DNS & Load balancing(dior8wsh)

[Yiannis An. Mandravellos]

> Oson afora to round robin - douleyei, kai douleyei akribws etsi. Den kserw
> wstoso ti kanei o resolver. Kanonika stelnei sto application ola ta
records poy
> antistoixoyn sto sxetiko DNS entry - einai douleia ths efarmoghs na
dialeksei
> kapoio. Epeidh h efarmogh dialegei synh8ws to prwto kai petaei ta
ypoloipa, o
> resolver synh8ws ta anakateyei prin ta steilei sthn efarmogh, to idio
kanei kai
> o DNS server. Kapoio apo ayta ta kommatia de doulepse sth sygkekrimenh
> periptwsh, h einai aplws 8ema cacheing. Se megalh klimaka pantws douleyei.

To provlima (plin tis pithanotitas na min yposthrizei round-robin h version
toy BIND poy xrhsimopoieitai) den vrisketai sto DNS kai to configuration toy
alla sthn dokimh(!) kai, malista, se dyo shmeia:

1on) Oi dyo IP poy dilothikan einai oi 192.168.0.1 kai 192.168.1.1. An
ypothesoyme oti h dokimh ginetai apo to idio mhxanhma, to netmask tha prepei
na einai 255.255.0.0 gia na mporei na exei epikoinonia kai me tis dyo
dieythynseis. Ayto, veveos, afoy doyleyei kai to round-robin. Sthn periptosh
mas, omos, poy den doyleyei, epistrefetai panta h proth kai den emfanizetai
to provlima ayto...

2on) Giati den doyleyei to round-robin? Epeidh h dokimh ginetai apo ena
mhxanhma! Skefteite ligo thn diadikasia enos IP connection. Dinoyme thn
entolh ping frummel.virtual.gr. To ping rotaei ton DNS poia IP antistoixei
se ayto to domain. O DNS epistrefei thn IP poy antistoixei. To ping, loipon,
tha arxisei na stelnei paketa se ayth thn IP (192.168.0.1). Thn epomenh fora
poy tha kanoyme ping - vlepe "sleep 1" sto script toy filoy - o DNS tha
epistrepsei thn *cached* plhroforia (dhladh thn idia IP). Ki an akoma exoyn
perasei meres kai den yparxei sthn cache, tha epistrepsei pali mia IP (isos
thn allh, 192.168.1.1). To ping, omos, tha kanei ping se mia dieyhynsh (thn
192.168.1.1), oxi mia sthn mia kai mia sthn allh.

En katakleidi:
Gia na dokimasoyme to round-robin, setaroyme ton DNS, kai dokimazoyme to
ping apo *dyo diaforetika* mhxanhmata. To ena tha prepei na kanei ping sthn
mia dieythynsh kai to allo sthn allh... An den doylepsei etsi, tote
psaxnoyme gia lathi sto configuration.

Elpizo na voithisa,

Yiannis An. Mandravellos




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