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.

8 files changed, 146 insertions, 0 deletions

diff --git a/Makefile.am b/Makefile.am
index 65b445ec5..b4bb6e1e3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -4109,2 +4109,3 @@ systemd_networkd_LDADD = \
 	libsystemd-dhcp.la \
+	libsystemd-label.la \
 	libsystemd-shared.la
@@ -4135,2 +4136,3 @@ test_network_LDADD = \
 	libsystemd-dhcp.la \
+	libsystemd-label.la \
 	libsystemd-shared.la
diff --git a/src/network/networkd-address.c b/src/network/networkd-address.c
index 8a7163005..7d06cf887 100644
--- a/src/network/networkd-address.c
+++ b/src/network/networkd-address.c
@@ -192,2 +192,39 @@ int address_configure(Address *address, Link *link,
 
+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,
diff --git a/src/network/networkd-gperf.gperf b/src/network/networkd-gperf.gperf
index f1594d56c..92954a836 100644
--- a/src/network/networkd-gperf.gperf
+++ b/src/network/networkd-gperf.gperf
@@ -27,2 +27,3 @@ 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
diff --git a/src/network/networkd-link.c b/src/network/networkd-link.c
index 25d92c8aa..5dd8e9100 100644
--- a/src/network/networkd-link.c
+++ b/src/network/networkd-link.c
@@ -380,2 +380,3 @@ static void dhcp_handler(sd_dhcp_client *client, int event, void *userdata) {
                 _cleanup_route_free_ Route *rt = NULL;
+                struct in_addr **nameservers;
 
@@ -422,2 +423,9 @@ static void dhcp_handler(sd_dhcp_client *client, int event, void *userdata) {
 
+                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);
diff --git a/src/network/networkd-manager.c b/src/network/networkd-manager.c
index c9ce1d6a1..f02eed1a5 100644
--- a/src/network/networkd-manager.c
+++ b/src/network/networkd-manager.c
@@ -21,2 +21,4 @@
 
+#include <resolv.h>
+
 #include "path-util.h"
@@ -25,2 +27,3 @@
 #include "udev-util.h"
+#include "mkdir.h"
 
@@ -278 +281,73 @@ int manager_rtnl_listen(Manager *m) {
 }
+
+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;
+}
diff --git a/src/network/networkd-network.c b/src/network/networkd-network.c
index f2546440c..f98731001 100644
--- a/src/network/networkd-network.c
+++ b/src/network/networkd-network.c
@@ -146,2 +146,4 @@ void network_free(Network *network) {
 
+        address_free(network->dns);
+
         while ((route = network->static_routes))
@@ -199,2 +201,8 @@ int network_apply(Manager *manager, Network *network, Link *link) {
 
+        if (network->dns) {
+                r = manager_update_resolv_conf(manager);
+                if (r < 0)
+                        return r;
+        }
+
         return 0;
diff --git a/src/network/networkd.c b/src/network/networkd.c
index 360afba98..055200cb0 100644
--- a/src/network/networkd.c
+++ b/src/network/networkd.c
@@ -62,2 +62,9 @@ int main(int argc, char *argv[]) {
 
+
+        /* write out empty resolv.conf to avoid a
+         * dangling symlink */
+        r = manager_update_resolv_conf(m);
+        if (r < 0)
+                goto out;
+
         sd_notify(false,
diff --git a/src/network/networkd.h b/src/network/networkd.h
index 55181f2bb..607feba0e 100644
--- a/src/network/networkd.h
+++ b/src/network/networkd.h
@@ -90,2 +90,3 @@ struct Network {
         LIST_HEAD(Route, static_routes);
+        Address *dns;
 
@@ -159,2 +160,3 @@ struct Link {
         Address *dhcp_address;
+        Address *dns;
 
@@ -197,2 +199,4 @@ int manager_rtnl_listen(Manager *m);
 
+int manager_update_resolv_conf(Manager *m);
+
 DEFINE_TRIVIAL_CLEANUP_FUNC(Manager*, manager_free);
@@ -260,2 +264,6 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(Address*, address_free);
 
+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,