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Message-ID: <db979c8f-8ee9-4717-9cd3-c3125e33580b@gmail.com>
Date: Mon, 26 Jan 2026 20:23:52 +0100
From: Justin Iurman <justin.iurman@...il.com>
To: Tom Herbert <tom@...bertland.com>
Cc: davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next v4 6/7] ipv6: Enforce Extension Header ordering
On 1/26/26 17:56, Tom Herbert wrote:
> On Fri, Jan 23, 2026 at 1:35 PM Justin Iurman <justin.iurman@...il.com> wrote:
>>
>> On 1/21/26 22:49, Tom Herbert wrote:
>>> RFC8200 highly recommends that different Extension Headers be send in
>>> a prescibed order and all Extension Header types occur at most once
>>> in a packet with the expection of Destination Options that may
>>> occur twice. This patch enforces the ordering be folowed in received
>>> packets. It also disregrards Destination Options before the Routing
>>> Header as those are unused in deployment asnd there is a proposal
>>> to deprecate them in draft-herbert-deprecate-destops-before-rh.
>>
>> I'll have to pushback on this specifically, at least as long as
>> draft-herbert-deprecate-destops-before-rh is not adopted and published
>> (to be clear, I'm not against it, just that it's still an individual
>> draft right now and there is no consensus). We're already at the edge of
>> remaining compliant with RFC8200 with this series (which is fine
>> considering its security gaps), so let's not rush by removing the
>> DestOps before the Routing header. It can be done later, this series
>> already mitigates DoS attacks quite well.
>
> Justin,
>
> If we need to wait for permission from IETF then pretty much most of
> this patch set isn't viable. For instance
> draft-iurman-6man-eh-occurrences is also not adopted and published
> either, and if we enforce the number of occurrences in implementation
> then we are non-conformant with RFC8200. But I think it is justified
> to protect users from an obvious DoS attack.
Hi Tom,
This is why I mentioned that, with this series, we're on the edge
regarding RFC8200 compliance. However, the text in Sec. 4.6 is somewhat
ambiguous and does not use normative language. Therefore, this series
does not actually violate RFC8200 compliance.
On the other hand, removing the DestOpt before RH would completely
violate RFC8200 and, IMO, is not necessary at the moment (because this
series already mitigates quite well DoS attacks).
> As for Destination Options before the Routing header, _no_one_ is
> using this. There is no deployment of that. The only routing header
> being deployed is SRv6 and the header has its own options, no routers
> support Destination Options before the Routing Header. But, if by some
> miracle someone is using them in their network, then they can simply
> set the sysctl to not enforce EH ordering. IMO, this approach is
> better than adding additional complexity to the stack just to detect a
> case that nobody cares about and a code path that is never exercised.
What if I want to enforce EH ordering in my network *AND* use the
DestOpt before RH? I don't think we should make it an exclusive
condition. And, as discussed offline, draft-bonica-6man-crh-helper-opt
defines a DestOpt before RH. Therefore, we have at least one use case
(still a draft, though) in limited domains. Let's wait and see if this
draft progresses, or if draft-herbert-deprecate-destops-before-rh gets
consensus.
Justin
> Tom
>
>>
>> FWIW, the rest of this patch makes sense.
>>
>>> The allowed order of Extension Headers is:
>>>
>>> IPv6 header
>>> Hop-by-Hop Options header
>>> Routing header
>>> Fragment header
>>> Authentication header
>>> Encapsulating Security Payload header
>>> Destination Options header
>>> Upper-Layer header
>>>
>>> Each Extension Header may be present only once in a packet.
>>>
>>> net.ipv6.enforce_ext_hdr_order is a sysctl to enable or disable
>>> enforcement of xtension Header order. If it is set to zero then
>>> Extension Header order and number of occurences is not checked
>>> in receive processeing (except for Hop-by-Hop Options that
>>> must be the first Extension Header and can only occur once in
>>> a packet.
>>>
>>> Signed-off-by: Tom Herbert <tom@...bertland.com>
>>> ---
>>> include/net/netns/ipv6.h | 1 +
>>> include/net/protocol.h | 16 ++++++++++++++++
>>> net/ipv6/af_inet6.c | 1 +
>>> net/ipv6/exthdrs.c | 2 ++
>>> net/ipv6/ip6_input.c | 14 ++++++++++++++
>>> net/ipv6/reassembly.c | 1 +
>>> net/ipv6/sysctl_net_ipv6.c | 7 +++++++
>>> net/ipv6/xfrm6_protocol.c | 2 ++
>>> 8 files changed, 44 insertions(+)
>>>
>>> diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
>>> index 34bdb1308e8f..2db56718ea60 100644
>>> --- a/include/net/netns/ipv6.h
>>> +++ b/include/net/netns/ipv6.h
>>> @@ -61,6 +61,7 @@ struct netns_sysctl_ipv6 {
>>> u8 fib_notify_on_flag_change;
>>> u8 icmpv6_error_anycast_as_unicast;
>>> u8 icmpv6_errors_extension_mask;
>>> + u8 enforce_ext_hdr_order;
>>> };
>>>
>>> struct netns_ipv6 {
>>> diff --git a/include/net/protocol.h b/include/net/protocol.h
>>> index b2499f88f8f8..70cc2d0fdc0c 100644
>>> --- a/include/net/protocol.h
>>> +++ b/include/net/protocol.h
>>> @@ -50,6 +50,21 @@ struct net_protocol {
>>> };
>>>
>>> #if IS_ENABLED(CONFIG_IPV6)
>>> +
>>> +/* Order of extension headers as prescribed in RFC8200. The ordering and
>>> + * number of extension headers in a packet can be enforced in IPv6 receive
>>> + * processing. Destination Options before the Routing Header is not included
>>> + * in the list as they are unused in deployment and it has been proposed that
>>> + * they be deprecated. See:
>>> + * www.ietf.org/archive/id/draft-herbert-deprecate-destops-before-rh-01.txt
>>> + */
>>> +#define IPV6_EXT_HDR_ORDER_HOP BIT(0)
>>> +#define IPV6_EXT_HDR_ORDER_ROUTING BIT(1)
>>> +#define IPV6_EXT_HDR_ORDER_FRAGMENT BIT(2)
>>> +#define IPV6_EXT_HDR_ORDER_AUTH BIT(3)
>>> +#define IPV6_EXT_HDR_ORDER_ESP BIT(4)
>>> +#define IPV6_EXT_HDR_ORDER_DEST BIT(5)
>>> +
>>> struct inet6_protocol {
>>> int (*handler)(struct sk_buff *skb);
>>>
>>> @@ -61,6 +76,7 @@ struct inet6_protocol {
>>>
>>> unsigned int flags; /* INET6_PROTO_xxx */
>>> u32 secret;
>>> + u32 ext_hdr_order;
>>> };
>>>
>>> #define INET6_PROTO_NOPOLICY 0x1
>>> diff --git a/net/ipv6/af_inet6.c b/net/ipv6/af_inet6.c
>>> index bd29840659f3..43097360ce64 100644
>>> --- a/net/ipv6/af_inet6.c
>>> +++ b/net/ipv6/af_inet6.c
>>> @@ -980,6 +980,7 @@ static int __net_init inet6_net_init(struct net *net)
>>> net->ipv6.sysctl.max_dst_opts_len = IP6_DEFAULT_MAX_DST_OPTS_LEN;
>>> net->ipv6.sysctl.max_hbh_opts_len = IP6_DEFAULT_MAX_HBH_OPTS_LEN;
>>> net->ipv6.sysctl.fib_notify_on_flag_change = 0;
>>> + net->ipv6.sysctl.enforce_ext_hdr_order = 1;
>>> atomic_set(&net->ipv6.fib6_sernum, 1);
>>>
>>> net->ipv6.sysctl.ioam6_id = IOAM6_DEFAULT_ID;
>>> diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
>>> index 394e3397e4d4..2af16694db46 100644
>>> --- a/net/ipv6/exthdrs.c
>>> +++ b/net/ipv6/exthdrs.c
>>> @@ -845,11 +845,13 @@ static int ipv6_rthdr_rcv(struct sk_buff *skb)
>>> static const struct inet6_protocol rthdr_protocol = {
>>> .handler = ipv6_rthdr_rcv,
>>> .flags = INET6_PROTO_NOPOLICY,
>>> + .ext_hdr_order = IPV6_EXT_HDR_ORDER_ROUTING,
>>> };
>>>
>>> static const struct inet6_protocol destopt_protocol = {
>>> .handler = ipv6_destopt_rcv,
>>> .flags = INET6_PROTO_NOPOLICY,
>>> + .ext_hdr_order = IPV6_EXT_HDR_ORDER_DEST,
>>> };
>>>
>>> static const struct inet6_protocol nodata_protocol = {
>>> diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
>>> index 168ec07e31cc..5254ead5c226 100644
>>> --- a/net/ipv6/ip6_input.c
>>> +++ b/net/ipv6/ip6_input.c
>>> @@ -366,6 +366,7 @@ void ip6_protocol_deliver_rcu(struct net *net, struct sk_buff *skb, int nexthdr,
>>> const struct inet6_protocol *ipprot;
>>> struct inet6_dev *idev;
>>> unsigned int nhoff;
>>> + u32 ext_hdrs = 0;
>>> SKB_DR(reason);
>>> bool raw;
>>>
>>> @@ -427,6 +428,19 @@ void ip6_protocol_deliver_rcu(struct net *net, struct sk_buff *skb, int nexthdr,
>>> goto discard;
>>> }
>>> }
>>> +
>>> + if (ipprot->ext_hdr_order &&
>>> + READ_ONCE(net->ipv6.sysctl.enforce_ext_hdr_order)) {
>>> + /* The protocol is an extension header and EH ordering
>>> + * is being enforced. Discard packet if we've already
>>> + * seen this EH or one that is lower in the order list
>>> + */
>>> + if (ipprot->ext_hdr_order <= ext_hdrs)
>>> + goto discard;
>>> +
>>> + ext_hdrs |= ipprot->ext_hdr_order;
>>> + }
>>> +
>>> if (!(ipprot->flags & INET6_PROTO_NOPOLICY)) {
>>> if (!xfrm6_policy_check(NULL, XFRM_POLICY_IN, skb)) {
>>> SKB_DR_SET(reason, XFRM_POLICY);
>>> diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
>>> index 25ec8001898d..91dba72c5a3c 100644
>>> --- a/net/ipv6/reassembly.c
>>> +++ b/net/ipv6/reassembly.c
>>> @@ -414,6 +414,7 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
>>> static const struct inet6_protocol frag_protocol = {
>>> .handler = ipv6_frag_rcv,
>>> .flags = INET6_PROTO_NOPOLICY,
>>> + .ext_hdr_order = IPV6_EXT_HDR_ORDER_FRAGMENT,
>>> };
>>>
>>> #ifdef CONFIG_SYSCTL
>>> diff --git a/net/ipv6/sysctl_net_ipv6.c b/net/ipv6/sysctl_net_ipv6.c
>>> index d2cd33e2698d..543b6acdb11d 100644
>>> --- a/net/ipv6/sysctl_net_ipv6.c
>>> +++ b/net/ipv6/sysctl_net_ipv6.c
>>> @@ -213,6 +213,13 @@ static struct ctl_table ipv6_table_template[] = {
>>> .proc_handler = proc_doulongvec_minmax,
>>> .extra2 = &ioam6_id_wide_max,
>>> },
>>> + {
>>> + .procname = "enforce_ext_hdr_order",
>>> + .data = &init_net.ipv6.sysctl.enforce_ext_hdr_order,
>>> + .maxlen = sizeof(u8),
>>> + .mode = 0644,
>>> + .proc_handler = proc_dou8vec_minmax,
>>> + },
>>> };
>>>
>>> static struct ctl_table ipv6_rotable[] = {
>>> diff --git a/net/ipv6/xfrm6_protocol.c b/net/ipv6/xfrm6_protocol.c
>>> index ea2f805d3b01..5826edf67f64 100644
>>> --- a/net/ipv6/xfrm6_protocol.c
>>> +++ b/net/ipv6/xfrm6_protocol.c
>>> @@ -197,12 +197,14 @@ static const struct inet6_protocol esp6_protocol = {
>>> .handler = xfrm6_esp_rcv,
>>> .err_handler = xfrm6_esp_err,
>>> .flags = INET6_PROTO_NOPOLICY,
>>> + .ext_hdr_order = IPV6_EXT_HDR_ORDER_ESP,
>>> };
>>>
>>> static const struct inet6_protocol ah6_protocol = {
>>> .handler = xfrm6_ah_rcv,
>>> .err_handler = xfrm6_ah_err,
>>> .flags = INET6_PROTO_NOPOLICY,
>>> + .ext_hdr_order = IPV6_EXT_HDR_ORDER_AUTH
>>> };
>>>
>>> static const struct inet6_protocol ipcomp6_protocol = {
>>
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