Add a mockable resolver backend and a DNS resolver/cache self-test

Route the getaddrinfo resolver through a swappable backend pair
(getaddrinfo/freeaddrinfo) that defaults to the libc resolver, so the
self-test can script DNS answers in-process with no network. The pair is
needed because a fake allocates its own addrinfo chain and must free it
with a matching deallocator.

Drive it from a new 'httrack -#D' debug subcommand backed by
htsdns_selftest.c: a scripted getaddrinfo checks address family,
single-address selection, the -@i4/-@i6 family filter, negative caching,
and that a cached host is resolved only once. tests/01_engine-dns.test
runs it.

No behavior change: the default backend is the libc resolver, one
indirect call on the cold resolve path. The seam stays internal (no
HTSEXT_API), so the exported ABI is unchanged. This is groundwork for the
multi-address record and connect fallback that fix the dead-IPv6 connect
stall; the dual-stack assertion pins today's single-address behavior and
will widen with that change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Xavier Roche <roche@httrack.com>

src/Makefile.am

@@ -56,7 +56,7 @@ whttrackrundir = $(bindir)
 whttrackrun_SCRIPTS = webhttrack
 
 libhttrack_la_SOURCES =  htscore.c htsparse.c htsback.c htscache.c \
-	htscache_selftest.c \
+	htscache_selftest.c htsdns_selftest.c \
 	htscatchurl.c htsfilters.c htsftp.c htshash.c coucal/coucal.c \
 	htshelp.c htslib.c htscoremain.c \
 	htsname.c htsrobots.c htstools.c htswizard.c \
@@ -66,7 +66,7 @@ libhttrack_la_SOURCES =  htscore.c htsparse.c htsback.c htscache.c \
 	md5.c \
 	minizip/ioapi.c minizip/mztools.c minizip/unzip.c minizip/zip.c \
 	hts-indextmpl.h htsalias.h htsback.h htsbase.h htssafe.h \
-	htsbasenet.h htsbauth.h htscache.h htscache_selftest.h htscatchurl.h  \
+	htsbasenet.h htsbauth.h htscache.h htscache_selftest.h htsdns_selftest.h htscatchurl.h  \
 	htsconfig.h htscore.h htsparse.h htscoremain.h htsdefines.h  \
 	htsfilters.h htsftp.h htsglobal.h htshash.h coucal/coucal.h \
 	htshelp.h htsindex.h htslib.h htsmd5.h \

src/htscoremain.c

@@ -47,6 +47,7 @@ Please visit our Website: http://www.httrack.com
 #include "htscharset.h"
 #include "htsencoding.h"
 #include "htscache_selftest.h"
+#include "htsdns_selftest.h"
 #include "htsmd5.h"
 
 #include <ctype.h>
@@ -2460,6 +2461,13 @@ static int hts_main_internal(int argc, char **argv, httrackp * opt) {
                   return 1;
                 }
                 break;
+              case 'D': { // DNS resolver/cache self-test (mock getaddrinfo)
+                const int err = dns_selftests(opt);
+
+                printf("dns-selftest: %s\n", err ? "FAIL" : "OK");
+                htsmain_free();
+                return err;
+              } break;
               case 'C':        // list cache files : httrack -#C '*spid*.gif' will attempt to find the matching file
                 {
                   int hasFilter = 0;

src/htsdns_selftest.c

@@ -0,0 +1,254 @@
+/* ------------------------------------------------------------ */
+/*
+HTTrack Website Copier, Offline Browser for Windows and Unix
+Copyright (C) 2026 Xavier Roche and other contributors
+
+SPDX-License-Identifier: GPL-3.0-or-later
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+Ethical use: we kindly ask that you NOT use this software to harvest email
+addresses or to collect any other private information about people. Doing so
+would dishonor our work and waste the many hours we have spent on it.
+
+Please visit our Website: http://www.httrack.com
+*/
+
+/* ------------------------------------------------------------ */
+/* File: htsdns_selftest.c subroutines:                         */
+/*       in-process self-test for the DNS resolver and cache    */
+/* Author: Xavier Roche                                         */
+/* ------------------------------------------------------------ */
+
+/* Routes the resolver through a scripted getaddrinfo (hts_resolver_backend)
+   instead of the network, so resolution and the DNS cache are testable for a
+   fixed set of scenarios (IPv4/IPv6/dual-stack, errors, family filter,
+   cache reuse) with no live DNS. */
+
+#define HTS_INTERNAL_BYTECODE
+
+#include "htsdns_selftest.h"
+
+#include "htscore.h"
+#include "htslib.h"
+#include "htsnet.h"
+
+#include <stdio.h>
+#include <string.h>
+
+#if HTS_INET6 != 0
+
+/* IPV6_resolver: 0 = v4+v6, 1 = v4 only, 2 = v6 only (htscoremain -@i). */
+extern int IPV6_resolver;
+
+/* One scripted host: either a getaddrinfo error, or an ordered address list. */
+typedef struct mock_addr {
+  int family;             /* AF_INET / AF_INET6 */
+  unsigned char addr[16]; /* 4 (v4) or 16 (v6) meaningful bytes */
+} mock_addr;
+
+typedef struct mock_host {
+  const char *name;
+  int gai_err; /* non-zero: getaddrinfo returns this */
+  int naddr;
+  mock_addr addr[3];
+  int calls; /* times the backend resolved this host */
+} mock_host;
+
+static mock_host mock_hosts[] = {
+    {"v4only.test", 0, 1, {{AF_INET, {1, 2, 3, 4}}}, 0},
+    {"v6only.test", 0, 1, {{AF_INET6, {0x20, 0x01, 0x0d, 0xb8, [15] = 1}}}, 0},
+    /* dual stack, IPv6 first (RFC 6724 order) then IPv4 */
+    {"dual.test",
+     0,
+     2,
+     {{AF_INET6, {0x20, 0x01, 0x0d, 0xb8, [15] = 2}}, {AF_INET, {5, 6, 7, 8}}},
+     0},
+    /* dual stack, IPv4 first: distinguishes "keep the first address" from
+       "prefer a family", so the selection contract is actually pinned. */
+    {"dual4.test",
+     0,
+     2,
+     {{AF_INET, {9, 10, 11, 12}},
+      {AF_INET6, {0x20, 0x01, 0x0d, 0xb8, [15] = 3}}},
+     0},
+    {"nodns.test", EAI_NONAME, 0, {{0}}, 0},
+};
+
+static mock_host *mock_find(const char *name) {
+  for (size_t i = 0; i < sizeof(mock_hosts) / sizeof(mock_hosts[0]); i++) {
+    if (strcmp(mock_hosts[i].name, name) == 0)
+      return &mock_hosts[i];
+  }
+  return NULL;
+}
+
+static void mock_reset_calls(void) {
+  for (size_t i = 0; i < sizeof(mock_hosts) / sizeof(mock_hosts[0]); i++)
+    mock_hosts[i].calls = 0;
+}
+
+/* Build one addrinfo node owning its sockaddr (freed by mock_freeaddrinfo). */
+static struct addrinfo *mock_mkai(const mock_addr *a) {
+  struct addrinfo *ai = calloct(1, sizeof(*ai));
+
+  ai->ai_family = a->family;
+  if (a->family == AF_INET) {
+    struct sockaddr_in *sin = calloct(1, sizeof(*sin));
+
+    sin->sin_family = AF_INET;
+    memcpy(&sin->sin_addr, a->addr, 4);
+    ai->ai_addr = (struct sockaddr *) sin;
+    ai->ai_addrlen = sizeof(*sin);
+  } else {
+    struct sockaddr_in6 *sin6 = calloct(1, sizeof(*sin6));
+
+    sin6->sin6_family = AF_INET6;
+    memcpy(&sin6->sin6_addr, a->addr, 16);
+    ai->ai_addr = (struct sockaddr *) sin6;
+    ai->ai_addrlen = sizeof(*sin6);
+  }
+  return ai;
+}
+
+static int mock_getaddrinfo(const char *node, const char *service,
+                            const struct addrinfo *hints,
+                            struct addrinfo **res) {
+  mock_host *const h = mock_find(node);
+  const int want = (hints != NULL) ? hints->ai_family : PF_UNSPEC;
+  struct addrinfo *head = NULL, *tail = NULL;
+
+  (void) service;
+  *res = NULL;
+  if (h == NULL)
+    return EAI_NONAME;
+  h->calls++; /* a real backend hit; a cached host skips this */
+  if (h->gai_err != 0)
+    return h->gai_err;
+  for (int i = 0; i < h->naddr; i++) {
+    if (want != PF_UNSPEC && want != h->addr[i].family)
+      continue; /* honor the requested family (v4/v6 only) */
+    struct addrinfo *const ai = mock_mkai(&h->addr[i]);
+
+    if (head == NULL)
+      head = ai;
+    else
+      tail->ai_next = ai;
+    tail = ai;
+  }
+  if (head == NULL)
+    return EAI_NONAME; /* filtered to empty, as the libc resolver does */
+  *res = head;
+  return 0;
+}
+
+static void mock_freeaddrinfo(struct addrinfo *res) {
+  while (res != NULL) {
+    struct addrinfo *const next = res->ai_next;
+
+    freet(res->ai_addr);
+    freet(res);
+    res = next;
+  }
+}
+
+static const hts_resolver_backend mock_backend = {mock_getaddrinfo,
+                                                  mock_freeaddrinfo};
+
+static int failures = 0;
+
+#define CHECK(cond)                                                            \
+  do {                                                                         \
+    if (!(cond)) {                                                             \
+      failures++;                                                              \
+      fprintf(stderr, "dns-selftest: FAIL at %s:%d: %s\n", __FILE__, __LINE__, \
+              #cond);                                                          \
+    }                                                                          \
+  } while (0)
+
+/* Resolve via the uncached entry point; return the address family, or
+   AF_UNSPEC if the host did not resolve. */
+static int resolve_family_nocache(const char *host) {
+  SOCaddr addr;
+  const char *err = NULL;
+
+  if (hts_dns_resolve_nocache2(host, &addr, &err) == NULL)
+    return AF_UNSPEC;
+  return SOCaddr_sinfamily(addr);
+}
+
+int dns_selftests(httrackp *opt) {
+  failures = 0;
+  hts_dns_set_resolver_backend(&mock_backend);
+
+  /* IPv4-only / IPv6-only hosts map to the right family. */
+  IPV6_resolver = 0;
+  CHECK(resolve_family_nocache("v4only.test") == AF_INET);
+  CHECK(resolve_family_nocache("v6only.test") == AF_INET6);
+
+  /* Dual-stack: the current resolver keeps only the *first* address. Both
+     orderings pin that (not a family preference); PR2 (multi-address) widens
+     it. */
+  CHECK(resolve_family_nocache("dual.test") == AF_INET6); /* v6 listed first */
+  CHECK(resolve_family_nocache("dual4.test") == AF_INET); /* v4 listed first */
+
+  /* Unknown host does not resolve. */
+  CHECK(resolve_family_nocache("nodns.test") == AF_UNSPEC);
+
+  /* Family filter (-@i4 / -@i6) selects v4 / v6 out of the dual-stack host. */
+  IPV6_resolver = 1;
+  CHECK(resolve_family_nocache("dual.test") == AF_INET);
+  IPV6_resolver = 2;
+  CHECK(resolve_family_nocache("dual.test") == AF_INET6);
+  IPV6_resolver = 0;
+
+  /* Cached driver resolves a host once and reuses the *same* address. */
+  mock_reset_calls();
+  {
+    SOCaddr a1, a2;
+    char ip1[64], ip2[64];
+    const char *err = NULL;
+
+    CHECK(hts_dns_resolve2(opt, "v4only.test", &a1, &err) != NULL);
+    CHECK(hts_dns_resolve2(opt, "v4only.test", &a2, &err) != NULL);
+    CHECK(mock_find("v4only.test")->calls == 1);
+    /* the cache returns the right address, not merely a hit for the key */
+    SOCaddr_inetntoa(ip1, sizeof(ip1), a1);
+    SOCaddr_inetntoa(ip2, sizeof(ip2), a2);
+    CHECK(strcmp(ip1, "1.2.3.4") == 0);
+    CHECK(strcmp(ip1, ip2) == 0);
+  }
+
+  /* A negative result is cached too: a second lookup does not re-resolve. */
+  {
+    SOCaddr a1, a2;
+    const char *err = NULL;
+
+    CHECK(hts_dns_resolve2(opt, "nodns.test", &a1, &err) == NULL);
+    CHECK(hts_dns_resolve2(opt, "nodns.test", &a2, &err) == NULL);
+    CHECK(mock_find("nodns.test")->calls == 1); /* resolved once, then cached */
+  }
+
+  hts_dns_set_resolver_backend(NULL);
+  return failures;
+}
+
+#else
+
+int dns_selftests(httrackp *opt) {
+  (void) opt;
+  return 0; /* resolver seam only exists in the IPv6 build */
+}
+
+#endif

src/htsdns_selftest.h

@@ -0,0 +1,51 @@
+/* ------------------------------------------------------------ */
+/*
+HTTrack Website Copier, Offline Browser for Windows and Unix
+Copyright (C) 2026 Xavier Roche and other contributors
+
+SPDX-License-Identifier: GPL-3.0-or-later
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+Ethical use: we kindly ask that you NOT use this software to harvest email
+addresses or to collect any other private information about people. Doing so
+would dishonor our work and waste the many hours we have spent on it.
+
+Please visit our Website: http://www.httrack.com
+*/
+
+/* ------------------------------------------------------------ */
+/* File: htsdns_selftest.h                                      */
+/* Author: Xavier Roche                                         */
+/* ------------------------------------------------------------ */
+
+#ifndef HTSDNS_SELFTEST_DEFH
+#define HTSDNS_SELFTEST_DEFH
+
+#ifdef HTS_INTERNAL_BYTECODE
+
+#ifndef HTS_DEF_FWSTRUCT_httrackp
+#define HTS_DEF_FWSTRUCT_httrackp
+typedef struct httrackp httrackp;
+#endif
+
+/* Drive the DNS resolver and cache through a scripted (mock) getaddrinfo,
+   asserting address family, single-address selection, negative caching, the
+   IPv4/IPv6 family filter, and that a cached host is resolved only once.
+   Returns the number of failed checks (0 == success). */
+int dns_selftests(httrackp *opt);
+
+#endif
+
+#endif

src/htslib.c

@@ -4815,8 +4815,21 @@ static SOCaddr* hts_ghbn(const t_dnscache *cache, const char *const iadr, SOCadd
   return NULL;
 }
 
-static SOCaddr* hts_dns_resolve_nocache2_(const char *const hostname,
-                                          SOCaddr *const addr, 
+#if HTS_INET6 != 0
+/* Active resolver backend; defaults to the libc resolver. The self-test
+   reroutes it to script DNS answers in-process (see
+   hts_dns_set_resolver_backend). */
+static const hts_resolver_backend hts_resolver_libc = {getaddrinfo,
+                                                       freeaddrinfo};
+static const hts_resolver_backend *hts_resolver = &hts_resolver_libc;
+
+void hts_dns_set_resolver_backend(const hts_resolver_backend *backend) {
+  hts_resolver = (backend != NULL) ? backend : &hts_resolver_libc;
+}
+#endif
+
+static SOCaddr *hts_dns_resolve_nocache2_(const char *const hostname,
+                                          SOCaddr *const addr,
                                           const char **error) {
   {
 #if HTS_INET6==0
@@ -4845,7 +4858,7 @@ static SOCaddr* hts_dns_resolve_nocache2_(const char *const hostname,
       hints.ai_family = PF_UNSPEC;
     hints.ai_socktype = SOCK_STREAM;
     hints.ai_protocol = IPPROTO_TCP;
-    if ( ( gerr = getaddrinfo(hostname, NULL, &hints, &res) ) == 0) {
+    if ((gerr = hts_resolver->getaddrinfo(hostname, NULL, &hints, &res)) == 0) {
       if (res != NULL) {
         if (res->ai_addr != NULL && res->ai_addrlen != 0) {
           SOCaddr_copyaddr2(*addr, res->ai_addr, res->ai_addrlen);
@@ -4857,7 +4870,7 @@ static SOCaddr* hts_dns_resolve_nocache2_(const char *const hostname,
       }
     }
     if (res) {
-      freeaddrinfo(res);
+      hts_resolver->freeaddrinfo(res);
     }
 #endif
   }

src/htsnet.h

@@ -304,6 +304,22 @@ static HTS_UNUSED void SOCaddr_inetntoa_(char *namebuf, size_t namebuflen,
 /** Length type for socket APIs (getsockname, accept, ...). */
 typedef socklen_t SOClen;
 
+#if HTS_INET6 != 0
+/** Resolver backend: getaddrinfo/freeaddrinfo as a swappable pair, so the
+    self-test can script DNS answers (families, multiplicity, errors)
+    in-process. The free function must match its getaddrinfo (a fake allocates
+    its own chain), hence the pair. */
+typedef struct hts_resolver_backend {
+  int (*getaddrinfo)(const char *node, const char *service,
+                     const struct addrinfo *hints, struct addrinfo **res);
+  void (*freeaddrinfo)(struct addrinfo *res);
+} hts_resolver_backend;
+
+/** Install a resolver backend for the process; NULL restores the libc default.
+    Test-only seam, not thread-safe; callers must serialize against resolves. */
+void hts_dns_set_resolver_backend(const hts_resolver_backend *backend);
+#endif
+
 #ifdef __cplusplus
 }
 #endif

tests/01_engine-dns.test

@@ -0,0 +1,15 @@
+#!/bin/bash
+#
+
+set -euo pipefail
+
+# DNS resolver/cache self-test: a mock getaddrinfo (no network) checks address
+# family, single-address selection, the -@i4/-@i6 family filter, and cache reuse.
+# The trailing token is required, like the other -# selftests, so a bare command
+# line isn't treated as "no arguments" and routed to the usage screen.
+out=$(httrack -#D run)
+
+test "$out" = "dns-selftest: OK" || {
+    echo "expected 'dns-selftest: OK', got: $out" >&2
+    exit 1
+}

tests/Makefile.am

@@ -29,6 +29,7 @@ TESTS = \
 	01_engine-cmdline.test \
 	01_engine-cookies.test \
 	01_engine-copyopt.test \
+	01_engine-dns.test \
 	01_engine-doitlog.test \
 	01_engine-entities.test \
 	01_engine-filter.test \