Re: [PATCH net] netfilter: nf_nat_masquerade: recalculate TCP TS offset when port is randomized

[Victor Nogueira <victor@mojatatu.com>]

Hi!

On 29/06/2026 06:34, xietangxin wrote:
> Problem observed in Kubernetes environments where MASQUERADE target with
> --random-fully is configured by default. after commit
> 165573e41f2f ("tcp: secure_seq: add back ports to TS offset") TCP short
> connection QPS dropped from ~20000 to ~10000. This added source and
> destination ports into TS offset calculation.
> 
> However, with MASQUERADE --random-fully, when multiple internal connections
> (e.g sport 10000,20000) are mapped to the same external port (e.g 30000),
> their TS offsets are calculated as ts_offset(10000) and ts_offset(20000).
> If the server reuses the TIME_WAIT slot from the first connection, there is
> a chance that ts_offset(20000) < ts_offset(10000), breaking TSval
> monotonicity for the same 4-tuple and causing RST packets:
>    Client -> Server 24870 -> 80 [SYN] TSval=2294041168
>    Server -> Client 80 -> 24870 [ACK] TSecr=2846236456
>    Client -> Server 24870 -> 80 [RST] Seq=855605690
> 
> After nf_nat_setup_info() successfully assigns a new randomized
> source port, recalculate the TS offset using the new port and
> update the SYN packet's TSval accordingly.
> 
> Test results on 4U4G VM with
> `./wrk -t8 -c200 -H "Connection: close" -d10s --latency http://5.5.5.5:80`
> Before:
>    random:10712 req/s, random-fully:10986 req/s
> After:
>    random:21463 req/s, random-fully:19181 req/s
> 
> Fixes: 165573e41f2f ("tcp: secure_seq: add back ports to TS offset")
> Cc: stable@vger.kernel.org
> Closes:https://lore.kernel.org/all/92935c00-e0be-4591-ac44-5978c7804d57@yeah.net/
> Signed-off-by: xietangxin <xietangxin@h-partners.com>
> [...]
> +
> +static void masquerade_update_tcp_ts_offset(struct nf_conn *ct, struct sk_buff *skb)
> +{
> [...]
> +
> +		if (nf_ct_l3num(ct) == NFPROTO_IPV4)
> +			st = secure_tcp_seq_and_ts_off(net, tuple->src.u3.ip, tuple->dst.u3.ip,
> +				tuple->src.u.tcp.port, tuple->dst.u.tcp.port);
> +		else
> +			st = secure_tcpv6_seq_and_ts_off(net, tuple->src.u3.ip6,
> +				tuple->dst.u3.ip6, tuple->src.u.tcp.port, tuple->dst.u.tcp.port);

This breaks the build when CONFIG_IPV6 is not set.

.config:4948:warning: override: reassigning to symbol NET
.config:4949:warning: override: reassigning to symbol NET_CORE
.config:4950:warning: override: reassigning to symbol NETDEVICES
.config:4951:warning: override: reassigning to symbol NETWORK_FILESYSTEMS
ERROR: modpost: "secure_tcpv6_seq_and_ts_off" [net/netfilter/nf_nat.ko] 
undefined!

cheers,
Victor