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From: Jan Engelhardt <jengelh@inai.de>
To: zenczykowski@gmail.com
Cc: maze@google.com, pablo@netfilter.org, fw@strlen.de,
	netdev@vger.kernel.org, netfilter-devel@vger.kernel.org
Subject: [PATCH v3] doc: document danger of applying REJECT to INVALID CTs
Date: Wed, 13 May 2020 11:17:54 +0200	[thread overview]
Message-ID: <20200513091754.32090-1-jengelh@inai.de> (raw)
In-Reply-To: <20200513043908.GA25216@f3>

Signed-off-by: Jan Engelhardt <jengelh@inai.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


  reply	other threads:[~2020-05-13  9:17 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-09  5:22 [PATCH] document danger of '-j REJECT'ing of '-m state INVALID' packets Maciej Żenczykowski
2020-05-09 10:52 ` Jan Engelhardt
2020-05-09 17:45   ` Maciej Żenczykowski
2020-05-09 18:02     ` Maciej Żenczykowski
2020-05-09 21:17     ` [PATCH] doc: document danger of applying REJECT to INVALID CTs Jan Engelhardt
2020-05-09 21:28       ` Maciej Żenczykowski
2020-05-09 21:31         ` Maciej Żenczykowski
2020-05-12 21:00         ` [PATCH v2] " Jan Engelhardt
2020-05-12 21:25           ` Maciej Żenczykowski
2020-05-13  4:39           ` Benjamin Poirier
2020-05-13  9:17             ` Jan Engelhardt [this message]
2020-05-13  9:28               ` [PATCH v3] " Maciej Żenczykowski
2020-05-13  9:39                 ` [PATCH v4] " Jan Engelhardt
2020-05-13 16:29                   ` Maciej Żenczykowski

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