Compressions des sessions TCP/IP avec des outils de type SSH
Version française du Compressed TCP/IP-Sessions using SSH-like tools
Sebastian Schreiber
<Schreib CHEZ SySS POINT de>
Adaptation française: Vincent Chartier
<vchartier CHEZ kyxar POINT fr>
Relecture de la version française: Claude Thomassin
<cthomassin CHEZ waika9 POINT com>
Préparation de la publication de la v.f.: Jean-Philippe Guérard
<jean TIRET philippe POINT guerard CHEZ tigreraye POINT org>
Version : 2000-02-02.fr.1.0
2003-12-31
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| Version v2000-02-02.fr.1.0 | 2003-07-23 | VC,CT,JPG |
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| Version v2000-02-02 | 2000-02-02 | SS |
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Résumé
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).
-------------------------------------------------------------------
Table des matières
1. Introduction
2. Compresser HTTP, FTP...
3. Compression du courrier électronique
3.1. Courrier entrant (POP3, IMAP4)
3.2. Courrier sortant (SMTP)
4. Notes sur les performances
5. Remerciements
6. Droits d'utilisation
A. GNU Free Documentation License
1. PREAMBLE
2. APPLICABILITY AND DEFINITIONS
3. VERBATIM COPYING
4. COPYING IN QUANTITY
5. MODIFICATIONS
6. COMBINING DOCUMENTS
7. COLLECTIONS OF DOCUMENTS
8. AGGREGATION WITH INDEPENDENT WORKS
9. TRANSLATION
10. TERMINATION
11. FUTURE REVISIONS OF THIS LICENSE
12. ADDENDUM: How to use this License for your
documents
1. Introduction
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 :
1. 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).
2. Vous accélérez les connexions à faible bande passante (Modem,
GSM, RNIS).
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.
2. Compresser HTTP, FTP...
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 :
ssh -C -L8080:mandataire:80 -f identifiant@nom_d_hote "sleep 10000"
nom_d_hote = la machine hébergée par le fournisseur d'accès
internet (FAI). Un accès via SSH à cette machine est nécessaire.
identifiant = mon nom d'utilisateur sur nom_d_hote.
mandataire = le serveur mandataire (proxy) de mon FAI.
Le navigateur de mon PC est configuré pour utiliser localhost:8080
comme serveur mandataire (localhost étant le nom d'hôte par défaut
permettant de se connecter à la machine locale). Mon portable se
connecte lui aussi sur le port 8080 de mon PC (en utilisant
l'adresse IP de mon PC au lieu de localhost). La connexion est
compressée et redirigée vers le véritable serveur mandataire via
SSH. L'infrastructure ressemble à celle-ci :
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
3. Compression du courrier électronique
3.1. Courrier entrant (POP3, IMAP4)
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 :
1. 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).
2. 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.
3. POP3 transfère les courriers électroniques sans aucune
compression, bien que les courriers électroniques soient
hautement compressible (d'un facteur de 3,5).
Vous pourriez compresser les connexions POP3 en redirigeant le
port 110 de la machine locale (localhost:110) 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 localhost:110 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.
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 :
scp -C identifiant@serveur-de-messagerie:/var/spool/mail/identifiant \
/tmp/nouveau_courrier
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 :
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
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 :
touch /var/spool/mail/identifiant.lock
Vous devrez le supprimer après coup. Une meilleure solution est de
déplacer le fichier identifiant dans le même répertoire :
ssh identifiant@serveur-de-messagerie \
"mv /var/spool/mail/identifiant /var/spool/mail/identifiant_a_rapatrier"
Après cela, vous pourrez utiliser formail à la place de procmail
pour filtrer /tmp/nouveau_courrier dans les répertoires
correspondants :
formail -s procmail < /tmp/nouveau_courrier
3.2. Courrier sortant (SMTP)
Envoyez des courriers électroniques au travers de connexions SSH
compressées et chiffrées, permet de :
o Économiser du trafic réseau.
o Sécuriser la connexion (cela n'a pas d'intérêt si le courrier
électronique transite par des réseaux non sécurisés par la
suite).
o 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.
Si vous avez un accès SSH au serveur de messagerie, vous aurez
besoin de la commande suivante :
ssh -C -L2525:serveur_de_messagerie:25 identifiant@serveur_de_messagerie
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 :
ssh -C -L2525:serveur_de_messagerie:25 identifiant@autre_server
Après cela, vous pourrez configurer votre client de messagerie
local (ou le serveur de messagerie, recherchez de la documentation
sur les relais de messagerie (smarthosts)) pour envoyer les
messages sortants vers le port 2525 de la machine locale
(localhost).
4. Notes sur les performances
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
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).
+--------------------------+---------+---------+-----------+-----------+
| | 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 |
+--------------------------+---------+---------+-----------+-----------+
5. Remerciements
Merci à Harald König <koenig CHEZ tat POINT physik POINT uni TIRET
tuebingen POINT de>, 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 :
http://www.tldp.org/HOWTO/Compressed-TCP.html
[http://www.tldp.org/HOWTO/Compressed-TCP.html].
La dernière version française de ce document est toujours
disponible à l'adresse
http://www.traduc.org/docs/howto/lecture/Compressed-TCP.html
[http://www.traduc.org/docs/howto/lecture/Compressed-TCP.html].
6. Droits d'utilisation
Copyright © 2002 Sebastian Schreiber pour la version originale.
Copyright © 2003-2004 Vincent Chartier, Claude Thomassin et
Jean-Philippe Guérard pour la version française.
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
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Permission is granted to copy, distribute and/or modify this
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distribute translations of the Document under the terms of section
Section 5, « MODIFICATIONS ». 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.
If a section in the Document is Entitled "Acknowledgements",
"Dedications", or "History", the requirement (section Section 5,
« MODIFICATIONS ») to Preserve its Title (section Section 2,
« APPLICABILITY AND DEFINITIONS ») will typically require changing
the actual title.
10. TERMINATION
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.
11. FUTURE REVISIONS OF THIS LICENSE
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/.
Each version of the License is given a distinguishing version
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have the option of following the terms and conditions either of
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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.
12. ADDENDUM: How to use this License for your documents
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:
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".
If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:
with the Invariant Sections being LIST THEIR TITLES, with the
Front-Cover Texts being LIST, and with the Back-Cover Texts
being LIST.
If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.
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.
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