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Systemd/src/resolve/resolved-resolv-conf.c

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14 KiB
C
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/* 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.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"
#include "tmpfile-util-label.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;
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);
for (;;) {
_cleanup_free_ char *line = NULL;
const char *a;
char *l;
r = read_line(f, LONG_LINE_MAX, &line);
if (r < 0) {
log_error_errno(r, "Failed to read /etc/resolv.conf: %m");
goto clear;
}
if (r == 0)
break;
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;
}
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