Bridging mini-HOWTO

Christopher Cole

               <[1]cole@coledd.com>

Vertaald door: Ellen Bokhorst

   <[2]bokkie@nl.linux.org>

   Maart 2001
   Wijzigingen
   Herziening 1.22 20-05-2002 Herzien door: tab
   Naar DocBook 4.1 geconverteerd en GFDL per Christopher Cole toegevoegd
   Herziening 1.21 07-03-2001 Herzien door: cc

   In dit document wordt beschreven hoe een ethernet bridge op te zetten.
   Wat is een ethernet bridge? Een ethernet bridge is een device dat
   datapackets bestuurt binnen een subnet in een poging de hoeveelheid
   verkeer te beperken. Een bridge wordt gewoonlijk tussen twee aparte
   groepen computers geplaatst welke binnen die groep communiceren, maar
   niet zoveel met de computers in de andere groep. Een goed voorbeeld
   hiervan is te zien in een cluster met Macintosh machines en een
   cluster unix machines. Beiden groepen machines neigen nogal babbelziek
   onder elkaar te zijn, en het verkeer dat ze op het netwerk produceren,
   veroorzaakt aanvaringen met de andere machines die met elkaar proberen
   te communiceren. Tussen deze groepen computers zou een bridge worden
   geplaatst. De taak van de bridge bestaat uit het één voor één
   bestuderen van de bestemming van de datapakketjes en te besluiten of
   het de pakketjes wel of niet aan de andere kant van het
   ethernetsegment zal doorgeven. Het resultaat is een sneller, stiller
   netwerk met minder aanvaringen.
     _________________________________________________________________

   Inhoudsopgave
   [3]Setup
   [4]Algemene Problemen
   [5]Copyright

Setup

     * Haal Bridge Config op: [6]BRCFG.tgz
     * BRCFG is ook te vinden op: [7]http://coledd.com/networking/bridge
     * Activeer meerdere ethernet devices op je machine door deze regel
       toe te voegen aan het bestand /etc/lilo.conf, en lilo uit te
       voeren:

append = "ether=0,0,eth1"

       Als je drie interfaces op je bridge hebt, gebruik dan in plaats
       daarvan deze regel:

append = "ether=0,0,eth1 ether=0,0,eth2"

       Er kunnen meer interfaces worden gevonden door meer ether
       statements toe te voegen. Standaard detecteert een Linux kernel
       een enkele ethernetkaart, en zodra er één wordt gevonden, stopt de
       detectie. De bovenstaande append opdracht vertelt de kernel te
       blijven zoeken naar meer ethernet devices nadat de eerste werd
       gevonden. Als alternatief kan daarvoor in de plaats een
       bootparameter worden gebruikt:

linux ether=0,0,eth1

       Of, gebruik bij 3 interfaces:

linux ether=0,0,eth1 ether=0,0,eth2

     * Hercompileer de kernel met BRIDGING geactiveerd.
     * Een bridge heeft geen IP-adres nodig. Het kan wel, maar voor een
       gewone bridge hoeft dat niet. Ga (bij een RedHat systeem) naar
       /etc/sysconfig/network-scripts/ en kopieer ifcfg-lo0 naar
       ifcfg-eth0 & ifcfg-eth1 om het IP-adres uit je bridge te
       verwijderen. Wijzig in deze twee nieuwe bestanden de regel met
       daarin DEVICE=lo in DEVICE=eth0 en DEVICE=eth1. Aangezien andere
       distributies hiervan kunnen afwijken, moet je wellicht aanvullende
       documentatie raadplegen. Als er meer dan 2 interfaces naar deze
       bridge zijn, zorg dan dat je de corresponderende configuraties ook
       daarin maakt.
     * Reboot zodat je de nieuwe kernel met BRIDGING draait, en zorg er
       ook voor dat er geen IP-adressen zijn verbonden met de
       netwerkinterfaces.
     * Plaats de ethernetkaarten in promiscuous modus zodra een backup
       van het systeem is gemaakt, zodat ze elk pakket dat de interfaces
       passeert bekijken:

ifconfig eth0 promisc ; ifconfig eth1 promisc

       Alle interfaces die zijn verbonden met netwerksegmenten aan de
       bridge worden in promiscuous modus geplaatst.
     * Zet bridging aan met behulp van het programma brcfg:

brcfg -ena

     * Verifieer dat er ander verkeer op elke interface is:

tcpdump -i eth0     (in een venster)
tcpdump -i eth1     (in een ander venster)


     * Start een sniffer of tcpdump op een andere machine om te
       verifiëren dat de bridge het segment correct scheidt.
     _________________________________________________________________

Algemene Problemen

   Vraag: [8]Ik krijg de melding ioctl(SIOCGIFBR) failed: Package not
          installed . Wat betekent dit?

   Vraag: [9]Machines aan de ene kant kunnen de andere kant niet pingen!
   Vraag: [10]Ik kan niet telnetten/ftp'en vanaf de bridge! Waarom niet?
   Vraag: [11]Wat heb ik nodig om de wijze van routing op te zetten?
   Vraag: [12]De bridge schijnt te werken, maar waarom toont traceroute
          de bridge niet als onderdeel van het pad?

   Vraag: [13]Is het noodzakelijk IP_FORWARD in de kernel te compileren?
   Vraag: [14]Waarom zijn de fysieke ethernetadressen voor poort 1 en
          poort 2 hetzelfde volgens het programma brcfg? Zouden ze niet
          verschillend moeten zijn?

   Vraag: [15]Bridging verschijnt niet als optie wanneer een make config
          op de kernel uitvoert. Hoe activeert men dit?

   Vraag: [16]Te veel hubs (4 of meer) zijn na elkaar in serie geketend,
          wat op een ethernet voor timing problemen zorgt. Welk effect
          heeft een bridge in een subnet dat met hubs in lagen werkt?

   Vraag: [17]Kan een bridge interface naar zowel 10Mb als 100Mb
          ethernetsegmenten? Zal een dergelijke configuratie de rest van
          het verkeer op de hoge snelheidslijn vertragen? 

   Vraag: Ik krijg de melding ioctl(SIOCGIFBR) failed: Package not
   installed . Wat betekent dit?

   Antwoord: Je hebt de bridging mogelijkheid niet in je kernel. Haal een
   2.0 of nieuwere kernel op, en compileer het met de optie BRIDGING
   geactiveerd.

   Vraag: Machines aan de ene kant kunnen de andere kant niet pingen!

   Antwoord: 

     Heb je bridging met behulp van het programm brcfg -ena geactiveerd?
   (brcfg zou bridging is ENABLED aan moeten geven)

     Heb je de interfaces in promiscuous modus geplaatst? (roep de
   opdracht ifconfig aan. De vlag PROMISC zou aan moeten staan bij beide
   interfaces.)

     Als je multiple-media interface-adapters gebruikt, zorg dan dat de
   juiste is geactiveerd. Wellicht dat je het config/setup programma moet
   gebruiken dat met de netwerkinterfacekaart werd meegeleverd.

   Vraag: Ik kan niet telnetten/ftp'en vanaf de bridge! Waarom niet?

   Antwoord: Dit komt doordat er geen IP-adres is verbonden met een van
   de bridge interfaces. Een bridge dient een transparant deel van een
   netwerk te zijn.

   Vraag: Wat heb ik nodig om de wijze van routing op te zetten?

   Antwoord: Niets! Alle routing intelligentie wordt afgehandeld door de
   bridging code in de kernel. Gebruik het programma brcfg in debug modus
   om de ethernetadressen te zien zoals ze door de bridge worden geleerd:
brcfg -deb

   Vraag: De bridge schijnt te werken, maar waarom toont traceroute de
   bridge niet als onderdeel van het pad?

   Antwoord: Vanwege de aard van een bridge hoort traceroute de bridge
   niet als onderdeel van het pad te tonen. Een bridge dient als een
   transparante component van het netwerk.

   Vraag: Is het noodzakelijk IP_FORWARD in de kernel te compileren?

   Antwoord: Nee. De bridging code in de kernel zorgt voor het transport
   van pakketjes. IP_FORWARD is voor een gateway die IP-adressen aan zijn
   interfaces heeft verbonden.

   Vraag: Waarom zijn de fysieke ethernetadressen voor poort 1 en poort 2
   hetzelfde volgens het programma brcfg? Zouden ze niet verschillend
   moeten zijn?

   Antwoord: Nee. Aan elke poort van een bridge wordt door de bridging
   code expres hetzelfde fysieke ethernetadres toegekend.

   Vraag: Bridging verschijnt niet als optie wanneer een make config op
   de kernel uitvoert. Hoe activeert men dit?

   Antwoord: Antwoord tijdens de kernelconfig "Y" op de vraag Prompt for
   development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL)
   [Y/n/?].

   Vraag: Te veel hubs (4 of meer) zijn na elkaar in serie geketend, wat
   op een ethernet voor timing problemen zorgt. Welk effect heeft een
   bridge in een subnet dat met hubs in lagen werkt?

   Antwoord: Een bridge reset de 3/4/5 hubs rule. Een bridge gaat niet zo
   om met pakketjes zoals een hub dat doet, en draagt daarom niet bij aan
   timing problemen op een netwerk.

   Vraag: Kan een bridge interface naar zowel 10Mb als 100Mb
   ethernetsegmenten? Zal een dergelijke configuratie de rest van het
   verkeer op de hoge snelheidslijn vertragen?

   Antwoord: Je, een bridge kan een 10Mb segment aan een 100Mb segment
   verbinden. Zolang als de netwerkkaart op het snelle netwerk 100Mb
   capabel is, zorgt TCP voor de rest. Ondanks dat het klopt dat de
   pakketjes van een host in het 100Mb netwerk die communiceert met een
   host in het 10Mb netwerk slechts met een snelheid van 10Mb's worden
   verplaatst, wordt de rest van het verkeer op het snelle ethernet niet
   vertraagd.
     _________________________________________________________________

Copyright

   Copyright © 2002 Christopher Cole

   Permission is granted to copy, distribute and/or modify this document
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   Documentation License".
     _________________________________________________________________

GNU Free Documentation License

   Version 1.1, March 2000

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References

   1. mailto:cole@coledd.com
   2. mailto:bokkie@nl.linux.org
   3. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN29
   4. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN78
   5. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN155
   6. ftp://ftp.tux.org/people/alan-cox/BRCFG.tgz
   7. http://coledd.com/networking/bridge/
   8. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN82
   9. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN89
  10. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN105
  11. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN112
  12. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN119
  13. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN126
  14. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN133
  15. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN139
  16. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN146
  17. file://localhost/tmp/Bridge-NL/Bridge-NL.html#AEN151
  18. http://www.gnu.org/copyleft/