networkd: don't touch global forwarding setting

This reverts commit 43c6d5abacaebf813845934ec8d5e5ee3c431854
(and a small part of 4046d8361c55c80ab8577aea52523b9e6eab0d0c)

It turns out we don't actually need to set the global ip_forward setting.
The only relevant setting is the one on each interface.

What the global toggle actually does is switch forwarding on/off for all
currently present interfaces and change the default for new ones.

That means that by setting the global ip_forward we

  - Introduce a race condition, because if the interface with IPForward=yes
    is brought up after one with IPForward=no, both will have forwarding
    enabled, because the global switch turns it on for all interfaces.
    If the other interface comes up first networkd correctly sets forward=0
    and it doesn't get overridden.

  - Change the forwarding setting for interfaces that networkd is not
    configured to touch, even if the user disabled forwarding via sysctl,
    either globally or per-interface

As forwarding works fine without this, as long as all relevant interfacest
individually set IPForward=yes:  just drop it

This means that non-networkd interfaces use the global default while
networkd interfaces default to off if IPForward isn't given.

2 files changed, 2 insertions, 30 deletions

diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index 4be9d1301..31484d843 100644
--- a/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@ -373,9 +373,7 @@
           globally turned on in the kernel, with the
           <filename>net.ipv4.ip_forward</filename> and
           <filename>net.ipv4.ip_forward</filename> sysctl
-          options. Also, if this option is enabled for at least one
-          interface, the global options in the kernel are also enabled
-          as necessary, to ensure IP forwarding can take place.</para>
+          options.</para>
           </listitem>
         </varlistentry>
         <varlistentry>
diff --git a/src/network/networkd-link.c b/src/network/networkd-link.c
index 5628ed92b..1c59e150d 100644
--- a/src/network/networkd-link.c
+++ b/src/network/networkd-link.c
@@ -1475,39 +1475,13 @@ static int link_enter_join_netdev(Link *link) {
 
 static int link_set_ipv4_forward(Link *link) {
         const char *p = NULL;
-        bool b;
         int r;
 
-        b = link_ipv4_forward_enabled(link);
-
         p = strjoina("/proc/sys/net/ipv4/conf/", link->ifname, "/forwarding");
-        r = write_string_file_no_create(p, one_zero(b));
+        r = write_string_file_no_create(p, one_zero(link_ipv4_forward_enabled(link)));
         if (r < 0)
                 log_link_warning_errno(link, r, "Cannot configure IPv4 forwarding for interface %s: %m", link->ifname);
 
-        if (b) {
-                _cleanup_free_ char *buf = NULL;
-
-                /* If IP forwarding is turned on for this interface,
-                 * then propagate this to the global setting. Given
-                 * that turning this on has side-effects on other
-                 * fields, we'll try to avoid doing this unless
-                 * necessary, hence check the previous value
-                 * first. Note that we never turn this option off
-                 * again, since all interfaces we manage do not do
-                 * forwarding anyway by default, and ownership rules
-                 * of this control are so unclear. */
-
-                r = read_one_line_file("/proc/sys/net/ipv4/ip_forward", &buf);
-                if (r < 0)
-                        log_link_warning_errno(link, r, "Cannot read /proc/sys/net/ipv4/ip_forward: %m");
-                else if (!streq(buf, "1")) {
-                        r = write_string_file_no_create("/proc/sys/net/ipv4/ip_forward", "1");
-                        if (r < 0)
-                                log_link_warning_errno(link, r, "Cannot write /proc/sys/net/ipv4/ip_forward: %m");
-                }
-        }
-
         return 0;
 }