Warn for tables with compat expressions in rules

While being able to "look inside" compat expressions using nft is a nice
feature, it is also (yet another) pitfall for unaware users, deceiving
them into assuming interchangeability (or at least compatibility)
between iptables-nft and nft.

In reality, which involves 'nft list ruleset | nft -f -', any correctly
translated compat expressions will turn into native nftables ones not
understood by (the version of) iptables-nft which created them in the
first place. Other compat expressions will vanish, potentially
compromising the firewall ruleset.

Emit a warning (as comment) to give users a chance to stop and
reconsider before shooting their own foot.

Signed-off-by: Phil Sutter <phil@nwl.cc>

3 files changed, 16 insertions, 3 deletions

diff --git a/include/rule.h b/include/rule.h
index ad9f9127..00a1bac5 100644
--- a/include/rule.h
+++ b/include/rule.h
@@ -169,6 +169,7 @@ struct table {
 	unsigned int		refcnt;
 	uint32_t		owner;
 	const char		*comment;
+	bool			has_xt_stmts;
 };
 
 extern struct table *table_alloc(void);
diff --git a/src/rule.c b/src/rule.c
index d1ee6c2e..1402210a 100644
--- a/src/rule.c
+++ b/src/rule.c
@@ -1233,6 +1233,11 @@ static void table_print(const struct table *table, struct output_ctx *octx)
 	const char *delim = "";
 	const char *family = family2str(table->handle.family);
 
+	if (table->has_xt_stmts)
+		fprintf(octx->error_fp,
+			"# Warning: table %s %s is managed by iptables-nft, do not touch!\n",
+			family, table->handle.table.name);
+
 	nft_print(octx, "table %s %s {", family, table->handle.table.name);
 	if (nft_output_handle(octx) || table->flags & TABLE_F_OWNER)
 		nft_print(octx, " #");
@@ -2387,9 +2392,14 @@ static int do_list_tables(struct netlink_ctx *ctx, struct cmd *cmd)
 static void table_print_declaration(struct table *table,
 				    struct output_ctx *octx)
 {
-	nft_print(octx, "table %s %s {\n",
-		  family2str(table->handle.family),
-		  table->handle.table.name);
+	const char *family = family2str(table->handle.family);
+
+	if (table->has_xt_stmts)
+		fprintf(octx->error_fp,
+			"# Warning: table %s %s is managed by iptables-nft, do not touch!\n",
+			family, table->handle.table.name);
+
+	nft_print(octx, "table %s %s {\n", family, table->handle.table.name);
 }
 
 static int do_list_chain(struct netlink_ctx *ctx, struct cmd *cmd,
diff --git a/src/xt.c b/src/xt.c
index 789de992..a5417352 100644
--- a/src/xt.c
+++ b/src/xt.c
@@ -238,6 +238,7 @@ void netlink_parse_match(struct netlink_parse_ctx *ctx,
 	stmt->xt.name = strdup(name);
 	stmt->xt.type = NFT_XT_MATCH;
 #endif
+	ctx->table->has_xt_stmts = true;
 	rule_stmt_append(ctx->rule, stmt);
 }
 
@@ -283,6 +284,7 @@ void netlink_parse_target(struct netlink_parse_ctx *ctx,
 	stmt->xt.name = strdup(name);
 	stmt->xt.type = NFT_XT_TARGET;
 #endif
+	ctx->table->has_xt_stmts = true;
 	rule_stmt_append(ctx->rule, stmt);
 }