networkd: generate resolv.conf

This adds support to generate a basic resolv.conf in /run/systemd/network.
This file will not take any effect unless a symlink is created from
/etc/resolv.conf.

Nameservers received over DHCP takes precedence over statically configured ones.

Note: /etc/resolv.conf is severely limited, so in the future we will likely
rather provide a much more powerfull nss plugin (or something to that effect),
but this should allow current users to function without any loss of
functionality.

Makefile.am

@@ -4107,6 +4107,7 @@ systemd_networkd_LDADD = \
 	libsystemd-id128-internal.la \
 	libsystemd-rtnl.la \
 	libsystemd-dhcp.la \
+	libsystemd-label.la \
 	libsystemd-shared.la
 
 nodist_systemunit_DATA += \
@@ -4133,6 +4134,7 @@ test_network_LDADD = \
 	libsystemd-daemon-internal.la \
 	libsystemd-rtnl.la \
 	libsystemd-dhcp.la \
+	libsystemd-label.la \
 	libsystemd-shared.la
 
 tests += \

src/network/networkd-address.c

@@ -190,6 +190,43 @@ int address_configure(Address *address, Link *link,
         return 0;
 }
 
+int config_parse_dns(const char *unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+        Address **dns = data;
+        _cleanup_address_free_ Address *n = NULL;
+        int r;
+
+        assert(filename);
+        assert(section);
+        assert(lvalue);
+        assert(rvalue);
+        assert(data);
+
+        r = address_new_dynamic(&n);
+        if (r < 0)
+                return r;
+
+        r = net_parse_inaddr(rvalue, &n->family, &n->in_addr);
+        if (r < 0) {
+                log_syntax(unit, LOG_ERR, filename, line, EINVAL,
+                           "DNS address is invalid, ignoring assignment: %s", rvalue);
+                return 0;
+        }
+
+        *dns = n;
+        n = NULL;
+
+        return 0;
+}
+
 int config_parse_address(const char *unit,
                 const char *filename,
                 unsigned line,

src/network/networkd-gperf.gperf

@@ -25,6 +25,7 @@ Network.Bridge,          config_parse_bridge,           0,       offsetof(Networ
 Network.DHCP,            config_parse_bool,             0,       offsetof(Network, dhcp)
 Network.Address,         config_parse_address,          0,       0
 Network.Gateway,         config_parse_gateway,          0,       0
+Network.DNS,             config_parse_dns,              0,       offsetof(Network, dns)
 Address.Address,         config_parse_address,          0,       0
 Address.Label,           config_parse_label,            0,       0
 Route.Gateway,           config_parse_gateway,          0,       0

src/network/networkd-link.c

@@ -378,6 +378,7 @@ static void dhcp_handler(sd_dhcp_client *client, int event, void *userdata) {
         if (event == DHCP_EVENT_IP_CHANGE || event == DHCP_EVENT_IP_ACQUIRE) {
                 _cleanup_address_free_ Address *addr = NULL;
                 _cleanup_route_free_ Route *rt = NULL;
+                struct in_addr **nameservers;
 
                 log_struct_link(LOG_INFO, link,
                                 "MESSAGE=%s: DHCPv4 address %u.%u.%u.%u/%u via %u.%u.%u.%u",
@@ -420,6 +421,13 @@ static void dhcp_handler(sd_dhcp_client *client, int event, void *userdata) {
                 addr = NULL;
                 rt = NULL;
 
+                r = sd_dhcp_client_get_dns(client, &nameservers);
+                if (r >= 0) {
+                        r = manager_update_resolv_conf(link->manager);
+                        if (r < 0)
+                                log_error("Failed to update resolv.conf");
+                }
+
                 link_enter_set_addresses(link);
         }
 

src/network/networkd-manager.c

@@ -19,10 +19,13 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  ***/
 
+#include <resolv.h>
+
 #include "path-util.h"
 #include "networkd.h"
 #include "libudev-private.h"
 #include "udev-util.h"
+#include "mkdir.h"
 
 const char* const network_dirs[] = {
         "/etc/systemd/network",
@@ -276,3 +279,75 @@ int manager_rtnl_listen(Manager *m) {
 
         return 0;
 }
+
+static void append_dns(FILE *f, struct in_addr *dns, unsigned char family, unsigned *count) {
+        char buf[INET6_ADDRSTRLEN];
+        const char *address;
+
+        address = inet_ntop(family, dns, buf, INET6_ADDRSTRLEN);
+        if (!address) {
+                log_warning("Invalid DNS address. Ignoring.");
+                return;
+        }
+
+        if (*count == MAXNS)
+                fputs("# Too many dynamic name servers configured, the "
+                      "following entries will be ignored\n", f);
+
+        fprintf(f, "nameserver %s\n", address);
+
+        (*count) ++;
+}
+
+int manager_update_resolv_conf(Manager *m) {
+        _cleanup_free_ char *temp_path = NULL;
+        _cleanup_fclose_ FILE *f = NULL;
+        Link *link;
+        Iterator i;
+        unsigned count = 0;
+        int r;
+
+        assert(m);
+
+        r = mkdir_safe_label("/run/systemd/network", 0755, 0, 0);
+        if (r < 0)
+                return r;
+
+        r = fopen_temporary("/run/systemd/network/resolv.conf", &f, &temp_path);
+        if (r < 0)
+                return r;
+
+        fchmod(fileno(f), 0644);
+
+        fputs("# This file is managed by systemd-networkd(8). Do not edit.\n", f);
+
+        HASHMAP_FOREACH(link, m->links, i) {
+                if (link->dhcp) {
+                        struct in_addr **nameservers;
+
+                        r = sd_dhcp_client_get_dns(link->dhcp, &nameservers);
+                        if (r >= 0) {
+                                unsigned j;
+
+                                for (j = 0; nameservers[j]; j++)
+                                        append_dns(f, nameservers[j], AF_INET, &count);
+                        }
+                }
+        }
+
+        HASHMAP_FOREACH(link, m->links, i)
+                if (link->network && link->network->dns)
+                        append_dns(f, &link->network->dns->in_addr.in,
+                                   link->network->dns->family, &count);
+
+        fflush(f);
+
+        if (ferror(f) || rename(temp_path, "/run/systemd/network/resolv.conf") < 0) {
+                r = -errno;
+                unlink("/run/systemd/network/resolv.conf");
+                unlink(temp_path);
+                return r;
+        }
+
+        return 0;
+}

src/network/networkd-network.c

@@ -144,6 +144,8 @@ void network_free(Network *network) {
 
         free(network->description);
 
+        address_free(network->dns);
+
         while ((route = network->static_routes))
                 route_free(route);
 
@@ -197,6 +199,12 @@ int network_apply(Manager *manager, Network *network, Link *link) {
         if (r < 0)
                 return r;
 
+        if (network->dns) {
+                r = manager_update_resolv_conf(manager);
+                if (r < 0)
+                        return r;
+        }
+
         return 0;
 }
 

src/network/networkd.c

@@ -60,6 +60,13 @@ int main(int argc, char *argv[]) {
         if (r < 0)
                 goto out;
 
+
+        /* write out empty resolv.conf to avoid a
+         * dangling symlink */
+        r = manager_update_resolv_conf(m);
+        if (r < 0)
+                goto out;
+
         sd_notify(false,
                   "READY=1\n"
                   "STATUS=Processing requests...");

src/network/networkd.h

@@ -88,6 +88,7 @@ struct Network {
 
         LIST_HEAD(Address, static_addresses);
         LIST_HEAD(Route, static_routes);
+        Address *dns;
 
         Hashmap *addresses_by_section;
         Hashmap *routes_by_section;
@@ -157,6 +158,7 @@ struct Link {
 
         Route *dhcp_route;
         Address *dhcp_address;
+        Address *dns;
 
         LinkState state;
 
@@ -195,6 +197,8 @@ int manager_udev_listen(Manager *m);
 
 int manager_rtnl_listen(Manager *m);
 
+int manager_update_resolv_conf(Manager *m);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(Manager*, manager_free);
 #define _cleanup_manager_free_ _cleanup_(manager_freep)
 
@@ -258,6 +262,10 @@ int address_drop(Address *address, Link *link, sd_rtnl_message_handler_t callbac
 DEFINE_TRIVIAL_CLEANUP_FUNC(Address*, address_free);
 #define _cleanup_address_free_ _cleanup_(address_freep)
 
+int config_parse_dns(const char *unit, const char *filename, unsigned line,
+                     const char *section, unsigned section_line, const char *lvalue,
+                     int ltype, const char *rvalue, void *data, void *userdata);
+
 int config_parse_address(const char *unit, const char *filename, unsigned line,
                          const char *section, unsigned section_line, const char *lvalue,
                          int ltype, const char *rvalue, void *data, void *userdata);