On Wed, Nov 17, 2004 at 06:47:51AM +0900, Le Thanh Phong wrote:
>
>
> The^m 1 die^`u la. trong LAN nu+~a, tre^n ly' thuye^'t em du+o+.c ho.c
> thi` ca'c router cu~ng nhu+ ca'c PC mie^~n la` co' card ma.ng. Khi 1
> packet to+'i mi`nh, eth card se~ kie^?m tra di.a chi? di'ch de^'n cu?a
> packet thi` se~ forward no' du+.a theo routing table. Gia? su+? ca'c ma'y
> trong LAN de^`u co' 1 rule la` packet co' di.a chi? di'ch la 0.0.0.0 thi`
> se~ forward qua 10.2.0.1 ...
Sao ko gia su la` ca'c ma'y trong LAN ko co' rule do'? Thu te
thi` chuc nang ipforward default la` OFF. Trong truong hop ca'c ma'y
trong LAN co' ca'i rule ky` qua'i a^'y, nghi~a la` cu+' nha^.n
ddu+o+.c go'i co' di'ch la` 0.0.0.0 thi` send dde^'n gateway thi`
gateway sau khi nhan duoc qua'i go'i na`y cu~ng discard luon because
no machine on the world has that weird address. Da^'y la` gia? su+?
go'i dda~ ddu+o+.c send di trong mang LAN. Tre^n thuc te khi may tinh
send mo^.t ip packet se~ co' 2 tru+o+`ng ho+.p :
1. Di'ch la` mo^.t ma'y trong cu`ng subnet. Card ma.ng se~ du`ng ddia
chi? MAC cua? ma'y di'ch de send go'i truc tiep de^'n ma'y ddi'ch.
2. Di'ch la` mot ma'y kha'c subnet. Card ma.ng se send goi nay den dia
chi MAC cua default gateway. Gateway thay la dia chi ip dich cua goi
ko phai la cua minh be`n look up trong ip routing table roi forward
tiep ( day chinh la` ipforward) To'm lai trong qua' trinh forward go'i
ip thi` chi dest MAC address thay doi from hop to hop, dest IP address
does not.
Tro lai truong hop ba'c ba?o ma'y ba'c send den dia chi? 0.0.0.0 thi`
bi`nh thuong kernel se refuse to send such packet vi` no' chang biet
du`ng MAC address na`o tuong ung cho ddia chi do' ca?\. Truong hop
dda(.c bie^.t la` ba('c du`ng raw ip but that's a different story.
layer packet co' MAC ddi'ch cua gateway
> Trong tru+o+`ng ho+.p cu?a em thi` em da~
> co^' ti`nh thay vi` forward qua 10.2.0.1 thi` em forward qua 1 ma'y ba^'t
> ki` na`o do' tre^n LAN vo+'i gateway la` 0.0.0.0 ... The^' nhu+ng die^`u
> la. la` ca'c packet de^`u bi. discard o+? da^u do' ... chu+' no' kho^ng
> forward giu`m mi`nh sang 10.2.0.1 ... Mong ca'c ba'c gia?i thi'ch the^m
> cho 'con tre?' die^`u ngo^. na`y !!! (Ca'i chuye^.n forward package la`
> no' na(`m o+? lo+'p thu+' 3 cu?a OSI ... hoa(.c la` layer thu+' 2 cu?a
> TCP/IP the^' thi` no' da^u di'nh gi` de^' App Layer tre^n ca'c ma'y PC
> da^u tu+'c la` ne^'u ai do' xa`i ZoneAlarm hay set firewall thi` da^u
> a?nh hu+o+?ng gi`)
>
>
To'm la.i, khi mot may co' the ping cac may co`n lai trong lan ma`
lai ko ping duoc mot ma'y khac (proxy server) thi` to+' ca' 10 an 1
la` do dda(.t sai default gateway. Anyway, your solution for the
problem is rather interesting.
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