networkd: Add EmitRouter= option for DHCP Server (#3251)

Add an option to disable appending DHCP option 3 (Router) to the DHCP
OFFER and ACK packets.
This commit adds the boolean option EmitRouter= for the [DHCPServer]
section in .network files.

Rationale: On embedded devices, it is very useful to have a DHCP server
running on an USB OTG ethernet gadget interface to avoid manual setup on
the client PCs, but it should only serve IP addresses, no route(r)s.
Otherwise, Windows clients experience network connectivity issues, due
to them using the address set in DHCP option 3 as default gateway.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>

9 files changed, 63 insertions, 22 deletions

diff --git a/NEWS b/NEWS
index ff2dd9abbf..a6e0581879 100644
--- a/NEWS
+++ b/NEWS
@@ -180,22 +180,26 @@ CHANGES WITH 230 in spe:
           service. Please leave PrivateDevices= off if you run into problems
           with this.
 
+        * The systemd-networkd DHCP server gained the option EmitRouter=, which
+          defaults to yes, to configure if the DHCP Option 3 (Router) should be
+          emitted.
+
         Contributions from: Alban Crequy, Alexander Kuleshov, Alex Crawford,
         Andrew Eikum, Beniamino Galvani, Benjamin Robin, Benjamin ROBIN, Biao
-        Lu, Bjørnar Ness, Calvin Owens, Christian Hesse, Colin Guthrie, Daniel
-        J Walsh, Daniel Mack, Dan Nicholson, daurnimator, David Herrmann, David
-        R. Hedges, Elias Probst, Emmanuel Gil Peyrot, EMOziko, Evgeny
-        Vereshchagin, Federico, Felipe Sateler, Filipe Brandenburger, Franck
-        Bui, frankheckenbach, Georgia Brikis, Harald Hoyer, Hendrik Brueckner,
-        Hristo Venev, Iago López Galeiras, Ian Kelling, Ismo Puustinen, Jakub
-        Wilk, Jaroslav Škarvada, Jeff Huang, Joel Holdsworth, kayrus, Klearchos
-        Chaloulos, Lennart Poettering, Lubomir Rintel, Lukas Nykryn, Lukáš
-        Nykrýn, Mantas Mikulėnas, Marcel Holtmann, Martin Pitt, Michael Biebl,
-        michaelolbrich, Michał Bartoszkiewicz, Michal Koutný, Michal Sekletar,
-        Mike Frysinger, Mike Gilbert, Mingcong Bai, Ming Lin, mulkieran,
-        muzena, Nalin Dahyabhai, Naohiro Aota, Nathan McSween, Nicolas
-        Braud-Santoni, Patrik Flykt, Peter Hutterer, Petr Lautrbach, Petros
-        Angelatos, Piotr Drąg, Rabin Vincent, Robert Węcławski, Ronny
+        Lu, Bjørnar Ness, Calvin Owens, Christian Hesse, Clemens Gruber, Colin
+        Guthrie, Daniel J Walsh, Daniel Mack, Dan Nicholson, daurnimator, David
+        Herrmann, David R. Hedges, Elias Probst, Emmanuel Gil Peyrot, EMOziko,
+        Evgeny Vereshchagin, Federico, Felipe Sateler, Filipe Brandenburger,
+        Franck Bui, frankheckenbach, Georgia Brikis, Harald Hoyer, Hendrik
+        Brueckner, Hristo Venev, Iago López Galeiras, Ian Kelling, Ismo
+        Puustinen, Jakub Wilk, Jaroslav Škarvada, Jeff Huang, Joel Holdsworth,
+        kayrus, Klearchos Chaloulos, Lennart Poettering, Lubomir Rintel, Lukas
+        Nykryn, Lukáš Nykrýn, Mantas Mikulėnas, Marcel Holtmann, Martin Pitt,
+        Michael Biebl, michaelolbrich, Michał Bartoszkiewicz, Michal Koutný,
+        Michal Sekletar, Mike Frysinger, Mike Gilbert, Mingcong Bai, Ming Lin,
+        mulkieran, muzena, Nalin Dahyabhai, Naohiro Aota, Nathan McSween,
+        Nicolas Braud-Santoni, Patrik Flykt, Peter Hutterer, Petr Lautrbach,
+        Petros Angelatos, Piotr Drąg, Rabin Vincent, Robert Węcławski, Ronny
         Chevalier, Samuel Tardieu, Stefan Schallenberg, Steven Siloti, Susant
         Sahani, Sylvain Plantefève, Taylor Smock, tblume, Tejun Heo, Thomas
         Blume, Thomas Haller, Thomas Hindoe Paaboel Andersen, Thomas
diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index 70e3804746..d917fe2c12 100644
--- a/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@ -982,6 +982,16 @@
       </varlistentry>
 
       <varlistentry>
+        <term><varname>EmitRouter=</varname></term>
+
+        <listitem><para>Similar to the <varname>EmitDNS=</varname>
+        setting described above, this setting configures whether the
+        DHCP lease should contain the router option. The same syntax,
+        propagation semantics and defaults apply as for
+        <varname>EmitDNS=</varname>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
         <term><varname>EmitTimezone=</varname></term>
         <term><varname>Timezone=</varname></term>
 
diff --git a/src/libsystemd-network/dhcp-server-internal.h b/src/libsystemd-network/dhcp-server-internal.h
index adb557167a..0c76956fad 100644
--- a/src/libsystemd-network/dhcp-server-internal.h
+++ b/src/libsystemd-network/dhcp-server-internal.h
@@ -63,6 +63,8 @@ struct sd_dhcp_server {
         struct in_addr *ntp, *dns;
         unsigned n_ntp, n_dns;
 
+        bool emit_router;
+
         Hashmap *leases_by_client_id;
         DHCPLease **bound_leases;
         DHCPLease invalid_lease;
diff --git a/src/libsystemd-network/sd-dhcp-server.c b/src/libsystemd-network/sd-dhcp-server.c
index 9adf8ec19d..fb335337c4 100644
--- a/src/libsystemd-network/sd-dhcp-server.c
+++ b/src/libsystemd-network/sd-dhcp-server.c
@@ -468,10 +468,12 @@ static int server_send_offer(sd_dhcp_server *server, DHCPRequest *req,
         if (r < 0)
                 return r;
 
-        r = dhcp_option_append(&packet->dhcp, req->max_optlen, &offset, 0,
-                               SD_DHCP_OPTION_ROUTER, 4, &server->address);
-        if (r < 0)
-                return r;
+        if (server->emit_router) {
+                r = dhcp_option_append(&packet->dhcp, req->max_optlen, &offset, 0,
+                                       SD_DHCP_OPTION_ROUTER, 4, &server->address);
+                if (r < 0)
+                        return r;
+        }
 
         r = dhcp_server_send_packet(server, req, packet, DHCP_OFFER, offset);
         if (r < 0)
@@ -505,10 +507,12 @@ static int server_send_ack(sd_dhcp_server *server, DHCPRequest *req,
         if (r < 0)
                 return r;
 
-        r = dhcp_option_append(&packet->dhcp, req->max_optlen, &offset, 0,
-                               SD_DHCP_OPTION_ROUTER, 4, &server->address);
-        if (r < 0)
-                return r;
+        if (server->emit_router) {
+                r = dhcp_option_append(&packet->dhcp, req->max_optlen, &offset, 0,
+                                       SD_DHCP_OPTION_ROUTER, 4, &server->address);
+                if (r < 0)
+                        return r;
+        }
 
         if (server->n_dns > 0) {
                 r = dhcp_option_append(
@@ -1158,3 +1162,14 @@ int sd_dhcp_server_set_ntp(sd_dhcp_server *server, const struct in_addr ntp[], u
 
         return 1;
 }
+
+int sd_dhcp_server_set_emit_router(sd_dhcp_server *server, int enabled) {
+        assert_return(server, -EINVAL);
+
+        if (enabled == server->emit_router)
+                return 0;
+
+        server->emit_router = enabled;
+
+        return 1;
+}
diff --git a/src/network/networkd-link.c b/src/network/networkd-link.c
index f3a79b0ec8..16a3609a0b 100644
--- a/src/network/networkd-link.c
+++ b/src/network/networkd-link.c
@@ -1020,6 +1020,12 @@ static int link_enter_set_addresses(Link *link) {
                                 log_link_warning_errno(link, r, "Failed to set NTP server for DHCP server, ignoring: %m");
                 }
 
+                r = sd_dhcp_server_set_emit_router(link->dhcp_server, link->network->dhcp_server_emit_router);
+                if (r < 0) {
+                        log_link_warning_errno(link, r, "Failed to set router emission for DHCP server: %m");
+                        return r;
+                }
+
                 if (link->network->dhcp_server_emit_timezone) {
                         _cleanup_free_ char *buffer = NULL;
                         const char *tz = NULL;
diff --git a/src/network/networkd-network-gperf.gperf b/src/network/networkd-network-gperf.gperf
index 4425ee4e2f..03e4e3b39f 100644
--- a/src/network/networkd-network-gperf.gperf
+++ b/src/network/networkd-network-gperf.gperf
@@ -95,6 +95,7 @@ DHCPServer.EmitDNS,                     config_parse_bool,
 DHCPServer.DNS,                         config_parse_dhcp_server_dns,                   0,                             0
 DHCPServer.EmitNTP,                     config_parse_bool,                              0,                             offsetof(Network, dhcp_server_emit_ntp)
 DHCPServer.NTP,                         config_parse_dhcp_server_ntp,                   0,                             0
+DHCPServer.EmitRouter,                  config_parse_bool,                              0,                             offsetof(Network, dhcp_server_emit_router)
 DHCPServer.EmitTimezone,                config_parse_bool,                              0,                             offsetof(Network, dhcp_server_emit_timezone)
 DHCPServer.Timezone,                    config_parse_timezone,                          0,                             offsetof(Network, dhcp_server_timezone)
 DHCPServer.PoolOffset,                  config_parse_uint32,                            0,                             offsetof(Network, dhcp_server_pool_offset)
diff --git a/src/network/networkd-network.c b/src/network/networkd-network.c
index 206c270e50..dd89b3770c 100644
--- a/src/network/networkd-network.c
+++ b/src/network/networkd-network.c
@@ -113,6 +113,7 @@ static int network_load_one(Manager *manager, const char *filename) {
 
         network->dhcp_server_emit_dns = true;
         network->dhcp_server_emit_ntp = true;
+        network->dhcp_server_emit_router = true;
         network->dhcp_server_emit_timezone = true;
 
         network->use_bpdu = true;
diff --git a/src/network/networkd-network.h b/src/network/networkd-network.h
index 4cd0fa4ab8..91099161ce 100644
--- a/src/network/networkd-network.h
+++ b/src/network/networkd-network.h
@@ -127,6 +127,7 @@ struct Network {
         bool dhcp_server_emit_ntp;
         struct in_addr *dhcp_server_ntp;
         unsigned n_dhcp_server_ntp;
+        bool dhcp_server_emit_router;
         bool dhcp_server_emit_timezone;
         char *dhcp_server_timezone;
         usec_t dhcp_server_default_lease_time_usec, dhcp_server_max_lease_time_usec;
diff --git a/src/systemd/sd-dhcp-server.h b/src/systemd/sd-dhcp-server.h
index fcef083ce6..d4517a26d6 100644
--- a/src/systemd/sd-dhcp-server.h
+++ b/src/systemd/sd-dhcp-server.h
@@ -51,6 +51,7 @@ int sd_dhcp_server_configure_pool(sd_dhcp_server *server, struct in_addr *addres
 int sd_dhcp_server_set_timezone(sd_dhcp_server *server, const char *timezone);
 int sd_dhcp_server_set_dns(sd_dhcp_server *server, const struct in_addr ntp[], unsigned n);
 int sd_dhcp_server_set_ntp(sd_dhcp_server *server, const struct in_addr dns[], unsigned n);
+int sd_dhcp_server_set_emit_router(sd_dhcp_server *server, int enabled);
 
 int sd_dhcp_server_set_max_lease_time(sd_dhcp_server *server, uint32_t t);
 int sd_dhcp_server_set_default_lease_time(sd_dhcp_server *server, uint32_t t);