Re: [PATCH] ipv6: allow bypassing cross-intf routing limits

[Michal Kazior]

On 17 April 2016 at 00:49, David Miller  wrote:
> From: Michal Kazior 
> Date: Thu, 14 Apr 2016 14:46:28 +0200
>
>> There are some use-cases to allow link-local
>> routing for bridging purposes.
>>
>> One of these is allowing transparent 802.11
>> bridging. Due to 802.11 framing limitations many
>> Access Points make it impossible to create bridges
>> on Client endpoints because they can't maintain
>> Destination/Source/Transmitter/Receiver address
>> distinction with only 3 addresses in frame header.
>>
>> The default behavior, i.e. link-local traffic
>> being non-routable, remains. The user has to
>> explicitly enable the bypass when defining a given
>> route.
>>
>> Signed-off-by: Michal Kazior 
>
> Sorry, whilst I realize your problem, I'm not going to add what is
> explicitly a violation of the way link-local addresses are meant to
> work and the very much intentional restrictions the RFCs place upon
> them (they MUST not be routed).
>
> I also didn't see any real discussions in response to your original
> proposals, not from even one person I know is knowledgable about ipv6
> and the implications your change would have, and that is extremely
> troubling.
>
> I tried to let your patches sit for several days in order to let that
> kind of discussion happen, but it didn't.

I totally understand. Thanks anyway.


> So, you'll need to find another way to achieve your goals.

Hmm.. I actually do have another idea in mind already.

I was wondering about about implementing a link device in a similar
fashion to macvlan, bridge, et al, i.e. the device would take an
interface (wlan interface in my case) as a slave and perform L2
address mangling (which should be enough to accommodate the addressing
deficiency in wireless client modes). The device would obviously need
to do some L3/L4 packet inspection and manipulation.

Would something like that be acceptable? Thoughts?


Michał

Re: [PATCH] ipv6: allow bypassing cross-intf routing limits

[David Miller]

From: Michal Kazior 
Date: Thu, 14 Apr 2016 14:46:28 +0200

> There are some use-cases to allow link-local
> routing for bridging purposes.
> 
> One of these is allowing transparent 802.11
> bridging. Due to 802.11 framing limitations many
> Access Points make it impossible to create bridges
> on Client endpoints because they can't maintain
> Destination/Source/Transmitter/Receiver address
> distinction with only 3 addresses in frame header.
> 
> The default behavior, i.e. link-local traffic
> being non-routable, remains. The user has to
> explicitly enable the bypass when defining a given
> route.
> 
> Signed-off-by: Michal Kazior 

Sorry, whilst I realize your problem, I'm not going to add what is
explicitly a violation of the way link-local addresses are meant to
work and the very much intentional restrictions the RFCs place upon
them (they MUST not be routed).

I also didn't see any real discussions in response to your original
proposals, not from even one person I know is knowledgable about ipv6
and the implications your change would have, and that is extremely
troubling.

I tried to let your patches sit for several days in order to let that
kind of discussion happen, but it didn't.

So, you'll need to find another way to achieve your goals.

[PATCH] ipv6: allow bypassing cross-intf routing limits

[Michal Kazior]

There are some use-cases to allow link-local
routing for bridging purposes.

One of these is allowing transparent 802.11
bridging. Due to 802.11 framing limitations many
Access Points make it impossible to create bridges
on Client endpoints because they can't maintain
Destination/Source/Transmitter/Receiver address
distinction with only 3 addresses in frame header.

The default behavior, i.e. link-local traffic
being non-routable, remains. The user has to
explicitly enable the bypass when defining a given
route.

Signed-off-by: Michal Kazior 
---
For more background see:

  http://www.spinics.net/lists/netdev/msg371022.html
  https://lists.openwrt.org/pipermail/openwrt-devel/2016-April/040783.html

 include/uapi/linux/rtnetlink.h |  8 ++--
 net/ipv6/ip6_output.c  | 11 +--
 net/ipv6/route.c   |  4 
 3 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/include/uapi/linux/rtnetlink.h b/include/uapi/linux/rtnetlink.h
index ca764b5da86d..a577eec0e56e 100644
--- a/include/uapi/linux/rtnetlink.h
+++ b/include/uapi/linux/rtnetlink.h
@@ -424,9 +424,13 @@ enum {
 #define RTAX_FEATURE_SACK  (1 << 1)
 #define RTAX_FEATURE_TIMESTAMP (1 << 2)
 #define RTAX_FEATURE_ALLFRAG   (1 << 3)
+#define RTAX_FEATURE_XFACE (1 << 4)
 
-#define RTAX_FEATURE_MASK  (RTAX_FEATURE_ECN | RTAX_FEATURE_SACK | \
-RTAX_FEATURE_TIMESTAMP | RTAX_FEATURE_ALLFRAG)
+#define RTAX_FEATURE_MASK  (RTAX_FEATURE_ECN | \
+RTAX_FEATURE_SACK | \
+RTAX_FEATURE_TIMESTAMP | \
+RTAX_FEATURE_ALLFRAG | \
+RTAX_FEATURE_XFACE)
 
 struct rta_session {
__u8proto;
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 9428345d3a07..9abb42acb6ad 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -283,6 +283,7 @@ static int ip6_forward_proxy_check(struct sk_buff *skb)
u8 nexthdr = hdr->nexthdr;
__be16 frag_off;
int offset;
+   int feat = dst_metric_raw(skb_dst(skb), RTAX_FEATURES);
 
if (ipv6_ext_hdr(nexthdr)) {
offset = ipv6_skip_exthdr(skb, sizeof(*hdr), , 
_off);
@@ -320,8 +321,11 @@ static int ip6_forward_proxy_check(struct sk_buff *skb)
 * The proxying router can't forward traffic sent to a link-local
 * address, so signal the sender and discard the packet. This
 * behavior is clarified by the MIPv6 specification.
+*
+* It's useful to allow an override for transparent traffic relay.
 */
-   if (ipv6_addr_type(>daddr) & IPV6_ADDR_LINKLOCAL) {
+   if ((ipv6_addr_type(>daddr) & IPV6_ADDR_LINKLOCAL) &&
+   !(feat & RTAX_FEATURE_XFACE)) {
dst_link_failure(skb);
return -1;
}
@@ -485,12 +489,15 @@ int ip6_forward(struct sk_buff *skb)
inet_putpeer(peer);
} else {
int addrtype = ipv6_addr_type(>saddr);
+   int feat = dst_metric_raw(dst, RTAX_FEATURES);
 
/* This check is security critical. */
if (addrtype == IPV6_ADDR_ANY ||
addrtype & (IPV6_ADDR_MULTICAST | IPV6_ADDR_LOOPBACK))
goto error;
-   if (addrtype & IPV6_ADDR_LINKLOCAL) {
+
+   if ((addrtype & IPV6_ADDR_LINKLOCAL) &&
+   !(feat & RTAX_FEATURE_XFACE)) {
icmpv6_send(skb, ICMPV6_DEST_UNREACH,
ICMPV6_NOT_NEIGHBOUR, 0);
goto error;
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index ed446639219c..560c99853907 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -629,8 +629,12 @@ static inline enum rt6_nud_state rt6_check_neigh(struct 
rt6_info *rt)
 static int rt6_score_route(struct rt6_info *rt, int oif,
   int strict)
 {
+   int feat = dst_metric_raw(>dst, RTAX_FEATURES);
int m;
 
+   if (feat & RTAX_FEATURE_XFACE)
+   strict &= ~RT6_LOOKUP_F_IFACE;
+
m = rt6_check_dev(rt, oif);
if (!m && (strict & RT6_LOOKUP_F_IFACE))
return RT6_NUD_FAIL_HARD;
-- 
2.1.4