diff --git a/CHANGES b/CHANGES
index a314b2fa3c..c7a501198d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,7 @@
+4.2.0-dev.317 | 2021-11-10 11:33:29 +0000
+
+  * Add case-insensitive search for find_str and rfind_str (Abdel)
+
 4.2.0-dev.314 | 2021-11-10 11:16:28 +0100
 
   * GH-1757: Add new hook `HookLoadFileExtended` that allows plugins
diff --git a/NEWS b/NEWS
index 2ece09bee0..d82680803e 100644
--- a/NEWS
+++ b/NEWS
@@ -38,6 +38,8 @@ New Functionality
 - The DNS analyzer has initial support for the SVCB and HTTPS types. The new events
   are ``dns_SVCB`` and ``dns_HTTPS``.
 
+- The ``find_str`` and ``rfind_str`` bifs now support case-insensitive searches.
+
 Changed Functionality
 ---------------------
 
diff --git a/VERSION b/VERSION
index 5296c1e837..1ac7ea7281 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-4.2.0-dev.314
+4.2.0-dev.317
diff --git a/src/strings.bif b/src/strings.bif
index 0d3812f267..e4a6e21851 100644
--- a/src/strings.bif
+++ b/src/strings.bif
@@ -1145,7 +1145,7 @@ function count_substr%(str: string, sub: string%) : count
 
 %%{
 
-static int64_t do_find_str(zeek::StringVal* str, zeek::StringVal* sub, int64_t start, int64_t end, bool rfind)
+static int64_t do_find_str(zeek::StringVal* str, zeek::StringVal* sub, int64_t start, int64_t end, bool rfind, bool case_sensitive)
 	{
 	// Don't bother if the start is after the end of the string.
 	if ( start > str->Len() )
@@ -1168,11 +1168,19 @@ static int64_t do_find_str(zeek::StringVal* str, zeek::StringVal* sub, int64_t s
 		return -1;
 
 	string s = str->ToStdString().substr(start, end_pos);
+	string sb = sub->ToStdString();
 	size_t pos = string::npos;
+
+	if ( ! case_sensitive )
+		{
+		transform(s.begin(), s.end(), s.begin(), ::tolower);
+		transform(sb.begin(), sb.end(), sb.begin(), ::tolower);
+		}
+
 	if ( rfind )
-		pos = s.rfind(sub->ToStdString());
+		pos = s.rfind(sb);
 	else
-		pos = s.find(sub->ToStdString());
+		pos = s.find(sb);
 
 	if ( pos == string::npos )
 		return -1;
@@ -1193,13 +1201,16 @@ static int64_t do_find_str(zeek::StringVal* str, zeek::StringVal* sub, int64_t s
 ## end: An optional position for the end of the substring. A value less than
 ##      zero (such as the default -1) means a search until the end of the
 ##      string.
+## case_sensitive: Set to false to perform a case-insensitive search.
+##                 (default: T). Note that case-insensitive searches use the
+##                 ``tolower`` libc function, which is locale-sensitive.
 ##
 ## Returns: The position of the substring. Returns -1 if the string wasn't
 ##          found. Prints an error if the starting position is after the ending
 ##          position.
-function find_str%(str: string, sub: string, start: count &default=0, end: int &default=-1%) : int
+function find_str%(str: string, sub: string, start: count &default=0, end: int &default=-1, case_sensitive: bool &default=T%) : int
 	%{
-	return zeek::val_mgr->Int(do_find_str(str, sub, start, end, false));
+	return zeek::val_mgr->Int(do_find_str(str, sub, start, end, false, case_sensitive));
 	%}
 
 ## The same as :zeek:see:`find_str`, but returns the highest index matching
@@ -1210,13 +1221,16 @@ function find_str%(str: string, sub: string, start: count &default=0, end: int &
 ## start: An optional position for the start of the substring.
 ## end: An optional position for the end of the substring. A value less than
 ##      zero (such as the default -1) means a search from the end of the string.
+## case_sensitive: Set to false to perform a case-insensitive search.
+##                 (default: T). Note that case-insensitive searches use the
+##                 ``tolower`` libc function, which is locale-sensitive.
 ##
 ## Returns: The position of the substring. Returns -1 if the string wasn't
 ##          found. Prints an error if the starting position is after the ending
 ##          position.
-function rfind_str%(str: string, sub: string, start: count &default=0, end: int &default=-1%) : int
+function rfind_str%(str: string, sub: string, start: count &default=0, end: int &default=-1, case_sensitive: bool &default=T%) : int
 	%{
-	return zeek::val_mgr->Int(do_find_str(str, sub, start, end, true));
+	return zeek::val_mgr->Int(do_find_str(str, sub, start, end, true, case_sensitive));
 	%}
 
 ## Returns whether a string starts with a substring.
diff --git a/testing/btest/Baseline/bifs.do_find_str/out b/testing/btest/Baseline/bifs.do_find_str/out
new file mode 100644
index 0000000000..2d6ffee887
--- /dev/null
+++ b/testing/btest/Baseline/bifs.do_find_str/out
@@ -0,0 +1,17 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+insensitive
+48
+48
+48
+-1
+-1
+48
+48
+sensitive
+-1
+-1
+-1
+-1
+-1
+-1
+48
diff --git a/testing/btest/bifs/do_find_str.zeek b/testing/btest/bifs/do_find_str.zeek
new file mode 100644
index 0000000000..30e33dcc9d
--- /dev/null
+++ b/testing/btest/bifs/do_find_str.zeek
@@ -0,0 +1,33 @@
+#
+# @TEST-EXEC: zeek -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+event zeek_init()
+	{
+	local a = "this is the concatenation of HTTP fields of the fOrM of the website that I am protecting";
+	local b = "form";
+	local c = "FORM";
+	local d = "FoRm";
+	local e = "om0";
+	local f = "f0rm";
+	local g = "fOrm";
+	local h = "fOrM";
+
+
+	print "insensitive";
+	print find_str(a, b, 0, -1, F);
+	print find_str(a, c, 0, -1, F);
+	print find_str(a, d, 0, -1, F);
+	print find_str(a, e, 0, -1, F);
+	print find_str(a, f, 0, -1, F);
+	print find_str(a, g, 0, -1, F);
+	print find_str(a, h, 0, -1, F);
+	print "sensitive";
+	print find_str(a, b, 0, -1);
+	print find_str(a, c, 0, -1);
+	print find_str(a, d, 0, -1);
+	print find_str(a, e, 0, -1);
+	print find_str(a, f, 0, -1);
+	print find_str(a, g, 0, -1);
+	print find_str(a, h, 0, -1);
+	}