{"id":15546,"date":"2026-09-05T13:18:25","date_gmt":"2026-09-05T11:18:25","guid":{"rendered":"https:\/\/www.lukaswojcik.com\/blog\/toolbox\/http3-alt-svc-discovery-checker\/"},"modified":"2026-09-05T13:18:25","modified_gmt":"2026-09-05T11:18:25","slug":"http3-alt-svc-discovery-checker","status":"publish","type":"page","link":"https:\/\/www.lukaswojcik.com\/blog\/en\/toolbox\/http3-alt-svc-discovery-checker\/","title":{"rendered":"HTTP\/3 &#038; Alt-Svc Discovery Checker"},"content":{"rendered":"<div class=\"gtm-analyser-container\" style=\"background: var(--bg-panel, #1e1e24); padding: 25px; border-radius: 8px; border: 1px solid var(--border, #2a2a35);\">\n<p style=\"color: var(--text-secondary, #a0a0b0); margin-bottom: 20px;\">A server that speaks HTTP\/3 has to tell clients about it: through an HTTPS record in DNS, which a browser reads before the first connection, or through an Alt-Svc header, which only helps from the second connection on. This check runs four requests against the same address from this server: HTTP\/1.1 only, HTTP\/2 as a first visit that learns the Alt-Svc header, HTTP\/3 only over QUIC without fallback, and a second visit with the cache from the first. Each row shows the negotiated version and the time to the first byte.<\/p>\n<div style=\"margin-bottom: 14px;\">\n        <label for=\"h3-url\" style=\"color: var(--text-secondary, #a0a0b0); display: block; font-size: 0.85rem; margin-bottom: 5px;\">Address (https:\/\/ is assumed; redirects are followed)<\/label><br \/>\n        <input id=\"h3-url\" type=\"text\" value=\"cloudflare.com\" class=\"form-control\" style=\"width: 100%; padding: 10px; background: var(--bg-body, #14141a); border: 1px solid var(--border, #2a2a35); color: var(--text-primary, #e8e8ee); border-radius: 6px; box-sizing: border-box; font-family: monospace;\">\n    <\/div>\n<p style=\"color: var(--text-secondary, #a0a0b0); font-size: 0.82rem; margin-bottom: 16px;\">Redirects and four short requests go from this server to the address entered; the response body is cut off after the first packet. The HTTPS record is asked of 1.1.1.1 and 8.8.8.8. Hosts in private, loopback and link-local networks are refused. The query is protected by reCAPTCHA v3; the calling IP address and the target are stored for one hour to limit the rate.<\/p>\n<p>    <button id=\"h3-btn\" class=\"button\" style=\"background: var(--accent, #7ee787); color: var(--on-accent, #0b1114); border: none; padding: 12px 24px; border-radius: 6px; font-weight: 700; cursor: pointer;\">Check HTTP\/3 discovery<\/button><\/p>\n<div id=\"h3-ausgabe\" style=\"margin-top: 22px;\"><\/div>\n<p style=\"color: var(--text-secondary, #a0a0b0); font-size: 0.8rem; margin: 24px 0 0;\">Limits worth knowing: only one of the resolved addresses is tested; the times are a single measurement from one server in Europe with a warm resolver and say nothing about a visitor&#039;s network; whether a real browser uses the HTTPS record depends on the browser and its DNS path; and a middlebox between this server and the target can block UDP 443 while a visitor elsewhere gets through.<\/p>\n<\/div>\n<p><script>\n(function () {\n    'use strict';<\/p>\n<p>    const T = {\"einleitung\":\"A server that speaks HTTP\\\/3 has to tell clients about it: through an HTTPS record in DNS, which a browser reads before the first connection, or through an Alt-Svc header, which only helps from the second connection on. This check runs four requests against the same address from this server: HTTP\\\/1.1 only, HTTP\\\/2 as a first visit that learns the Alt-Svc header, HTTP\\\/3 only over QUIC without fallback, and a second visit with the cache from the first. Each row shows the negotiated version and the time to the first byte.\",\"label_url\":\"Address (https:\\\/\\\/ is assumed; redirects are followed)\",\"datenschutz\":\"Redirects and four short requests go from this server to the address entered; the response body is cut off after the first packet. The HTTPS record is asked of 1.1.1.1 and 8.8.8.8. Hosts in private, loopback and link-local networks are refused. The query is protected by reCAPTCHA v3; the calling IP address and the target are stored for one hour to limit the rate.\",\"knopf\":\"Check HTTP\\\/3 discovery\",\"laeuft\":\"Running the four requests...\",\"grenze\":\"Limits worth knowing: only one of the resolved addresses is tested; the times are a single measurement from one server in Europe with a warm resolver and say nothing about a visitor\\u0027s network; whether a real browser uses the HTTPS record depends on the browser and its DNS path; and a middlebox between this server and the target can block UDP 443 while a visitor elsewhere gets through.\",\"h_entdeckung\":\"Discovery\",\"zeile_url\":\"Measured address\",\"zeile_ip\":\"Resolved addresses\",\"zeile_server\":\"Server header\",\"zeile_dns\":\"DNS HTTPS record\",\"zeile_altsvc\":\"Alt-Svc header\",\"zeile_weg\":\"First visit\",\"zeile_dauer\":\"Took\",\"zeile_limit\":\"Checks left this hour\",\"dns_vorhanden\":\"present: alpn {alpn}, TTL {ttl} s\",\"dns_keiner\":\"none\",\"dns_fehler\":\"not readable (no answer from the resolvers)\",\"dns_alias\":\"alias record pointing to {ziel}\",\"dns_ech_zusatz\":\"+ ECH configuration\",\"dns_port_zusatz\":\"(port {port})\",\"weg_dns\":\"HTTP\\\/3 from the first connection: the HTTPS record announces it before any TCP handshake.\",\"weg_alt_svc\":\"HTTP\\\/2 on the first connection, HTTP\\\/3 from the second: only the Alt-Svc header announces it.\",\"weg_unangekuendigt\":\"QUIC answers, but nothing announces it. Browsers never try it on their own.\",\"weg_unerreichbar\":\"HTTP\\\/3 is announced, but QUIC did not answer from here. Browsers fall back to HTTP\\\/2 after a delay.\",\"weg_keine\":\"No HTTP\\\/3: nothing announces it and QUIC does not answer.\",\"h_protokolle\":\"Four requests, one address\",\"spalte_versuch\":\"Request\",\"spalte_ausgehandelt\":\"Negotiated\",\"spalte_status\":\"Status\",\"spalte_connect\":\"Connect\",\"spalte_tls\":\"TLS\",\"spalte_ttfb\":\"First byte\",\"spalte_gesamt\":\"Total\",\"versuch_h1\":\"HTTP\\\/1.1 only\",\"versuch_h2\":\"HTTP\\\/2, first visit\",\"versuch_h3\":\"HTTP\\\/3 only (QUIC, no fallback)\",\"versuch_zweiter\":\"Second visit with Alt-Svc cache\",\"fehlgeschlagen\":\"failed\",\"curl_refused\":\"connection refused, nothing listens on UDP 443\",\"curl_timeout\":\"timed out, UDP 443 is probably dropped on the way\",\"curl_tls\":\"TLS handshake failed\",\"curl_dns\":\"name did not resolve\",\"curl_sonstiges\":\"transport error\",\"h_befunde\":\"Findings\",\"stufe_hoch\":\"High\",\"stufe_mittel\":\"Medium\",\"stufe_niedrig\":\"Low\",\"stufe_info\":\"Information\",\"stufe_gut\":\"In order\",\"keine_befunde\":\"Nothing to report.\",\"urteil_gut\":\"Discovery and transport are consistent.\",\"urteil_warnung\":\"Works, but the announcement and the transport do not match, or a current protocol is missing.\",\"urteil_kritisch\":\"The address could not be fetched at all.\",\"ja\":\"yes\",\"nein\":\"no\",\"wert_keine\":\"none\",\"fehler_kein_token\":\"The check could not be started because reCAPTCHA did not load.\",\"fehler_captcha\":\"reCAPTCHA classified the request as automated. Reloading the page normally helps.\",\"fehler_zu_viele\":\"The limit of {limit} checks per hour for this address has been reached.\",\"fehler_adresse\":\"That address cannot be used.\",\"fehler_zu_viele_spruenge\":\"The address redirects more than five times; the chain was not followed further.\",\"fehler_nur_http\":\"The redirect chain ends on a plain http:\\\/\\\/ address. HTTP\\\/3 exists only over TLS, so there is nothing to measure.\",\"fehler_zaehler\":\"The rate counter is unavailable, so nothing was checked.\",\"fehler_pruefdienst\":\"The reCAPTCHA service could not be reached.\",\"fehler_aufbau\":\"The service is not configured correctly. The fault is on this side.\",\"fehler_eingabe\":\"The request could not be read.\",\"fehler_methode\":\"Wrong request method.\",\"fehler_netz\":\"The service could not be reached.\",\"fehler_antwort\":\"The answer could not be read.\",\"fehler_unbekannt\":\"Something went wrong that has no message of its own.\",\"grund_leer\":\"Nothing was entered.\",\"grund_zu_lang\":\"The input is too long.\",\"grund_unlesbar\":\"It does not parse as an address.\",\"grund_schema\":\"Only http and https addresses are checked.\",\"grund_benutzerinfo\":\"Inputs with credentials in them are refused.\",\"grund_kein_punkt\":\"The host name needs at least one dot.\",\"grund_hostform\":\"Enter a host name, not an IP address.\",\"grund_port\":\"Only the ports 80, 443, 8080 and 8443 are checked.\",\"grund_gesperrter_bereich\":\"That host resolves into a private or reserved network. Those are never contacted.\",\"grund_kein_dns\":\"The host name does not resolve.\",\"grund_abruf\":\"The address did not answer.\",\"befund_weiterleitung\":\"Redirected {n} time(s) to {ziel} || The measurements refer to the final address of the chain, because that is the one a browser ends up talking to.\",\"befund_abruf_fehlgeschlagen\":\"The address does not answer over TCP || Neither HTTP\\\/1.1 nor HTTP\\\/2 produced a response: {fehler}. Without a TCP path there is nothing to compare HTTP\\\/3 against.\",\"befund_https_dns_fehler\":\"The HTTPS record could not be read || The resolvers gave no usable answer ({grund}). This says nothing about the record itself; the assessment continues with the Alt-Svc header alone.\",\"befund_https_alias\":\"HTTPS record in alias mode, pointing to {ziel} || An alias record (priority 0) delegates to another name. A client follows it and reads the service parameters there; from this check only the delegation is visible.\",\"befund_https_eintrag_h3\":\"HTTPS record announces HTTP\\\/3 (alpn {alpn}, TTL {ttl} s) || The record is read in parallel with the A and AAAA lookups, so a browser that supports it knows about QUIC before the first TCP handshake and can use HTTP\\\/3 on the very first connection. It also tells the browser to go straight to https. Target: {ziel}.\",\"befund_https_eintrag_ohne_h3\":\"HTTPS record without h3 (alpn {alpn}) || The record exists and still does useful work, such as the https upgrade, but it does not announce HTTP\\\/3. A browser learns about QUIC only from the Alt-Svc header, and thus only for the second connection. Target: {ziel}.\",\"befund_https_ech\":\"ECH configuration present || The record carries an Encrypted Client Hello key. Browsers that support ECH hide the server name in the TLS handshake from anyone watching the wire.\",\"befund_https_hints\":\"Address hints in the record: {liste} || ipv4hint and ipv6hint let a client start connecting before the A and AAAA answers arrive. They are hints, not authoritative addresses.\",\"befund_https_port\":\"The record points to port {port} || A different port than the one measured; clients honouring the record connect there instead.\",\"befund_https_unbekannte_params\":\"Unknown service parameters: keys {liste} || Parameters this check does not decode. Clients ignore keys they do not understand unless they are marked mandatory.\",\"befund_https_eintrag_fehlt\":\"No HTTPS record for {host} || Without it, a browser cannot know about HTTP\\\/3 before the first connection. The first visit runs over TCP; only the Alt-Svc header can move the second one to QUIC. Cloudflare and Google publish this record; on other setups it is one DNS entry of type HTTPS with alpn=\\u0022h3,h2\\u0022.\",\"befund_alt_svc_clear\":\"Alt-Svc: clear || The server tells clients to forget every alternative service they cached for this origin.\",\"befund_alt_svc_h3\":\"Alt-Svc announces {liste} for {ma} s || Header: {roh}. A browser remembers this for the given time (ma, default 86400 s) and tries QUIC on the next connection to this origin. The first connection is unaffected.\",\"befund_alt_svc_nur_draft\":\"Only draft versions announced ({liste}) || h3-29 and the other draft identifiers belong to the pre-standard QUIC drafts. Current browsers only speak the final h3; without that entry the announcement is ignored.\",\"befund_alt_svc_ma_kurz\":\"Short Alt-Svc lifetime: {ma} s || A browser forgets the alternative after this time and starts the next visit over TCP again. The usual value is 86400 s or more.\",\"befund_alt_svc_anderer_host\":\"Alt-Svc points to a different host || {roh}. Several browsers ignore alternatives on other hosts, or require extra checks before using them.\",\"befund_alt_svc_ohne_h3\":\"Alt-Svc without h3 || Header: {roh}. There is an Alt-Svc header, but none of its entries announces HTTP\\\/3.\",\"befund_alt_svc_fehlt\":\"No Alt-Svc header from {host} || Nothing tells clients that another transport exists. Together with a missing HTTPS record this means HTTP\\\/3, if the server speaks it at all, is never used.\",\"befund_h3_erreichbar\":\"HTTP\\\/3 answers over QUIC (status {status}, first byte after {ttfb} ms) || A connection with QUIC only, no fallback, reached the server and got a response.\",\"befund_h3_angekuendigt_unerreichbar\":\"HTTP\\\/3 is announced but does not answer || Announced via {quelle}, yet a QUIC-only connection failed: {fehler}. Typical causes: UDP 443 not open on the firewall or load balancer, the HTTP\\\/3 listener not enabled, or a path that drops UDP. Browsers mark the origin as broken for QUIC and fall back, which costs a delay on every fresh attempt.\",\"befund_h3_nicht_erreichbar\":\"No HTTP\\\/3 over QUIC ({fehler}) || Nothing announces it and nothing answers on UDP 443. That is consistent; the server simply does not speak HTTP\\\/3.\",\"befund_h2_erreichbar\":\"HTTP\\\/2 negotiated (status {status}, first byte after {ttfb} ms) || ALPN in the TLS handshake selected h2.\",\"befund_h2_fehlt\":\"No HTTP\\\/2: the TLS connection negotiated HTTP\\\/{version} || Every current browser offers h2 via ALPN. Without it, each request needs its own connection or waits in line; enabling HTTP\\\/2 is a one-line change in every current web server.\",\"befund_h1_fehlt\":\"HTTP\\\/1.1 refused ({fehler}) || Unusual: the server answered HTTP\\\/2 but not a plain HTTP\\\/1.1 request.\",\"befund_zweiter_besuch_h3\":\"Second visit uses HTTP\\\/3 (first byte after {ttfb} ms) || With the Alt-Svc information cached from the first connection, the next connection went over QUIC. This is what a returning browser does.\",\"befund_zweiter_besuch_kein_h3\":\"Second visit stays on HTTP\\\/{version} despite the announcement || QUIC answers and Alt-Svc announces it, but a client with the cached header did not switch. The announced port or host may differ from what actually answers.\",\"befund_zweiter_besuch_bleibt\":\"Second visit stays on HTTP\\\/{version} || Without an Alt-Svc announcement there is nothing for a cached second visit to switch to.\",\"befund_ttfb_vergleich\":\"First byte: HTTP\\\/2 {h2} ms, HTTP\\\/3 {h3} ms ({delta} ms) || One measurement from this server. HTTP\\\/3 saves a round trip on a fresh connection and reacts better to packet loss; on a clean, short path the difference is small, and a single sample can point either way.\"};<\/p>\n<p>    var ENDPUNKT = '\/lw-h3check.php';\n    var SITEKEY = '6LcrPkEtAAAAAPo1QCOf-IIM2fCL0UfdJz4y2iSY';\n    var STUFEN = ['hoch', 'mittel', 'niedrig', 'info', 'gut'];\n    var VERSUCHE = ['h1', 'h2', 'h3', 'zweiter'];\n    var WEGE = ['dns', 'alt_svc', 'unangekuendigt', 'unerreichbar', 'keine'];<\/p>\n<p>    function liste(w) { return Array.isArray(w) ? w : []; }\n    function zahl(w) { return (typeof w === 'number' && isFinite(w)) ? w : null; }\n    function text(schluessel, daten) {\n        var t = T[schluessel];\n        if (typeof t !== 'string') { return null; }\n        return t.replace(\/\\{([a-z_0-9]+)\\}\/g, function (m, k) {\n            return (daten && daten[k] !== undefined && daten[k] !== null) ? String(daten[k]) : m;\n        });\n    }<\/p>\n<p>    function abruf(r) {\n        r = (r && typeof r === 'object') ? r : {};\n        return { ok: !!r.ok, version: r.version ? String(r.version) : null, status: zahl(r.status), fehler: r.fehler ? String(r.fehler) : null,\n                 fehlerKurz: r.fehler_kurz ? String(r.fehler_kurz) : null,\n                 connect: zahl(r.connect_ms), tls: zahl(r.tls_ms), ttfb: zahl(r.ttfb_ms), gesamt: zahl(r.gesamt_ms) };\n    }<\/p>\n<p>    function bewerten(antwort) {\n        var a = antwort || {};\n        if (a.fehler) { return { fehler: String(a.fehler), grund: a.grund || null, limit: zahl(a.limit), kette: liste(a.kette) }; }\n        var befunde = liste(a.befunde).map(function (f) {\n            return { key: String(f.key || ''), stufe: STUFEN.indexOf(f.stufe) === -1 ? 'info' : f.stufe, daten: (f.daten && typeof f.daten === 'object') ? f.daten : {} };\n        });\n        var gruppen = {};\n        STUFEN.forEach(function (s) { gruppen[s] = befunde.filter(function (f) { return f.stufe === s; }); });\n        var abrufe = {};\n        VERSUCHE.forEach(function (v) { abrufe[v] = abruf(a.abrufe ? a.abrufe[v] : null); });\n        var dns = (a.dns && typeof a.dns === 'object') ? a.dns : {};\n        var alt = (a.alt_svc && typeof a.alt_svc === 'object') ? a.alt_svc : {};\n        return {\n            fehler: null, url: a.url ? String(a.url) : '', host: a.host ? String(a.host) : '', ip: a.ip ? String(a.ip) : '', ips: liste(a.ips), server: a.server ? String(a.server) : null,\n            urteil: (a.urteil === 'gut' || a.urteil === 'warnung' || a.urteil === 'kritisch') ? a.urteil : 'kritisch',\n            weg: WEGE.indexOf(a.weg) === -1 ? 'keine' : a.weg,\n            dns: { fehler: dns.fehler ? String(dns.fehler) : null, alpn: liste(dns.alpn).map(String), h3: !!dns.h3, ech: !!dns.ech, alias: dns.alias ? String(dns.alias) : null,\n                   ttl: zahl(dns.ttl), ziel: dns.ziel ? String(dns.ziel) : '', port: zahl(dns.port), n: zahl(dns.n) || 0, aufloeser: dns.aufloeser ? String(dns.aufloeser) : null },\n            altSvc: { roh: alt.roh ? String(alt.roh) : null, h3: liste(alt.h3).map(String), ma: zahl(alt.ma), clear: !!alt.clear },\n            abrufe: abrufe, befunde: befunde, gruppen: gruppen,\n            kette: liste(a.kette), dauer: zahl(a.dauer_ms), limitRest: zahl(a.limit_rest)\n        };\n    }<\/p>\n<p>    function skriptLaden() {\n        return new Promise(function (auf) {\n            if (window.grecaptcha) { auf(true); return; }\n            var s = document.createElement('script');\n            s.src = 'https:\/\/www.google.com\/recaptcha\/api.js?render=' + SITEKEY;\n            s.onload = function () { auf(true); }; s.onerror = function () { auf(false); };\n            document.head.appendChild(s);\n            setTimeout(function () { auf(!!window.grecaptcha); }, 8000);\n        });\n    }\n    function tokenHolen() {\n        return skriptLaden().then(function (da) {\n            if (!da || !window.grecaptcha || !window.grecaptcha.ready) { return null; }\n            return new Promise(function (auf) {\n                var fertig = false;\n                function einmal(w) { if (!fertig) { fertig = true; auf(w); } }\n                setTimeout(function () { einmal(null); }, 12000);\n                try {\n                    window.grecaptcha.ready(function () {\n                        if (fertig) { return; }\n                        if (!window.grecaptcha.execute) { einmal(null); return; }\n                        window.grecaptcha.execute(SITEKEY, { action: 'h3check' }).then(function (t) { einmal(t || null); }, function () { einmal(null); });\n                    });\n                } catch (e) { einmal(null); }\n            });\n        }, function () { return null; });\n    }\n    function abfragen(url) {\n        return tokenHolen().then(function (token) {\n            return fetch(ENDPUNKT, { method: 'POST', headers: { 'Content-Type': 'application\/json' }, body: JSON.stringify({ url: url, token: token || '' }) })\n                .then(function (r) { return r.json().then(function (j) { return j; }, function () { return { fehler: 'antwort' }; }); }, function () { return { fehler: 'netz' }; });\n        });\n    }<\/p>\n<p>    window.LW_TEST = window.LW_TEST || {};\n    window.LW_TEST.H3 = { bewerten: bewerten, darstellen: null, text: text, ENDPUNKT: ENDPUNKT, STUFEN: STUFEN, VERSUCHE: VERSUCHE, WEGE: WEGE, T: T };<\/p>\n<p>    var btn = document.getElementById('h3-btn');\n    var out = document.getElementById('h3-ausgabe');\n    if (!btn || !out) { return; }<\/p>\n<p>    function el(tag, stil, txt) { var e = document.createElement(tag); if (stil) { e.setAttribute('style', stil); } if (txt !== undefined && txt !== null) { e.textContent = txt; } return e; }\n    function wert(id) { var e = document.getElementById(id); return e ? e.value : ''; }\n    var UEBERSCHRIFT = 'font-family: \"Nunito Sans\", sans-serif; font-weight: 700; color: var(--text-primary, #e8e8ee); font-size: 0.95rem; margin: 22px 0 8px;';\n    var ZELLE = 'padding: 5px 12px 5px 0; color: var(--text-secondary, #a0a0b0); font-size: 0.87rem;';\n    var ZELLE_WERT = 'padding: 5px 12px 5px 0; color: var(--text-primary, #e8e8ee); font-size: 0.87rem; font-family: monospace; word-break: break-all;';\n    var KASTEN = 'background: var(--bg-body, #14141a); border: 1px solid var(--border, #2a2a35); border-radius: 8px; padding: 14px 16px; margin: 0 0 10px;';\n    var GUT = ' color: #7ee787;', WARN = ' color: #ffa94d;', ROT = ' color: #ff7b72;', GRAU = ' color: #8a8a99;';\n    var FARBEN = { hoch: '#ff7b72', mittel: '#ffa94d', niedrig: '#e3b341', info: '#8a8a99', gut: '#7ee787' };\n    var URTEIL = { gut: GUT, warnung: WARN, kritisch: ROT };\n    function gitter(sp) { return el('div', 'display: grid; grid-template-columns: ' + sp + '; gap: 0 18px; align-items: baseline;'); }\n    function paar(g, n, w, stil) { g.appendChild(el('div', ZELLE, n)); g.appendChild(el('div', ZELLE_WERT + (stil || ''), w)); }\n    function ms(v) { return v === null ? '-' : v + ' ms'; }<\/p>\n<p>    function dnsText(d) {\n        if (d.fehler) { return T.dns_fehler; }\n        if (d.n === 0) { return T.dns_keiner; }\n        if (d.alias && !d.alpn.length) { return text('dns_alias', { ziel: d.alias }); }\n        var s = text('dns_vorhanden', { alpn: d.alpn.length ? d.alpn.join(', ') : '-', ttl: d.ttl === null ? 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