diff --git a/src/network/kernel/qdnslookup.cpp b/src/network/kernel/qdnslookup.cpp
index 2d9ada7f411..04662acece6 100644
--- a/src/network/kernel/qdnslookup.cpp
+++ b/src/network/kernel/qdnslookup.cpp
@@ -977,10 +977,12 @@ void QDnsLookupRunnable::run()
     QDnsLookupReply reply;
 
     // Validate input.
-    if (requestName.isEmpty()) {
+    if (qsizetype n = requestName.size(); n > MaxDomainNameLength || n == 0) {
         reply.error = QDnsLookup::InvalidRequestError;
         reply.errorString = tr("Invalid domain name");
         emit finished(reply);
+        if (n)
+            qWarning("QDnsLookup: domain name being looked up is too long (%lld bytes)", n);
         return;
     }
 
diff --git a/src/network/kernel/qdnslookup_p.h b/src/network/kernel/qdnslookup_p.h
index 3b0c476c2a7..522cb7d0738 100644
--- a/src/network/kernel/qdnslookup_p.h
+++ b/src/network/kernel/qdnslookup_p.h
@@ -31,6 +31,7 @@ QT_BEGIN_NAMESPACE
 
 //#define QDNSLOOKUP_DEBUG
 
+constexpr qsizetype MaxDomainNameLength = 255;
 constexpr quint16 DnsPort = 53;
 
 class QDnsLookupRunnable;