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Systemd/src/resolve/resolved-etc-hosts.c
Lennart Poettering 9ca875e80c resolved: beef up logic for suppressing "localhost" entry in /etc/hosts 
Either suppress the entry entirely, or not at all. But do not suppress
the "localhost" names we recognize, leaving the ones we do not in place.

On Fedora, where "localhost4.localdomain4" is among those listed in
/etc/hosts for 127.0.0.1 we'd thus otherwise drop the "localhost" but
keep the "localhost4.localdomain4" and then on reverse lookups only
return that, which is highly confusing.
2020-12-03 09:06:27 +09:00

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/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include "fd-util.h"
#include "fileio.h"
#include "hostname-util.h"
#include "resolved-dns-synthesize.h"
#include "resolved-etc-hosts.h"
#include "socket-netlink.h"
#include "stat-util.h"
#include "string-util.h"
#include "strv.h"
#include "time-util.h"
/* Recheck /etc/hosts at most once every 2s */
#define ETC_HOSTS_RECHECK_USEC (2*USEC_PER_SEC)
static void etc_hosts_item_free(EtcHostsItem *item) {
strv_free(item->names);
free(item);
}
static void etc_hosts_item_by_name_free(EtcHostsItemByName *item) {
free(item->name);
free(item->addresses);
free(item);
}
void etc_hosts_free(EtcHosts *hosts) {
hosts->by_address = hashmap_free_with_destructor(hosts->by_address, etc_hosts_item_free);
hosts->by_name = hashmap_free_with_destructor(hosts->by_name, etc_hosts_item_by_name_free);
hosts->no_address = set_free_free(hosts->no_address);
}
void manager_etc_hosts_flush(Manager *m) {
etc_hosts_free(&m->etc_hosts);
m->etc_hosts_stat = (struct stat) {};
}
static int parse_line(EtcHosts *hosts, unsigned nr, const char *line) {
_cleanup_free_ char *address_str = NULL;
struct in_addr_data address = {};
bool found = false;
EtcHostsItem *item;
int r;
assert(hosts);
assert(line);
r = extract_first_word(&line, &address_str, NULL, EXTRACT_RELAX);
if (r < 0)
return log_error_errno(r, "/etc/hosts:%u: failed to extract address: %m", nr);
assert(r > 0); /* We already checked that the line is not empty, so it should contain *something* */
r = in_addr_ifindex_from_string_auto(address_str, &address.family, &address.address, NULL);
if (r < 0) {
log_warning_errno(r, "/etc/hosts:%u: address '%s' is invalid, ignoring: %m", nr, address_str);
return 0;
}
r = in_addr_is_null(address.family, &address.address);
if (r < 0) {
log_warning_errno(r, "/etc/hosts:%u: address '%s' is invalid, ignoring: %m", nr, address_str);
return 0;
}
if (r > 0)
/* This is an 0.0.0.0 or :: item, which we assume means that we shall map the specified hostname to
* nothing. */
item = NULL;
else {
/* If this is a normal address, then simply add entry mapping it to the specified names */
item = hashmap_get(hosts->by_address, &address);
if (!item) {
r = hashmap_ensure_allocated(&hosts->by_address, &in_addr_data_hash_ops);
if (r < 0)
return log_oom();
item = new(EtcHostsItem, 1);
if (!item)
return log_oom();
*item = (EtcHostsItem) {
.address = address,
};
r = hashmap_put(hosts->by_address, &item->address, item);
if (r < 0) {
free(item);
return log_oom();
}
}
}
for (;;) {
_cleanup_free_ char *name = NULL;
EtcHostsItemByName *bn;
r = extract_first_word(&line, &name, NULL, EXTRACT_RELAX);
if (r < 0)
return log_error_errno(r, "/etc/hosts:%u: couldn't extract hostname: %m", nr);
if (r == 0)
break;
found = true;
r = dns_name_is_valid_ldh(name);
if (r <= 0) {
log_warning_errno(r, "/etc/hosts:%u: hostname \"%s\" is not valid, ignoring.", nr, name);
continue;
}
if (!item) {
/* Optimize the case where we don't need to store any addresses, by storing
* only the name in a dedicated Set instead of the hashmap */
r = set_ensure_consume(&hosts->no_address, &dns_name_hash_ops, TAKE_PTR(name));
if (r < 0)
return r;
continue;
}
r = strv_extend(&item->names, name);
if (r < 0)
return log_oom();
bn = hashmap_get(hosts->by_name, name);
if (!bn) {
r = hashmap_ensure_allocated(&hosts->by_name, &dns_name_hash_ops);
if (r < 0)
return log_oom();
bn = new0(EtcHostsItemByName, 1);
if (!bn)
return log_oom();
r = hashmap_put(hosts->by_name, name, bn);
if (r < 0) {
free(bn);
return log_oom();
}
bn->name = TAKE_PTR(name);
}
if (!GREEDY_REALLOC(bn->addresses, bn->n_allocated, bn->n_addresses + 1))
return log_oom();
bn->addresses[bn->n_addresses++] = &item->address;
}
if (!found)
log_warning("/etc/hosts:%u: line is missing any hostnames", nr);
return 0;
}
static void strip_localhost(EtcHosts *hosts) {
static const struct in_addr_data local_in_addrs[] = {
{
.family = AF_INET,
#if __BYTE_ORDER == __LITTLE_ENDIAN
/* We want constant expressions here, that's why we don't use htole32() here */
.address.in.s_addr = UINT32_C(0x0100007F),
#else
.address.in.s_addr = UINT32_C(0x7F000001),
#endif
},
{
.family = AF_INET6,
.address.in6 = IN6ADDR_LOOPBACK_INIT,
},
};
EtcHostsItem *item;
assert(hosts);
/* Removes the 'localhost' entry from what we loaded. But only if the mapping is exclusively between
* 127.0.0.1 and localhost (or aliases to that we recognize). If there's any other name assigned to
* it, we leave the entry in.
*
* This way our regular synthesizing can take over, but only if it would result in the exact same
* mappings. */
for (size_t j = 0; j < ELEMENTSOF(local_in_addrs); j++) {
bool all_localhost, in_order;
char **i;
item = hashmap_get(hosts->by_address, local_in_addrs + j);
if (!item)
continue;
/* Check whether all hostnames the loopback address points to are localhost ones */
all_localhost = true;
STRV_FOREACH(i, item->names)
if (!is_localhost(*i)) {
all_localhost = false;
break;
}
if (!all_localhost) /* Not all names are localhost, hence keep the entries for this address. */
continue;
/* Now check if the names listed for this address actually all point back just to this
* address (or the other loopback address). If not, let's stay away from this too. */
in_order = true;
STRV_FOREACH(i, item->names) {
EtcHostsItemByName *n;
bool all_local_address;
n = hashmap_get(hosts->by_name, *i);
if (!n) /* No reverse entry? Then almost certainly the entry already got deleted from
* the previous iteration of this loop, i.e. via the other protocol */
break;
/* Now check if the addresses of this item are all localhost addresses */
all_local_address = true;
for (size_t m = 0; m < n->n_addresses; m++)
if (!in_addr_is_localhost(n->addresses[m]->family, &n->addresses[m]->address)) {
all_local_address = false;
break;
}
if (!all_local_address) {
in_order = false;
break;
}
}
if (!in_order)
continue;
STRV_FOREACH(i, item->names) {
EtcHostsItemByName *n;
n = hashmap_remove(hosts->by_name, *i);
if (n)
etc_hosts_item_by_name_free(n);
}
assert_se(hashmap_remove(hosts->by_address, local_in_addrs + j) == item);
etc_hosts_item_free(item);
}
}
int etc_hosts_parse(EtcHosts *hosts, FILE *f) {
_cleanup_(etc_hosts_free) EtcHosts t = {};
unsigned nr = 0;
int r;
for (;;) {
_cleanup_free_ char *line = NULL;
char *l;
r = read_line(f, LONG_LINE_MAX, &line);
if (r < 0)
return log_error_errno(r, "Failed to read /etc/hosts: %m");
if (r == 0)
break;
nr++;
l = strchr(line, '#');
if (l)
*l = '\0';
l = strstrip(line);
if (isempty(l))
continue;
r = parse_line(&t, nr, l);
if (r < 0)
return r;
}
strip_localhost(&t);
etc_hosts_free(hosts);
*hosts = t;
t = (EtcHosts) {}; /* prevent cleanup */
return 0;
}
static int manager_etc_hosts_read(Manager *m) {
_cleanup_fclose_ FILE *f = NULL;
struct stat st;
usec_t ts;
int r;
assert_se(sd_event_now(m->event, clock_boottime_or_monotonic(), &ts) >= 0);
/* See if we checked /etc/hosts recently already */
if (m->etc_hosts_last != USEC_INFINITY && m->etc_hosts_last + ETC_HOSTS_RECHECK_USEC > ts)
return 0;
m->etc_hosts_last = ts;
if (m->etc_hosts_stat.st_mode != 0) {
if (stat("/etc/hosts", &st) < 0) {
if (errno != ENOENT)
return log_error_errno(errno, "Failed to stat /etc/hosts: %m");
manager_etc_hosts_flush(m);
return 0;
}
/* Did the mtime or ino/dev change? If not, there's no point in re-reading the file. */
if (stat_inode_unmodified(&m->etc_hosts_stat, &st))
return 0;
}
f = fopen("/etc/hosts", "re");
if (!f) {
if (errno != ENOENT)
return log_error_errno(errno, "Failed to open /etc/hosts: %m");
manager_etc_hosts_flush(m);
return 0;
}
/* Take the timestamp at the beginning of processing, so that any changes made later are read on the next
* invocation */
r = fstat(fileno(f), &st);
if (r < 0)
return log_error_errno(errno, "Failed to fstat() /etc/hosts: %m");
r = etc_hosts_parse(&m->etc_hosts, f);
if (r < 0)
return r;
m->etc_hosts_stat = st;
m->etc_hosts_last = ts;
return 1;
}
int manager_etc_hosts_lookup(Manager *m, DnsQuestion* q, DnsAnswer **answer) {
bool found_a = false, found_aaaa = false;
struct in_addr_data k = {};
EtcHostsItemByName *bn;
DnsResourceKey *t;
const char *name;
unsigned i;
int r;
assert(m);
assert(q);
assert(answer);
if (!m->read_etc_hosts)
return 0;
(void) manager_etc_hosts_read(m);
name = dns_question_first_name(q);
if (!name)
return 0;
r = dns_name_address(name, &k.family, &k.address);
if (r > 0) {
EtcHostsItem *item;
DnsResourceKey *found_ptr = NULL;
item = hashmap_get(m->etc_hosts.by_address, &k);
if (!item)
return 0;
/* We have an address in /etc/hosts that matches the queried name. Let's return successful. Actual data
* we'll only return if the request was for PTR. */
DNS_QUESTION_FOREACH(t, q) {
if (!IN_SET(t->type, DNS_TYPE_PTR, DNS_TYPE_ANY))
continue;
if (!IN_SET(t->class, DNS_CLASS_IN, DNS_CLASS_ANY))
continue;
r = dns_name_equal(dns_resource_key_name(t), name);
if (r < 0)
return r;
if (r > 0) {
found_ptr = t;
break;
}
}
if (found_ptr) {
char **n;
r = dns_answer_reserve(answer, strv_length(item->names));
if (r < 0)
return r;
STRV_FOREACH(n, item->names) {
_cleanup_(dns_resource_record_unrefp) DnsResourceRecord *rr = NULL;
rr = dns_resource_record_new(found_ptr);
if (!rr)
return -ENOMEM;
rr->ptr.name = strdup(*n);
if (!rr->ptr.name)
return -ENOMEM;
r = dns_answer_add(*answer, rr, 0, DNS_ANSWER_AUTHENTICATED);
if (r < 0)
return r;
}
}
return 1;
}
bn = hashmap_get(m->etc_hosts.by_name, name);
if (bn) {
r = dns_answer_reserve(answer, bn->n_addresses);
if (r < 0)
return r;
} else {
/* Check if name was listed with no address. If yes, continue to return an answer. */
if (!set_contains(m->etc_hosts.no_address, name))
return 0;
}
DNS_QUESTION_FOREACH(t, q) {
if (!IN_SET(t->type, DNS_TYPE_A, DNS_TYPE_AAAA, DNS_TYPE_ANY))
continue;
if (!IN_SET(t->class, DNS_CLASS_IN, DNS_CLASS_ANY))
continue;
r = dns_name_equal(dns_resource_key_name(t), name);
if (r < 0)
return r;
if (r == 0)
continue;
if (IN_SET(t->type, DNS_TYPE_A, DNS_TYPE_ANY))
found_a = true;
if (IN_SET(t->type, DNS_TYPE_AAAA, DNS_TYPE_ANY))
found_aaaa = true;
if (found_a && found_aaaa)
break;
}
for (i = 0; bn && i < bn->n_addresses; i++) {
_cleanup_(dns_resource_record_unrefp) DnsResourceRecord *rr = NULL;
if ((!found_a && bn->addresses[i]->family == AF_INET) ||
(!found_aaaa && bn->addresses[i]->family == AF_INET6))
continue;
r = dns_resource_record_new_address(&rr, bn->addresses[i]->family, &bn->addresses[i]->address, bn->name);
if (r < 0)
return r;
r = dns_answer_add(*answer, rr, 0, DNS_ANSWER_AUTHENTICATED);
if (r < 0)
return r;
}
return found_a || found_aaaa;
}
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