networkd: stop managing per-interface IP forwarding settings

As it turns out the kernel does not support per-interface IPv6 packet
forwarding controls (unlike as it does for IPv4), but only supports a
global option (#1597). Also, the current per-interface management of the
setting isn't really useful, as you want it to propagate to at least one
more interface than the one you configure it on. This created much grief
(#1411, #1808).

Hence, let's roll this logic back and simplify this again, so that we
can expose the same behaviour on IPv4 and IPv6 and things start to work
automatically again for most folks: if a network with this setting set
is set up we propagate the setting into the global setting, but this is
strictly one-way: we never reset it again, and we do nothing for network
interfaces where this setting is not enabled.

Fixes: #1808, #1597.

2 files changed, 39 insertions, 23 deletions

diff --git a/src/network/networkd-link.c b/src/network/networkd-link.c
index 4a84e49699..c37532bb73 100644
--- a/src/network/networkd-link.c
+++ b/src/network/networkd-link.c
@@ -111,16 +111,26 @@ static bool link_ipv4_forward_enabled(Link *link) {
         if (!link->network)
                 return false;
 
+        if (link->network->ip_forward == _ADDRESS_FAMILY_BOOLEAN_INVALID)
+                return false;
+
         return link->network->ip_forward & ADDRESS_FAMILY_IPV4;
 }
 
 static bool link_ipv6_forward_enabled(Link *link) {
+
+        if (!socket_ipv6_is_supported())
+                return false;
+
         if (link->flags & IFF_LOOPBACK)
                 return false;
 
         if (!link->network)
                 return false;
 
+        if (link->network->ip_forward == _ADDRESS_FAMILY_BOOLEAN_INVALID)
+                return false;
+
         return link->network->ip_forward & ADDRESS_FAMILY_IPV6;
 }
 
@@ -1851,45 +1861,43 @@ static int link_enter_join_netdev(Link *link) {
 }
 
 static int link_set_ipv4_forward(Link *link) {
-        const char *p = NULL, *v;
         int r;
 
-        if (link->flags & IFF_LOOPBACK)
-                return 0;
-
-        if (link->network->ip_forward == _ADDRESS_FAMILY_BOOLEAN_INVALID)
+        if (!link_ipv4_forward_enabled(link))
                 return 0;
 
-        p = strjoina("/proc/sys/net/ipv4/conf/", link->ifname, "/forwarding");
-        v = one_zero(link_ipv4_forward_enabled(link));
+        /* We propagate the forwarding flag from one interface to the
+         * global setting one way. This means: as long as at least one
+         * interface was configured at any time that had IP forwarding
+         * enabled the setting will stay on for good. We do this
+         * primarily to keep IPv4 and IPv6 packet forwarding behaviour
+         * somewhat in sync (see below). */
 
-        r = write_string_file(p, v, WRITE_STRING_FILE_VERIFY_ON_FAILURE);
+        r = write_string_file("/proc/sys/net/ipv4/ip_forward", "1", WRITE_STRING_FILE_VERIFY_ON_FAILURE);
         if (r < 0)
-                log_link_warning_errno(link, r, "Cannot configure IPv4 forwarding for interface %s: %m", link->ifname);
+                log_link_warning_errno(link, r, "Cannot turn on IPv4 packet forwarding, ignoring: %m");
 
         return 0;
 }
 
 static int link_set_ipv6_forward(Link *link) {
-        const char *p = NULL, *v = NULL;
         int r;
 
-        /* Make this a NOP if IPv6 is not available */
-        if (!socket_ipv6_is_supported())
-                return 0;
-
-        if (link->flags & IFF_LOOPBACK)
-                return 0;
-
-        if (link->network->ip_forward == _ADDRESS_FAMILY_BOOLEAN_INVALID)
+        if (!link_ipv6_forward_enabled(link))
                 return 0;
 
-        p = strjoina("/proc/sys/net/ipv6/conf/", link->ifname, "/forwarding");
-        v = one_zero(link_ipv6_forward_enabled(link));
+        /* On Linux, the IPv6 stack does not not know a per-interface
+         * packet forwarding setting: either packet forwarding is on
+         * for all, or off for all. We hence don't bother with a
+         * per-interface setting, but simply propagate the interface
+         * flag, if it is set, to the global flag, one-way. Note that
+         * while IPv4 would allow a per-interface flag, we expose the
+         * same behaviour there and also propagate the setting from
+         * one to all, to keep things simple (see above). */
 
-        r = write_string_file(p, v, WRITE_STRING_FILE_VERIFY_ON_FAILURE);
+        r = write_string_file("/proc/sys/net/ipv6/conf/all/forwarding", "1", WRITE_STRING_FILE_VERIFY_ON_FAILURE);
         if (r < 0)
-                log_link_warning_errno(link, r, "Cannot configure IPv6 forwarding for interface: %m");
+                log_link_warning_errno(link, r, "Cannot configure IPv6 packet forwarding, ignoring: %m");
 
         return 0;
 }
diff --git a/src/network/networkd-util.c b/src/network/networkd-util.c
index df091393f6..2545621a93 100644
--- a/src/network/networkd-util.c
+++ b/src/network/networkd-util.c
@@ -79,10 +79,18 @@ int config_parse_address_family_boolean_with_kernel(
         assert(rvalue);
         assert(data);
 
+        /* This function is mostly obsolete now. It simply redirects
+         * "kernel" to "no". In older networkd versions we used to
+         * distuingish IPForward=off from IPForward=kernel, where the
+         * former would explicitly turn off forwarding while the
+         * latter would simply not touch the setting. But that logic
+         * is gone, hence silently accept the old setting, but turn it
+         * to "no". */
+
         s = address_family_boolean_from_string(rvalue);
         if (s < 0) {
                 if (streq(rvalue, "kernel"))
-                        s = _ADDRESS_FAMILY_BOOLEAN_INVALID;
+                        s = ADDRESS_FAMILY_NO;
                 else {
                         log_syntax(unit, LOG_ERR, filename, line, 0, "Failed to parse IPForward= option, ignoring: %s", rvalue);
                         return 0;