From 9ca875e80c38d5bd9898cab61a612ad16d527a5a Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Tue, 10 Nov 2020 23:30:25 +0100
Subject: [PATCH] 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.
---
 src/resolve/resolved-etc-hosts.c | 95 ++++++++++++++++++++++++++++++--
 1 file changed, 91 insertions(+), 4 deletions(-)

diff --git a/src/resolve/resolved-etc-hosts.c b/src/resolve/resolved-etc-hosts.c
index dda2014017..2cb90d201a 100644
--- a/src/resolve/resolved-etc-hosts.c
+++ b/src/resolve/resolved-etc-hosts.c
@@ -113,10 +113,6 @@ static int parse_line(EtcHosts *hosts, unsigned nr, const char *line) {
                         continue;
                 }
 
-                if (is_localhost(name))
-                        /* Suppress the "localhost" line that is often seen */
-                        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 */
@@ -163,6 +159,95 @@ static int parse_line(EtcHosts *hosts, unsigned nr, const char *line) {
         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;
@@ -193,6 +278,8 @@ int etc_hosts_parse(EtcHosts *hosts, FILE *f) {
                         return r;
         }
 
+        strip_localhost(&t);
+
         etc_hosts_free(hosts);
         *hosts = t;
         t = (EtcHosts) {}; /* prevent cleanup */