resolved: read the system /etc/resolv.conf unless we wrote it ourselves

This way we integrate nicely with foreign network management stacks,
such as NM.

src/resolve/resolved-conf.c

@@ -94,19 +94,26 @@ int config_parse_dnsv(
         else
                 l = &m->dns_servers;
 
-        /* Empty assignment means clear the list */
         if (isempty(rvalue)) {
+
+                /* Empty assignment means clear the list */
                 while (*l)
                         dns_server_free(*l);
 
-                return 0;
+        } else {
+
+                /* Otherwise add to the list */
+                r = manager_parse_dns_server(m, ltype, rvalue);
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, -r, "Failed to parse DNS server string '%s'. Ignoring.", rvalue);
+                        return 0;
+                }
         }
 
-        r = manager_parse_dns_server(m, ltype, rvalue);
-        if (r < 0) {
-                log_syntax(unit, LOG_ERR, filename, line, -r, "Failed to parse DNS server string '%s'. Ignoring.", rvalue);
-                return 0;
-        }
+        /* If we have a manual setting, then we stop reading
+         * /etc/resolv.conf */
+        if (ltype == DNS_SERVER_SYSTEM)
+                m->read_resolv_conf = false;
 
         return 0;
 }

src/resolve/resolved-link.c

@@ -150,13 +150,13 @@ static int link_update_dns_servers(Link *l) {
 
         assert(l);
 
-        LIST_FOREACH(servers, s, l->dns_servers)
-                s->marked = true;
-
         r = sd_network_get_dns(l->ifindex, &nameservers);
         if (r < 0)
                 goto clear;
 
+        LIST_FOREACH(servers, s, l->dns_servers)
+                s->marked = true;
+
         STRV_FOREACH(nameserver, nameservers) {
                 union in_addr_union a;
                 int family;

src/resolve/resolved-manager.c

@@ -184,7 +184,6 @@ fail:
         return 0;
 }
 
-
 static int manager_rtnl_listen(Manager *m) {
         _cleanup_rtnl_message_unref_ sd_rtnl_message *req = NULL, *reply = NULL;
         sd_rtnl_message *i;
@@ -410,6 +409,7 @@ int manager_new(Manager **ret) {
         m->hostname_fd = -1;
 
         m->llmnr_support = SUPPORT_YES;
+        m->read_resolv_conf = true;
 
         r = manager_parse_dns_server(m, DNS_SERVER_FALLBACK, DNS_SERVERS);
         if (r < 0)
@@ -520,6 +520,110 @@ Manager *manager_free(Manager *m) {
         return NULL;
 }
 
+int manager_read_resolv_conf(Manager *m) {
+        _cleanup_fclose_ FILE *f = NULL;
+        struct stat st, own;
+        char line[LINE_MAX];
+        DnsServer *s, *nx;
+        usec_t t;
+        int r;
+
+        assert(m);
+
+        /* Reads the system /etc/resolv.conf, if it exists and is not
+         * symlinked to our own resolv.conf instance */
+
+        if (!m->read_resolv_conf)
+                return 0;
+
+        r = stat("/etc/resolv.conf", &st);
+        if (r < 0) {
+                if (errno != ENOENT)
+                        log_warning("Failed to open /etc/resolv.conf: %m");
+                r = -errno;
+                goto clear;
+        }
+
+        /* Have we already seen the file? */
+        t = timespec_load(&st.st_mtim);
+        if (t == m->resolv_conf_mtime)
+                return 0;
+
+        m->resolv_conf_mtime = t;
+
+        /* Is it symlinked to our own file? */
+        if (stat("/run/systemd/resolve/resolv.conf", &own) >= 0 &&
+            st.st_dev == own.st_dev &&
+            st.st_ino == own.st_ino) {
+                r = 0;
+                goto clear;
+        }
+
+        f = fopen("/etc/resolv.conf", "re");
+        if (!f) {
+                if (errno != ENOENT)
+                        log_warning("Failed to open /etc/resolv.conf: %m");
+                r = -errno;
+                goto clear;
+        }
+
+        if (fstat(fileno(f), &st) < 0) {
+                log_error("Failed to stat open file: %m");
+                r = -errno;
+                goto clear;
+        }
+
+        LIST_FOREACH(servers, s, m->dns_servers)
+                s->marked = true;
+
+        FOREACH_LINE(line, f, r = -errno; goto clear) {
+                union in_addr_union address;
+                int family;
+                char *l;
+                const char *a;
+
+                truncate_nl(line);
+
+                l = strstrip(line);
+                if (*l == '#' || *l == ';')
+                        continue;
+
+                a = first_word(l, "nameserver");
+                if (!a)
+                        continue;
+
+                r = in_addr_from_string_auto(a, &family, &address);
+                if (r < 0) {
+                        log_warning("Failed to parse name server %s.", a);
+                        continue;
+                }
+
+                LIST_FOREACH(servers, s, m->dns_servers)
+                        if (s->family == family && in_addr_equal(family, &s->address, &address) > 0)
+                                break;
+
+                if (s)
+                        s->marked = false;
+                else {
+                        r = dns_server_new(m, NULL, DNS_SERVER_SYSTEM, NULL, family, &address);
+                        if (r < 0)
+                                goto clear;
+                }
+        }
+
+        LIST_FOREACH_SAFE(servers, s, nx, m->dns_servers)
+                if (s->marked)
+                        dns_server_free(s);
+
+        return 0;
+
+clear:
+        while (m->dns_servers)
+                dns_server_free(m->dns_servers);
+
+        return r;
+}
+
 static void write_resolve_conf_server(DnsServer *s, FILE *f, unsigned *count) {
         _cleanup_free_ char *t  = NULL;
         int r;
@@ -553,6 +657,9 @@ int manager_write_resolv_conf(Manager *m) {
 
         assert(m);
 
+        /* Read the system /etc/resolv.conf first */
+        manager_read_resolv_conf(m);
+
         r = fopen_temporary(path, &f, &temp_path);
         if (r < 0)
                 return r;
@@ -953,11 +1060,11 @@ bool manager_known_dns_server(Manager *m, int family, const union in_addr_union
         assert(in_addr);
 
         LIST_FOREACH(servers, s, m->dns_servers)
-                if (s->family == family && in_addr_equal(family, &s->address, in_addr))
+                if (s->family == family && in_addr_equal(family, &s->address, in_addr) > 0)
                         return true;
 
         LIST_FOREACH(servers, s, m->fallback_dns_servers)
-                if (s->family == family && in_addr_equal(family, &s->address, in_addr))
+                if (s->family == family && in_addr_equal(family, &s->address, in_addr) > 0)
                         return true;
 
         return false;
@@ -985,6 +1092,9 @@ DnsServer *manager_get_dns_server(Manager *m) {
         Link *l;
         assert(m);
 
+        /* Try to read updates resolv.conf */
+        manager_read_resolv_conf(m);
+
         if (!m->current_dns_server)
                 manager_set_dns_server(m, m->dns_servers);
 

src/resolve/resolved-manager.h

@@ -78,6 +78,9 @@ struct Manager {
         LIST_HEAD(DnsServer, fallback_dns_servers);
         DnsServer *current_dns_server;
 
+        bool read_resolv_conf;
+        usec_t resolv_conf_mtime;
+
         LIST_HEAD(DnsScope, dns_scopes);
         DnsScope *unicast_scope;
 
@@ -111,6 +114,7 @@ struct Manager {
 int manager_new(Manager **ret);
 Manager* manager_free(Manager *m);
 
+int manager_read_resolv_conf(Manager *m);
 int manager_write_resolv_conf(Manager *m);
 
 bool manager_known_dns_server(Manager *m, int family, const union in_addr_union *in_addr);

src/shared/util.c

@@ -140,26 +140,38 @@ char* endswith(const char *s, const char *postfix) {
         return (char*) s + sl - pl;
 }
 
-bool first_word(const char *s, const char *word) {
+char* first_word(const char *s, const char *word) {
         size_t sl, wl;
+        const char *p;
 
         assert(s);
         assert(word);
 
+        /* Checks if the string starts with the specified word, either
+         * followed by NUL or by whitespace. Returns a pointer to the
+         * NUL or the first character after the whitespace. */
+
         sl = strlen(s);
         wl = strlen(word);
 
         if (sl < wl)
-                return false;
+                return NULL;
 
         if (wl == 0)
-                return true;
+                return (char*) s;
 
         if (memcmp(s, word, wl) != 0)
-                return false;
+                return NULL;
 
-        return s[wl] == 0 ||
-                strchr(WHITESPACE, s[wl]);
+        p = s + wl;
+        if (*p == 0)
+                return (char*) p;
+
+        if (!strchr(WHITESPACE, *p))
+                return NULL;
+
+        p += strspn(p, WHITESPACE);
+        return (char*) p;
 }
 
 int close_nointr(int fd) {

src/shared/util.h

@@ -169,7 +169,7 @@ static inline const char *startswith_no_case(const char *s, const char *prefix)
 
 char *endswith(const char *s, const char *postfix) _pure_;
 
-bool first_word(const char *s, const char *word) _pure_;
+char *first_word(const char *s, const char *word) _pure_;
 
 int close_nointr(int fd);
 int safe_close(int fd);