▁Linux ▁kernel ▁main line ▁in ▁kernel ▁version ▁ 3 . 1 3 . ▁ ▁Over view ▁X tables ▁allows ▁the ▁system ▁administrator ▁to ▁define ▁tables ▁containing ▁ch ains ▁of ▁rules ▁for ▁the ▁treatment ▁of ▁packets . ▁Each ▁table ▁is ▁associated ▁with ▁a ▁different ▁kind ▁of ▁packet ▁processing . ▁Pack ets ▁are ▁processed ▁by ▁sequ entially ▁travers ing ▁the ▁rules ▁in ▁ch ains . ▁A ▁rule ▁in ▁a ▁chain ▁can ▁cause ▁a ▁got o ▁or ▁jump ▁to ▁another ▁chain , ▁and ▁this ▁can ▁be ▁repeated ▁to ▁whatever ▁level ▁of ▁nest ing ▁is ▁desired . ▁( A ▁jump ▁is ▁like ▁a ▁“ call ”, ▁i . e . ▁the ▁point ▁that ▁was ▁jump ed ▁from ▁is ▁remembered .) ▁Every ▁network ▁packet ▁arriv ing ▁at ▁or ▁leaving ▁from ▁the ▁computer ▁travers es ▁at ▁least ▁one ▁chain . ▁ ▁The ▁origin ▁of ▁the ▁packet ▁determ ines ▁which ▁chain ▁it ▁travers es ▁initially . ▁There ▁are ▁five ▁pre defined ▁ch ains ▁( mapping ▁to ▁the ▁five ▁available ▁Net filter ▁hook s ), ▁though ▁a ▁table ▁may ▁not ▁have ▁all ▁ch ains . ▁Pre defined ▁ch ains ▁have ▁a ▁policy , ▁for ▁example ▁D ROP , ▁which ▁is ▁applied ▁to ▁the ▁packet ▁if ▁it ▁reaches ▁the ▁end ▁of ▁the ▁chain . ▁The ▁system ▁administrator ▁can ▁create ▁as ▁many ▁other ▁ch ains ▁as ▁desired . ▁These ▁ch ains ▁have ▁no ▁policy ; ▁if ▁a ▁packet ▁reaches ▁the ▁end ▁of ▁the ▁chain ▁it ▁is ▁returned ▁to ▁the ▁chain ▁which ▁called ▁it . ▁A ▁chain ▁may ▁be ▁empty . ▁▁ ▁PR ER