[PATCH v3 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

[PATCH v3 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

[Praveen Chaudhary]

For IPv4, default route is learned via DHCPv4 and user is allowed to change
metric using config etc/network/interfaces. But for IPv6, default route can
be learned via RA, for which, currently a fixed metric value 1024 is used.

Ideally, user should be able to configure metric on default route for IPv6
similar to IPv4. This fix adds sysctl for the same.

Signed-off-by: Praveen Chaudhary <pchaudhary@linkedin.com>
Signed-off-by: Zhenggen Xu <zxu@linkedin.com>

Changes in v1.
1.) Correct the call to rt6_add_dflt_router.

Changes in v2.
1.) Replace accept_ra_defrtr_metric to ra_defrtr_metric.
2.) Change Type to __u32 instead of __s32.
3.) Change description in Documentation/networking/ip-sysctl.rst.
4.) Use proc_douintvec instead of proc_dointvec.
5.) Code style in ndisc_router_discovery().
6.) Change Type to u32 instead of unsigned int.

Changes in v3:
1.) Removed '---' and '```' from description.
2.) Remove stray ' after accept_ra_defrtr.
3.) Fix tab in net/ipv6/addrconf.c.

Logs:

For IPv4:

Config in etc/network/interfaces:
auto eth0
iface eth0 inet dhcp
    metric 4261413864

IPv4 Kernel Route Table:
$ ip route list
default via 172.21.47.1 dev eth0 metric 4261413864

FRR Table, if a static route is configured:
[In real scenario, it is useful to prefer BGP learned default route over DHCPv4 default route.]
Codes: K - kernel route, C - connected, S - static, R - RIP,
       O - OSPF, I - IS-IS, B - BGP, P - PIM, E - EIGRP, N - NHRP,
       T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
       > - selected route, * - FIB route

S>* 0.0.0.0/0 [20/0] is directly connected, eth0, 00:00:03
K   0.0.0.0/0 [254/1000] via 172.21.47.1, eth0, 6d08h51m

i.e. User can prefer Default Router learned via Routing Protocol in IPv4.
Similar behavior is not possible for IPv6, without this fix.

After fix [for IPv6]:
sudo sysctl -w net.ipv6.conf.eth0.net.ipv6.conf.eth0.ra_defrtr_metric=1996489705

IP monitor: [When IPv6 RA is received]
default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  pref high

Kernel IPv6 routing table
$ ip -6 route list
default via fe80::be16:65ff:feb3:ce8e dev eth0 proto ra metric 1996489705 expires 21sec hoplimit 64 pref high

FRR Table, if a static route is configured:
[In real scenario, it is useful to prefer BGP learned default route over IPv6 RA default route.]
Codes: K - kernel route, C - connected, S - static, R - RIPng,
       O - OSPFv3, I - IS-IS, B - BGP, N - NHRP, T - Table,
       v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
       > - selected route, * - FIB route

S>* ::/0 [20/0] is directly connected, eth0, 00:00:06
K   ::/0 [119/1001] via fe80::xx16:xxxx:feb3:ce8e, eth0, 6d07h43m

If the metric is changed later, the effect will be seen only when next IPv6
RA is received, because the default route must be fully controlled by RA msg.
Below metric is changed from 1996489705 to 1996489704.

$ sudo sysctl -w net.ipv6.conf.eth0.ra_defrtr_metric=1996489704
net.ipv6.conf.eth0.ra_defrtr_metric = 1996489704

IP monitor:
[On next IPv6 RA msg, Kernel deletes prev route and installs new route with updated metric]

Deleted default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  expires 3sec hoplimit 64 pref high
default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489704  pref high
---
 Documentation/networking/ip-sysctl.rst | 12 ++++++++++++
 include/linux/ipv6.h                   |  1 +
 include/net/ip6_route.h                |  3 ++-
 include/uapi/linux/ipv6.h              |  1 +
 include/uapi/linux/sysctl.h            |  1 +
 net/ipv6/addrconf.c                    | 10 ++++++++++
 net/ipv6/ndisc.c                       | 14 ++++++++++----
 net/ipv6/route.c                       |  5 +++--
 8 files changed, 40 insertions(+), 7 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index dd2b12a32b73..6a644e794605 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -1871,6 +1871,18 @@ accept_ra_defrtr - BOOLEAN
 		- enabled if accept_ra is enabled.
 		- disabled if accept_ra is disabled.
 
+ra_defrtr_metric - INTEGER
+	Route metric for default route learned in Router Advertisement. This value
+	will be assigned as metric for the default route learned via IPv6 Router
+	Advertisement. Takes affect only if accept_ra_defrtr is enabled.
+
+	Possible values are:
+		0:
+			default value will be used for route metric
+			i.e. IP6_RT_PRIO_USER 1024.
+		1 to 0xFFFFFFFF:
+			current value will be used for route metric.
+
 accept_ra_from_local - BOOLEAN
 	Accept RA with source-address that is found on local machine
 	if the RA is otherwise proper and able to be accepted.
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index dda61d150a13..9d1f29f0c512 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -31,6 +31,7 @@ struct ipv6_devconf {
 	__s32		max_desync_factor;
 	__s32		max_addresses;
 	__s32		accept_ra_defrtr;
+	__u32		ra_defrtr_metric;
 	__s32		accept_ra_min_hop_limit;
 	__s32		accept_ra_pinfo;
 	__s32		ignore_routes_with_linkdown;
diff --git a/include/net/ip6_route.h b/include/net/ip6_route.h
index 2a5277758379..f51a118bfce8 100644
--- a/include/net/ip6_route.h
+++ b/include/net/ip6_route.h
@@ -174,7 +174,8 @@ struct fib6_info *rt6_get_dflt_router(struct net *net,
 				     struct net_device *dev);
 struct fib6_info *rt6_add_dflt_router(struct net *net,
 				     const struct in6_addr *gwaddr,
-				     struct net_device *dev, unsigned int pref);
+				     struct net_device *dev, unsigned int pref,
+				     u32 defrtr_usr_metric);
 
 void rt6_purge_dflt_routers(struct net *net);
 
diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
index 13e8751bf24a..70603775fe91 100644
--- a/include/uapi/linux/ipv6.h
+++ b/include/uapi/linux/ipv6.h
@@ -189,6 +189,7 @@ enum {
 	DEVCONF_ACCEPT_RA_RT_INFO_MIN_PLEN,
 	DEVCONF_NDISC_TCLASS,
 	DEVCONF_RPL_SEG_ENABLED,
+	DEVCONF_RA_DEFRTR_METRIC,
 	DEVCONF_MAX
 };
 
diff --git a/include/uapi/linux/sysctl.h b/include/uapi/linux/sysctl.h
index 458179df9b27..1e05d3caa712 100644
--- a/include/uapi/linux/sysctl.h
+++ b/include/uapi/linux/sysctl.h
@@ -571,6 +571,7 @@ enum {
 	NET_IPV6_ACCEPT_SOURCE_ROUTE=25,
 	NET_IPV6_ACCEPT_RA_FROM_LOCAL=26,
 	NET_IPV6_ACCEPT_RA_RT_INFO_MIN_PLEN=27,
+	NET_IPV6_RA_DEFRTR_METRIC=28,
 	__NET_IPV6_MAX
 };
 
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index eff2cacd5209..9f207f7dc70c 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -205,6 +205,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
 	.max_desync_factor	= MAX_DESYNC_FACTOR,
 	.max_addresses		= IPV6_MAX_ADDRESSES,
 	.accept_ra_defrtr	= 1,
+	.ra_defrtr_metric	= 0,
 	.accept_ra_from_local	= 0,
 	.accept_ra_min_hop_limit= 1,
 	.accept_ra_pinfo	= 1,
@@ -260,6 +261,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
 	.max_desync_factor	= MAX_DESYNC_FACTOR,
 	.max_addresses		= IPV6_MAX_ADDRESSES,
 	.accept_ra_defrtr	= 1,
+	.ra_defrtr_metric	= 0,
 	.accept_ra_from_local	= 0,
 	.accept_ra_min_hop_limit= 1,
 	.accept_ra_pinfo	= 1,
@@ -5475,6 +5477,7 @@ static inline void ipv6_store_devconf(struct ipv6_devconf *cnf,
 	array[DEVCONF_MAX_DESYNC_FACTOR] = cnf->max_desync_factor;
 	array[DEVCONF_MAX_ADDRESSES] = cnf->max_addresses;
 	array[DEVCONF_ACCEPT_RA_DEFRTR] = cnf->accept_ra_defrtr;
+	array[DEVCONF_RA_DEFRTR_METRIC] = cnf->ra_defrtr_metric;
 	array[DEVCONF_ACCEPT_RA_MIN_HOP_LIMIT] = cnf->accept_ra_min_hop_limit;
 	array[DEVCONF_ACCEPT_RA_PINFO] = cnf->accept_ra_pinfo;
 #ifdef CONFIG_IPV6_ROUTER_PREF
@@ -6667,6 +6670,13 @@ static const struct ctl_table addrconf_sysctl[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname	= "ra_defrtr_metric",
+		.data		= &ipv6_devconf.ra_defrtr_metric,
+		.maxlen		= sizeof(u32),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec,
+	},
 	{
 		.procname	= "accept_ra_min_hop_limit",
 		.data		= &ipv6_devconf.accept_ra_min_hop_limit,
diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index 76717478f173..7a5b0ce6e6ea 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -1173,6 +1173,7 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 	struct neighbour *neigh = NULL;
 	struct inet6_dev *in6_dev;
 	struct fib6_info *rt = NULL;
+	u32 defrtr_usr_metric;
 	struct net *net;
 	int lifetime;
 	struct ndisc_options ndopts;
@@ -1303,18 +1304,23 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 			return;
 		}
 	}
-	if (rt && lifetime == 0) {
+	/* Set default route metric if specified by user */
+	defrtr_usr_metric = in6_dev->cnf.ra_defrtr_metric;
+	if (defrtr_usr_metric == 0)
+		defrtr_usr_metric = IP6_RT_PRIO_USER;
+	/* delete the route if lifetime is 0 or if metric needs change */
+	if (rt && ((lifetime == 0) || (rt->fib6_metric != defrtr_usr_metric)))  {
 		ip6_del_rt(net, rt, false);
 		rt = NULL;
 	}
 
-	ND_PRINTK(3, info, "RA: rt: %p  lifetime: %d, for dev: %s\n",
-		  rt, lifetime, skb->dev->name);
+	ND_PRINTK(3, info, "RA: rt: %p  lifetime: %d, metric: %d, for dev: %s\n",
+		  rt, lifetime, defrtr_usr_metric, skb->dev->name);
 	if (!rt && lifetime) {
 		ND_PRINTK(3, info, "RA: adding default router\n");
 
 		rt = rt6_add_dflt_router(net, &ipv6_hdr(skb)->saddr,
-					 skb->dev, pref);
+					 skb->dev, pref, defrtr_usr_metric);
 		if (!rt) {
 			ND_PRINTK(0, err,
 				  "RA: %s failed to add default route\n",
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 188e114b29b4..64fe5b51b0c2 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -4252,11 +4252,12 @@ struct fib6_info *rt6_get_dflt_router(struct net *net,
 struct fib6_info *rt6_add_dflt_router(struct net *net,
 				     const struct in6_addr *gwaddr,
 				     struct net_device *dev,
-				     unsigned int pref)
+				     unsigned int pref,
+				     u32 defrtr_usr_metric)
 {
 	struct fib6_config cfg = {
 		.fc_table	= l3mdev_fib_table(dev) ? : RT6_TABLE_DFLT,
-		.fc_metric	= IP6_RT_PRIO_USER,
+		.fc_metric	= defrtr_usr_metric ? : IP6_RT_PRIO_USER,
 		.fc_ifindex	= dev->ifindex,
 		.fc_flags	= RTF_GATEWAY | RTF_ADDRCONF | RTF_DEFAULT |
 				  RTF_UP | RTF_EXPIRES | RTF_PREF(pref),

base-commit: 139711f033f636cc78b6aaf7363252241b9698ef
-- 
2.29.0

Re: [PATCH v3 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

[David Ahern]

On 1/19/21 2:29 PM, Praveen Chaudhary wrote:
> For IPv4, default route is learned via DHCPv4 and user is allowed to change
> metric using config etc/network/interfaces. But for IPv6, default route can
> be learned via RA, for which, currently a fixed metric value 1024 is used.
> 
> Ideally, user should be able to configure metric on default route for IPv6
> similar to IPv4. This fix adds sysctl for the same.
> 
> Signed-off-by: Praveen Chaudhary <pchaudhary@linkedin.com>
> Signed-off-by: Zhenggen Xu <zxu@linkedin.com>
> 
> Changes in v1.
> 1.) Correct the call to rt6_add_dflt_router.
> 
> Changes in v2.
> 1.) Replace accept_ra_defrtr_metric to ra_defrtr_metric.
> 2.) Change Type to __u32 instead of __s32.
> 3.) Change description in Documentation/networking/ip-sysctl.rst.
> 4.) Use proc_douintvec instead of proc_dointvec.
> 5.) Code style in ndisc_router_discovery().
> 6.) Change Type to u32 instead of unsigned int.
> 
> Changes in v3:
> 1.) Removed '---' and '```' from description.
> 2.) Remove stray ' after accept_ra_defrtr.
> 3.) Fix tab in net/ipv6/addrconf.c.
> 
> Logs:
> 
> For IPv4:
> 
> Config in etc/network/interfaces:
> auto eth0
> iface eth0 inet dhcp
>     metric 4261413864
> 
> IPv4 Kernel Route Table:
> $ ip route list
> default via 172.21.47.1 dev eth0 metric 4261413864
> 
> FRR Table, if a static route is configured:
> [In real scenario, it is useful to prefer BGP learned default route over DHCPv4 default route.]
> Codes: K - kernel route, C - connected, S - static, R - RIP,
>        O - OSPF, I - IS-IS, B - BGP, P - PIM, E - EIGRP, N - NHRP,
>        T - Table, v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
>        > - selected route, * - FIB route
> 
> S>* 0.0.0.0/0 [20/0] is directly connected, eth0, 00:00:03
> K   0.0.0.0/0 [254/1000] via 172.21.47.1, eth0, 6d08h51m
> 
> i.e. User can prefer Default Router learned via Routing Protocol in IPv4.
> Similar behavior is not possible for IPv6, without this fix.
> 
> After fix [for IPv6]:
> sudo sysctl -w net.ipv6.conf.eth0.net.ipv6.conf.eth0.ra_defrtr_metric=1996489705
> 
> IP monitor: [When IPv6 RA is received]
> default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  pref high
> 
> Kernel IPv6 routing table
> $ ip -6 route list
> default via fe80::be16:65ff:feb3:ce8e dev eth0 proto ra metric 1996489705 expires 21sec hoplimit 64 pref high
> 
> FRR Table, if a static route is configured:
> [In real scenario, it is useful to prefer BGP learned default route over IPv6 RA default route.]
> Codes: K - kernel route, C - connected, S - static, R - RIPng,
>        O - OSPFv3, I - IS-IS, B - BGP, N - NHRP, T - Table,
>        v - VNC, V - VNC-Direct, A - Babel, D - SHARP,
>        > - selected route, * - FIB route
> 
> S>* ::/0 [20/0] is directly connected, eth0, 00:00:06
> K   ::/0 [119/1001] via fe80::xx16:xxxx:feb3:ce8e, eth0, 6d07h43m
> 
> If the metric is changed later, the effect will be seen only when next IPv6
> RA is received, because the default route must be fully controlled by RA msg.
> Below metric is changed from 1996489705 to 1996489704.
> 
> $ sudo sysctl -w net.ipv6.conf.eth0.ra_defrtr_metric=1996489704
> net.ipv6.conf.eth0.ra_defrtr_metric = 1996489704
> 
> IP monitor:
> [On next IPv6 RA msg, Kernel deletes prev route and installs new route with updated metric]
> 
> Deleted default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  expires 3sec hoplimit 64 pref high
> default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489704  pref high
> ---
>  Documentation/networking/ip-sysctl.rst | 12 ++++++++++++
>  include/linux/ipv6.h                   |  1 +
>  include/net/ip6_route.h                |  3 ++-
>  include/uapi/linux/ipv6.h              |  1 +
>  include/uapi/linux/sysctl.h            |  1 +
>  net/ipv6/addrconf.c                    | 10 ++++++++++
>  net/ipv6/ndisc.c                       | 14 ++++++++++----
>  net/ipv6/route.c                       |  5 +++--
>  8 files changed, 40 insertions(+), 7 deletions(-)
> 

LGTM. I can't think of a better way to do this than a sysctl. Shame that
the metric/priority is not an RA option.

Reviewed-by: David Ahern <dsahern@kernel.org>

Re: [PATCH v3 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

[David Ahern]

On 1/22/21 9:02 PM, Maciej Żenczykowski wrote:
> Why can't we get rid of the special case for 0 and simply make 1024 the
> default value?

That would work too.

> 
> As for making it an RA option: it's not clear how that would work, the
> use case I see for this is for example two connections to the internet,
> of which one is clearly better (higher throughput, lower latency, lower
> packet loss, etc) then the other.
> 
> The upstream routers would have to somehow coordinate with each other
> the metric values... that seems impossible to achieve in practice -
> unless they do something like report expected down/up
> bandwidth, latency, etc...  While some sort of policy on the machine
> itself seems much more feasible (for example wired interface > wireless
> interface > cell interface or something like that)

I was thinking the admin of the network controls the RAs and knows which
paths are preferred over the admin of the node receiving the RA (not
practical for a mobile setup with cell vs wifi, but is for a DC which is
the driving use case).

But it takes an extension to IPv6/ndisc to add metric as an RA option,
so not realistic in a reasonable time frame.

Re: [PATCH v3 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

[Jakub Kicinski]

On Fri, 22 Jan 2021 22:16:41 -0700 David Ahern wrote:
> On 1/22/21 9:02 PM, Maciej Żenczykowski wrote:
> > Why can't we get rid of the special case for 0 and simply make 1024 the
> > default value?  
> 
> That would work too.

Should we drop it then? Easier to bring it back than to change the
interpretation later. It doesn't seem to serve any clear purpose right
now.

(Praveen if you post v4 please take a look at the checkpatch --strict
warnings and address the ones which make sense, e.g. drop the brackets
around comparisons, those are just noise, basic grasp of C operator
precedence can be assumed in readers of kernel code).

Re: [PATCH v3 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

[David Ahern]

On 1/23/21 1:00 PM, Jakub Kicinski wrote:
> On Fri, 22 Jan 2021 22:16:41 -0700 David Ahern wrote:
>> On 1/22/21 9:02 PM, Maciej Żenczykowski wrote:
>>> Why can't we get rid of the special case for 0 and simply make 1024 the
>>> default value?  
>>
>> That would work too.
> 
> Should we drop it then? Easier to bring it back than to change the
> interpretation later. It doesn't seem to serve any clear purpose right
> now.
> 
> (Praveen if you post v4 please take a look at the checkpatch --strict
> warnings and address the ones which make sense, e.g. drop the brackets
> around comparisons, those are just noise, basic grasp of C operator
> precedence can be assumed in readers of kernel code).
> 

let's do a v4.

Praveen: set the initial value to IP6_RT_PRIO_USER, do not allow 0,
remove the checks on value and don't forget to update documentation.

Oh and cc me on the next otherwise the review depends on me finding time
to scan netdev.

Re: [PATCH v3 net-next 1/1] Allow user to set metric on default route learned via Router Advertisement.

[praveen chaudhary]

> On Jan 23, 2021, at 5:13 PM, David Ahern <dsahern@gmail.com> wrote:
> 
> On 1/23/21 1:00 PM, Jakub Kicinski wrote:
>> On Fri, 22 Jan 2021 22:16:41 -0700 David Ahern wrote:
>>> On 1/22/21 9:02 PM, Maciej Żenczykowski wrote:
>>>> Why can't we get rid of the special case for 0 and simply make 1024 the
>>>> default value?  
>>> 
>>> That would work too.
>> 
>> Should we drop it then? Easier to bring it back than to change the
>> interpretation later. It doesn't seem to serve any clear purpose right
>> now.
>> 
>> (Praveen if you post v4 please take a look at the checkpatch --strict
>> warnings and address the ones which make sense, e.g. drop the brackets
>> around comparisons, those are just noise, basic grasp of C operator
>> precedence can be assumed in readers of kernel code).
>> 
> 
> let's do a v4.
> 
> Praveen: set the initial value to IP6_RT_PRIO_USER, do not allow 0,
> remove the checks on value and don't forget to update documentation.
> 

Sure, I will respin V4, with above mentioned changes. Also, I will address checkpatch --strict warnings.

I wanted to set initial value to IP6_RT_PRIO_USER in v1, but avoided till review for 2 simple coding reasons:
1.) IP6_RT_PRIO_USER must be exposed in net/ipv6/addrconf.c by including include/uapi/linux/ipv6_route.h.
2.) If rt6_add_dflt_router() will be called from other files in future, IP6_RT_PRIO_USER should be included in all those files as well, because caller will pass most probably default value.

> Oh and cc me on the next otherwise the review depends on me finding time
> to scan netdev.

Sure, I will cc you and will add “Reviewed by” as well. I will also send you the lkml link to v4.
Thanks Jakub and you for reviewing this over the weekend.