diff --git a/lib/util.c b/lib/util.c
index 1f0191a0..d43b8e97 100644
--- a/lib/util.c
+++ b/lib/util.c
@@ -1,3 +1,6 @@
+#define _DARWIN_C_SOURCE /* strnstr */
+#define _GNU_SOURCE      /* strnlen */
+
 #include <assert.h>
 #include <errno.h>
 #include <stdlib.h>
@@ -41,3 +44,25 @@ zalloc(size_t sz)
         return p;
 }
 #endif
+
+char *
+xstrnstr(const char *haystack, const char *needle, size_t len)
+{
+#if defined(__APPLE__)
+        return strnstr(haystack, needle, len);
+#else
+        size_t haystack_len = strnlen(haystack, len);
+        size_t needle_len = strlen(needle);
+        if (needle_len > haystack_len) {
+                return NULL;
+        }
+        size_t max_offset = haystack_len - needle_len;
+        size_t i;
+        for (i = 0; i <= max_offset; i++) {
+                if (!memcmp(&haystack[i], needle, needle_len)) {
+                        return (char *)&haystack[i]; /* discard const */
+                }
+        }
+        return NULL;
+#endif
+}
diff --git a/lib/util.h b/lib/util.h
index 302c7fb3..82178d32 100644
--- a/lib/util.h
+++ b/lib/util.h
@@ -19,3 +19,5 @@ void *__must_check zalloc(size_t sz) __malloc_like __alloc_size(1);
 #define ZERO(p) memset(p, 0, sizeof(*p))
 
 #define HOWMANY(a, b) ((a + (b - 1)) / b)
+
+char *xstrnstr(const char *haystack, const char *needle, size_t len);