QDnsLookup/Unix: modernize the setting of IPv6 server addresses

Instead of using #if, this now uses SFINAE to detect the presence of the
glibc extensions to set IPv6 nameserver addresses. It's also possible
that this fixes some bugs that have always been there, but never checked
because we don't have a way to unit-test explicit name servers.

To that effect, this commit adds a manual unit test that mimics the BIND
tool "dig". When running:

 ./qdnslookup qt-project.org any @dns.google

it printed for me:

; <<>> QDnsLookup 6.6.0
;; status: NoError
;; QUESTION:
;qt-project.org                 IN ANY

;; ANSWER:
qt-project.org            3600  IN MX   10 mx.qt-project.org
qt-project.org            3600  IN NS   ns14.cloudns.net
qt-project.org            3600  IN NS   ns11.cloudns.net
qt-project.org            3600  IN NS   ns12.cloudns.net
qt-project.org            3600  IN NS   ns13.cloudns.net
qt-project.org            3600  IN A    52.18.144.254
qt-project.org            3600  IN TXT  "v=spf1 mx ip4:193.209.87.4 include:spf.protection.outlook.com ~all"

;; Query time: 241 ms
;; SERVER: 2001:4860:4860::8844#53

strace confirms the DNS queries were sent to the correct address.

Change-Id: I3e3bfef633af4130a03afffd175d56a92371ed16
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

1 files changed, 68 insertions, 38 deletions

diff --git a/src/network/kernel/qdnslookup_unix.cpp b/src/network/kernel/qdnslookup_unix.cpp
index ceab943a04..240ace3c3c 100644
--- a/src/network/kernel/qdnslookup_unix.cpp
+++ b/src/network/kernel/qdnslookup_unix.cpp
@@ -1,4 +1,5 @@
 // Copyright (C) 2012 Jeremy Lainé <jeremy.laine@m4x.org>
+// Copyright (C) 2023 Intel Corporation.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 #include "qdnslookup_p.h"
@@ -6,7 +7,7 @@
 #include <qscopedpointer.h>
 #include <qurl.h>
 #include <qvarlengtharray.h>
-#include <private/qnativesocketengine_p.h>      // for SetSALen
+#include <private/qnativesocketengine_p.h>      // for setSockAddr
 #include <private/qtnetwork-config_p.h>
 
 QT_REQUIRE_CONFIG(res_ninit);
@@ -23,6 +24,67 @@ QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
 
+template <typename T> void setNsMap(T &ext, std::enable_if_t<sizeof(T::nsmap) != 0, uint16_t> v)
+{
+    // Set nsmap[] to indicate that nsaddrs[0] is an IPv6 address
+    // See: https://sourceware.org/ml/libc-hacker/2002-05/msg00035.html
+    // Unneeded since glibc 2.22 (2015), but doesn't hurt to set it
+    // See: https://sourceware.org/git/?p=glibc.git;a=commit;h=2212c1420c92a33b0e0bd9a34938c9814a56c0f7
+    ext.nsmap[0] = v;
+}
+template <typename T> void setNsMap(T &, ...)
+{
+    // fallback
+}
+
+template <bool Condition>
+using EnableIfIPv6 = std::enable_if_t<Condition, const QHostAddress *>;
+
+template <typename State>
+bool setIpv6NameServer(State *state,
+                       EnableIfIPv6<sizeof(std::declval<State>()._u._ext.nsaddrs) != 0> addr,
+                       quint16 port)
+{
+    // glibc-like API to set IPv6 name servers
+    struct sockaddr_in6 *ns = state->_u._ext.nsaddrs[0];
+
+    // nsaddrs will be NULL if no nameserver is set in /etc/resolv.conf
+    if (!ns) {
+        // Memory allocated here will be free()'d in res_close() as we
+        // have done res_init() above.
+        ns = static_cast<struct sockaddr_in6*>(calloc(1, sizeof(struct sockaddr_in6)));
+        Q_CHECK_PTR(ns);
+        state->_u._ext.nsaddrs[0] = ns;
+    }
+
+    setNsMap(state->_u._ext, MAXNS + 1);
+    state->_u._ext.nscount6 = 1;
+    setSockaddr(ns, *addr, port);
+    return true;
+}
+
+template <typename State> bool setIpv6NameServer(State *, const void *, quint16)
+{
+    // fallback
+    return false;
+}
+
+static const char *applyNameServer(res_state state, const QHostAddress &nameserver, quint16 port)
+{
+    if (nameserver.isNull())
+        return nullptr;
+
+    state->nscount = 1;
+    state->nsaddr_list[0].sin_family = AF_UNSPEC;
+    if (nameserver.protocol() == QAbstractSocket::IPv6Protocol) {
+        if (!setIpv6NameServer(state, &nameserver, port))
+            return QDnsLookupPrivate::msgNoIpV6NameServerAdresses;
+    } else {
+        setSockaddr(&state->nsaddr_list[0], nameserver, port);
+    }
+    return nullptr;
+}
+
 void QDnsLookupRunnable::query(const int requestType, const QByteArray &requestName, const QHostAddress &nameserver, QDnsLookupReply *reply)
 {
     // Initialize state.
@@ -35,43 +97,11 @@ void QDnsLookupRunnable::query(const int requestType, const QByteArray &requestN
     auto guard = qScopeGuard([&] { res_nclose(&state); });
 
     //Check if a nameserver was set. If so, use it
-    if (!nameserver.isNull()) {
-        if (nameserver.protocol() == QAbstractSocket::IPv4Protocol) {
-            state.nsaddr_list[0].sin_addr.s_addr = htonl(nameserver.toIPv4Address());
-            state.nscount = 1;
-        } else if (nameserver.protocol() == QAbstractSocket::IPv6Protocol) {
-#if defined(Q_OS_LINUX)
-            struct sockaddr_in6 *ns;
-            ns = state._u._ext.nsaddrs[0];
-            // nsaddrs will be NULL if no nameserver is set in /etc/resolv.conf
-            if (!ns) {
-                // Memory allocated here will be free'd in res_close() as we
-                // have done res_init() above.
-                ns = (struct sockaddr_in6*) calloc(1, sizeof(struct sockaddr_in6));
-                Q_CHECK_PTR(ns);
-                state._u._ext.nsaddrs[0] = ns;
-            }
-#ifndef __UCLIBC__
-            // Set nsmap[] to indicate that nsaddrs[0] is an IPv6 address
-            // See: https://sourceware.org/ml/libc-hacker/2002-05/msg00035.html
-            state._u._ext.nsmap[0] = MAXNS + 1;
-#endif
-            state._u._ext.nscount6 = 1;
-            ns->sin6_family = AF_INET6;
-            ns->sin6_port = htons(53);
-            SetSALen::set(ns, sizeof(*ns));
-
-            Q_IPV6ADDR ipv6Address = nameserver.toIPv6Address();
-            for (int i=0; i<16; i++) {
-                ns->sin6_addr.s6_addr[i] = ipv6Address[i];
-            }
-#else
-            qWarning("%s", QDnsLookupPrivate::msgNoIpV6NameServerAdresses);
-            reply->error = QDnsLookup::ResolverError;
-            reply->errorString = tr(QDnsLookupPrivate::msgNoIpV6NameServerAdresses);
-            return;
-#endif
-        }
+    if (const char *msg = applyNameServer(&state, nameserver, DnsPort)) {
+        qWarning("%s", msg);
+        reply->error = QDnsLookup::ResolverError;
+        reply->errorString = tr(msg);
+        return;
     }
 #ifdef QDNSLOOKUP_DEBUG
     state.options |= RES_DEBUG;