From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-dl1-f66.google.com (mail-dl1-f66.google.com [74.125.82.66]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E90AC2DB7BC for ; Thu, 8 Jan 2026 17:16:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=74.125.82.66 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767892565; cv=none; b=KkV1tigyVVnSQkqQo9tiJYfJHM/m5vzeuh9Fy6by5H3aM0E0uN30k8JAvi55qNtr/jyp1rQjYUm+9xoW6yVCjKRIs3bl0eejY02fMO9HSyVTGUM/3eYSPO074/QEtSPf5XFuk5eCghbSOeW45LQwfkMfNLAPKtXRu/mupfXNsm0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767892565; c=relaxed/simple; bh=iVn7YA8nn01ehzj/XTgcwn2kQO76KBY0zuM655UBDEQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=s5ujIpP1mScJ2gV7ZY4EQSK2fjadxQCEfN/mGbI7h1Am4iCV6zZd1wzbGpRRM3krt4t4+C0ac2MmWC+00B1HO2OIqgso3Nb06fWYIZGNQjbZl17nAdibUMnDimQAkoLqf3LP195R52D2tGs67Kw99a5qowlKudoplrr7mQd1idk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=herbertland.com; spf=pass smtp.mailfrom=herbertland.com; dkim=pass (2048-bit key) header.d=herbertland.com header.i=@herbertland.com header.b=TRb3mIOD; arc=none smtp.client-ip=74.125.82.66 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=herbertland.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=herbertland.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=herbertland.com header.i=@herbertland.com header.b="TRb3mIOD" Received: by mail-dl1-f66.google.com with SMTP id a92af1059eb24-11b992954d4so3201914c88.1 for ; Thu, 08 Jan 2026 09:16:03 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=herbertland.com; s=google; t=1767892563; x=1768497363; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=ymnZ12ZVg5d56UvSdMCpk88bIArGImL4sE/PnHSiNIQ=; b=TRb3mIOD02aXwh683WoTy8T7VvvxXeqOawpzQQen9Czcw1a3m/NFVuON4oAR6wlpL6 NXFk4bu5ipX2kk90NVraMyL5wYDYTxmacAM3xDr5XlcbKzzqG8QSP7K/mRl01lGyyxYE Iu84WSyx4n41oVMr1DajtbuLxtHIXIBAyepSIP4/yzjdX60RYLOQQpcKGXXeUUVE1iFJ Ai+EQ6iaEReBdYjpNo5hvfjBkbivRa+OxNNTFunr1HUeGdEMeylx9Ts+dm8HlBdutfyI sYsH/ldFt7XmEbRpz4oKHpx3DcDB9JbnuMplc3iklpE9LhdrkVVRYLRcNl041ET698bL Zhnw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1767892563; x=1768497363; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to; bh=ymnZ12ZVg5d56UvSdMCpk88bIArGImL4sE/PnHSiNIQ=; b=RORHTv6xYll6m/S/0u2/EBqjbHsYFe9NV8HmflcJxK8Jy74Xn6coJwWHmZLANksgiz xStDujFXfH5kSxWkAh50VKov6ICfHI/sADcUmGy9Qz3SCUHHQYLT1UIKBq4gcXwxgLC7 LHO1rcWWR990PAUu4+6ZfaV8kHtcykzAwH/1qcLFu+6nGfWziow7vhtZrngjSgD3LrdV dU3VnJRrVvd/vD8JaQZbLOuZak0JRTo9PCjEMlIPTQT+Loi7RyhVOcOq8kFgTL8C3BJo BeptVPOss5QWZcIAmvOidVm6uXR3hFXsgqwL/Uq8qPdvGwOC0bSelwgoDWZP7xrcjfAa Mxag== X-Forwarded-Encrypted: i=1; AJvYcCXLf5t0IyCWBnvU1kDzuyJLtOk4yg+b/ozu8yjNYaLqZDfxycsSqTC1ZJqtvYNEq01Y7lnDFUY=@vger.kernel.org X-Gm-Message-State: AOJu0YzM4YD+X259RnI5O/ikt3gHmpsyukpg/UsdfN/wT93nxG95pRfc 8DbvPPHlTrF8lWci9SW0fYH5bWTH+9vFhJueZSRWD8d52bwJkA1o6jthjatbUklJxRkAmc5+DlG 0+ZJ8pw== X-Gm-Gg: AY/fxX6LOPfSv4km5qSHYlJG9PCq1dMQxvUzhPrGrqdxKniz2/WLOZ/QDbqeqYVkQ3C gJKYUsTiAxmQGt24j5eQVL4EtRNBelfPTZYdfVUdTYa1tHlXsjsff3xBlASxgGK+WH+rFgT/Vsc ZFO3h1Vs6LmYgVqzx2xG6qSBdKULjV+HKyDtR3vKDt7RfLIWt++7ndBKSo9c15KDx1daDHmkop9 qKfeFqAxYfjkGWeG3mpH67aedPDBcabq0nnH48UhAq6ImIpT6J5WgKcyDy9sbtcJB0j6nRSKBhg nKFfUIiAwnFxkArbI89ezxwuaYrgalXPwxhhXkIJybYGFds3W65pcWTlFMU+W81Z83EHE8lLGE6 lLIuj6ppIJTT2HVMWUxu4Khcsn9tCnHVbMHtaqe/ozclboPVApErhxHu4blVSxjTVZgI/3ljsiC Hh5FP7X6FCSHXhhBdISGzoYkfv9IcGw97TPwOoJp7MaO1rQYnINuyyqnOm X-Google-Smtp-Source: AGHT+IH/P63oPD2Zjgbk6vQ5uk1IV5bufCRAoon/NN+MlFYjFEMytYxxH8BoZy1vBED78EauWwhmdQ== X-Received: by 2002:a05:7022:387:b0:119:e56c:189d with SMTP id a92af1059eb24-121f8afbcdfmr6181418c88.5.1767892562923; Thu, 08 Jan 2026 09:16:02 -0800 (PST) Received: from pong.herbertland.com ([2601:646:8980:b330:812d:d4cb:feac:3d09]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-121f248bb6esm14029259c88.12.2026.01.08.09.16.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 08 Jan 2026 09:16:02 -0800 (PST) From: Tom Herbert To: davem@davemloft.net, kuba@kernel.org, netdev@vger.kernel.org Cc: Tom Herbert Subject: [PATCH net-next v2 4/4] ipv6: Document defaults for max_{dst|hbh}_opts_number sysctls Date: Thu, 8 Jan 2026 09:14:56 -0800 Message-ID: <20260108171456.47519-5-tom@herbertland.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20260108171456.47519-1-tom@herbertland.com> References: <20260108171456.47519-1-tom@herbertland.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit In the descriptions of max_dst_opts_number and max_hbh_opts_number sysctls add text about how a zero setting means that a packet with any Destination or Hop-by-Hop options is dropped. Report the default for max_dst_opts_number to be zero (i.e. packets with Destination options are dropped by default), and add a justification. Report the default for max_hbh_opts_number to be one. Signed-off-by: Tom Herbert --- Documentation/networking/ip-sysctl.rst | 38 ++++++++++++++++++-------- 1 file changed, 27 insertions(+), 11 deletions(-) diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index bc9a01606daf..de078f7f6a17 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -2474,20 +2474,36 @@ mld_qrv - INTEGER Minimum: 1 (as specified by RFC6636 4.5) max_dst_opts_number - INTEGER - Maximum number of non-padding TLVs allowed in a Destination - options extension header. If this value is less than zero - then unknown options are disallowed and the number of known - TLVs allowed is the absolute value of this number. - - Default: 8 + Maximum number of non-padding TLVs allowed in a Destination + options extension header. If this value is zero then receive + Destination Options processing is disabled in which case packets + with the Destination Options extension header are dropped. If + this value is less than zero then unknown options are disallowed + and the number of known TLVs allowed is the absolute value of + this number. + + The default is zero which means the all received packets with + Destination Options extension header are dropped. The rationale is that + for the vast majority of hosts, Destination Options serve no purpose. + In the thirty years of IPv6 no broadly useful IPv6 Destination options + have been defined, they have no security or even checksum protection, + latest data shows the Destination have drop rates on the Internet + from ten percent to more than thirty percent (depending on the size of + the extension header). They also have the potential to be used as a + Denial of Service attack. + + Default: 0 max_hbh_opts_number - INTEGER Maximum number of non-padding TLVs allowed in a Hop-by-Hop - options extension header. If this value is less than zero - then unknown options are disallowed and the number of known - TLVs allowed is the absolute value of this number. - - Default: 8 + options extension header. If this value is zero then receive + Hop-by-Hop Options processing is disabled in which case packets + with the Hop-by-Hop Options extension header are dropped. + If this value is less than zero then unknown options are disallowed + and the number of known TLVs allowed is the absolute value of this + number. + + Default: 1 max_dst_opts_length - INTEGER Maximum length allowed for a Destination options extension -- 2.43.0