netdev - [PATCH v4] doc: document danger of applying REJECT to INVALID CTs

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Date:   Wed, 13 May 2020 11:39:44 +0200
From:   Jan Engelhardt <jengelh@...i.de>
To:     zenczykowski@...il.com
Cc:     maze@...gle.com, pablo@...filter.org, fw@...len.de,
        netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: [PATCH v4] doc: document danger of applying REJECT to INVALID CTs

Signed-off-by: Jan Engelhardt <jengelh@...i.de>
---

4th time is the charm?!

 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