path: root/src/resolve/resolved-resolv-conf.c

/* SPDX-License-Identifier: LGPL-2.1+ */

#include <resolv.h>
#include <stdio_ext.h>

#include "alloc-util.h"
#include "dns-domain.h"
#include "fd-util.h"
#include "fileio-label.h"
#include "fileio.h"
#include "ordered-set.h"
#include "resolved-conf.h"
#include "resolved-dns-server.h"
#include "resolved-resolv-conf.h"
#include "string-util.h"
#include "strv.h"

/* A resolv.conf file containing the DNS server and domain data we learnt from uplink, i.e. the full uplink data */
#define PRIVATE_UPLINK_RESOLV_CONF "/run/systemd/resolve/resolv.conf"

/* A resolv.conf file containing the domain data we learnt from uplink, but our own DNS server address. */
#define PRIVATE_STUB_RESOLV_CONF "/run/systemd/resolve/stub-resolv.conf"

/* A static resolv.conf file containing no domains, but only our own DNS server address */
#define PRIVATE_STATIC_RESOLV_CONF ROOTLIBEXECDIR "/resolv.conf"

int manager_check_resolv_conf(const Manager *m) {
        const char *path;
        struct stat st;
        int r;

        assert(m);

        /* This warns only when our stub listener is disabled and /etc/resolv.conf is a symlink to
         * PRIVATE_STATIC_RESOLV_CONF or PRIVATE_STUB_RESOLV_CONF. */

        if (m->dns_stub_listener_mode != DNS_STUB_LISTENER_NO)
                return 0;

        r = stat("/etc/resolv.conf", &st);
        if (r < 0) {
                if (errno == ENOENT)
                        return 0;

                return log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
        }

        FOREACH_STRING(path,
                       PRIVATE_STUB_RESOLV_CONF,
                       PRIVATE_STATIC_RESOLV_CONF) {

                struct stat own;

                /* Is it symlinked to our own uplink file? */
                if (stat(path, &own) >= 0 &&
                    st.st_dev == own.st_dev &&
                    st.st_ino == own.st_ino) {
                        log_warning("DNSStubListener= is disabled, but /etc/resolv.conf is a symlink to %s "
                                    "which expects DNSStubListener= to be enabled.", path);
                        return -EOPNOTSUPP;
                }
        }

        return 0;
}

static bool file_is_our_own(const struct stat *st) {
        const char *path;

        assert(st);

        FOREACH_STRING(path,
                       PRIVATE_UPLINK_RESOLV_CONF,
                       PRIVATE_STUB_RESOLV_CONF,
                       PRIVATE_STATIC_RESOLV_CONF) {

                struct stat own;

                /* Is it symlinked to our own uplink file? */
                if (stat(path, &own) >= 0 &&
                    st->st_dev == own.st_dev &&
                    st->st_ino == own.st_ino)
                        return true;
        }

        return false;
}

int manager_read_resolv_conf(Manager *m) {
        _cleanup_fclose_ FILE *f = NULL;
        struct stat st;
        char line[LINE_MAX];
        unsigned n = 0;
        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)
                        return 0;

                r = log_warning_errno(errno, "Failed to stat /etc/resolv.conf: %m");
                goto clear;
        }

        /* Have we already seen the file? */
        if (timespec_load(&st.st_mtim) == m->resolv_conf_mtime)
                return 0;

        if (file_is_our_own(&st))
                return 0;

        f = fopen("/etc/resolv.conf", "re");
        if (!f) {
                if (errno == ENOENT)
                        return 0;

                r = log_warning_errno(errno, "Failed to open /etc/resolv.conf: %m");
                goto clear;
        }

        if (fstat(fileno(f), &st) < 0) {
                r = log_error_errno(errno, "Failed to stat open file: %m");
                goto clear;
        }

        if (file_is_our_own(&st))
                return 0;

        dns_server_mark_all(m->dns_servers);
        dns_search_domain_mark_all(m->search_domains);

        FOREACH_LINE(line, f, r = -errno; goto clear) {
                const char *a;
                char *l;

                n++;

                l = strstrip(line);
                if (IN_SET(*l, '#', ';', 0))
                        continue;

                a = first_word(l, "nameserver");
                if (a) {
                        r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
                        if (r < 0)
                                log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);

                        continue;
                }

                a = first_word(l, "domain");
                if (!a) /* We treat "domain" lines, and "search" lines as equivalent, and add both to our list. */
                        a = first_word(l, "search");
                if (a) {
                        r = manager_parse_search_domains_and_warn(m, a);
                        if (r < 0)
                                log_warning_errno(r, "Failed to parse search domain string '%s', ignoring.", a);
                }

                log_syntax(NULL, LOG_DEBUG, "/etc/resolv.conf", n, 0, "Ignoring resolv.conf line: %s", l);
        }

        m->resolv_conf_mtime = timespec_load(&st.st_mtim);

        /* Flush out all servers and search domains that are still
         * marked. Those are then ones that didn't appear in the new
         * /etc/resolv.conf */
        dns_server_unlink_marked(m->dns_servers);
        dns_search_domain_unlink_marked(m->search_domains);

        /* Whenever /etc/resolv.conf changes, start using the first
         * DNS server of it. This is useful to deal with broken
         * network managing implementations (like NetworkManager),
         * that when connecting to a VPN place both the VPN DNS
         * servers and the local ones in /etc/resolv.conf. Without
         * resetting the DNS server to use back to the first entry we
         * will continue to use the local one thus being unable to
         * resolve VPN domains. */
        manager_set_dns_server(m, m->dns_servers);

        /* Unconditionally flush the cache when /etc/resolv.conf is
         * modified, even if the data it contained was completely
         * identical to the previous version we used. We do this
         * because altering /etc/resolv.conf is typically done when
         * the network configuration changes, and that should be
         * enough to flush the global unicast DNS cache. */
        if (m->unicast_scope)
                dns_cache_flush(&m->unicast_scope->cache);

        /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
         * might now talk to a very different DNS server that just happens to have the same IP address as an old one
         * (think 192.168.1.1). */
        dns_server_reset_features_all(m->dns_servers);

        return 0;

clear:
        dns_server_unlink_all(m->dns_servers);
        dns_search_domain_unlink_all(m->search_domains);
        return r;
}

static void write_resolv_conf_server(DnsServer *s, FILE *f, unsigned *count) {
        assert(s);
        assert(f);
        assert(count);

        if (!dns_server_string(s)) {
                log_warning("Out of memory, or invalid DNS address. Ignoring server.");
                return;
        }

        /* Check if the DNS server is limited to particular domains;
         * resolv.conf does not have a syntax to express that, so it must not
         * appear as a global name server to avoid routing unrelated domains to
         * it (which is a privacy violation, will most probably fail anyway,
         * and adds unnecessary load) */
        if (dns_server_limited_domains(s)) {
                log_debug("DNS server %s has route-only domains, not using as global name server", dns_server_string(s));
                return;
        }

        if (*count == MAXNS)
                fputs("# Too many DNS servers configured, the following entries may be ignored.\n", f);
        (*count)++;

        fprintf(f, "nameserver %s\n", dns_server_string(s));
}

static void write_resolv_conf_search(
                OrderedSet *domains,
                FILE *f) {
        unsigned length = 0, count = 0;
        Iterator i;
        char *domain;

        assert(domains);
        assert(f);

        fputs("search", f);

        ORDERED_SET_FOREACH(domain, domains, i) {
                if (++count > MAXDNSRCH) {
                        fputs("\n# Too many search domains configured, remaining ones ignored.", f);
                        break;
                }
                length += strlen(domain) + 1;
                if (length > 256) {
                        fputs("\n# Total length of all search domains is too long, remaining ones ignored.", f);
                        break;
                }
                fputc(' ', f);
                fputs(domain, f);
        }

        fputs("\n", f);
}

static int write_uplink_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
        Iterator i;

        fputs("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
              "#\n"
              "# This is a dynamic resolv.conf file for connecting local clients directly to\n"
              "# all known uplink DNS servers. This file lists all configured search domains.\n"
              "#\n"
              "# Third party programs must not access this file directly, but only through the\n"
              "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
              "# replace this symlink by a static file or a different symlink.\n"
              "#\n"
              "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
              "# operation for /etc/resolv.conf.\n"
              "\n", f);

        if (ordered_set_isempty(dns))
                fputs("# No DNS servers known.\n", f);
        else {
                unsigned count = 0;
                DnsServer *s;

                ORDERED_SET_FOREACH(s, dns, i)
                        write_resolv_conf_server(s, f, &count);
        }

        if (!ordered_set_isempty(domains))
                write_resolv_conf_search(domains, f);

        return fflush_and_check(f);
}

static int write_stub_resolv_conf_contents(FILE *f, OrderedSet *dns, OrderedSet *domains) {
        fputs_unlocked("# This file is managed by man:systemd-resolved(8). Do not edit.\n"
                       "#\n"
                       "# This is a dynamic resolv.conf file for connecting local clients to the\n"
                       "# internal DNS stub resolver of systemd-resolved. This file lists all\n"
                       "# configured search domains.\n"
                       "#\n"
                       "# Run \"resolvectl status\" to see details about the uplink DNS servers\n"
                       "# currently in use.\n"
                       "#\n"
                       "# Third party programs must not access this file directly, but only through the\n"
                       "# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,\n"
                       "# replace this symlink by a static file or a different symlink.\n"
                       "#\n"
                       "# See man:systemd-resolved.service(8) for details about the supported modes of\n"
                       "# operation for /etc/resolv.conf.\n"
                       "\n"
                       "nameserver 127.0.0.53\n", f);

        if (!ordered_set_isempty(domains))
                write_resolv_conf_search(domains, f);

        return fflush_and_check(f);
}

int manager_write_resolv_conf(Manager *m) {

        _cleanup_ordered_set_free_ OrderedSet *dns = NULL, *domains = NULL;
        _cleanup_free_ char *temp_path_uplink = NULL, *temp_path_stub = NULL;
        _cleanup_fclose_ FILE *f_uplink = NULL, *f_stub = NULL;
        int r;

        assert(m);

        /* Read the system /etc/resolv.conf first */
        (void) manager_read_resolv_conf(m);

        /* Add the full list to a set, to filter out duplicates */
        r = manager_compile_dns_servers(m, &dns);
        if (r < 0)
                return log_warning_errno(r, "Failed to compile list of DNS servers: %m");

        r = manager_compile_search_domains(m, &domains, false);
        if (r < 0)
                return log_warning_errno(r, "Failed to compile list of search domains: %m");

        r = fopen_temporary_label(PRIVATE_UPLINK_RESOLV_CONF, PRIVATE_UPLINK_RESOLV_CONF, &f_uplink, &temp_path_uplink);
        if (r < 0)
                return log_warning_errno(r, "Failed to open private resolv.conf file for writing: %m");

        (void) __fsetlocking(f_uplink, FSETLOCKING_BYCALLER);
        (void) fchmod(fileno(f_uplink), 0644);

        r = fopen_temporary_label(PRIVATE_STUB_RESOLV_CONF, PRIVATE_STUB_RESOLV_CONF, &f_stub, &temp_path_stub);
        if (r < 0)
                return log_warning_errno(r, "Failed to open private stub-resolv.conf file for writing: %m");

        (void) __fsetlocking(f_stub, FSETLOCKING_BYCALLER);
        (void) fchmod(fileno(f_stub), 0644);

        r = write_uplink_resolv_conf_contents(f_uplink, dns, domains);
        if (r < 0) {
                log_error_errno(r, "Failed to write private resolv.conf contents: %m");
                goto fail;
        }

        if (rename(temp_path_uplink, PRIVATE_UPLINK_RESOLV_CONF) < 0) {
                r = log_error_errno(errno, "Failed to move private resolv.conf file into place: %m");
                goto fail;
        }

        r = write_stub_resolv_conf_contents(f_stub, dns, domains);
        if (r < 0) {
                log_error_errno(r, "Failed to write private stub-resolv.conf contents: %m");
                goto fail;
        }

        if (rename(temp_path_stub, PRIVATE_STUB_RESOLV_CONF) < 0) {
                r = log_error_errno(errno, "Failed to move private stub-resolv.conf file into place: %m");
                goto fail;
        }

        return 0;

fail:
        (void) unlink(PRIVATE_UPLINK_RESOLV_CONF);
        (void) unlink(temp_path_uplink);
        (void) unlink(PRIVATE_STUB_RESOLV_CONF);
        (void) unlink(temp_path_stub);

        return r;
}