Hi Phil,
On Wed, Jul 28, 2021 at 03:43:47AM +0200, Phil Sutter wrote:
> You might want to check iptables commit ccf154d7420c0 ("xtables: Don't
> use native nftables comments") for reference, it does the opposite of
> what you want to do.
I went ahead and looked through this commit and also found found the
code that initially added this functionality; commit d64ef34a9961
("iptables-compat: use nft built-in comments support ").
Additionally I found some other commits that moved code to nft native
implementations of the xtables counterpart so that proved helpful.
After a couple days of research I did end up figuring out what to do
and have added a (mostly complete) native nft log support in
iptables-nft. It all seems to work without any kernel changes
required. The only problem I'm now faced with is that since we want to
take the string passed into the iptables-nft command and add it to the
nftnl expression (`NFTNL_EXPR_LOG_PREFIX`) I'm not entirely sure where
to get the original sized string from aside from `argv` in the `struct
iptables_command_state`. I would get it from the `struct
xt_nflog_info`, but that's going to store the truncated version and we
would like to be able to store 128 characters of the string as opposed
to 64.
Any recommendations about how I might do this safely?
An example of the program running with my patch:
kyle@debian:~/netfilter/iptables$ sudo /usr/local/sbin/iptables-nft -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-N test-chain
kyle@debian:~/netfilter/iptables$ sudo /usr/local/sbin/iptables-nft -A
test-chain -j NFLOG --nflog-prefix "this string is hard coded for
testing so what I put here doesn't end up in the prefix"
kyle@debian:~/netfilter/iptables$ sudo /usr/local/sbin/iptables-nft -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-N test-chain
-A test-chain -j NFLOG --nflog-prefix "iff the value at the end is 12
then this string is truncated 12"
kyle@debian:~/netfilter/iptables$ sudo nft list ruleset
table ip filter {
chain test-chain {
counter packets 0 bytes 0 log prefix "iff the value at the end is
12 then this string is truncated 123"
}
[...]
}
See below for the patch:
>From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Kbowman <kbowman@xxxxxxxxxxxxxx>
Date: Thu, 29 Jul 2021 15:12:28 -0500
Subject: [PATCH] iptables-nft: use nft built-in logging instead of xt_NFLOG
Replaces the use of xt_NFLOG with the nft built-in log statement.
This additionally adds support for using longer log prefixes of 128
characters in size. A caveat to this is that the string will be
truncated when the rule is printed via iptables-nft but will remain
untruncated in nftables.
Some changes have also been made to nft_is_expr_compatible() since
xt_NFLOG does not support log level or log flags. With the new changes
this means that when a log is used and sets either NFTNL_EXPR_LOG_LEVEL
or NFTNL_LOG_FLAGS to a value aside from their default (log level
defaults to 4, log flags will not be set) this will produce a
compatibility error.
---
iptables/nft-shared.c | 45 +++++++++++++++++++++++++++++++++++++++++++
iptables/nft.c | 38 ++++++++++++++++++++++++++++++++++++
iptables/nft.h | 1 +
3 files changed, 84 insertions(+)
diff --git a/iptables/nft-shared.c b/iptables/nft-shared.c
index 4253b081..b5259db0 100644
--- a/iptables/nft-shared.c
+++ b/iptables/nft-shared.c
@@ -22,6 +22,7 @@
#include <linux/netfilter/xt_comment.h>
#include <linux/netfilter/xt_limit.h>
+#include <linux/netfilter/xt_NFLOG.h>
#include <libmnl/libmnl.h>
#include <libnftnl/rule.h>
@@ -595,6 +596,48 @@ static void nft_parse_limit(struct nft_xt_ctx *ctx, struct nftnl_expr *e)
ctx->h->ops->parse_match(match, ctx->cs);
}
+static void nft_parse_log(struct nft_xt_ctx *ctx, struct nftnl_expr *e)
+{
+ __u16 group = nftnl_expr_get_u16(e, NFTNL_EXPR_LOG_GROUP);
+ __u16 qthreshold = nftnl_expr_get_u16(e, NFTNL_EXPR_LOG_QTHRESHOLD);
+ __u32 snaplen = nftnl_expr_get_u32(e, NFTNL_EXPR_LOG_SNAPLEN);
+ const char *prefix = nftnl_expr_get_str(e, NFTNL_EXPR_LOG_PREFIX);
+ struct xtables_target *target;
+ struct xt_entry_target *t;
+ size_t target_size;
+
+ void *data = ctx->cs;
+
+ target = xtables_find_target("NFLOG", XTF_TRY_LOAD);
+ if (target == NULL)
+ return;
+
+ target_size = XT_ALIGN(sizeof(struct xt_entry_target)) + target->size;
+
+ t = xtables_calloc(1, target_size);
+ t->u.target_size = target_size;
+ strcpy(t->u.user.name, target->name);
+ t->u.user.revision = target->revision;
+
+ target->t = t;
+
+ struct xt_nflog_info *info = xtables_malloc(sizeof(struct xt_nflog_info));
+ info->group = group;
+ info->len = snaplen;
+ info->threshold = qthreshold;
+
+ /* Here, because we allow 128 characters in nftables but only 64
+ * characters in xtables (in xt_nflog_info specifically), we may
+ * end up truncating the string when parsing it.
+ */
+ strncpy(info->prefix, prefix, sizeof(info->prefix));
+ info->prefix[sizeof(info->prefix) - 1] = '\0';
+
+ memcpy(&target->t->data, info, target->size);
+
+ ctx->h->ops->parse_target(target, data);
+}
+
static void nft_parse_lookup(struct nft_xt_ctx *ctx, struct nft_handle *h,
struct nftnl_expr *e)
{
@@ -644,6 +687,8 @@ void nft_rule_to_iptables_command_state(struct nft_handle *h,
nft_parse_limit(&ctx, expr);
else if (strcmp(name, "lookup") == 0)
nft_parse_lookup(&ctx, h, expr);
+ else if (strcmp(name, "log") == 0)
+ nft_parse_log(&ctx, expr);
expr = nftnl_expr_iter_next(iter);
}
diff --git a/iptables/nft.c b/iptables/nft.c
index f1deb82f..dce8fe0b 100644
--- a/iptables/nft.c
+++ b/iptables/nft.c
@@ -39,6 +39,7 @@
#include <linux/netfilter/nf_tables_compat.h>
#include <linux/netfilter/xt_limit.h>
+#include <linux/netfilter/xt_NFLOG.h>
#include <libmnl/libmnl.h>
#include <libnftnl/gen.h>
@@ -1340,6 +1341,8 @@ int add_action(struct nftnl_rule *r, struct iptables_command_state *cs,
ret = add_verdict(r, NF_DROP);
else if (strcmp(cs->jumpto, XTC_LABEL_RETURN) == 0)
ret = add_verdict(r, NFT_RETURN);
+ else if (strcmp(cs->jumpto, "NFLOG") == 0)
+ ret = add_log(r, cs);
else
ret = add_target(r, cs->target->t);
} else if (strlen(cs->jumpto) > 0) {
@@ -1352,6 +1355,36 @@ int add_action(struct nftnl_rule *r, struct iptables_command_state *cs,
return ret;
}
+int add_log(struct nftnl_rule *r, struct iptables_command_state *cs)
+{
+ struct nftnl_expr *expr;
+ struct xt_nflog_info *info = (struct xt_nflog_info *)cs->target->t->data;
+
+ expr = nftnl_expr_alloc("log");
+ if (!expr)
+ return -ENOMEM;
+
+ if (info->prefix != NULL) {
+ //char prefix[NF_LOG_PREFIXLEN] = {};
+
+ // get prefix here from somewhere...
+ // maybe in cs->argv?
+ nftnl_expr_set_str(expr, NFTNL_EXPR_LOG_PREFIX, "iff the value at the end is 12 then this string is truncated 123");
+ }
+ if (info->group) {
+ nftnl_expr_set_u16(expr, NFTNL_EXPR_LOG_GROUP, info->group);
+ if (info->flags & XT_NFLOG_F_COPY_LEN)
+ nftnl_expr_set_u32(expr, NFTNL_EXPR_LOG_SNAPLEN,
+ info->len);
+ if (info->threshold)
+ nftnl_expr_set_u16(expr, NFTNL_EXPR_LOG_QTHRESHOLD,
+ info->threshold);
+ }
+
+ nftnl_rule_add_expr(r, expr);
+ return 0;
+}
+
static void nft_rule_print_debug(struct nftnl_rule *r, struct nlmsghdr *nlh)
{
#ifdef NLDEBUG
@@ -3487,6 +3520,11 @@ static int nft_is_expr_compatible(struct nftnl_expr *expr, void *data)
nftnl_expr_get_u32(expr, NFTNL_EXPR_LIMIT_FLAGS) == 0)
return 0;
+ if (!strcmp(name, "log") &&
+ nftnl_expr_get_u32(expr, NFTNL_EXPR_LOG_LEVEL) == 4 &&
+ !nftnl_expr_is_set(expr, NFTNL_EXPR_LOG_FLAGS))
+ return 0;
+
return -1;
}
diff --git a/iptables/nft.h b/iptables/nft.h
index 4ac7e009..28dc81b7 100644
--- a/iptables/nft.h
+++ b/iptables/nft.h
@@ -193,6 +193,7 @@ int add_match(struct nft_handle *h, struct nftnl_rule *r, struct xt_entry_match
int add_target(struct nftnl_rule *r, struct xt_entry_target *t);
int add_jumpto(struct nftnl_rule *r, const char *name, int verdict);
int add_action(struct nftnl_rule *r, struct iptables_command_state *cs, bool goto_set);
+int add_log(struct nftnl_rule *r, struct iptables_command_state *cs);
char *get_comment(const void *data, uint32_t data_len);
enum nft_rule_print {
--
2.32.0
