diff --git a/src/basic/hashmap.h b/src/basic/hashmap.h
index ad85093d69..233f1d7a1e 100644
--- a/src/basic/hashmap.h
+++ b/src/basic/hashmap.h
@@ -415,6 +415,11 @@ static inline int set_dump_sorted(Set *h, void ***ret, size_t *ret_n) {
  * the entries were inserted.
  * It is safe to remove the current entry.
  */
+#define _HASHMAP_BASE_FOREACH(e, h, i) \
+        for (Iterator i = ITERATOR_FIRST; _hashmap_iterate((h), &i, (void**)&(e), NULL); )
+#define HASHMAP_BASE_FOREACH(e, h) \
+        _HASHMAP_BASE_FOREACH(e, h, UNIQ_T(i, UNIQ))
+
 #define _HASHMAP_FOREACH(e, h, i) \
         for (Iterator i = ITERATOR_FIRST; hashmap_iterate((h), &i, (void**)&(e), NULL); )
 #define HASHMAP_FOREACH(e, h) \
@@ -425,6 +430,11 @@ static inline int set_dump_sorted(Set *h, void ***ret, size_t *ret_n) {
 #define ORDERED_HASHMAP_FOREACH(e, h) \
         _ORDERED_HASHMAP_FOREACH(e, h, UNIQ_T(i, UNIQ))
 
+#define _HASHMAP_BASE_FOREACH_KEY(e, k, h, i) \
+        for (Iterator i = ITERATOR_FIRST; _hashmap_iterate((h), &i, (void**)&(e), (const void**) &(k)); )
+#define HASHMAP_BASE_FOREACH_KEY(e, k, h) \
+        _HASHMAP_BASE_FOREACH_KEY(e, k, h, UNIQ_T(i, UNIQ))
+
 #define _HASHMAP_FOREACH_KEY(e, k, h, i) \
         for (Iterator i = ITERATOR_FIRST; hashmap_iterate((h), &i, (void**)&(e), (const void**) &(k)); )
 #define HASHMAP_FOREACH_KEY(e, k, h) \
diff --git a/src/network/networkd-route.c b/src/network/networkd-route.c
index 587bca2b26..6cefd3a9bf 100644
--- a/src/network/networkd-route.c
+++ b/src/network/networkd-route.c
@@ -1945,7 +1945,9 @@ int network_add_ipv4ll_route(Network *network) {
         if (!network->ipv4ll_route)
                 return 0;
 
-        section_line = hashmap_find_free_section_line(network->routes_by_section);
+        r = hashmap_by_section_find_unused_line(network->routes_by_section, network->filename, &section_line);
+        if (r < 0)
+                return r;
 
         /* IPv4LLRoute= is in [Network] section. */
         r = route_new_static(network, network->filename, section_line, &n);
@@ -1978,7 +1980,9 @@ int network_add_default_route_on_device(Network *network) {
         if (!network->default_route_on_device)
                 return 0;
 
-        section_line = hashmap_find_free_section_line(network->routes_by_section);
+        r = hashmap_by_section_find_unused_line(network->routes_by_section, network->filename, &section_line);
+        if (r < 0)
+                return r;
 
         /* DefaultRouteOnDevice= is in [Network] section. */
         r = route_new_static(network, network->filename, section_line, &n);
diff --git a/src/shared/conf-parser.c b/src/shared/conf-parser.c
index 68c4b2ca1a..63030d4a2d 100644
--- a/src/shared/conf-parser.c
+++ b/src/shared/conf-parser.c
@@ -748,9 +748,13 @@ static int config_section_compare_func(const ConfigSection *x, const ConfigSecti
 
 DEFINE_HASH_OPS(config_section_hash_ops, ConfigSection, config_section_hash_func, config_section_compare_func);
 
-int config_section_new(const char *filename, unsigned line, ConfigSection **s) {
+int config_section_new(const char *filename, unsigned line, ConfigSection **ret) {
         ConfigSection *cs;
 
+        assert(filename);
+        assert(line > 0);
+        assert(ret);
+
         cs = malloc0(offsetof(ConfigSection, filename) + strlen(filename) + 1);
         if (!cs)
                 return -ENOMEM;
@@ -758,21 +762,31 @@ int config_section_new(const char *filename, unsigned line, ConfigSection **s) {
         strcpy(cs->filename, filename);
         cs->line = line;
 
-        *s = TAKE_PTR(cs);
-
+        *ret = TAKE_PTR(cs);
         return 0;
 }
 
-unsigned hashmap_find_free_section_line(Hashmap *hashmap) {
+int _hashmap_by_section_find_unused_line(
+                HashmapBase *entries_by_section,
+                const char *filename,
+                unsigned *ret) {
+
         ConfigSection *cs;
         unsigned n = 0;
         void *entry;
 
-        HASHMAP_FOREACH_KEY(entry, cs, hashmap)
-                if (n < cs->line)
-                        n = cs->line;
+        HASHMAP_BASE_FOREACH_KEY(entry, cs, entries_by_section) {
+                if (filename && !streq(cs->filename, filename))
+                        continue;
+                n = MAX(n, cs->line);
+        }
 
-        return n + 1;
+        /* overflow? */
+        if (n >= UINT_MAX)
+                return -EFBIG;
+
+        *ret = n + 1;
+        return 0;
 }
 
 #define DEFINE_PARSER(type, vartype, conv_func)                         \
diff --git a/src/shared/conf-parser.h b/src/shared/conf-parser.h
index 902717965c..140883969c 100644
--- a/src/shared/conf-parser.h
+++ b/src/shared/conf-parser.h
@@ -136,9 +136,24 @@ static inline ConfigSection* config_section_free(ConfigSection *cs) {
 }
 DEFINE_TRIVIAL_CLEANUP_FUNC(ConfigSection*, config_section_free);
 
-int config_section_new(const char *filename, unsigned line, ConfigSection **s);
+int config_section_new(const char *filename, unsigned line, ConfigSection **ret);
 extern const struct hash_ops config_section_hash_ops;
-unsigned hashmap_find_free_section_line(Hashmap *hashmap);
+int _hashmap_by_section_find_unused_line(
+                HashmapBase *entries_by_section,
+                const char *filename,
+                unsigned *ret);
+static inline int hashmap_by_section_find_unused_line(
+                Hashmap *entries_by_section,
+                const char *filename,
+                unsigned *ret) {
+        return _hashmap_by_section_find_unused_line(HASHMAP_BASE(entries_by_section), filename, ret);
+}
+static inline int ordered_hashmap_by_section_find_unused_line(
+                OrderedHashmap *entries_by_section,
+                const char *filename,
+                unsigned *ret) {
+        return _hashmap_by_section_find_unused_line(HASHMAP_BASE(entries_by_section), filename, ret);
+}
 
 static inline bool section_is_invalid(ConfigSection *section) {
         /* If this returns false, then it does _not_ mean the section is valid. */