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>

7 files changed, 35 insertions, 8 deletions

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);