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From b9844d3dd9d7fdf81d475b81d06a6e9ec821f91d Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Mon, 9 Nov 2020 22:22:56 +0100
Subject: [PATCH] resolved: let's preferably route reverse lookups for local
subnets to matching interfaces
Let's preferably route traffic for reverse lookups to LLMNR/mDNS/DNS on
the matching interface if the IP address is in the local subnet. Also,
if looking up an IP address of our own host, let's avoid doing
LLMNR/mDNS at all.
This is useful if "~." is a routing domain to DNS, as it means, local
reverse lookups still go to LLMNR/mDNS, too.
(cherry picked from commit 13eb76ef06f5d50bbeb58df1744057e41ef2647e)
Resolves #1739689
---
src/resolve/resolved-dns-scope.c | 85 +++++++++++++++++++++++++++++++-
src/resolve/resolved-link.c | 12 +++--
src/resolve/resolved-link.h | 1 +
3 files changed, 92 insertions(+), 6 deletions(-)
diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
index 38ea7fea0a..8b65813428 100644
--- a/src/resolve/resolved-dns-scope.c
+++ b/src/resolve/resolved-dns-scope.c
@@ -417,6 +417,65 @@ int dns_scope_socket_tcp(DnsScope *s, int family, const union in_addr_union *add
return dns_scope_socket(s, SOCK_STREAM, family, address, server, port, ret_socket_address);
}
+static DnsScopeMatch match_subnet_reverse_lookups(
+ DnsScope *s,
+ const char *domain,
+ bool exclude_own) {
+
+ union in_addr_union ia;
+ LinkAddress *a;
+ int f, r;
+
+ assert(s);
+ assert(domain);
+
+ /* Checks whether the specified domain is a reverse address domain (i.e. in the .in-addr.arpa or
+ * .ip6.arpa area), and if so, whether the address matches any of the local subnets of the link the
+ * scope is associated with. If so, our scope should consider itself relevant for any lookup in the
+ * domain, since it apparently refers to hosts on this link's subnet.
+ *
+ * If 'exclude_own' is true this will return DNS_SCOPE_NO for any IP addresses assigned locally. This
+ * is useful for LLMNR/mDNS as we never want to look up our own hostname on LLMNR/mDNS but always use
+ * the locally synthesized one. */
+
+ if (!s->link)
+ return _DNS_SCOPE_INVALID; /* No link, hence no local addresses to check */
+
+ r = dns_name_address(domain, &f, &ia);
+ if (r < 0)
+ log_debug_errno(r, "Failed to determine whether '%s' is an address domain: %m", domain);
+ if (r <= 0)
+ return _DNS_SCOPE_INVALID;
+
+ if (s->family != AF_UNSPEC && f != s->family)
+ return _DNS_SCOPE_INVALID; /* Don't look for IPv4 addresses on LLMNR/mDNS over IPv6 and vice versa */
+
+ LIST_FOREACH(addresses, a, s->link->addresses) {
+
+ if (a->family != f)
+ continue;
+
+ /* Equals our own address? nah, let's not use this scope. The local synthesizer will pick it up for us. */
+ if (exclude_own &&
+ in_addr_equal(f, &a->in_addr, &ia) > 0)
+ return DNS_SCOPE_NO;
+
+ if (a->prefixlen == UCHAR_MAX) /* don't know subnet mask */
+ continue;
+
+ /* Check if the address is in the local subnet */
+ r = in_addr_prefix_covers(f, &a->in_addr, a->prefixlen, &ia);
+ if (r < 0)
+ log_debug_errno(r, "Failed to determine whether link address covers lookup address '%s': %m", domain);
+ if (r > 0)
+ /* Note that we only claim zero labels match. This is so that this is at the same
+ * priority a DNS scope with "." as routing domain is. */
+ return DNS_SCOPE_YES + 0;
+ }
+
+ return _DNS_SCOPE_INVALID;
+}
+
DnsScopeMatch dns_scope_good_domain(DnsScope *s, int ifindex, uint64_t flags, const char *domain) {
DnsSearchDomain *d;
@@ -455,6 +514,7 @@ DnsScopeMatch dns_scope_good_domain(DnsScope *s, int ifindex, uint64_t flags, co
case DNS_PROTOCOL_DNS: {
DnsServer *dns_server;
+ DnsScopeMatch m;
/* Never route things to scopes that lack DNS servers */
dns_server = dns_scope_get_dns_server(s);
@@ -485,10 +545,23 @@ DnsScopeMatch dns_scope_good_domain(DnsScope *s, int ifindex, uint64_t flags, co
dns_name_endswith(domain, "local") == 0)
return DNS_SCOPE_MAYBE;
+ /* If the IP address to look up matches the local subnet, then implicity synthesizes
+ * DNS_SCOPE_YES_BASE + 0 on this interface, i.e. preferably resolve IP addresses via the DNS
+ * server belonging to this interface. */
+ m = match_subnet_reverse_lookups(s, domain, false);
+ if (m >= 0)
+ return m;
+
return DNS_SCOPE_NO;
}
- case DNS_PROTOCOL_MDNS:
+ case DNS_PROTOCOL_MDNS: {
+ DnsScopeMatch m;
+
+ m = match_subnet_reverse_lookups(s, domain, true);
+ if (m >= 0)
+ return m;
+
if ((s->family == AF_INET && dns_name_endswith(domain, "in-addr.arpa") > 0) ||
(s->family == AF_INET6 && dns_name_endswith(domain, "ip6.arpa") > 0) ||
(dns_name_endswith(domain, "local") > 0 && /* only resolve names ending in .local via mDNS */
@@ -497,8 +570,15 @@ DnsScopeMatch dns_scope_good_domain(DnsScope *s, int ifindex, uint64_t flags, co
return DNS_SCOPE_MAYBE;
return DNS_SCOPE_NO;
+ }
+
+ case DNS_PROTOCOL_LLMNR: {
+ DnsScopeMatch m;
+
+ m = match_subnet_reverse_lookups(s, domain, true);
+ if (m >= 0)
+ return m;
- case DNS_PROTOCOL_LLMNR:
if ((s->family == AF_INET && dns_name_endswith(domain, "in-addr.arpa") > 0) ||
(s->family == AF_INET6 && dns_name_endswith(domain, "ip6.arpa") > 0) ||
(dns_name_is_single_label(domain) && /* only resolve single label names via LLMNR */
@@ -507,6 +587,7 @@ DnsScopeMatch dns_scope_good_domain(DnsScope *s, int ifindex, uint64_t flags, co
return DNS_SCOPE_MAYBE;
return DNS_SCOPE_NO;
+ }
default:
assert_not_reached("Unknown scope protocol");
diff --git a/src/resolve/resolved-link.c b/src/resolve/resolved-link.c
index ff2be12415..c42fe5b5f4 100644
--- a/src/resolve/resolved-link.c
+++ b/src/resolve/resolved-link.c
@@ -776,10 +776,13 @@ int link_address_new(Link *l, LinkAddress **ret, int family, const union in_addr
if (!a)
return -ENOMEM;
- a->family = family;
- a->in_addr = *in_addr;
+ *a = (LinkAddress) {
+ .family = family,
+ .in_addr = *in_addr,
+ .link = l,
+ .prefixlen = UCHAR_MAX,
+ };
- a->link = l;
LIST_PREPEND(addresses, l->addresses, a);
l->n_addresses++;
@@ -1077,7 +1080,8 @@ int link_address_update_rtnl(LinkAddress *a, sd_netlink_message *m) {
if (r < 0)
return r;
- sd_rtnl_message_addr_get_scope(m, &a->scope);
+ (void) sd_rtnl_message_addr_get_prefixlen(m, &a->prefixlen);
+ (void) sd_rtnl_message_addr_get_scope(m, &a->scope);
link_allocate_scopes(a->link);
link_add_rrs(a->link, false);
diff --git a/src/resolve/resolved-link.h b/src/resolve/resolved-link.h
index 063d3f35c3..8d52b10950 100644
--- a/src/resolve/resolved-link.h
+++ b/src/resolve/resolved-link.h
@@ -24,6 +24,7 @@ struct LinkAddress {
int family;
union in_addr_union in_addr;
+ unsigned char prefixlen;
unsigned char flags, scope;
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