright resulting from the compilation is not used to limit the legal
rights of the compilation's users beyond what the individual works
permit. When the Document is included in an aggregate, this License does
not apply to the other works in the aggregate which are not themselves
derivative works of the Document.</para>
<para>
If the Cover Text requirement of section <xref linkend="gfdl-3"
xrefstyle="labelnumber"/> is applicable to these copies of the Document,
then if the Document is less than one half of the entire aggregate, the
Document's Cover Texts may be placed on covers that bracket the Document
within the aggregate, or the electronic equivalent of covers if the
Document is in electronic form. Otherwise they must appear on printed
covers that bracket the whole aggregate.
</para>
</section>
<section id="gfdl-8"><title>TRANSLATION</title>
<para>
Translation is considered a kind of modification, so you may distribute
translations of the Document under the terms of section <xref
linkend="gfdl-4" xrefstyle="labelnumber"/>. Replacing Invariant
Sections with translations requires special permission from their
copyright holders, but you may include translations of some or all
Invariant Sections in addition to the original versions of these
Invariant Sections. You may include a translation of this License, and
all the license notices in the Document, and any Warranty Disclaimers,
provided that you also include the original English version of this
License and the original versions of those notices and disclaimers. In
case of a disagreement between the translation and the original version
of this License or a notice or disclaimer, the original version will
prevail.
</para>
<para>
If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section <xref
linkend="gfdl-4" xrefstyle="labelnumber"/>) to Preserve its Title
(section <xref linkend="gfdl-1" xrefstyle="labelnumber"/>) will
typically require changing the actual title.
</para>
</section>
<section id="gfdl-9"><title>TERMINATION</title>
<para>You may not copy, modify, sublicense, or distribute the Document
except as expressly provided for under this License. Any other attempt
to copy, modify, sublicense or distribute the Document is void, and will
automatically terminate your rights under this License. However,
parties who have received copies, or rights, from you under this License
will not have their licenses terminated so long as such parties remain
in full compliance.</para>
</section>
<section id="gfdl-10"><title>FUTURE REVISIONS OF THIS LICENSE</title>
<para>The Free Software Foundation may publish new, revised versions of
the GNU Free Documentation License from time to time. Such new versions
will be similar in spirit to the present version, but may differ in
detail to address new problems or concerns. See
http://www.gnu.org/copyleft/.</para>
<para>Each version of the License is given a distinguishing version
number. If the Document specifies that a particular numbered version of
this License "or any later version" applies to it, you have the option
of following the terms and conditions either of that specified version
or of any later version that has been published (not as a draft) by the
Free Software Foundation. If the Document does not specify a version
number of this License, you may choose any version ever published (not
as a draft) by the Free Software Foundation.</para>
</section>
<section id="gfdl-addendum"><title>ADDENDUM: How to use this License for
your documents</title>
<para>To use this License in a document you have written, include a copy
of the License in the document and put the following copyright and
license notices just after the title page:</para>
<blockquote id="copyright-sample"><para>
Copyright © YEAR YOUR NAME.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".
</para></blockquote>
<para>If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:</para>
<blockquote id="inv-cover-sample"><para>
with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST.
</para></blockquote>
<para>If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.</para>
<para>If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of free
software license, such as the GNU General Public License, to permit
their use in free software.</para>
</section>
</appendix>
</article>
