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30 w(W)12 b(I)7 b(N)i(D)2199 2200 y Fq(and)44 b Fj(C)6 b(LT)p 2564 2200 V 41 w(S)f(E)g(Q)50 b(>)h(S)5 b(V)18 b(R)p 3134 2200 V 31 w(AC)6 b(K)50 b Fq(the)45 b(pac)n(k)n(et)e(is)2199 2299 y(acceptable,)i(the)d(data)g (ma)n(y)f(b)r(e)h(stored)f(for)h(later)2199 2399 y(use)23 b(\(dep)r(ending)g(on)f(the)i(implemen)n(tation\))f(but)g(not)2199 2499 y(sen)n(t)49 b(to)g(the)g(user)f(since)h(the)g(b)r(eginning)g(of)g (the)2199 2598 y(stream)27 b(\(sequence)g(n)n(um)n(b)r(er)h Fj(S)5 b(V)18 b(R)p 3342 2598 V 31 w(AC)6 b(K)g Fq(\))28 b(is)g(miss-)2199 2698 y(ing.)2093 2853 y(2.)41 b(If)26 b Fj(C)6 b(LT)p 2467 2853 V 41 w(S)f(E)g(Q)22 b(>)h(S)5 b(V)19 b(R)p 2982 2853 V 30 w(AC)6 b(K)21 b Fq(+)14 b Fj(S)5 b(V)19 b(R)p 3496 2853 V 30 w(W)12 b(I)7 b(N)i(D)28 b Fq(or)2199 2953 y Fj(C)6 b(LT)p 2386 2953 V 41 w(S)f(E)g(Q)23 b(<)g(S)5 b(V)18 b(R)p 2901 2953 V 31 w(AC)6 b(K)34 b Fq(the)28 b(pac)n(k)n(et)f(is)h(not)g(ac-)2199 3052 y(ceptable)e(and)g (will)g(b)r(e)h(dropp)r(ed.)36 b(The)26 b(data)f(is)h(lost.)2116 3208 y(In)g(b)r(oth)g(case)f(data)g(exc)n(hange)g(is)g(not)h(p)r (ossible)f(ev)n(en)h(if)1992 3307 y(the)h(state)h(exists.)1992 3535 y Fn(2.3)112 b(The)38 b(attac)m(k)1992 3688 y Fq(The)21 b(prop)r(osed)f(attac)n(k)g(consists)g(of)h(creating)f(a)h(desync)n (hro-)1992 3788 y(nized)34 b(state)f(on)h(b)r(oth)g(ends)g(of)g(the)g (TCP)f(connection)h(so)1992 3888 y(that)23 b(the)g(t)n(w)n(o)f(p)r(oin) n(ts)g(cannot)h(exc)n(hange)e(data)h(an)n(y)g(longer.)1992 3987 y(A)30 b(third)g(part)n(y)f(host)h(is)g(then)h(used)f(to)g(create) f(acceptable)1992 4087 y(pac)n(k)n(ets)c(for)g(b)r(oth)i(ends)f(whic)n (h)h(mimics)f(the)h(real)e(pac)n(k)n(ets.)2116 4186 y(Assume)32 b(that)h(the)g(TCP)f(session)f(is)h(in)h(a)f(desync)n(hro-)1992 4286 y(nized)27 b(state)h(and)f(that)h(the)g(clien)n(t)g(sends)f(a)g (pac)n(k)n(et)g(with)2429 4442 y Fj(S)5 b(E)g(G)p 2621 4442 V 30 w(S)g(E)g(Q)82 b Fq(=)h Fj(C)6 b(LT)p 3250 4442 V 40 w(S)f(E)g(Q)2412 4566 y(S)g(E)g(G)p 2604 4566 V 30 w(AC)h(K)89 b Fq(=)83 b Fj(C)6 b(LT)p 3250 4566 V 40 w(AC)g(K)1992 4722 y Fq(Since)42 b Fj(C)6 b(LT)p 2410 4722 V 41 w(S)f(E)g(Q)48 b Fg(6)p Fq(=)f Fj(S)5 b(V)19 b(R)p 2975 4722 V 30 w(AC)6 b(K)49 b Fq(the)43 b(data)f(will)h(not)1992 4821 y(b)r(e)37 b(accepted)g(and)g(the)h(pac)n (k)n(et)e(is)i(dropp)r(ed.)66 b(The)37 b(third)1992 4921 y(part)n(y)g(then)j(sends)e(the)h(same)f(pac)n(k)n(et)g(but)h(c)n (hanges)f(the)1992 5020 y Fj(S)5 b(E)g(G)p 2184 5020 V 29 w(S)g(E)g(Q)22 b Fq(and)g Fj(S)5 b(E)g(G)p 2766 5020 V 30 w(AC)h(K)28 b Fq(\(and)22 b(the)g(c)n(hec)n(ksum\))f(suc)n(h) 1992 5120 y(that)2410 5275 y Fj(S)5 b(E)g(G)p 2602 5275 V 30 w(S)g(E)g(Q)83 b Fq(=)f Fj(S)5 b(V)19 b(R)p 3237 5275 V 30 w(AC)6 b(K)q(;)2394 5400 y(S)f(E)g(G)p 2586 5400 V 30 w(AC)h(K)89 b Fq(=)82 b Fj(S)5 b(V)19 b(R)p 3237 5400 V 30 w(S)5 b(E)g(Q)p eop 4 3 bop 615 86 2671 4 v 613 178 4 92 v 935 150 a Ft(Serv)n(er)1541 b(Clien)n(t)p 3284 178 V 613 269 V 3284 269 V 613 360 V 916 333 a Fs(LISTEN)1483 b(CLOSED)p 3284 360 V 613 560 4 200 v 1552 479 a Fh( )1650 364 y Ff(\032)1754 433 y Fi(S)t(Y)16 b(N)t(;)1754 524 y(C)5 b(LT)p 1926 524 24 4 v 38 w(S)t(E)t(Q)2122 532 y Fm(0)p 3284 560 4 200 v 613 651 4 92 v 916 623 a Fs(LISTEN)1446 b(SYN-SENT)p 3284 651 V 613 925 4 274 v 1607 715 a Fi(S)t(Y)17 b(N)t(;)1607 806 y(S)t(V)g(R)p 1783 806 24 4 v 28 w(S)t(E)t(Q)1979 814 y Fm(0)2013 806 y Fi(;)1607 897 y(C)5 b(LT)p 1779 897 V 38 w(S)t(E)t(Q)1975 905 y Fm(0)2027 897 y Fs(+)16 b(1)2183 667 y Ff(\))2272 807 y Fh(!)p 3284 925 4 274 v 613 1016 4 92 v 760 989 a Fs(SYN-RECEIVED)1212 b(EST)-6 b(ABLISHED)p 3284 1016 V 613 1381 4 366 v 2489 1080 a Fi(S)t(V)17 b(R)p 2665 1080 24 4 v 29 w(S)t(E)t(Q)k Fs(=)2655 1171 y Fi(C)5 b(LT)p 2828 1171 V 39 w(S)t(E)t(Q)3024 1179 y Fm(0)3075 1171 y Fs(+)17 b(1)2489 1263 y Fi(C)5 b(LT)p 2662 1263 V 39 w(AC)g(K)26 b Fs(=)2655 1354 y Fi(S)t(V)18 b(R)p 2832 1354 V 28 w(S)t(E)t(Q)3028 1362 y Fm(0)3079 1354 y Fs(+)f(1)p 3284 1381 4 366 v 613 1655 4 274 v 1552 1537 a Fh( )1650 1397 y Ff(\()1758 1445 y Fi(AC)5 b(K)q(;)1758 1537 y(C)g(LT)p 1931 1537 24 4 v 39 w(S)t(E)t(Q)2127 1545 y Fm(0)2178 1537 y Fs(+)17 b(1)1758 1628 y Fi(S)t(V)g(R)p 1934 1628 V 28 w(S)t(E)t(Q)2130 1636 y Fm(0)2182 1628 y Fs(+)f(1)p 3284 1655 4 274 v 613 1747 4 92 v 787 1719 a(EST)-6 b(ABLISHED)p 3284 1747 V 613 2112 4 366 v 706 1811 a Fi(S)t(V)17 b(R)p 882 1811 24 4 v 28 w(S)t(E)t(Q)k Fs(=)872 1902 y Fi(S)t(V)c(R)p 1048 1902 V 28 w(S)t(E)t(Q)1244 1910 y Fm(0)1295 1902 y Fs(+)g(1)706 1993 y Fi(S)t(V)g(R)p 882 1993 V 28 w(AC)5 b(K)27 b Fs(=)872 2085 y Fi(C)5 b(LT)p 1044 2085 V 38 w(S)t(E)t(Q)1240 2093 y Fm(0)1292 2085 y Fs(+)16 b(1)p 3284 2112 4 366 v 615 2115 2671 4 v 1165 2264 a Fq(Figure)27 b(1:)36 b(Example)27 b(of)g(a)h(connection)f(op)r(ening)0 2602 y(whic)n(h)33 b(is)g(acceptable)f(b)n(y)h(the)g(serv)n(er.)51 b(The)33 b(data)g(is)g(pro-)0 2701 y(cessed)27 b(b)n(y)g(the)h(serv)n (er.)125 2794 y(If)166 b Fj(C)6 b(LT)p 532 2794 25 4 v 40 w(T)12 b(O)p 688 2794 V 32 w(S)5 b(V)18 b(R)p 904 2794 V 31 w(O)r(F)12 b(F)g(S)5 b(E)g(T)177 b Fq(refers)165 b(to)0 2885 y Fj(S)5 b(V)19 b(R)p 192 2885 V 30 w(AC)6 b(K)261 b Fg(\000)254 b Fj(C)6 b(LT)p 1181 2885 V 41 w(S)f(E)g(Q)381 b Fq(and)0 2977 y Fj(S)5 b(V)19 b(R)p 192 2977 V 30 w(T)12 b(O)p 347 2977 V 32 w(C)6 b(LT)p 559 2977 V 40 w(O)r(F)12 b(F)g(S)5 b(E)g(T)65 b Fq(refers)53 b(to)g Fj(C)6 b(LT)p 1579 2977 V 41 w(AC)g(K)42 b Fg(\000)0 3068 y Fj(S)5 b(V)19 b(R)p 192 3068 V 30 w(S)5 b(E)g(Q)18 b Fq(then)h(the)g(\014rst)g(part)n(y)e(attac)n(k)n(er)g(has)h(to)g (rewrite)0 3159 y(the)28 b(TCP)f(pac)n(k)n(et)g(from)g(the)h(clien)n(t) g(to)f(the)h(serv)n(er)e(as:)32 3325 y Fi(S)t(E)t(G)p 209 3325 24 4 v 28 w(S)t(E)t(Q)21 b Fh( )g Fi(S)t(E)t(G)p 701 3325 V 28 w(S)t(E)t(Q)16 b Fs(+)h Fi(C)5 b(LT)p 1163 3325 V 39 w(T)11 b(O)p 1307 3325 V 29 w(S)t(V)17 b(R)p 1505 3325 V 29 w(O)r(F)11 b(F)g(S)t(E)t(T)17 3492 y(S)t(E)t(G)p 194 3492 V 28 w(AC)5 b(K)26 b Fh( )21 b Fi(S)t(E)t(G)p 701 3492 V 28 w(AC)5 b(K)22 b Fh(\000)17 b Fi(S)t(V)g(R)p 1182 3492 V 28 w(T)11 b(O)p 1326 3492 V 30 w(C)5 b(LT)p 1521 3492 V 38 w(O)r(F)11 b(F)g(S)t(E)t(T)50 3628 y Fq(Considering)48 b(that)i(the)f(attac)n(k)n(er)f(can)h(listen)h(to)f(an)n(y)0 3727 y(pac)n(k)n(et)40 b(exc)n(hanged)f(b)r(et)n(w)n(een)h(the)h(t)n(w) n(o)f(p)r(oin)n(ts)h(and)f(can)0 3827 y(forge)f(an)n(y)h(kind)h(of)g (IP)f(pac)n(k)n(et)f(\(therefore)h(masquerad-)0 3926 y(ing)j(as)f(either)h(the)h(clien)n(t)f(or)f(the)i(serv)n(er\))e(then)h (ev)n(ery-)0 4026 y(thing)33 b(acts)f(as)f(if)i(the)g(connection)f(go)r (es)g(through)f(the)i(at-)0 4126 y(tac)n(k)n(er)39 b(mac)n(hine.)77 b(This)41 b(one)g(can)f(add)h(or)f(remo)n(v)n(e)f(an)n(y)0 4225 y(data)48 b(to)g(the)g(stream.)98 b(F)-7 b(or)48 b(instance)g(if)h(the)f(connec-)0 4325 y(tion)35 b(is)f(a)h(remote)f (login)g(using)g(telnet)i(the)f(attac)n(k)n(er)e(can)0 4425 y(include)41 b(an)n(y)f(command)g(on)g(b)r(ehalf)h(of)f(the)h (user)f(\()p Fe(echo)0 4524 y(merit.edu)g(lpj)i(>&)h(~)14 b(/.rhosts)h Fq(is)k(an)f(example)g(of)h(suc)n(h)0 4624 y(a)43 b(command\))h(and)f(\014lter)g(out)h(an)n(y)f(un)n(w)n(an)n(ted) g(ec)n(ho)f(so)0 4723 y(that)34 b(the)h(user)e(will)h(not)g(b)r(e)g(a)n (w)n(are)e(of)i(the)g(in)n(truder.)56 b(Of)0 4823 y(course)46 b(in)h(this)h(case)e Fj(C)6 b(LT)p 954 4823 25 4 v 41 w(T)12 b(O)p 1110 4823 V 31 w(S)5 b(V)19 b(R)p 1326 4823 V 30 w(O)r(F)12 b(F)g(S)5 b(E)g(T)59 b Fq(and)0 4923 y Fj(S)5 b(V)19 b(R)p 192 4923 V 30 w(T)12 b(O)p 347 4923 V 32 w(C)6 b(LT)p 559 4923 V 40 w(O)r(F)12 b(F)g(S)5 b(E)g(T)40 b Fq(ha)n(v)n(e)28 b(to)g(c)n(hange.)39 b(The)29 b(new)0 5022 y(v)-5 b(alues)27 b(are)g(let)h(as)f(an)g(exercise)f(for)h (the)h(reader.)1551 4992 y Fp(3)1992 2602 y Fn(2.4)112 b(\\TCP)37 b(Ac)m(k)f(storm")1992 2934 y Fq(A)43 b(\015a)n(w)g(of)g (the)h(attac)n(k)e(is)h(the)h(generation)d(of)j(a)f Fd(lot)g Fq(of)1992 3033 y(TCP)d(A)n(CK)h(pac)n(k)n(ets.)77 b(When)42 b(receiving)e(an)h(unaccept-)1992 3133 y(able)30 b(pac)n(k)n(et)g(the)h (host)f(ac)n(kno)n(wledges)f(it)i(b)n(y)f(sending)h(the)1992 3233 y(exp)r(ected)41 b(sequence)g(n)n(um)n(b)r(er)g(\(As)h(the)g(Ac)n (knolegemen)n(t)1992 3332 y(n)n(um)n(b)r(er.)93 b(C.f.)g(in)n(tro)r (duction)47 b(ab)r(out)f(TCP\))g(and)g(us-)1992 3432 y(ing)36 b(its)i(o)n(wn)e(sequence)h(n)n(um)n(b)r(er.)65 b(This)37 b(pac)n(k)n(et)g(is)g(itself)1992 3532 y(unacceptable)29 b(and)h(will)g(generate)f(an)h(ac)n(kno)n(wledgemen)n(t)1992 3631 y(pac)n(k)n(et)j(whic)n(h)h(in)h(turn)f(will)h(generate)e(an)h(ac) n(kno)n(wledge-)1992 3731 y(men)n(t)24 b(pac)n(k)n(et)e(etc...)36 b(creating)23 b(a)g(supp)r(osedly)h(endless)f(lo)r(op)1992 3830 y(for)k(ev)n(ery)f(data)h(pac)n(k)n(et)g(sen)n(t.)2116 4026 y(Since)40 b(these)g(pac)n(k)n(ets)e(do)h(not)h(carry)e(data)h (they)h(are)1992 4126 y(not)34 b(retransmitted)g(if)h(the)g(pac)n(k)n (et)f(is)g(lost.)58 b(This)35 b(means)1992 4225 y(that)25 b(if)h(one)f(of)h(the)g(pac)n(k)n(ets)e(in)i(the)g(lo)r(op)f(is)g (dropp)r(ed)h(then)1992 4325 y(the)h(lo)r(op)g(ends.)36 b(F)-7 b(ortunately)27 b(\(or)g(unfortunately?\))37 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(can)f(stop)h(the)208 4474 y(RST)20 b(generation)f(b)n(y)h(either)g(p)r (erforming)f(the)h(attac)n(k)208 4573 y(when)33 b(the)i(clien)n(t)e(is) h(do)n(wn)f(or)g(b)n(y)g(o)n(v)n(er\015o)n(wing)e(the)208 4673 y(clien)n(t's)40 b(TCP)g(queue)h(so)e(the)i(SYN/A)n(CK)g(pac)n(k)n (et)208 4772 y(will)27 b(b)r(e)h(lost.)125 4937 y Fg(\017)41 b Fq(The)55 b(attac)n(k)n(er)e(cannot)h(receiv)n(e)g(data)h(from)f(the) 208 5037 y(serv)n(er.)62 b(But)37 b(he)g(can)g(send)g(data)f(whic)n(h)h (is)f(some-)208 5137 y(time)28 b(enough)f(to)g(compromise)f(a)i(host.) 125 5300 y(The)22 b(are)f(four)h(principal)g(di\013erences)h(b)r(et)n (w)n(een)f(Morris')0 5400 y(attac)n(k)27 b(and)g(the)h(presen)n(t)f (one:)2116 882 y Fg(\017)41 b Fq(Morris's)18 b(relies)h(on)g(the)h Fd(truste)l(d)h(hosts)f Fq(iden)n(ti\014cation)2199 982 y(sc)n(heme)j(whereas)g(the)h(presen)n(t)f(attac)n(k)g(lets)h(the)g (user)2199 1082 y(conduct)31 b(the)h(iden)n(ti\014cation)f(stage)f(of)h (the)h(connec-)2199 1181 y(tion.)2116 1361 y Fg(\017)41 b Fq(The)21 b(presen)n(t)g(attac)n(k)f(is)h(a)f(full)i(duplex)f(TCP)g (stream.)2199 1461 y(The)28 b(attac)n(k)n(er)e(can)h(send)g(and)h (receiv)n(e)e(data.)2116 1640 y Fg(\017)41 b Fq(The)119 b(presen)n(t)f(attac)n(k)g(uses)g(the)i(ether-)2199 1740 y(net)109 b(sni\013er)g(to)f(predict)h(\(or)f(just)h(get\))2199 1839 y Fj(S)5 b(V)19 b(R)p 2391 1839 V 30 w(S)5 b(E)g(Q)2603 1851 y Fp(0)2640 1839 y Fq(.)2116 2019 y Fg(\017)41 b Fq(The)19 b(presen)n(t)f(attac)n(k)g(can)h(b)r(e)g(used)g(against)f(an) n(y)g(kind)2199 2119 y(of)28 b(host)f(b)r(esides)h(Unix)f(hosts.)1992 2295 y(Morris')22 b(attac)n(k)i(can)f(easily)h(b)r(e)g(exten)n(ted)h (in)f(regard)f(of)h(the)1992 2395 y(presen)n(t)i(attac)n(k:)2116 2571 y Fg(\017)41 b Fq(The)27 b(sni\013er)g(is)f(used)h(to)g(get)g(the) g(serv)n(er's)e(initial)i(se-)2199 2670 y(quence)19 b(n)n(um)n(b)r(er.) 34 b(Morris')19 b(attac)n(k)f(can)h(then)i(b)r(e)e(p)r(er-)2199 2770 y(formed)26 b(against)f(the)i(serv)n(er.)35 b(The)26 b(attac)n(k)n(er)f(do)h(not)2199 2870 y(need)i(to)f(w)n(ait)g(for)h(a)f (clien)n(t)g(to)h(connect.)2116 3049 y Fg(\017)41 b Fq(Considering)32 b(that)h(the)g(clien)n(t)f(will)h(not)g(send)g(RST)2199 3149 y(pac)n(k)n(ets)j(\(for)i(example)f(it)g(is)h(do)n(wn\))f(the)h (attac)n(k)n(er)2199 3249 y(can)19 b(establish)g(a)g(full)h(duplex)f (TCP)g(connection)g(with)2199 3348 y(the)35 b(serv)n(er.)56 b(It)34 b(can)h(send)f(data)g(and)g(receiv)n(e)g(data)2199 3448 y(on)h(b)r(ehalf)g(of)f(the)i(clien)n(t.)58 b(Of)35 b(course)e(the)j(crac)n(k)n(er)2199 3547 y(still)k(has)e(to)h(pass)g (the)h(iden)n(ti\014cation)f(barrier.)70 b(If)2199 3647 y(the)24 b(iden)n(ti\014cation)f(is)h(based)f(on)g(trusted)g(hosts)g (\(lik)n(e)2199 3747 y Fd(NFS)g Fq(or)g Fd(rlo)l(gin)p Fq(\))h(the)g(crac)n(k)n(er)d(has)h(full)i(access)e(to)i(the)2199 3846 y(host's)j(services.)2116 4023 y(Stev)n(en)37 b(M.)h(Bello)n(vin)f (in)h([Bello)n(vin89)n(])g(also)e(presen)n(ts)1992 4122 y(ho)n(w)e(ICMP)h(pac)n(k)n(ets)f(can)g(b)r(e)i(used)f(to)g(disable)f (one)h(side)1992 4222 y(of)30 b(the)g(connection.)45 b(In)30 b(this)h(case)e(the)i(attac)n(k)n(er)e(gets)g(full)1992 4321 y(con)n(trol)g(of)i(the)g(session)f(\(p)r(eople)h(ha)n(v)n(e)e (referred)h(to)h('TCP)1992 4421 y(session)d(hijac)n(king'\),)h(but)g (this)h(is)f(to)r(o)g(easily)f(detected)h(b)n(y)1992 4521 y(the)e(user.)1992 4813 y Fr(8)134 b(Conclusion)1992 5001 y Fq(Although)36 b(easy)f(to)i(detect)f(when)h(used)f(on)g(a)g(lo) r(cal)g(net-)1992 5101 y(w)n(ork,)d(the)g(attac)n(k)f(presen)n(ted)h (here)g(is)g(quite)g(e\016cien)n(t)g(on)1992 5201 y(long)k(distance,)42 b(lo)n(w)37 b(bandwidth,)42 b(high)d(dela)n(y)f(net)n(w)n(orks)1992 5300 y(\(usually)31 b(W)-9 b(AN\).)33 b(It)f(can)g(b)r(e)g(carried)f (with)h(the)h(same)e(re-)1992 5400 y(sources)38 b(as)h(for)h(a)g (passiv)n(e)e(sni\016ng)i(attac)n(k)f(whic)n(h)h(ha)n(v)n(e)p eop 13 12 bop 0 83 a Fq(o)r(ccurred)32 b(so)h(frequen)n(tly)f(on)h(the)h (In)n(ternet.)53 b(This)33 b(attac)n(k)0 183 y(has)21 b(also)f(the)i(dangerous)e(adv)-5 b(an)n(tage)20 b(of)h(b)r(eing)h(in)n (visible)f(to)0 282 y(the)g(user.)34 b(While)22 b(crac)n(king)d(in)n (to)h(a)h(host)f(on)h(the)g(In)n(ternet)g(is)0 382 y(b)r(ecoming)j (more)f(and)i(more)e(frequen)n(t,)i(the)g(stealthfulness)0 482 y(of)30 b(the)g(attac)n(k)f(is)h(no)n(w)f(a)g(v)n(ery)g(imp)r (ortan)n(t)h(parameter)e(for)0 581 y(the)h(success)g(of)g(the)g(attac)n (k)f(and)h(mak)n(es)f(it)i(more)e(di\016cult)0 681 y(to)f(detect.)125 780 y(When)f(ev)n(eryb)r(o)r(dy's)f(atten)n(tion)h(in)h(the)g(In)n (ternet)f(is)g(fo-)0 880 y(cused)k(on)f(the)h(emerging)f(new)h(IPv6)e (proto)r(col)h(to)h(replace)0 980 y(the)c(curren)n(t)e(IPv4,)h (increasing)f(attac)n(ks)g(and)h(the)h(need)f(for)0 1079 y(secure)31 b(systems)g(press)g(us)h(to)g(dev)n(elop)f(and)g(use)h(a)f (secure)0 1179 y(transp)r(ort)d(la)n(y)n(er)f(for)h(the)h(In)n(ternet)f (comm)n(unit)n(y)-7 b(.)40 b(Options)0 1279 y(should)24 b(b)r(e)i(a)n(v)-5 b(ailable)23 b(to)i(send)f(signed)g(and)h(ev)n(en)n (tually)f(en-)0 1378 y(crypted)e(data)g(to)h(pro)n(vide)e(priv)-5 b(acy)e(.)35 b(And)23 b(since)f(the)h(signa-)0 1478 y(ture)30 b(of)h(the)g(data)f(implies)g(reliabilit)n(y)g(the)h(signature)e(can)0 1577 y(b)r(e)f(substituted)g(to)g(the)g(curren)n(t)f(TCP)g(c)n(hec)n (ksum.)125 1677 y(This)j(pap)r(er)g(do)r(es)g(not)g(attempt)h(to)g (explain)f(all)g(cases)0 1777 y(of)j(activ)n(e)f(attac)n(ks)g(using)h (a)g(sni\013er.)53 b(It)33 b(is)g(more)f(a)h(w)n(arn-)0 1876 y(ing)38 b(for)f(p)r(eople)h(using)g(s/k)n(ey)f(or)g(Kerb)r(eros)f (against)h(the)0 1976 y(danger)30 b(of)g(someone)g(sni\016ng)h(the)g (ethernet.)47 b(It)31 b(pro)n(vides)0 2076 y(a)36 b(few)g(ideas)g(and)g (starting)g(p)r(oin)n(ts)g(whic)n(h)g(can)g(b)r(e)g(more)0 2175 y(deeply)26 b(studied.)37 b(The)27 b(metho)r(d)g(presen)n(ted)e (has)h(b)r(een)h(suc-)0 2275 y(cessfully)c(used)f(during)h(our)f(test)h (ev)n(en)f(with)h(a)g(v)n(ery)e(simple)0 2374 y(attac)n(k)n(er's)26 b(soft)n(w)n(are.)0 2642 y Fr(References)0 2824 y Fq([Bello)n(vin89])40 b Fd("Se)l(curity)26 b(Pr)l(oblems)i(in)f(the)g(TCP/IP)i(Pr)l(o-)88 2923 y(to)l(c)l(ol)36 b(Suite")p Fq(,)e(Bello)n(vin,)h(S.,)h(Computer)e (Comm)n(unica-)88 3023 y(tions)27 b(Review,)h(April)f(1989.)1992 83 y([Kerb)r(eros])39 b Fd("Kerb)l(er)l(os:)e(A)n(n)26 b(A)n(uthentic)l(ation)f(Servic)l(e)i(for)2079 183 y(Op)l(en)k(Network) h(Systems")p Fq(,)e(Steiner,)g(J.,)g(Neuman,)h(C.,)2079 282 y(Sc)n(hiller,)e(J.,)g(USENIX)g(Conference)g(Pro)r(ceeding,)e(Dal-) 2079 382 y(las,)g(T)-7 b(exas,)27 b(F)-7 b(ebruary)26 b(1989.)1992 573 y([Morris85])39 b Fd("A)73 b(We)l(akness)h(in)g(the)g (4.2BSD)h(UNIX)2079 672 y(TCP/IP)24 b(Softwar)l(e")p Fq(,)d(Morris,)e(R.,)i(Computing)e(Science)2079 772 y(T)-7 b(ec)n(hnical)18 b(Rep)r(ort)h(No)f(117,)h(A)-7 b(T&T)19 b(Bell)g(Lab)r(oratories,)2079 872 y(Murra)n(y)26 b(Hill,)i(New)g (Jersey)-7 b(,)26 b(1985.)1992 1063 y([PGP])40 b Fd(Pr)l(etty)55 b(Go)l(o)l(d)h(Privacy)i Fq(V)-7 b(ersion)54 b(2.6.1,)61 b(Philip)2079 1162 y(Zimmermann,)27 b(August)h(1994.)1992 1353 y([RF)n(C)f(793])40 b(Request)k(F)-7 b(or)42 b(Commen)n(t)i(793,)h Fd(\\T)-6 b(r)l(ansmis-)2079 1453 y(sion)29 b(Contr)l(ol)g(Pr)l(oto)l (c)l(ol")p Fq(,)e(Septem)n(b)r(er)f(1981,)e(J.)i(P)n(ostel.)1992 1644 y([RF)n(C)h(854])40 b(Request)31 b(F)-7 b(or)30 b(Commen)n(t)h(854,)f Fd(\\T)-6 b(elnet)33 b(Pr)l(o-)2079 1743 y(to)l(c)l(ol)66 b(Sp)l(e)l(ci\014c)l(ation)-8 b(")p Fq(,)77 b(Ma)n(y)66 b(1983,)74 b(J.)67 b(P)n(ostel,)75 b(J.)2079 1843 y(Reynolds)1992 2034 y([SKEY])41 b Fd("The)46 b(S/Key)g(One-time)f(Passwor)l(d)i(System")p Fq(,)2079 2134 y(Haller,)32 b(N.,)g(Pro)r(ceeding)e(of)h(the)h(Symp)r(osium)f(on) g(Net-)2079 2233 y(w)n(ork)20 b(&)h(Distributed)h(Systems,)h(Securit)n (y)-7 b(,)22 b(In)n(ternet)g(So-)2079 2333 y(ciet)n(y)-7 b(,)28 b(San)f(Diego,)g(CA,)h(F)-7 b(ebruary)27 b(1994.)1992 2524 y([TCPcrypt])40 b Fd("Public)67 b(Key)f(Encryption)g(Supp)l(ort)g (for)2079 2623 y(TCP")p Fq(,)26 b(Jonc)n(hera)n(y)-7 b(,)22 b(L.,)i(W)-7 b(ork)23 b(in)h(progress,)e(Ma)n(y)h(1995.)1992 2814 y([TCPDUMP])41 b Fk(tcp)s(dump\(8\))g Fq(V)-7 b(ersion)42 b(2.2.1,)j(V)-7 b(an)42 b(Ja-)2079 2914 y(cobson,)f(Craig)c(Leres,)j (Stev)n(en)f(Berk)n(eley)-7 b(,)40 b(Univ)n(ersit)n(y)2079 3013 y(of)28 b(California,)e(Berk)n(eley)-7 b(,)26 b(CA.)p eop end userdict /end-hook known{end-hook}if