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.

3 files changed, 57 insertions, 42 deletions

diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index 5994869d97..e6dedb027d 100644
--- a/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@ -363,29 +363,28 @@
         </varlistentry>
         <varlistentry>
           <term><varname>IPForward=</varname></term>
-          <listitem><para>Configures IP forwarding for the network
-          interface. If enabled, incoming packets on the network
-          interface will be forwarded to other interfaces according to
-          the routing table. Takes either a boolean argument, or the
-          values <literal>ipv4</literal> or <literal>ipv6</literal>,
-          which only enables IP forwarding for the specified address
-          family, or <literal>kernel</literal>, which preserves existing sysctl settings.
-          This controls the
-          <filename>net.ipv4.conf.&lt;interface&gt;.forwarding</filename>
-          and
-          <filename>net.ipv6.conf.&lt;interface&gt;.forwarding</filename>
-          sysctl options of the network interface (see <ulink
+          <listitem><para>Configures IP packet forwarding for the
+          system. If enabled, incoming packets on any network
+          interface will be forwarded to any other interfaces
+          according to the routing table. Takes either a boolean
+          argument, or the values <literal>ipv4</literal> or
+          <literal>ipv6</literal>, which only enable IP packet
+          forwarding for the specified address family.  This controls
+          the <filename>net.ipv4.ip_forward</filename> and
+          <filename>net.ipv6.conf.all.forwarding</filename> sysctl
+          options of the network interface (see <ulink
           url="https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt">ip-sysctl.txt</ulink>
           for details about sysctl options). Defaults to
           <literal>no</literal>.</para>
 
-         <para>Note: unless this option is turned on, or set to <literal>kernel</literal>,
-          no IP forwarding is done on this interface, even if this is
-          globally turned on in the kernel, with the
-          <filename>net.ipv4.ip_forward</filename>,
-          <filename>net.ipv4.conf.all.forwarding</filename>, and
-          <filename>net.ipv6.conf.all.forwarding</filename> sysctl
-          options.</para>
+          <para>Note: this setting controls a global kernel option,
+          and does so one way only: if a network that has this setting
+          enabled is set up the global setting is turned on.  However,
+          it is never turned off again, even after all networks with
+          this setting enabled are shut down again.</para>
+
+          <para>To allow IP packet forwarding only between specific
+          network interfaces use a firewall.</para>
           </listitem>
         </varlistentry>
         <varlistentry>
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;