[PATCH v1 net-next 0/1] Allow user to set metric on default route learned via Router Advertisement.

Allow user to set metric on default route learned via Router Advertisement.

Note: RFC 4191 does not say anything for metric for IPv6 default route.

Fix:
For IPv4, default route is learned via DHCPv4 and user is allowed to change
metric using config in 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.

Logs:
----------------------------------------------------------------
For IPv4:
----------------------------------------------------------------

Config in etc/network/interfaces
----------------------------------------------------------------

auto eth0
iface eth0 inet dhcp
    metric 4261413864

IPv4 Kernel Route Table:
----------------------------------------------------------------

$ sudo route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.11.44.1     0.0.0.0         UG    -33553432 0        0 eth0

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,
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.accept_ra_defrtr_metric=0x770003e9

IP monitor:
----------------------------------------------------------------

default via fe80::xx16:xxxx:feb3:ce8e dev eth0 proto ra metric 1996489705  pref high

Kernel IPv6 routing table
----------------------------------------------------------------

Destination                    Next Hop                   Flag Met Ref Use If
::/0                           fe80::xx16:xxxx:feb3:ce8e  UGDAe 1996489705 0
 0 eth0

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 IPv6 RA is received, because the default route must be fully controlled by RA msg.

admin@lnos-x1-a-asw03:~$ sudo sysctl -w net.ipv6.conf.eth0.accept_ra_defrtr_metric=0x770003e8
net.ipv6.conf.eth0.accept_ra_defrtr_metric = 0x770003e8

IP monitor: when metric is changed after learning Default Route from previous IPv6 RA msg:

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

Praveen Chaudhary (1):
  Allow user to set metric on default route learned via Router
    Advertisement.

 Documentation/networking/ip-sysctl.rst | 18 ++++++++++++++++++
 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, 46 insertions(+), 7 deletions(-)


base-commit: 139711f033f636cc78b6aaf7363252241b9698ef
-- 
2.29.0

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<redacted>
Signed-off-by: Zhenggen Xu<redacted>

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

---
 Documentation/networking/ip-sysctl.rst | 18 ++++++++++++++++++
 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, 46 insertions(+), 7 deletions(-)

diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index dd2b12a32b73..384159081d91 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst

@@ -1871,6 +1871,24 @@ accept_ra_defrtr - BOOLEAN
 		- enabled if accept_ra is enabled.
 		- disabled if accept_ra is disabled.
 
+accept_ra_defrtr_metric - INTEGER
+	Route metric for default route learned in Router Advertisement. This
+	value will be assigned as metric for the route learned via IPv6 Router
+	Advertisement.
+
+	Possible values are:
+		0:
+			Use default value i.e. IP6_RT_PRIO_USER	1024.
+		0xFFFFFFFF to -1:
+			-ve values represent high route metric, value will be treated as
+			unsigned value. This behaviour is inline with current IPv4 metric
+			shown with commands such as "route -n" or "ip route list".
+		1 to 0x7FFFFFF:
+			+ve values will be used as is for route metric.
+
+	Functional default: enabled if accept_ra_defrtr is enabled.
+				disabled if accept_ra_defrtr is disabled.
+
 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..19af90c77200 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;
+	__s32		accept_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..a470bdab2420 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,
+				     unsigned int 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..945de5de5144 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_ACCEPT_RA_DEFRTR_METRIC,
 	DEVCONF_MAX
 };
 

diff --git a/include/uapi/linux/sysctl.h b/include/uapi/linux/sysctl.h
index 458179df9b27..5e79c196e33c 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_ACCEPT_RA_DEFRTR_METRIC=28,
 	__NET_IPV6_MAX
 };
 

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index eff2cacd5209..702ec4a33936 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,
+	.accept_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,
+	.accept_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_ACCEPT_RA_DEFRTR_METRIC] = cnf->accept_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	= "accept_ra_defrtr_metric",
+		.data		= &ipv6_devconf.accept_ra_defrtr_metric,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
 	{
 		.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..96ab202e95f9 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c

@@ -1180,6 +1180,7 @@ static void ndisc_router_discovery(struct sk_buff *skb)
 	unsigned int pref = 0;
 	__u32 old_if_flags;
 	bool send_ifinfo_notify = false;
+	unsigned int defrtr_usr_metric = 0;
 
 	__u8 *opt = (__u8 *)(ra_msg + 1);
 

@@ -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.accept_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..5f177ae97e42 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,
+				     unsigned int 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 ? 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),

-- 
2.29.0

On 1/12/21 6:50 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.

This is a single patch set, so the details you have in the cover letter
should be in this description here. Also, please just 'ip' commands in
the patch description; 'route' command is a dinosaur that needs to be
retired.

Drop the 'accept' part; ra_defrtr_metric is sufficiently long. Since the
value is not from the RA, it is not really about accepting data from the RA.

+	Route metric for default route learned in Router Advertisement. This
+	value will be assigned as metric for the route learned via IPv6 Router
+	Advertisement.
+
+	Possible values are:
+		0:
+			Use default value i.e. IP6_RT_PRIO_USER	1024.
+		0xFFFFFFFF to -1:
+			-ve values represent high route metric, value will be treated as
+			unsigned value. This behaviour is inline with current IPv4 metric
+			shown with commands such as "route -n" or "ip route list".
+		1 to 0x7FFFFFF:
+			+ve values will be used as is for route metric.

route metric is a u32, so these ranges should not be needed. 'ip route
list' shows metric as a positive number only.

+
+	Functional default: enabled if accept_ra_defrtr is enabled.
+				disabled if accept_ra_defrtr is disabled.

Alignment problem, but I think this can be moved above to the
description and changed to something like 'only takes affect if
accept_ra_defrtr' is enabled.

__u32 and drop the 'accept_' prefix.

u32 for consistency

Drop 'ACCEPT_'

Drop 'ACCEPT_'

proc_douintvec

net coding style is reverse xmas tree - ie., longest to shortest. yes,
this function is old school, but new entries can be added in the right
place. Since you set before use, the initialization to 0 is not
necessary. And u32 for consistency.

u32

 {
 	struct fib6_config cfg = {
 		.fc_table	= l3mdev_fib_table(dev) ? : RT6_TABLE_DFLT,
-		.fc_metric	= IP6_RT_PRIO_USER,
+		.fc_metric	= defrtr_usr_metric ? defrtr_usr_metric : IP6_RT_PRIO_USER,

you can abbreviate that as:
		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),

Hi David

Thanks a lot for Review,

I have raised v2 after addressing your review comments from
https://lkml.org/lkml/2021/1/14/1400.

List of changes in v2:
---
1.) Replace accept_ra_defrtr_metric with 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.
---

Thanks a lot again for help.