[PATCH v3] ipv6: add `force_forwarding` sysctl to enable per-interface forwarding
From: Gabriel Goller
Date: Wed Jul 02 2025 - 03:46:50 EST
It is currently impossible to enable ipv6 forwarding on a per-interface
basis like in ipv4. To enable forwarding on an ipv6 interface we need to
enable it on all interfaces and disable it on the other interfaces using
a netfilter rule. This is especially cumbersome if you have lots of
interface and only want to enable forwarding on a few. According to the
sysctl docs [0] the `net.ipv6.conf.all.forwarding` enables forwarding
for all interfaces, while the interface-specific
`net.ipv6.conf.<interface>.forwarding` configures the interface
Host/Router configuration.
Introduce a new sysctl flag `force_forwarding`, which can be set on every
interface. The ip6_forwarding function will then check if the global
forwarding flag OR the force_forwarding flag is active and forward the
packet.
To preserver backwards-compatibility reset the flag (on all interfaces)
to 0 if the net.ipv6.conf.all.forwarding flag is set to 0.
[0]: https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
Signed-off-by: Gabriel Goller <g.goller@xxxxxxxxxxx>
---
Changelog:
v2 -> v3:
* remove forwarding=0 setting force_forwarding=0 globally.
* add min and max (0 and 1) value to sysctl.
v1 -> v2:
* rename from `do_forwarding` to `force_forwarding`.
* add global `force_forwarding` flag which will enable
`force_forwarding` on every interface like the
`ipv4.all.forwarding` flag.
* `forwarding`=0 will disable global and per-interface
`force_forwarding`.
* export option as NETCONFA_FORCE_FORWARDING.
Documentation/networking/ip-sysctl.rst | 5 ++
include/linux/ipv6.h | 1 +
include/uapi/linux/ipv6.h | 1 +
include/uapi/linux/netconf.h | 1 +
include/uapi/linux/sysctl.h | 1 +
net/ipv6/addrconf.c | 90 ++++++++++++++++++++++++++
net/ipv6/ip6_output.c | 3 +-
7 files changed, 101 insertions(+), 1 deletion(-)
diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst
index 0f1251cce314..f709aed44cde 100644
--- a/Documentation/networking/ip-sysctl.rst
+++ b/Documentation/networking/ip-sysctl.rst
@@ -2292,6 +2292,11 @@ conf/all/forwarding - BOOLEAN
proxy_ndp - BOOLEAN
Do proxy ndp.
+force_forwarding - BOOLEAN
+ Enable forwarding on this interface only -- regardless of the setting on
+ ``conf/all/forwarding``. When setting ``conf.all.forwarding`` to 0,
+ the `force_forwarding` flag will be reset on all interfaces.
+
fwmark_reflect - BOOLEAN
Controls the fwmark of kernel-generated IPv6 reply packets that are not
associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
diff --git a/include/linux/ipv6.h b/include/linux/ipv6.h
index 5aeeed22f35b..5380107e466c 100644
--- a/include/linux/ipv6.h
+++ b/include/linux/ipv6.h
@@ -19,6 +19,7 @@ struct ipv6_devconf {
__s32 forwarding;
__s32 disable_policy;
__s32 proxy_ndp;
+ __s32 force_forwarding;
__cacheline_group_end(ipv6_devconf_read_txrx);
__s32 accept_ra;
diff --git a/include/uapi/linux/ipv6.h b/include/uapi/linux/ipv6.h
index cf592d7b630f..d4d3ae774b26 100644
--- a/include/uapi/linux/ipv6.h
+++ b/include/uapi/linux/ipv6.h
@@ -199,6 +199,7 @@ enum {
DEVCONF_NDISC_EVICT_NOCARRIER,
DEVCONF_ACCEPT_UNTRACKED_NA,
DEVCONF_ACCEPT_RA_MIN_LFT,
+ DEVCONF_FORCE_FORWARDING,
DEVCONF_MAX
};
diff --git a/include/uapi/linux/netconf.h b/include/uapi/linux/netconf.h
index fac4edd55379..1c8c84d65ae3 100644
--- a/include/uapi/linux/netconf.h
+++ b/include/uapi/linux/netconf.h
@@ -19,6 +19,7 @@ enum {
NETCONFA_IGNORE_ROUTES_WITH_LINKDOWN,
NETCONFA_INPUT,
NETCONFA_BC_FORWARDING,
+ NETCONFA_FORCE_FORWARDING,
__NETCONFA_MAX
};
#define NETCONFA_MAX (__NETCONFA_MAX - 1)
diff --git a/include/uapi/linux/sysctl.h b/include/uapi/linux/sysctl.h
index 8981f00204db..63d1464cb71c 100644
--- a/include/uapi/linux/sysctl.h
+++ b/include/uapi/linux/sysctl.h
@@ -573,6 +573,7 @@ enum {
NET_IPV6_ACCEPT_RA_FROM_LOCAL=26,
NET_IPV6_ACCEPT_RA_RT_INFO_MIN_PLEN=27,
NET_IPV6_RA_DEFRTR_METRIC=28,
+ NET_IPV6_FORCE_FORWARDING=29,
__NET_IPV6_MAX
};
diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index ba2ec7c870cc..cca78f75cf0c 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -239,6 +239,7 @@ static struct ipv6_devconf ipv6_devconf __read_mostly = {
.ndisc_evict_nocarrier = 1,
.ra_honor_pio_life = 0,
.ra_honor_pio_pflag = 0,
+ .force_forwarding = 0,
};
static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
@@ -303,6 +304,7 @@ static struct ipv6_devconf ipv6_devconf_dflt __read_mostly = {
.ndisc_evict_nocarrier = 1,
.ra_honor_pio_life = 0,
.ra_honor_pio_pflag = 0,
+ .force_forwarding = 0,
};
/* Check if link is ready: is it up and is a valid qdisc available */
@@ -857,6 +859,15 @@ static void addrconf_forward_change(struct net *net, __s32 newf)
idev = __in6_dev_get_rtnl_net(dev);
if (idev) {
int changed = (!idev->cnf.forwarding) ^ (!newf);
+ /*
+ * With the introduction of force_forwarding, we need to be backwards
+ * compatible, so that means we need to set the force_forwarding flag
+ * on every interface to 0 if net.ipv6.conf.all.forwarding is set to 0.
+ * This allows the global forwarding flag to disable forwarding for
+ * all interfaces.
+ */
+ if (newf == 0)
+ WRITE_ONCE(idev->cnf.force_forwarding, newf);
WRITE_ONCE(idev->cnf.forwarding, newf);
if (changed)
@@ -5719,6 +5730,7 @@ static void ipv6_store_devconf(const struct ipv6_devconf *cnf,
array[DEVCONF_ACCEPT_UNTRACKED_NA] =
READ_ONCE(cnf->accept_untracked_na);
array[DEVCONF_ACCEPT_RA_MIN_LFT] = READ_ONCE(cnf->accept_ra_min_lft);
+ array[DEVCONF_FORCE_FORWARDING] = READ_ONCE(cnf->force_forwarding);
}
static inline size_t inet6_ifla6_size(void)
@@ -6747,6 +6759,77 @@ static int addrconf_sysctl_disable_policy(const struct ctl_table *ctl, int write
return ret;
}
+/* called with RTNL locked */
+static void addrconf_force_forward_change(struct net *net, __s32 newf)
+{
+ struct net_device *dev;
+ struct inet6_dev *idev;
+
+ for_each_netdev(net, dev) {
+ idev = __in6_dev_get_rtnl_net(dev);
+ if (idev) {
+ int changed = (!idev->cnf.force_forwarding) ^ (!newf);
+
+ WRITE_ONCE(idev->cnf.force_forwarding, newf);
+ if (changed) {
+ inet6_netconf_notify_devconf(dev_net(dev), RTM_NEWNETCONF,
+ NETCONFA_FORCE_FORWARDING,
+ dev->ifindex, &idev->cnf);
+ }
+ }
+ }
+}
+
+static int addrconf_sysctl_force_forwarding(const struct ctl_table *ctl, int write,
+ void *buffer, size_t *lenp, loff_t *ppos)
+{
+ int *valp = ctl->data;
+ int ret;
+ int old, new;
+
+ // get extra params from table
+ struct inet6_dev *idev = ctl->extra1;
+ struct net *net = ctl->extra2;
+
+ // copy table and change extra params to min/max so we can use proc_douintvec_minmax
+ struct ctl_table lctl;
+
+ lctl = *ctl;
+ lctl.extra1 = SYSCTL_ZERO;
+ lctl.extra2 = SYSCTL_ONE;
+
+ old = *valp;
+ ret = proc_douintvec_minmax(&lctl, write, buffer, lenp, ppos);
+ new = *valp;
+
+ if (write && old != new) {
+ if (!rtnl_net_trylock(net))
+ return restart_syscall();
+
+ if (valp == &net->ipv6.devconf_dflt->force_forwarding) {
+ inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
+ NETCONFA_FORCE_FORWARDING,
+ NETCONFA_IFINDEX_DEFAULT,
+ net->ipv6.devconf_dflt);
+ } else if (valp == &net->ipv6.devconf_all->force_forwarding) {
+ inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
+ NETCONFA_FORCE_FORWARDING,
+ NETCONFA_IFINDEX_ALL,
+ net->ipv6.devconf_all);
+
+ addrconf_force_forward_change(net, new);
+ } else {
+ inet6_netconf_notify_devconf(net, RTM_NEWNETCONF,
+ NETCONFA_FORCE_FORWARDING,
+ idev->dev->ifindex,
+ &idev->cnf);
+ }
+ rtnl_net_unlock(net);
+ }
+
+ return ret;
+}
+
static int minus_one = -1;
static const int two_five_five = 255;
static u32 ioam6_if_id_max = U16_MAX;
@@ -7217,6 +7300,13 @@ static const struct ctl_table addrconf_sysctl[] = {
.extra1 = SYSCTL_ZERO,
.extra2 = SYSCTL_TWO,
},
+ {
+ .procname = "force_forwarding",
+ .data = &ipv6_devconf.force_forwarding,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = addrconf_sysctl_force_forwarding,
+ },
};
static int __addrconf_sysctl_register(struct net *net, char *dev_name,
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 7bd29a9ff0db..c15ed4197416 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -509,7 +509,8 @@ int ip6_forward(struct sk_buff *skb)
u32 mtu;
idev = __in6_dev_get_safely(dev_get_by_index_rcu(net, IP6CB(skb)->iif));
- if (READ_ONCE(net->ipv6.devconf_all->forwarding) == 0)
+ if ((idev && READ_ONCE(idev->cnf.force_forwarding) == 0) &&
+ READ_ONCE(net->ipv6.devconf_all->forwarding) == 0)
goto error;
if (skb->pkt_type != PACKET_HOST)
--
2.39.5