luan van loc thu dien tu theo giao thuc smtp

7/24/2019 Luan Van Loc Thu Dien Tu Theo Giao Thuc SMTP

1/23

HC VIN CNG NGH BU CHNH VIN THNG

---------------------------------------

inh Quang nh

NGHIN CU XY DNG H THNG

LC NI DUNG TH IN T

GI THEO GIAO THC SMTP

Chuyn ngnh: Truyn d liu v mng my tnh

M s: 60.48.15

TM TT LUN VN THC S

H NI- 2013


2/23

Lun vn c hon thnh ti:HC VIN CNG NGH BU CHNH VIN THNG

Ngi hng dn khoa hc: TS.Nguyn Thnh Phc

Phn bin 1:

Phn bin 2: ..

Lun vn s c bo v trc Hi ng chm lun vn thc s ti Hc vin Cng ngh Bu

chnh Vin thng

Vo lc: ....... gi ....... ngy ....... thng ....... .. nm ...............

C th tm hiu lunvn ti:

- Th vin ca Hc vin Cng ngh Bu chnh Vin thng

H NI- 2013


3/23

1

M US pht trin khng ngng ca khoa hc cng ngh, c bit l tc bng pht

mnh m ca cng ngh thng tin khin cho Internet tr thnh mt cng c hu ch

khng th thiu trong i sng hin nay. Khng ch l kho ti nguyn thng tin khng l,

Internet cn cung cp cho ngi s dng cc cng c khai thc thng tin tin li, nhanh

chng v hin i. Mt trong s cc dch v mng c ngi dng s dng rng ri nht l

dch v th in t (e-Mail). Dch v ny cho php ngi s dng trao i th tn mt cch

d dng, ng thi c th ph bin kin thc, thng tin, thng bo v ni dung chnh sch

ca mt vn trong mt c quan, t chc Vi tnh cht d s dng, khng ph thuc

vo khng gian v thi gian, th in t tr thnh mt phng tin trao i thng tin

quan trng i vi nhiu ngi.

Cng vi s pht trin tin li ca Internet, vic ly cp thng tin, thm nhp bt hp

php, ph hoi thng qua Internet cng gia tng v s lng, loi hnh v k thut. Bn cnh

, cc i tngc t tng chnh tr cc oan, cc th lc phn ng trong nc v

ngoi nc tn dng trit nhng kh nng ca mng Internet phc v cho mc ch

tuyn truyn, pht tn ti liu phn ng v thc hin cc hnh vi phn ng khc nhm

chng ph nh nc cng ha x hi ch ngha Vit Nam. Do , vic u tranh chng cc

hot ng s dng cc dch v trn mng phc v cho mc ch xu tr thnh mt yucu cp thit ca lc lng Cng an.

Trc nhu cu thc t , ti nghin cu v mnh dn chn ti: Nghin cu

xy dng h thng lc ni dung th in t gi theo giao thc SMTP lm bo co tt

nghip nhm xut mt gii php kim sot cc ni dung ca th in t gi i trn mng

Internet.

Mc tiu ca ti

Xy dng h thng lc nhm tm ra nhng th c gi trn mng Internet theo giao

thc SMTP c ni dung lin quan n cng tc nghip v Cng an cn quan tm; phc

v yu cu nghip v trong lc lng.

Phm vi nghin cu

Nhng th c ni dung lin quan n cc vn m cng tc Cng an quan tm (ni

dung ca bc th c gi c th hin th di dng text .doc, .docx, .pdf, .txt; th c ni

dung thuc cc a ch cn theo di v nhng cm t m cng tc nghip v cng an phiquan tm).


4/23

2

Gii hn ni dung nhng th c vit bng ngn ng ting Vit vi cc loi font

ch ting Vit hin c (TCVN3, Unicode, VNI, UTF8). ng thi, th c gi i trn

mng theo giao thc SMTP.

Phng php nghin cu

-Nghin cu mt th c gi theo giao thc SMTP phn tch ra cc phn trong

ni dung bc th nh: a ch gi, nhn, ch bc th, ni dung bc th, cc file vn bn

nh km Trn c s s dng phng php so snh a chui (multi matching) tm

kim cc cm t cn quan tm v i snh xy dng chnh sch lc theo yu cu nghip

v.

-Nghin cu cc font ch ting Vit v phng php chuyn i cc font ch, t

chuyn i cc font ch sang mt font tiu chun ri p dng phng php tm kim v sosnh a chui thc hin vic lc ni dung th.

Ngoi phn li m u, kt lun, ti liu tham kho v phlc, lun vn c chia

thnh 3 chng:

Chng 1: Tng quan v h thng th in t

Chng ny ni v cu trc ca mt th in t, mt h thng th tn in t v cc

giao thc c s dng cho dch v th in t. Ngoi ra, chng ny cng gii thiu giao

thc SMTP, i su vo nghin cu vic gi mt th in t qua giao thc SMTP, nghin

cu proxy cho SMTP.

Chng 2: Phng php v cc k thut s dng cho h thng lc ni dung th

in t

Chng ny trnh by v nhng thng tin cn kim sot i vi mt th in t;

mt s bin php kim sot ni dung th in t; cc k thut s dng phc v cho vic

lc th: k thut tm kim, so snh chui (multi matching), x l phng ting Vit(Unicode, TCVN3,VNI, UTF8)

Chng 3: Lc ni dung th in t gi theo giao thc SMTP

Chng ny xut phng php lc v m hnh, xy dng h thng lc ni dung

th in t gi theo giao thc SMTP.


5/23

3

SMTP (via Internet )

System Call

SMTP

System Call

Vit System Call

c/ Vit POP/IMAP

SMTP( Direct connection )

Chng1- TNG QUAN V H THNG TH IN T

1.1 L thuyt chung v th in t

H thng thin t cho php ngi dng trao i th in t vi nhau. H thng

ny bao gm mt hoc nhiu my ch th tn (mail server) , trn c ci t mt phnmm mail server qun l ti khon ca ngi dng, thc hin vic trao i th gia

nhng ngi dng v trao i th vi cc my ch th tn khc.

1.1.1Cu trc mt h thng th in t

thc hin vic trao i th vi ngi dng, gia my ch (mail server) v cc

my khch (mail client) thng nht s dng chung mt b giao thc gi v nhn th, trong

c quy nh c th v cng lm vic, quy trnh thao tc, cc cu lnh trao i, cu trc

ca th in t... Hnh 1.1 trnh by m hnh mt h thng th tn in t vi giao thc gi

thSMTP v giao thc nhn th POP hoc IMAP.

H thng ny bao gm bn phn t chnh: MUA (Mail User Agent), MTA (MailTransfer Agent), MDA (Mail Delivery Agent), MRA (Mail Retrieval Agent).

MUA

gi

MDAcc b

TCPMDA

MUA

nhn

MTA chuyn tip

MTA nhn

Hp th

MailRetreival

Agent

Hnh 1.1: M hnh mt h thng th tn in t


6/23

4

Mail User Agent (MUA):l chng trnh phn mm ca my client c ngi

dng s dng gi, nhn, son tho, x l th in t.

Mail Tranfer Agent (MTA): l mt chng trnh th ca my ch, cho php

truyn ti th in t t my ny sang my khc. Mail Delivery Agent (MDA):l chng trnh m MTA s dng chuynth

vo hp th ca ngi dng hoc truyn ti th ti mt MTA khc. Mi MTA s dng

mt hoc nhiu MDA, mi MDA c s dng cho mt loi yu cu phn pht th ring.

Mail Retrieval Agent (MRA): l mt chng trnh hoc mt dch v c chc

nng ly th in t v t mt hp th trn mt my ch xa v a chng ti mt MUA.

Cc MRA truy vn cc th v cc phn header t nhng hp th xa v phn pht chng

ti cc MUA trn my ca ngi dng.

1.1.2Cu trc mt th in t

Th in t thng c hai phnchnh: phn u(Header) v phn thn (Body) l vn

bn cha ni dung ca th. Khi gi i, ton b th in t c gi trong ni dung

(content). Ngoi ra, h thng th cn to thm mt phn na c gi l b th (envelope),

phn ny cha cc thng tin cn thit cho vic chuyn th n ni nhn. Hnh 1.2 trnh by

cu trc mt th in t.

Hnh 1.2: Cu trc ca mt th in t

B th

Tiu

Phnthn

Ni dung


7/23

5

1.1.3Mt sgiao thc s dng gi v nhn th in t

H thng Mail c xy dng da trn mt s giao thc: Simple Mail Transfer

Protocol (SMTP), Post Office Protocol (POP), Multipurpose Internet Mail Extensions

(MIME) vInteractive Mail Access Protocol(IMAP) c nh dng trong RFC 1176 lmt giao thc quan trng thay thPOP, n cung cp nhiu c ch tm kim vn bn,

phn tch messaget xa m ta khng tm thy trongPOP.

1.1.3.1 Mt sgiao thc s dng gi th in t

1.1.3.1.1 Giao thc SMTP (Simple Mail Transfer Protocol)

SMTP l giao thc tin cy, chu trch nhim phn pht th in t. N chuyn th

in t t h thng mng ny sang h thng mng khc, chuyn th trong h thng mng

ni b. Giao thc ny s c trnh by c th phn Xy dng proxy cho SMTP.

1.1.3.1.2 Giao thc X.400

X.400 l giao thc c ITU-T v ISO nh ngha v c ng dng rng ri

Chu u, Canada. X.400 cung cp tnh nng iu khin v phn phi e-Mail, s dng nh

dng nh phn, do khng cn m ha ni dung khi phn pht th trn mng Internet.

1.1.3.2 Mt sgiao thc s dng nhn th in t

C hai giao thc chnh thng c dng bi cc ng dng my th khch truy

cp th tn t cc my ch: Post Office Protocol (POP) v Internet Message Access Protocol

(IMAP).

1.1.3.2.1 Giao thcPOP

POP l giao thc c thit kt h tr tin trnh th offline, trong tin trnh ny

th c phn pht ti mt my ch. Mt my tnh c nhn ngi dng gi nh k mt

chng trnh th khch ckt ni ti my ch v ti tt c th treo ti my tnh cangi dng. Cch truy cp offline l mt loi dch v store-to-forward, c s dng

chuyn th (theo n t hng) t my ch th (v tr a v) ti my ca ngi c th,

thng l mt PC hoc Mac.

1.1.3.2.2 Giao thcIMAP (Internet Message Access Protocol)

IMAP l mt giao thc chun cho vic truy cp th in t t my ch th cc b.

N l mt giao thc ch/khch trong th in t c nhn v duy tr bi my ch th.

Vi nhng yu cu ny ch mt trao i d liu nh lm vic tt thm ch qua mt kt nichm nh l mt modem. Ch khi ngi dng yu cu c mt th in t c th th n s


8/23

6

SMTPCommands/Replies

And Mail

SMTP khch SMTP ch

c ti v tmy ch th . Ngi dngc th to v ch tc cc th mc hoc cc hp

th trn my ch, xa nhng tin nhn

1.2Giao thc SMTP

1.2.1

Gii thiu c bn v SMTP

SMTP (Simple Mail Transfer Protocol) l giao thc quy nh vic truyn th ch yu

trn Internet, c s dng nh mt c ch chung cho vic chuyn ti th in t gia cc

my tnh vi nhau trong giao thc TCP/IP. Khi mt tin trnh SMTP thc hin, SMTP client

m mt kt ni TCP ti mt tin trnh SMTP server nm trn mt my ch xa v c gng

gi mail thng qua kt ni. SMTP server lng nghe mt kt ni TCP trn mt cng 25.

Hnh 1.3: M hnh truyn th s dng giao thc SMTP

Khi SMTP client c mt thng ip ctruyn i, n thit lp mt knh truyn hai

chiu ti mt SMTP server. Trch nhim ca SMTP client l chuyn giao nhng thng ip

th cho mt hoc nhiu SMTP server (hoc bo nhng li sai khi thc hin).

Hot ng ca giao thcSMTP trong h thng th in t

Mi quan h gia SMTP v h thng th in tc trnh by trong hnh 1.4:

Hnh 1.4: Hot ng ca giao thc SMTP trong h thng th in t

Ngidn

SMTPkhch

SMTPch

Hthng

Hthng


9/23

7

Client lin quan n th i, Server lin quan nnhn th. H thng th cc b cha

hp th (mailbox) ca mi user. Mailbox c 2 phn: phn cc b v phn ton cc.

Sau khi tho bc th trong khun dng chun, h thng th cc b xc nh tn ngi

nhn hp th cc b hay phi gi ra ngoi. bc thc gi i,Client SMTP phi bit

a ch IP ca ni nhn thng qua DNS v gi qua cng a ch SMTP (25) bt u thit

lp kt ni server SMTP ni nhn. Khi mi ni c thit lp, Client bt u chuyn th

n Server bi cc lnh ca SMTP.

1.2.2M hnh giao thc SMTP

M hnh SMTP h tr c hai phng php truyn pht th end-to-end (khng c cc

MTA trung gian) v store-and-forward. Phng php end-to-end c s dng gia cc

mng ni b ca cc t chc v phng phpstore-and-forwardc la chncho cc h

iu hnh gia cc t chc c mng s dng giao thc TCP/IP v SMTP c s.

Mt tin trnh SMTP c bn c th truyn ti th in t ti mt tin trnh khc trn

cng mt mng hoc ti mt mng khc thng qua mt tin trnh truyn tip hoc qua cng

ni c th ti c c hai mng. Mt m hnh n gin cc thnh phn ca h thng SMTP

c trnh by trong hnh 1.5:

SMTP Commands,Replies and Mail

Kt ni giao thcTCP, cng 25

User

( ngi gi)

UA(User Agent)

UA(User Agent)

Queue of mailHng i th

UserMailboxes

MTA client(Mail Transfer Agent)

MTA Server(Mail Transfer Agent)

User(ngi nhn)

Hnh 1.5: M hnh giao thc SMTP


10/23

8

SMTP Commands,Replies and Mail

Thng qua Internet

1.2.3H thng chuyn tip th theo giao thc SMTP

Ngi dng lm vic vi UA (User Agent). Vic trao i th s dng giao thc TCP

c thc hin nh mt MTA. MTA gi truyn th qua mng ti cng 25 ca giao thc

TCP ca MTA nhn. Vic truyn thng tin gia my ch gi v my ch nhn mngngoi th vic chuyn tip c th phc tp (xem Hnh 1.6). Vic thm mt MTA vo pha

ngi gi v mt MTA vo pha ngi nhn, cc MTA khc thc hin nh my ch v my

khch, c th chuyn tip th in t qua mng.

H thng cc MTA relay cho php nhng ni khng s dng b giao thc TCP/IP

gi th in t ti nhng ngi dng nhng ni khc c th hoc khng th s dng b

giao thc TCP/IP.

UA(UserAgent)

LocalMTA

User(nginhn)

UA(User Agent)

Queue of mail

i thgi

Relay MTALocalMTA

LocalMTA

Queue ofmail

Relay MTA LocalMTA

LocalMTA

Queue ofmail

User

Mailboxes

User(nginhn)

LocalMTA

My ch gi My ch nhn

Hnh 1.6: M hnh SMTP vi cc MTA chuyn tip


11/23

9

S gi tr qu tnh phn pht th

Giao thc SMTP cho php gi tr s phn pht, v th in t c th c gi tr ti

v tr ngi gi, ch ngi nhn, hoc cc my ch trung gian.

1.3

Xy dngProxy cho SMTP

1.3.1Khi nim Proxy

Proxy l mt my ch trung gian cho php kt ni t my c nhn (client) ti cc

my ch chy web trn Internet (theo ngha trc tip), khng cho php cc packet (gi tin)

i trc tip gia h thng s dng v Internet m phi i gin tip thng qua cc dual home

host hoc qua s kt hp gia basion host (pho i phng ng) v screening router.

Hnh 1.7: M hnh proxy

Cc dch v Proxy khng cho php kt ni trc tip, chng buc tt c cc gi tin trn

mng phi c kim tra v lc theo quy tc thch hp. Thay v trao i thng tin trc tip

vi dch v thc s, ngi dng phi trao i thng tin vi Proxy server.

1.3.2Proxy tng ng dng

Proxy tng ng dng l mt my chtrung gian cho php kt ni t my trm khai

thc dch v (client) ti cc my ch cung cp dch v (server), khng cho php cc packet

(gi tin) i trc tip t client ti server m phi i gin tip thng qua cc proxy.

ProxyServer ging nh cu ni trung gian gia Client v Server. Khi c yu cu kt niClient n Server tht, ProxyServer s quyt nh p ng hay khng p ng.

Mt dch v proxy c ba b phn ring bit, l: proxy server, proxy client, v b

phn tch giao thc. Trong , proxy server lm nhim v chuyn cc yu cu c chp

nhn ca client ti server thc v chuyn p ng ca yu cu ny ti client thc.

Hnh 1.8 trnh by m hnh trao i thng tin gia Client v Server qua Proxy.


12/23

10

SMTPSMTP

POP,IMAP

POP,IMAP

SMTP

SMTP

Hnh 1.8: M hnh trao i thng tin gia Client v Server qua Proxy

1.3.3Xy dng proxy cho SMTP

SMTP proxy l proxy thuc lp ng dng s dng cho dch v th tn kim sot

th in t c gi i qua giao thc SMTP. N gm hai thnh phn thc hin nhng chc

nng cch ring bit: Proxy Server v Proxy Client. Hnh 1.9 trnh by mt m hnh proxy

cho giao thc SMTP.

Hnh 1.9: M hnh hot ng ca SMTP Proxy

Clientthc

Server ClientSMTPServerthc

SMTP Prox

Spool


13/23

11

Chng 2- PHNG PHP V CC K THUT S DNG CHO H THNGLC NI DUNG TH IN T

2.1 Nhng thng tin cn kim sot ca mt th in t

Da trn mc tiu ca h thng lc l kim sot cc ni dung ca mt th in t

c gi i, nhng thng tin cn kim sot l:

a ch ngi gi (trong trng from), ngi nhn (trong trngto);

Ch , tiu (trong trng Subject);

Cc a ch trong trng CC v Bcc: y l trng gm nhiu a ch ngi

nhn;

Ni dung ca bc th c vit trong phn bodyc ngi gi son tho;

Cc file nh km (c th c nn, m ho, hoc nh dng .doc,docx,txt,pdf.).

2.2 Mt s bin php kim sotni dung th in t

2.2.1Lc th rc

Th rc (spam)l th in t, tin nhn c gi n ngi nhn m ngi nhn

khng mong mun hoc khng c trch nhim phi tip nhn theo quy nh ca php lut.

Vn th rc l vn gy nhc nhi trong x hi trong nhng nm gn y.Nhiucng trnh nghin cu v phng php lc th rc c u t v tin hnh t kh lu.

nh gi hiu qu ca mt cng c lc th rc ngi ta thng da trn hai o sau:

False PositiveT l th thng b lc nhm thnh th rc.

False NegativeT l th rc b lc nhm thnh th thng.

Tt c nhng cng c lc c gi tr ngy nay thng s dng mt trong s nhng

phng php hoc kt hp ca cc phng php sau:

Phng php lc Bayesian

Phng php lc SpamAssassin

Phng php dng danh sch trng/en

Phng php lc th rc dng chui hi p (Challenge/Response filters)

Phng php lc da vo v tr ca cc b lc (Filter Placement)

Phng php lc da trn xc nhn danh tnh ca ngi gi


14/23

12

2.2.2Lc theo t kha

Phng php lc theo t kha l mt phng php truyn thng trong vic lc th.

Ngi ta da vo nhng t hay cm t c trong u ca th (subject) v ni dung ca

th lc.Phng php nyc u im l:- Tnh thch nghi:Ngi dng c th d dng bin i b lcv thit lp chnh sch

lc theo mc chca mnh n c th lcra nhng th c ni dung cn quan tm phc

v mc ch ca ngi dng.

- Tnh m rng: phng php ny cng c th s dng cho vic lc th rc. Khi mt

th mi c gi i, cc t hay cm t ny s xc nh l th rc hay khng.

Phng php ny c bit hiu qu vi mc ch kim sot ni dung thng tin, a

ch ca th nhm phc v mc ch ca ngi dng. V d mt s th in t c nhng ni

dung mang tnh cht phn ng nh sau: bn cht cp ca cng ng Vit Nam

Nhng ni dung ny cn c kim sot, ngn chn v tm ra ngun gc ngi pht tn,

cng nh c gi n ngi nhn no.

2.3 Mt s thutton s dng so snh chui(multi matching)

2.3.1 Khi nim so snh chui

So snh chui l qu trnh tm kim mt hoc nhiu v tr xut hin ca mu

(pattern), tm kim t kha trongvn bn. Phn ny s tm hiu v so snh ba gii thut c

bn trong so snh chuibao gm Nave, Knutt MorrisPratt v BoyerMoore. Kthut

ny c s dng lc ni dung theo phng php lc t kha.

2.3.2 Mt sthut ton so snh chui

2.3.2.1 Thut ton Nave

y l gii thut c bn v n gin nht, s dng nguyn l vt cn kim tra tt

c cc kh nng xut hin ca mt chui k t c trong vnbn.

Hnh 2.1: Minh ha thut ton Nave String Matcher


15/23

13

Thut ton Knutt Morris Pratt c trnh by di y c ci thin, tt n

nhiu so vi Nave v tn dng c cc thng tin hu ch khi tm kim.

2.3.2.2 Thut ton KnuthMorrisPratt

Thut ton Knuth-Morris-Pratt c xy dng da trn thut ton Nave vi tngli dng li nhng thng tin ca ln th trc cho ln sau.

Hnh 2.2: Minh ha hm tin t

(a)

So snh mu P = ababaca vi vn bn T, cho ra kt qu 5 k t u tinc khp ni thnh cng. (b)Vi nhng kt qu bit v vn bn v 5 k t c ghp

ni, c th thy dch chuyn s + 1 s khng hp l, nhng s = s + 2 c kh nng hp l.

(c) Thng tin hu ch ny s gip gim s ln th, v n c th c tnh ton bng cch so

snh chnh cc k t trong chui mu.

Mc d gim c s ln so snh trn mi k t, thut ton KMP vn c gng

kim tra mi k t mt ln. Do , thi gian x l cha c ci thin nhiu. iu quan

trng l phi gim bt s k t cn so khp. y cng chnh l tng ca thut ton

BoyerMoore c trnh by tip theo.

2.3.2.3 Thut ton BoyerMoore

Thut ton Boyer - Moore l thut ton tm kim chui rt c hiu qu trong thc

tin, hot ng theo quy tc kim tra cc k t ca mu t phi sang tri v khi pht hin s

khc nhau u tin, thut ton s tin hnh dch chuyn.


16/23

14

Bng cch loi b ti a s k t cn so snh, tc x l ca thut ton BM

c tng ln ng k. Tuy nhin, khi di ca vn bn ln v s lng vn bn nhiu th

qu trnh tm kim s rt tn km. Trn thc t, rt nhiu nghin cu xut thut ton mi

v ci tin thut ton c c tin hnh nhm gii quyt bi ton so snh a mu . Bn

cnh cc thut ton so snh a mu in hnh nh Aho Corasick, Wu Manber (WB)

c cho l thut ton ti u hn c trong qu trnh tm kim v so snh chui. Phn tip

theo s trnh by chi tit hn v thut ton ny.

2.3.2.4 Thutton WuManber

a) Gii thiu thut ton:

Thut ton WM c xy dng vi hai c ch li, l c ch lc da trn cng

ngh bm v c ch dch chuyn khi k t da trn cng ngh dch chuyn k t xu ca

thut ton Boyer Moore.

phc v vic tnh ton dch chuyn, giai on tin x l s xc nh kch

thc ca ca s i snh; thng thi thit lp ba bng d liu c bn gm bng SHIFT lu

trkhong cch dch chuyn ca cc khik txut hin trong vn bn; bng HASH cha

danh sch lin kt cc mu c chui hu t ging nhau bn trong cas i snh; bng

PREFIX chadanh sch lin kt cc mu c chuitin t ging nhaubn trong casisnh.

Qu trnh so khp mu c thc hin thng qua tnh ton gi tr bm, thit lp

danh sch nhng mu c chui hu t ging nhau, sau so snh vi gi tr bm ca khi

k t bn trong ca s i snh hin thi. Khi mt chui mu c so khp, ca s tip tc

c dch chuyn sang phi. Cng ngh dch chuyn k t xu c ng dng dch

chuyn ca s i snh, tuy nhin y s dch chuyn theo khi k t, thay v tng k t

n l nh trong Boyer Moore.

Nhm khc phc nh hng ca cc mu ngn, thut ton High Concurrence Wu

Manber (HCWM) c xut vi nhiu ci tin ng k. Bng cch tin hnh chia tt

cmu thnh nhiu b mu khc nhau ty theo dicachng; thut ton HCWM sau

ln lt x l cc b mu theo tng cch khc nhau.Bn cnh , cu trc d liu c lp

c s dng cho cc bmu khc nhau to ra stng tranh cao, c th ng thi x

l ccb mu, nh tng cng tc sosnh mu ca HCWM, lm cho thut ton ny

t hiu sut cao hn nhiu so vi WM.


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Server Client

SMTP PROXY

M unh thng lc ni

dung th

Chng 3- LC NI DUNG CHO TH IN T GI THEOGIAO THC SMTP c th kim sot c vic gi th in t qua giao thc SMTP, cn phi c mt

h thng lc ni dung th nm gia ng i ca my gi th v cc mail server. Chnh

sch lc th da trn vic kim tra cc phn envelope, contentv cc file nh km ca th

in t pht hin nhng th vi phm chnh sch bo m an ninh an ton thng tin.

3.1 M hnh h thng lc ni dung Th in t qua giao thc SMTP

M hnh mt h thng lc th in t c trnh by trong Hnh 3.2.

Hnh 3.1: M hnh h thng lcni dung th in t s dng giao thc SMTP

B lc th in t c gi i theo giao thc SMTP gm cc m un chnh sau: m

un Firewall, m un SMTP proxy, m un x l font ting Vit, m un lc v m unqun tr.

Server Client

M un x l font ting Vit

M un Qun Tr b lc

M unlc

Chnh schlc

CSDL

Lu gi thvi phm

E-mailClient Mail

Server

FIRE WALL


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3.1.1M unFirewall

M un nythc hinchc nng nh hng li gi tin ca th in t c gi t

my Client ti mt SMTP server theo cng 25. Cc gi tin c a ch cng ch TCP l 25

s c nh hng li i ti m un SMTPproxy server vi mt cng ch c xc

nh.

3.1.2M unSMTP proxy

Khi h thng cha c c ch lc ni dung, m un SMTP proxy c chc nng

nh mt Proxy bnh thng nhn th t pha my ngi gi ri chuyn tip th ti my

mail server truyn pht th ti a ch ch.

Khi h thng c c ch lc ni dung, th hot ng ca cc thnh phn ca Proxy

c s thay i. Cc thnh phn ny s ng vai tr nh ngi trung gian tip nhn cc yu

cu t my Client cng nh tip nhn cc yu cu t Mail server chuyn n m un x

l font ting Vit.

3.1.3M un x l font ting Vit

Khi nhn th t SMTPproxy server gi n, m un ny s phn tch phn envelope

vcontentca bc th.Mt th c ngi dng to ra v c gi i n c vit di

nhiu dng font ch khc nhau. Hin nay, c nhiu cch m ha cc k t ting Vit khc

nhau, dn ti c nhiu bng m khc nhau c s dng. Theo thng k, c ti trn 40 bng

m ting Vit khc nhau c s dng, do , vic khai thc ti liu cng nh x l d liu

rt phc tp.

Tuy nhin, hin nay vic s dng ting Vit trn my tnh vn cha c s thng nht

cao v chun m ting Vit, gy kh khn ln cho vic thu thp, khai thc v x l ting

Vit. i hi cc h thng x l vn bn ting Vit cn phi c bc tin x l nhn

dng v quy chun cc k t v mt bng m chung.M un x l font ting Vit c chc nng:

+ Chuyn cc t kha c m ha di nhiu dng font ch khc nhau v

mt mt dng quy chun cc k t v mt bng m chung c h thng thit lp t

trc, sau chuyn ti m un lc thc hin vic lc th.

+ Chuyn i m ca ni dung bc th v nh dng ban u ca ngi dng

son tho chuyn tip cho SMTP proxy client gi th i n a ch nhn.


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3.1.4M un lc

y l phn quan trng nht ca h thnglc ni dung th in t. Khi ni dung bc

th c m un x l font ting Vit thc hin, m un lcs s dng cc k thut tm

kim v so snh chui (multi matching) thc hini snh ccphna ch gi i, a

ch nhn th, v cc tkha caphn tiu bc thv trong ni dungvi cc tv cm

tkha cm c trong c s d liu ca b lc (t in lc) kim tra nhng ni dung

c trng khp hay khng? Nu mt trong cc thng tin i snh trng nhau th b lc s

thc hin vic ghi li nht k v thc hin chnh sch lc.

3.1.5M un Qun tr

Cho php ngi qun tr h thng iu khin b lc, thit lp cc chnh sch lc v

cc cng c lc nh: thit lp cc a ch cn lc, cc t kha cn lc, kt xut nht k viphm, lu gi th vi phm v.v...

3.2 Lu hot ng ca h thng lc th in t

3.2.1 Lu tip nhn v gi mt th khng vi phm

Lu tip nhn v gi mt th khng vi phm c biu din Hnh 3.2.

Khi SMTP proxy khi to kt ni, my mail client s gi yu cu kt ni ti SMTP

proxy server. Sau khi SMTP proxy server xc nhn v cho php mail client gi th, hthng s tin hnh x l font ting Vit. Tip theo, h thng lc th s kim tra th gi ic

vi phm chnh sch lc hay khng.Nu th c gi khng vi phm chnh sch lc, th

s c chuyn ngay ti Proxy SMTP client.


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Khng vi phm

Vi phm

Gi th Proxy SMTPServer

Proxy SMTPkhi to

E-mail Client

X l font tingVit

Kim tra th

vi phm?

Proxy Client

SMTP SERVER

Thc hin theoyu cu nghip

v

Hnh 3.2: Lu tip nhn v gi th in t khng vi phm

Ngc li, nu th vi phm chnh sch lc th th s c x l theo cc chnh

sch vi phm c trnh by phn 3.2.2.

3.2.2 Lu x l th vi phm

Hnh 3.3 th hin cc bc x l i vi mt th vi phm chnh sch an ninh an ton

thng tin.


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Vi phm

Th gi i

D liu cn i snh

Khn vi h m

Hnh 3.3: Lu x l th vi phm

Th c SMTP proxy tip nhn t mail client s c lu vo mt vng nh. Ti

y h thng lc sx l font ting Vit trc khily a ch v t kha cn ngn chnc

trong CSDL ca h thng lc i snh vi a ch v ni dung ca th (nh ni phn

trn). Khi c s trng khpth th vi phm chnh sch lc. H thng s ghi li nht k

vi phm, ng thi thay i ni dung th ri chuyn tip th ti Proxy client chuyn

th ti SMTP server.

CSDLCn i snh

Proxy SMTP

server

B nh

X l font ting Vit

Kim trath

vi hm?

SMTP SERVER

Ghi nht k

Client proxy

Thay i ni dungth


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3.2.3 Thit lpchnh sch lc

Chnh sch lc th in t c thit lp theo cc tiu ch: lc theo a ch v lc

theo t kha.

Lc theo a chbao gm:lc a ch ngi gi v a ch ngi nhn.

Lc theo a ch ngi gi.

Lc theo a ch nhn gm c cc trng: To: ;Cc: ;Bcc: . Nu a ch gi i

trng vi a ch c trong CSDL ca h thng lc cn chn th b lc s thay i trng a

ch ngi nhn bng trng a ch ngi gi ng thi thay ni dung bc th bng thng

bo Th vi phm an ninh an ton thng tin.

Lc theo t khac thc hinlc t tiu bc th (subject) v phn ni dung

ca bc th. Nhng t cn kim sot s c thit lp trong t in lc, h thng s tin

hnh so snh vi ni dung th, nu vi phm s b h thng ngn chn hoc gi thng bo

ngi qun tr c bin php x l kp thi.

3.3 Ci t h thng v hot ng ca phn mm lc th in t

3.3.1 Ci t Firewall

3.3.2 Ci t h thng lc th in t

ci t h thng lc th in t cn thao tc theo th t cc bc nh sau:

Bc 1: Chpb ci temailrelay.tar vo trong th mc/opt dnglnh/tarxvzf

emailrelay.tar gii nn b ci t vo trong th mcc la chn ci t

cho b lc mail.Sa file cu hnh ca b lc theo chnh sch lc v cng lc

c n nh. Trong :

Th mc/usr/local/emailrelay/etccha file cu hnh ca chng trnh.

Th mc/usr/local/emailrelay/libexec cha cc file th vin v file chy cachng trnh.

Th mc/usr/local/emailrelay/tmp cha cc file nht k.

Th mc/usr/local/emailrelay/var/spool lu gi th tm thi, ch h thng

chuyn i.

Bc 2: Kch hot h thng lc th bng vic nhlnh:

/usr/local/emailrelay/libexec/emailrelay/init/emailrelay start

Bc 3: Kim tra hot ng ca h thng:psef | grep emailrelay


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KT LUN

Lc ni dung th in t theo giao thc SMTPnhm mc ch kim sot ni dung

th in t c gi trn Internet l mt hng mi trong vic m bo an ninh an ton

thng tin. Hng tip cn ti c bit hiu qutrong vic thu thp thng tin, theo di itng phc v cho cc hot ng nghip v ca lc lng Cng an. cng nh gp phn

khng nh trong vic hn ch vic pht tn ti liu, tuyn truyn, kch ng v ni xu

ng, lnh o, v Nh nc. ti cng p ng c mt phnyu cu thc tin t ra

v vn kim sot ni dung thng tin trn Internet.

Kha lun h thng ha c mt s vn l thuyt v h thng th in t, gii

thiu mt s bin php kim sot ni dung th in t, mt s phng php c ng dng

phbin. Tuy nhin vi mc tiu ti t ra th vic s dng phng php lc t kha sdng k thut multi matching thu c cc kt qu thc s kh quan v p ng c

nhu cu rt cp thit hin nay i vi mt s vn cn quan tm.

Tuy nhin, do thi gian nghin cu c hnchng trnh lc mi ch thc hin c

cc chc nng lc c bn da theo cu trc ca mt bc th in t nh lc theo cc trng

a ch ngi nhn, ngi gi, tiu th v ni dung th theo t kha ting vit.

Vi cc kt qu t c, ti ang tin hnh th nghim ti phng th nghim

An ninh an ton thng tin, Cc Tin hc nghip v, B Cng an. Nu c iu kin, thi gian

v c s cho php, tc gimong mun c nghin cu pht trin hon thin cc chc

nng lc ca chng trnh nh lc ni dung cc tp nh km vi cc nh dng khc nhau

Trong thi gian ti s tin hnh th nghim tch hp vi cc m un lc khc nh:

lc th in t s dng giao thc khc nh: giao thc POP, IMAP; lc ni dung trang web,

kim tral hng an ninh trang web a h thng vo hot ng trong thc t.

luan van loc thu dien tu theo giao thuc smtp

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