[PATCH 6.12 523/567] netfilter: nft_ct: add seqadj extension for natted connections

Greg Kroah-Hartman
2026-01-06T17:43:17Z

From: Andrii Melnychenko <a.melnychenko@vyos.io>

[ Upstream commit 90918e3b6404c2a37837b8f11692471b4c512de2 ]

Sequence adjustment may be required for FTP traffic with PASV/EPSV modes.
due to need to re-write packet payload (IP, port) on the ftp control
connection. This can require changes to the TCP length and expected
seq / ack_seq.

The easiest way to reproduce this issue is with PASV mode.
Example ruleset:
table inet ftp_nat {
ct helper ftp_helper {
type "ftp" protocol tcp
l3proto inet
}

chain prerouting {
type filter hook prerouting priority 0; policy accept;
tcp dport 21 ct state new ct helper set "ftp_helper"
}
}
table ip nat {
chain prerouting {
type nat hook prerouting priority -100; policy accept;
tcp dport 21 dnat ip prefix to ip daddr map {
192.168.100.1 : 192.168.13.2/32 }
}

chain postrouting {
type nat hook postrouting priority 100 ; policy accept;
tcp sport 21 snat ip prefix to ip saddr map {
192.168.13.2 : 192.168.100.1/32 }
}
}

Note that the ftp helper gets assigned *after* the dnat setup.

The inverse (nat after helper assign) is handled by an existing
check in nf_nat_setup_info() and will not show the problem.

Topoloy:

+-------------------+ +----------------------------------+
| FTP: 192.168.13.2 | <-> | NAT: 192.168.13.3, 192.168.100.1 |
+-------------------+ +----------------------------------+
|
+-----------------------+
| Client: 192.168.100.2 |
+-----------------------+

ftp nat changes do not work as expected in this case:
Connected to 192.168.100.1.
[..]
ftp> epsv
EPSV/EPRT on IPv4 off.
ftp> ls
227 Entering passive mode (192,168,100,1,209,129).
421 Service not available, remote server has closed connection.

Kernel logs:
Missing nfct_seqadj_ext_add() setup call
WARNING: CPU: 1 PID: 0 at net/netfilter/nf_conntrack_seqadj.c:41
[..]
__nf_nat_mangle_tcp_packet+0x100/0x160 [nf_nat]
nf_nat_ftp+0x142/0x280 [nf_nat_ftp]
help+0x4d1/0x880 [nf_conntrack_ftp]
nf_confirm+0x122/0x2e0 [nf_conntrack]
nf_hook_slow+0x3c/0xb0
..

Fix this by adding the required extension when a conntrack helper is assigned
to a connection that has a nat binding.

Fixes: 1a64edf54f55 ("netfilter: nft_ct: add helper set support")
Signed-off-by: Andrii Melnychenko <a.melnychenko@vyos.io>
Signed-off-by: Florian Westphal <fw@strlen.de>
[Harshit: Clean cherry-pick, apply it to stable-6.12.y]
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/netfilter/nft_ct.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/net/netfilter/nft_ct.c
+++ b/net/netfilter/nft_ct.c
@@ -22,6 +22,7 @@
#include <net/netfilter/nf_conntrack_timeout.h>
#include <net/netfilter/nf_conntrack_l4proto.h>
#include <net/netfilter/nf_conntrack_expect.h>
+#include <net/netfilter/nf_conntrack_seqadj.h>

struct nft_ct_helper_obj {
struct nf_conntrack_helper *helper4;
@@ -1173,6 +1174,10 @@ static void nft_ct_helper_obj_eval(struc
if (help) {
rcu_assign_pointer(help->helper, to_assign);
set_bit(IPS_HELPER_BIT, &ct->status);
+
+ if ((ct->status & IPS_NAT_MASK) && !nfct_seqadj(ct))
+ if (!nfct_seqadj_ext_add(ct))
+ regs->verdict.code = NF_DROP;
}
}