netdev - Re: [PATCH v3] doc: document danger of applying REJECT to INVALID CTs

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Message-ID: <CANP3RGfNH1m=-rFdkAmGUt3vxFqaGmJnW+RKP-faU6WwOKWoZg@mail.gmail.com>
Date:   Wed, 13 May 2020 02:28:25 -0700
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Jan Engelhardt <jengelh@...i.de>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Linux NetDev <netdev@...r.kernel.org>,
        Netfilter Development Mailing List 
        <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH v3] doc: document danger of applying REJECT to INVALID CTs

you still missed: succesful -> successful

On Wed, May 13, 2020 at 2:17 AM Jan Engelhardt <jengelh@...i.de> wrote:
>
> Signed-off-by: Jan Engelhardt <jengelh@...i.de>
> ---
>
> Spello fix near "indiscriminately".
>
>  extensions/libip6t_REJECT.man | 20 ++++++++++++++++++++
>  extensions/libipt_REJECT.man  | 20 ++++++++++++++++++++
>  2 files changed, 40 insertions(+)
>
> diff --git a/extensions/libip6t_REJECT.man b/extensions/libip6t_REJECT.man
> index 0030a51f..7387436c 100644
> --- a/extensions/libip6t_REJECT.man
> +++ b/extensions/libip6t_REJECT.man
> @@ -30,3 +30,23 @@ TCP RST packet to be sent back.  This is mainly useful for blocking
>  hosts (which won't accept your mail otherwise).
>  \fBtcp\-reset\fP
>  can only be used with kernel versions 2.6.14 or later.
> +.PP
> +\fIWarning:\fP You should not indiscriminately apply the REJECT target to
> +packets whose connection state is classified as INVALID; instead, you should
> +only DROP these.
> +.PP
> +Consider a source host transmitting a packet P, with P experiencing so much
> +delay along its path that the source host issues a retransmission, P_2, with
> +P_2 being succesful in reaching its destination and advancing the connection
> +state normally. It is conceivable that the late-arriving P may be considered to
> +be not associated with any connection tracking entry. Generating a reject
> +packet for this packet would then terminate the healthy connection.
> +.PP
> +So, instead of:
> +.PP
> +-A INPUT ... -j REJECT
> +.PP
> +do consider using:
> +.PP
> +-A INPUT ... -m conntrack --ctstate INVALID -j DROP
> +-A INPUT ... -j REJECT
> diff --git a/extensions/libipt_REJECT.man b/extensions/libipt_REJECT.man
> index 8a360ce7..618a766c 100644
> --- a/extensions/libipt_REJECT.man
> +++ b/extensions/libipt_REJECT.man
> @@ -30,3 +30,23 @@ TCP RST packet to be sent back.  This is mainly useful for blocking
>  hosts (which won't accept your mail otherwise).
>  .IP
>  (*) Using icmp\-admin\-prohibited with kernels that do not support it will result in a plain DROP instead of REJECT
> +.PP
> +\fIWarning:\fP You should not indiscriminately apply the REJECT target to
> +packets whose connection state is classified as INVALID; instead, you should
> +only DROP these.
> +.PP
> +Consider a source host transmitting a packet P, with P experiencing so much
> +delay along its path that the source host issues a retransmission, P_2, with
> +P_2 being succesful in reaching its destination and advancing the connection
> +state normally. It is conceivable that the late-arriving P may be considered to
> +be not associated with any connection tracking entry. Generating a reject
> +packet for this packet would then terminate the healthy connection.
> +.PP
> +So, instead of:
> +.PP
> +-A INPUT ... -j REJECT
> +.PP
> +do consider using:
> +.PP
> +-A INPUT ... -m conntrack --ctstate INVALID -j DROP
> +-A INPUT ... -j REJECT
> --
> 2.26.2
>
Maciej Żenczykowski, Kernel Networking Developer @ Google