{"id":52324,"date":"2019-11-06T00:00:00","date_gmt":"2019-11-05T21:00:00","guid":{"rendered":"https:\/\/prohoster.info\/blog\/blog_prohoster\/hvatit-ispolzovat-smehotvorno-malyj-ttl-dlya-dns"},"modified":"2021-01-02T13:04:14","modified_gmt":"2021-01-02T11:04:14","slug":"hvatit-ispolzovat-smehotvorno-malyj-ttl-dlya-dns","status":"publish","type":"post","link":"https:\/\/prohoster.info\/ro\/blog\/administrirovanie\/hvatit-ispolzovat-smehotvorno-malyj-ttl-dlya-dns","title":{"rendered":"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"<p>O laten\u021b\u0103 DNS sc\u0103zut\u0103 este un factor cheie pentru o func\u021bionare rapid\u0103 pe internet. Pentru a o minimiza, este important s\u0103 alegem cu aten\u021bie serverele DNS \u0219i <noindex><a rel=\"nofollow\" href=\"https:\/\/github.com\/DNSCrypt\/dnscrypt-resolvers\/blob\/master\/v2\/relays.md\">riluri anonime<\/a><\/noindex>. Dar mai \u00eent\u00e2i trebuie s\u0103 elimin\u0103m cererile inutile.<\/p>\n<p>De aceea, DNS a fost creat ini\u021bial ca un protocol cu un grad ridicat de cache. Administratorii de zone stabilesc timpul de via\u021b\u0103 (TTL) pentru \u00eenregistr\u0103rile individuale, iar rezolvatoarele folosesc aceste informa\u021bii pentru a stoca \u00eenregistr\u0103rile \u00een memorie, evit\u00e2nd astfel traficul nedorit.<\/p>\n<p>Este eficient cachingul? Cu ceva timp \u00een urm\u0103, o mic\u0103 cercetare a ar\u0103tat c\u0103 nu este perfect. S\u0103 ne uit\u0103m la situa\u021bia actual\u0103.<br \/>\n<noindex><a rel=\"nofollow\" name=\"habracut\"><\/a><\/noindex><br \/>\nPentru a aduna informa\u021bii, am patch-uit <noindex><a rel=\"nofollow\" href=\"https:\/\/github.com\/jedisct1\/encrypted-dns-server\">Server DNS criptat<\/a><\/noindex> pentru a p\u0103stra valoarea TTL pentru r\u0103spuns. Aceasta este definit\u0103 ca fiind TTL minim al \u00eenregistr\u0103rilor sale, pentru fiecare cerere primit\u0103. Acest lucru ofer\u0103 o bun\u0103 imagine de ansamblu asupra distribu\u021biei TTL \u00een traficul real \u0219i ia \u00een considerare popularitatea cererilor individuale. Versiunea patch-uit\u0103 a serverului a func\u021bionat c\u00e2teva ore.<\/p>\n<p>Setul de date rezultat con\u021bine 1&nbsp;583&nbsp;579 de \u00eenregistr\u0103ri (name, qtype, TTL, timestamp). Iat\u0103 distribu\u021bia general\u0103 a TTL-ului (axa X reprezint\u0103 TTL-ul \u00een secunde):<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/ce19a1ceb07e1fd2f3b2284f86271eac.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Dac\u0103 nu lu\u0103m \u00een considerare o u\u0219oar\u0103 excrescen\u021b\u0103 la 86&nbsp;400 (\u00een principal pentru \u00eenregistr\u0103rile SOA), este destul de evident c\u0103 TTL-urile se afl\u0103 \u00eentr-un interval sc\u0103zut. S\u0103 ne uit\u0103m mai aproape:<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/abbe48952b616e1c5bdb18c2dc103dc7.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Bine, TTL-urile de peste 1 or\u0103 sunt statistic nesemnificative. Atunci s\u0103 ne concentr\u0103m pe intervalul 0\u20133600:<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/f8a88267e1868a55d9234f66ad8c5dee.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Majoritatea TTL-urilor sunt \u00eentre 0 \u0219i 15 minute:<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/aab650c806097513a5924e5262211ea5.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Cea mai mare parte se \u00eencadreaz\u0103 \u00eentre 0 \u0219i 5 minute:<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/736af4b33b745fb0d5070200fa511426.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Acest lucru nu este prea bun.<\/p>\n<p>Distribu\u021bia acumulativ\u0103 face problema \u0219i mai evident\u0103:<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/a3a4f03188dd6dbbf4558076b827f6a5.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>\u00cen jum\u0103tate din r\u0103spunsurile DNS, TTL-ul este de 1 minut sau mai pu\u021bin, iar la trei sferturi&nbsp;\u2014 5 minute sau mai pu\u021bin.<\/p>\n<p>Dar a\u0219teapt\u0103, de fapt, lucrurile sunt chiar mai rele. Acesta este TTL-ul de la serverele autoritative. Cu toate acestea, rezolvatoarele clientului (de exemplu, routerele, cache-urile locale) primesc TTL de la rezolvatoarele superioare, iar acesta scade cu fiecare secund\u0103.<\/p>\n<p>Astfel, clientul poate folosi, de fapt, fiecare \u00eenregistrare, \u00een medie, timp de jum\u0103tate din TTL-ul original, dup\u0103 care va trimite o nou\u0103 cerere.<\/p>\n<p>Poate c\u0103 aceste TTL-uri foarte sc\u0103zute se refer\u0103 doar la cereri neobi\u0219nuite, nu la site-uri web populare \u0219i API-uri? S\u0103 vedem:<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/2680f9669a90f449a593edcc84ca0dc8.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Axa X&nbsp;\u2014 este TTL-ul, axa Y&nbsp;\u2014 popularitatea cererilor.<\/p>\n<p>Din p\u0103cate, cele mai populare cereri sunt de asemenea cele mai pu\u021bin cache-uite.<\/p>\n<p>S\u0103 ne apropiem:<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/c670382b195169779743f43fd7b15827.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Verdict: things are really bad. It was already bad before, and now it has gotten even worse. DNS caching has become practically useless. As fewer people use their provider's DNS resolver (for valid reasons), the increased latency becomes more noticeable.<\/p>\n<p>DNS caching has only become useful for content that no one visits.<\/p>\n<p>Also note that the software can <noindex><a rel=\"nofollow\" href=\"https:\/\/00f.net\/2011\/11\/17\/how-long-does-a-dns-ttl-last\/\">interpret low TTLs<\/a><\/noindex> differently.<\/p>\n<h1>Why is that?<\/h1>\n<p>Why is such a low TTL set for DNS records?<\/p>\n<ul>\n<li>Outdated load balancers remain with default settings.<\/li>\n<li>Exist\u0103 mituri c\u0103 echilibrarea \u00eenc\u0103rc\u0103rii DNS depinde de TTL (nu este adev\u0103rat&nbsp;\u2014 din vremurile Netscape Navigator, clien\u021bii aleg o adres\u0103 IP aleatorie din setul RR \u0219i \u00eencearc\u0103 transparent altceva, dac\u0103 nu se pot conecta)<\/li>\n<li>Administrators want to apply changes immediately, so it's easier to plan.<\/li>\n<li>The DNS server or load balancer administrator sees their task as effectively deploying the configuration requested by users, not speeding up websites and services.<\/li>\n<li>Low TTLs provide peace of mind.<\/li>\n<li>People initially set low TTLs for testing and then forget to change them.<\/li>\n<\/ul>\n<p>I haven't included 'failure handling' on the list, as it is becoming less relevant. If it's necessary to redirect users to another network just to display an error page when everything else has completely broken, a delay of more than one minute is probably acceptable.<\/p>\n<p>Moreover, a one-minute TTL means that if authoritative DNS servers are blocked for more than a minute, no one else can access dependent services. And redundancy won't help if the cause is a configuration error or a hack. On the other hand, with reasonable TTLs, many clients will continue using the previous configuration and may never notice anything.<\/p>\n<p>Low TTLs are largely blamed on CDN services and load balancers, especially when they combine CNAMEs with low TTLs and records with equally low (but independent) TTLs:<\/p>\n<pre>$ drill raw.githubusercontent.com\nraw.githubusercontent.com.\t9\tIN\tCNAME\tgithub.map.fastly.net.\ngithub.map.fastly.net.\t20\tIN\tA\t151.101.128.133\ngithub.map.fastly.net.\t20\tIN\tA\t151.101.192.133\ngithub.map.fastly.net.\t20\tIN\tA\t151.101.0.133\ngithub.map.fastly.net.\t20\tIN\tA\t151.101.64.133<\/pre>\n<p>Ori de c\u00e2te ori expir\u0103 CNAME sau orice \u00eenregistrare A, trebuie trimis\u0103 o nou\u0103 solicitare. Ambele au un TTL de 30 de secunde, dar nu coincide. TTL-ul mediu real va fi de 15 secunde.<\/p>\n<p>Dar a\u0219teapt\u0103! E \u0219i mai r\u0103u. Unele rezolvatoare se comport\u0103 foarte prost \u00een aceast\u0103 situa\u021bie cu dou\u0103 TTL-uri sc\u0103zute corelate:<\/p>\n<pre>$ drill raw.githubusercontent.com @4.2.2.2\nraw.githubusercontent.com.\t1\tIN\tCNAME\tgithub.map.fastly.net.\ngithub.map.fastly.net.\t1\tIN\tA\t151.101.16.133<\/pre>\n<p>Rezolvatorul Level3 func\u021bioneaz\u0103 probabil pe BIND. Dac\u0103 continui s\u0103 trimi\u021bi aceast\u0103 solicitare, va returna mereu un TTL egal cu 1. Practic, <code>raw.githubusercontent.com<\/code> nu este niciodat\u0103 cache-uit.<\/p>\n<p>Iat\u0103 un alt exemplu de astfel de situa\u021bie cu un domeniu foarte popular:<\/p>\n<pre>$ drill detectportal.firefox.com @1.1.1.1\ndetectportal.firefox.com.\t25\tIN\tCNAME\tdetectportal.prod.mozaws.net.\ndetectportal.prod.mozaws.net.\t26\tIN\tCNAME\tdetectportal.firefox.com-v2.edgesuite.net.\ndetectportal.firefox.com-v2.edgesuite.net.\t10668\tIN\tCNAME\ta1089.dscd.akamai.net.\na1089.dscd.akamai.net.\t10\tIN\tA\t104.123.50.106\na1089.dscd.akamai.net.\t10\tIN\tA\t104.123.50.88<\/pre>\n<p>Cel pu\u021bin trei \u00eenregistr\u0103ri CNAME. Of! Una are un TTL decent, dar acesta este complet inutil. \u00cen alte CNAME, TTL-ul ini\u021bial este de 60 de secunde, dar pentru domeniile <code>akamai.net<\/code> TTL-ul maxim este de 20 de secunde, \u0219i niciunul dintre ele nu este sincronizat.<\/p>\n<p>Ce zici de domeniile care interogheaz\u0103 constant dispozitivele Apple?<\/p>\n<pre>$ drill 1-courier.push.apple.com @4.2.2.2\n1-courier.push.apple.com.\t1253\tIN\tCNAME\t1.courier-push-apple.com.akadns.net.\n1.courier-push-apple.com.akadns.net.\t1\tIN\tCNAME\tgb-courier-4.push-apple.com.akadns.net.\ngb-courier-4.push-apple.com.akadns.net.\t1\tIN\tA\t17.57.146.84\ngb-courier-4.push-apple.com.akadns.net.\t1\tIN\tA\t17.57.146.85<\/pre>\n<p>Aceea\u0219i problem\u0103 ca la Firefox, \u0219i TTL-ul va r\u0103m\u00e2ne de multe ori blocat la 1 secund\u0103 folosind rezolvatorul Level3.<\/p>\n<p>Dropbox?<\/p>\n<pre>$ drill client.dropbox.com @8.8.8.8\nclient.dropbox.com.\t7\tIN\tCNAME\tclient.dropbox-dns.com.\nclient.dropbox-dns.com.\t59\tIN\tA\t162.125.67.3\n\n$ drill client.dropbox.com @4.2.2.2\nclient.dropbox.com.\t1\tIN\tCNAME\tclient.dropbox-dns.com.\nclient.dropbox-dns.com.\t1\tIN\tA\t162.125.64.3<\/pre>\n<p>La \u00eenregistrare <code>safebrowsing.googleapis.com<\/code> valoarea TTL este de 60 de secunde, la fel ca \u0219i pentru domeniile Facebook. \u0218i, din nou, din perspectiva clientului, aceste valori se reduc la jum\u0103tate.<\/p>\n<h1>Ce zici de stabilirea unui TTL minim?<\/h1>\n<p>Folosind numele, tipul de solicitare, TTL-ul \u0219i timestamp-ul ini\u021bial salvat, am scris un script pentru a simula 1,5 milioane de solicit\u0103ri care trec printr-un rezolvator de cache, pentru a evalua volumul de cereri inutile trimise din cauza unei \u00eenregistr\u0103ri de cache expirate.<\/p>\n<p>47,4% dintre solicit\u0103ri au fost f\u0103cute dup\u0103 expirarea \u00eenregistr\u0103rii existente. Acest procent este inacceptabil de ridicat.<\/p>\n<p>Care va fi impactul asupra caching-ului dac\u0103 se stabile\u0219te un TTL minim?<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/3abd417d312450519b45f66865b23763.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Axa X reprezint\u0103 valorile minime TTL. \u00cenregistr\u0103rile cu TTL ini\u021bial mai mari dec\u00e2t aceast\u0103 valoare nu sunt afectate.<\/p>\n<p>Axa Y reprezint\u0103 procentul cererilor de la client care deja are o \u00eenregistrare cache, dar perioada de valabilitate a acesteia a expirat \u0219i face o nou\u0103 cerere.<\/p>\n<p>Ponderea cererilor \"\u00een plus\" scade de la 47% la 36% doar prin setarea unui TTL minim de 5 minute. C\u00e2nd se seteaz\u0103 un TTL minim de 15 minute, num\u0103rul acestor cereri scade la 29%. Un TTL minim de 1 or\u0103 reduce acest num\u0103r la 17%. O diferen\u021b\u0103 semnificativ\u0103!<\/p>\n<p>Ce spui de a nu schimba nimic pe server, ci \u00een schimb s\u0103 stabile\u0219ti TTL-uri minime \u00een cache-urile DNS ale clien\u021bilor (routere, rezolutori local)?<\/p>\n<p><img decoding=\"async\" alt=\"Este suficient s\u0103 folose\u0219ti un TTL ridicol de mic pentru DNS.\" src=\"\/wp-content\/uploads\/2019\/11\/107a2bd53797d421d65ac7260bf78dfe.jpg\" style=\"display:block;margin: 0 auto;\"><\/p>\n<p>Num\u0103rul de cereri necesare scade de la 47% la 34% prin setarea unui TTL minim de 5 minute, la 25% cu un minim de 15 minute \u0219i la 13% cu un minim de 1 or\u0103. Este posibil ca valoarea optim\u0103 s\u0103 fie de 40 de minute.<\/p>\n<p>Impactul acestei modific\u0103ri minime este uria\u0219.<\/p>\n<h1>Care sunt consecin\u021bele?<\/h1>\n<p>Desigur, serviciul poate fi mutat la un nou furnizor de cloud, pe un nou server, pe o nou\u0103 re\u021bea, cer\u00e2nd clien\u021bilor s\u0103 utilizeze ultimele \u00eenregistr\u0103ri DNS. \u0218i un TTL suficient de mic ajut\u0103 la realizarea acestei tranzi\u021bii \u00eentr-un mod lin \u0219i subtil. Dar cu trecerea la o nou\u0103 infrastructur\u0103, nimeni nu se a\u0219teapt\u0103 ca clien\u021bii s\u0103 treac\u0103 la noile \u00eenregistr\u0103ri DNS \u00een decurs de 1 minut, 5 minute sau 15 minute. Stabilirea unei durate minime de via\u021b\u0103 de 40 de minute \u00een loc de 5 nu va \u00eempiedica utilizatorii s\u0103 acceseze serviciul.<\/p>\n<p>Cu toate acestea, acest lucru va permite s\u0103 se reduc\u0103 semnificativ \u00eent\u00e2rzierile \u0219i s\u0103 se creasc\u0103 confiden\u021bialitatea \u0219i fiabilitatea, evit\u00e2nd cererile inutile.<\/p>\n<p>Desigur, RFC-uri spun c\u0103 trebuie s\u0103 respect\u0103m strict TTL. Dar realitatea este c\u0103 sistemul DNS a devenit mult prea ineficient.<\/p>\n<p>Dac\u0103 lucrezi cu servere DNS autoritative, te rug\u0103m s\u0103 verifici TTL-urile tale. Ai nevoie cu adev\u0103rat de valori at\u00e2t de ridicul de mici?<\/p>\n<p>Desigur, exist\u0103 motive \u00eentemeiate pentru a stabili TTL-uri mici pentru \u00eenregistr\u0103rile DNS. Dar nu pentru 75% din traficul DNS care practic nu se schimb\u0103.<\/p>\n<p>\u0218i dac\u0103 din anumite motive trebuie s\u0103 folose\u0219ti TTL-uri mici pentru DNS, asigur\u0103-te c\u0103 pe site-ul t\u0103u nu este activat\u0103 caching-ul. Din acelea\u0219i motive.<\/p>\n<p>Dac\u0103 ai un cache DNS local \u00een func\u021biune, cum ar fi <noindex><a rel=\"nofollow\" href=\"https:\/\/github.com\/dnscrypt\/dnscrypt-proxy\">dnscrypt-proxy<\/a><\/noindex>, care permite setarea unui TTL minim, folosi\u021bi aceast\u0103 func\u021bie. Este \u00een regul\u0103. Nu va fi nimic r\u0103u. Seta\u021bi un TTL minim de aproximativ \u00eentre 40 de minute (2400 de secunde) \u0219i 1 or\u0103. O gam\u0103 destul de rezonabil\u0103.<\/p>\n<p>Sursa: <a content=\"nofollow\" rel=\"nofollow\" href=\"https:\/\/habr.com\/ru\/post\/474450\/\">habr.com<\/a><\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"<p>\u041d\u0438\u0437\u043a\u0430\u044f \u0437\u0430\u0434\u0435\u0440\u0436\u043a\u0430 DNS \u2014 \u043a\u043b\u044e\u0447\u0435\u0432\u043e\u0439 \u0444\u0430\u043a\u0442\u043e\u0440 \u0434\u043b\u044f \u0431\u044b\u0441\u0442\u0440\u043e\u0439 \u0440\u0430\u0431\u043e\u0442\u044b \u0432 \u0438\u043d\u0442\u0435\u0440\u043d\u0435\u0442\u0435. \u0427\u0442\u043e\u0431\u044b \u0435\u0451 \u043c\u0438\u043d\u0438\u043c\u0438\u0437\u0438\u0440\u043e\u0432\u0430\u0442\u044c, \u0432\u0430\u0436\u043d\u043e \u0442\u0449\u0430\u0442\u0435\u043b\u044c\u043d\u043e \u043f\u043e\u0434\u043e\u0431\u0440\u0430\u0442\u044c DNS-\u0441\u0435\u0440\u0432\u0435\u0440\u044b \u0438 \u0430\u043d\u043e\u043d\u0438\u043c\u043d\u044b\u0435 \u0440\u0438\u043b\u0435\u0438. \u041d\u043e \u043f\u0435\u0440\u0432\u044b\u043c \u0434\u0435\u043b\u043e\u043c \u0441\u043b\u0435\u0434\u0443\u0435\u0442 \u0438\u0437\u0431\u0430\u0432\u0438\u0442\u044c\u0441\u044f \u043e\u0442 \u0431\u0435\u0441\u043f\u043e\u043b\u0435\u0437\u043d\u044b\u0445 \u0437\u0430\u043f\u0440\u043e\u0441\u043e\u0432. \u0418\u043c\u0435\u043d\u043d\u043e \u043f\u043e\u044d\u0442\u043e\u043c\u0443 DNS \u0438\u0437\u043d\u0430\u0447\u0430\u043b\u044c\u043d\u043e \u0441\u043e\u0437\u0434\u0430\u0432\u0430\u043b\u0441\u044f \u043a\u0430\u043a \u0441\u0438\u043b\u044c\u043d\u043e \u043a\u044d\u0448\u0438\u0440\u0443\u0435\u043c\u044b\u0439 \u043f\u0440\u043e\u0442\u043e\u043a\u043e\u043b. \u0410\u0434\u043c\u0438\u043d\u0438\u0441\u0442\u0440\u0430\u0442\u043e\u0440\u044b \u0437\u043e\u043d \u0443\u0441\u0442\u0430\u043d\u0430\u0432\u043b\u0438\u0432\u0430\u044e\u0442 \u0432\u0440\u0435\u043c\u044f \u0436\u0438\u0437\u043d\u0438 (TTL) \u0434\u043b\u044f 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