resolved: add "proxy-only" stub on 127.0.0.54

This beefs up the DNS stub logic to listen on two IP addresses:
127.0.0.53 (as before) + 127.0.0.54 (new). When the latter is contact
our stub will operate in "bypass" mode only, i.e we'll try to pass DNS
requests as unmodified upstream as we can (and not do mDNS/LLMNR and
such, also no DNSSEC validation – but we'll still do DNS-over-TLS
wrapping).

This is supposed to be useful for container environments or tethering:
this stub could be exposed (via NAT redirect) to clients of this system
and we'll try to stay out of the way with doing too much DNS magic
ourselves, but still expose whatever the current DNS server is from
upstream under a stable address/port.

How to use this:

  # iptables -t nat -I PREROUTING -p udp -i <interface> --dport 53 -j DNAT --to 127.0.0.54:53
  # echo 1 > /proc/sys/net/ipv4/conf/<interface>/route_localnet

5 files changed, 162 insertions, 52 deletions

diff --git a/src/resolve/resolved-conf.c b/src/resolve/resolved-conf.c
index 3afaaad937..453f1175e1 100644
--- a/src/resolve/resolved-conf.c
+++ b/src/resolve/resolved-conf.c
@@ -35,7 +35,7 @@ static int manager_add_dns_server_by_string(Manager *m, DnsServerType type, cons
         if (r < 0)
                 return r;
 
-        /* Silently filter out 0.0.0.0 and 127.0.0.53 (our own stub DNS listener) */
+        /* Silently filter out 0.0.0.0, 127.0.0.53, 127.0.0.54 (our own stub DNS listener) */
         if (!dns_server_address_valid(family, &address))
                 return 0;
 
diff --git a/src/resolve/resolved-dns-stub.c b/src/resolve/resolved-dns-stub.c
index 4c5429c77b..09fd7d2418 100644
--- a/src/resolve/resolved-dns-stub.c
+++ b/src/resolve/resolved-dns-stub.c
@@ -21,6 +21,7 @@
 #define ADVERTISE_EXTRA_DATAGRAM_SIZE_MAX DNS_PACKET_UNICAST_SIZE_LARGE_MAX
 
 static int manager_dns_stub_fd_extra(Manager *m, DnsStubListenerExtra *l, int type);
+static int manager_dns_stub_fd(Manager *m, int family, const union in_addr_union *listen_address, int type);
 
 static void dns_stub_listener_extra_hash_func(const DnsStubListenerExtra *a, struct siphash *state) {
         assert(a);
@@ -483,6 +484,34 @@ static int dns_stub_finish_reply_packet(
         return 0;
 }
 
+static bool address_is_proxy(int family, const union in_addr_union *a) {
+        assert(a);
+
+        /* Returns true if the specified address is the DNS "proxy" stub, i.e. where we unconditionally enable bypass mode */
+
+        if (family != AF_INET)
+                return false;
+
+        return be32toh(a->in.s_addr) == INADDR_DNS_PROXY_STUB;
+}
+
+static int find_socket_fd(
+                Manager *m,
+                DnsStubListenerExtra *l,
+                int family,
+                const union in_addr_union *listen_address,
+                int type) {
+
+        assert(m);
+
+        /* Finds the right socket to use for sending. If we know the extra listener, otherwise go via the
+         * address to send from */
+        if (l)
+                return manager_dns_stub_fd_extra(m, l, type);
+
+        return manager_dns_stub_fd(m, family, listen_address, type);
+}
+
 static int dns_stub_send(
                 Manager *m,
                 DnsStubListenerExtra *l,
@@ -498,15 +527,22 @@ static int dns_stub_send(
 
         if (s)
                 r = dns_stream_write_packet(s, reply);
-        else
-                /* Note that it is essential here that we explicitly choose the source IP address for this packet. This
-                 * is because otherwise the kernel will choose it automatically based on the routing table and will
-                 * thus pick 127.0.0.1 rather than 127.0.0.53. */
+        else {
+                int fd;
+
+                fd = find_socket_fd(m, l, p->family, &p->sender, SOCK_DGRAM);
+                if (fd < 0)
+                        return fd;
+
+                /* Note that it is essential here that we explicitly choose the source IP address for this
+                 * packet. This is because otherwise the kernel will choose it automatically based on the
+                 * routing table and will thus pick 127.0.0.1 rather than 127.0.0.53. */
                 r = manager_send(m,
-                                 manager_dns_stub_fd_extra(m, l, SOCK_DGRAM),
-                                 l ? p->ifindex : LOOPBACK_IFINDEX, /* force loopback iface if this is the main listener stub */
+                                 fd,
+                                 l || address_is_proxy(p->family, &p->destination) ? p->ifindex : LOOPBACK_IFINDEX, /* force loopback iface if this is the main listener stub */
                                  p->family, &p->sender, p->sender_port, &p->destination,
                                  reply);
+        }
         if (r < 0)
                 return log_debug_errno(r, "Failed to send reply packet: %m");
 
@@ -841,9 +877,11 @@ static int dns_stub_stream_complete(DnsStream *s, int error) {
 }
 
 static void dns_stub_process_query(Manager *m, DnsStubListenerExtra *l, DnsStream *s, DnsPacket *p) {
+        uint64_t protocol_flags = SD_RESOLVED_PROTOCOLS_ALL;
         _cleanup_(dns_query_freep) DnsQuery *q = NULL;
         Hashmap **queries_by_packet;
         DnsQuery *existing;
+        bool bypass = false;
         int r;
 
         assert(m);
@@ -851,6 +889,7 @@ static void dns_stub_process_query(Manager *m, DnsStubListenerExtra *l, DnsStrea
         assert(p->protocol == DNS_PROTOCOL_DNS);
 
         if (!l && /* l == NULL if this is the main stub */
+            !address_is_proxy(p->family, &p->destination) && /* don't restrict needlessly for 127.0.0.54 */
             (in_addr_is_localhost(p->family, &p->sender) <= 0 ||
              in_addr_is_localhost(p->family, &p->destination) <= 0)) {
                 log_warning("Got packet on unexpected (i.e. non-localhost) IP range, ignoring.");
@@ -907,19 +946,34 @@ static void dns_stub_process_query(Manager *m, DnsStubListenerExtra *l, DnsStrea
                 return;
         }
 
-        if (DNS_PACKET_DO(p) && DNS_PACKET_CD(p)) {
+        if (address_is_proxy(p->family, &p->destination)) {
+                _cleanup_free_ char *dipa = NULL;
+
+                r = in_addr_to_string(p->family, &p->destination, &dipa);
+                if (r < 0) {
+                        log_error_errno(r, "Failed to format destination address: %m");
+                        return;
+                }
+
+                log_debug("Got request to DNS proxy address 127.0.0.54, enabling bypass logic.");
+                bypass = true;
+                protocol_flags = SD_RESOLVED_DNS|SD_RESOLVED_NO_ZONE; /* Turn off mDNS/LLMNR for proxy stub. */
+        } else if ((DNS_PACKET_DO(p) && DNS_PACKET_CD(p))) {
                 log_debug("Got request with DNSSEC checking disabled, enabling bypass logic.");
+                bypass = true;
+        }
 
+        if (bypass)
                 r = dns_query_new(m, &q, NULL, NULL, p, 0,
-                                  SD_RESOLVED_PROTOCOLS_ALL|
+                                  protocol_flags|
                                   SD_RESOLVED_NO_CNAME|
                                   SD_RESOLVED_NO_SEARCH|
                                   SD_RESOLVED_NO_VALIDATE|
                                   SD_RESOLVED_REQUIRE_PRIMARY|
                                   SD_RESOLVED_CLAMP_TTL);
-        } else
+        else
                 r = dns_query_new(m, &q, p->question, p->question, NULL, 0,
-                                  SD_RESOLVED_PROTOCOLS_ALL|
+                                  protocol_flags|
                                   SD_RESOLVED_NO_SEARCH|
                                   (DNS_PACKET_DO(p) ? SD_RESOLVED_REQUIRE_PRIMARY : 0)|
                                   SD_RESOLVED_CLAMP_TTL);
@@ -1085,26 +1139,32 @@ static int set_dns_stub_common_tcp_socket_options(int fd) {
         return 0;
 }
 
-static int manager_dns_stub_fd(Manager *m, int type) {
-        union sockaddr_union sa = {
-                .in.sin_family = AF_INET,
-                .in.sin_addr.s_addr = htobe32(INADDR_DNS_STUB),
-                .in.sin_port = htobe16(53),
-        };
+static int manager_dns_stub_fd(
+                Manager *m,
+                int family,
+                const union in_addr_union *listen_addr,
+                int type) {
+
+        sd_event_source **event_source;
         _cleanup_close_ int fd = -1;
+        union sockaddr_union sa;
         int r;
 
-        assert(IN_SET(type, SOCK_DGRAM, SOCK_STREAM));
+        if (type == SOCK_DGRAM)
+                event_source = address_is_proxy(family, listen_addr) ? &m->dns_proxy_stub_udp_event_source : &m->dns_stub_udp_event_source;
+        else if (type == SOCK_STREAM)
+                event_source = address_is_proxy(family, listen_addr) ? &m->dns_proxy_stub_tcp_event_source : &m->dns_stub_tcp_event_source;
+        else
+                return -EPROTONOSUPPORT;
 
-        sd_event_source **event_source = type == SOCK_DGRAM ? &m->dns_stub_udp_event_source : &m->dns_stub_tcp_event_source;
         if (*event_source)
                 return sd_event_source_get_io_fd(*event_source);
 
-        fd = socket(AF_INET, type | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
+        fd = socket(family, type | SOCK_CLOEXEC | SOCK_NONBLOCK, 0);
         if (fd < 0)
                 return -errno;
 
-        r = set_dns_stub_common_socket_options(fd, AF_INET);
+        r = set_dns_stub_common_socket_options(fd, family);
         if (r < 0)
                 return r;
 
@@ -1114,12 +1174,30 @@ static int manager_dns_stub_fd(Manager *m, int type) {
                         return r;
         }
 
-        /* Make sure no traffic from outside the local host can leak to onto this socket */
-        r = socket_bind_to_ifindex(fd, LOOPBACK_IFINDEX);
-        if (r < 0)
-                return r;
+        /* Set slightly different socket options for the non-proxy and the proxy binding. The former we want
+         * to be accessible only from the local host, for the latter it's OK if people use NAT redirects or
+         * so to redirect external traffic to it. */
+
+        if (!address_is_proxy(family, listen_addr)) {
+                /* Make sure no traffic from outside the local host can leak to onto this socket */
+                r = socket_bind_to_ifindex(fd, LOOPBACK_IFINDEX);
+                if (r < 0)
+                        return r;
+
+                r = socket_set_ttl(fd, family, 1);
+                if (r < 0)
+                        return r;
+        } else if (type == SOCK_DGRAM) {
+                r = socket_disable_pmtud(fd, family);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to disable UDP PMTUD, ignoring: %m");
+
+                r = socket_set_recvfragsize(fd, family, true);
+                if (r < 0)
+                        log_debug_errno(r, "Failed to enable fragment size reception, ignoring: %m");
+        }
 
-        r = setsockopt_int(fd, IPPROTO_IP, IP_TTL, 1);
+        r = sockaddr_set_in_addr(&sa, family, listen_addr, 53);
         if (r < 0)
                 return r;
 
@@ -1153,11 +1231,9 @@ static int manager_dns_stub_fd_extra(Manager *m, DnsStubListenerExtra *l, int ty
         int r;
 
         assert(m);
+        assert(l);
         assert(IN_SET(type, SOCK_DGRAM, SOCK_STREAM));
 
-        if (!l)
-                return manager_dns_stub_fd(m, type);
-
         sd_event_source **event_source = type == SOCK_DGRAM ? &l->udp_event_source : &l->tcp_event_source;
         if (*event_source)
                 return sd_event_source_get_io_fd(*event_source);
@@ -1251,38 +1327,63 @@ fail:
 }
 
 int manager_dns_stub_start(Manager *m) {
-        const char *t = "UDP";
-        int r = 0;
+        int r;
 
         assert(m);
 
         if (m->dns_stub_listener_mode == DNS_STUB_LISTENER_NO)
                 log_debug("Not creating stub listener.");
-        else
+        else {
+                static const struct {
+                        uint32_t addr;
+                        int socket_type;
+                } stub_sockets[] = {
+                        { INADDR_DNS_STUB,       SOCK_DGRAM  },
+                        { INADDR_DNS_STUB,       SOCK_STREAM },
+                        { INADDR_DNS_PROXY_STUB, SOCK_DGRAM  },
+                        { INADDR_DNS_PROXY_STUB, SOCK_STREAM },
+                };
+
                 log_debug("Creating stub listener using %s.",
                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_UDP ? "UDP" :
                           m->dns_stub_listener_mode == DNS_STUB_LISTENER_TCP ? "TCP" :
                           "UDP/TCP");
 
-        if (FLAGS_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_UDP))
-                r = manager_dns_stub_fd(m, SOCK_DGRAM);
+                for (size_t i = 0; i < ELEMENTSOF(stub_sockets); i++) {
+                        union in_addr_union a = {
+                                .in.s_addr = htobe32(stub_sockets[i].addr),
+                        };
 
-        if (r >= 0 &&
-            FLAGS_SET(m->dns_stub_listener_mode, DNS_STUB_LISTENER_TCP)) {
-                t = "TCP";
-                r = manager_dns_stub_fd(m, SOCK_STREAM);
-        }
+                        if (m->dns_stub_listener_mode == DNS_STUB_LISTENER_UDP && stub_sockets[i].socket_type == SOCK_STREAM)
+                                continue;
+                        if (m->dns_stub_listener_mode == DNS_STUB_LISTENER_TCP && stub_sockets[i].socket_type == SOCK_DGRAM)
+                                continue;
+
+                        r = manager_dns_stub_fd(m, AF_INET, &a, stub_sockets[i].socket_type);
+                        if (r < 0) {
+                                _cleanup_free_ char *busy_socket = NULL;
+
+                                if (asprintf(&busy_socket,
+                                             "%s socket " IPV4_ADDRESS_FMT_STR ":53",
+                                             stub_sockets[i].socket_type == SOCK_DGRAM ? "UDP" : "TCP",
+                                             IPV4_ADDRESS_FMT_VAL(a.in)) < 0)
+                                        return log_oom();
+
+                                if (IN_SET(r, -EADDRINUSE, -EPERM)) {
+                                        log_warning_errno(r,
+                                                          r == -EADDRINUSE ? "Another process is already listening on %s.\n"
+                                                          "Turning off local DNS stub support." :
+                                                          "Failed to listen on %s: %m.\n"
+                                          "Turning off local DNS stub support.",
+                                                          busy_socket);
+                                        manager_dns_stub_stop(m);
+                                        break;
+                                }
 
-        if (IN_SET(r, -EADDRINUSE, -EPERM)) {
-                log_warning_errno(r,
-                                  r == -EADDRINUSE ? "Another process is already listening on %s socket 127.0.0.53:53.\n"
-                                                     "Turning off local DNS stub support." :
-                                                     "Failed to listen on %s socket 127.0.0.53:53: %m.\n"
-                                                     "Turning off local DNS stub support.",
-                                  t);
-                manager_dns_stub_stop(m);
-        } else if (r < 0)
-                return log_error_errno(r, "Failed to listen on %s socket 127.0.0.53:53: %m", t);
+                                return log_error_errno(r, "Failed to listen on %s: %m", busy_socket);
+                        }
+                }
+        }
 
         if (!ordered_set_isempty(m->dns_extra_stub_listeners)) {
                 DnsStubListenerExtra *l;
@@ -1305,6 +1406,8 @@ void manager_dns_stub_stop(Manager *m) {
 
         m->dns_stub_udp_event_source = sd_event_source_disable_unref(m->dns_stub_udp_event_source);
         m->dns_stub_tcp_event_source = sd_event_source_disable_unref(m->dns_stub_tcp_event_source);
+        m->dns_proxy_stub_udp_event_source = sd_event_source_disable_unref(m->dns_proxy_stub_udp_event_source);
+        m->dns_proxy_stub_tcp_event_source = sd_event_source_disable_unref(m->dns_proxy_stub_tcp_event_source);
 }
 
 static const char* const dns_stub_listener_mode_table[_DNS_STUB_LISTENER_MODE_MAX] = {
diff --git a/src/resolve/resolved-manager.h b/src/resolve/resolved-manager.h
index 1371c41b92..35e0068a83 100644
--- a/src/resolve/resolved-manager.h
+++ b/src/resolve/resolved-manager.h
@@ -140,6 +140,10 @@ struct Manager {
         sd_event_source *dns_stub_udp_event_source;
         sd_event_source *dns_stub_tcp_event_source;
 
+        /* Local DNS proxy stub on 127.0.0.54:53 */
+        sd_event_source *dns_proxy_stub_udp_event_source;
+        sd_event_source *dns_proxy_stub_tcp_event_source;
+
         Hashmap *polkit_registry;
 
         VarlinkServer *varlink_server;
diff --git a/src/shared/resolve-util.c b/src/shared/resolve-util.c
index 6541d231ed..820f9bb530 100644
--- a/src/shared/resolve-util.c
+++ b/src/shared/resolve-util.c
@@ -31,12 +31,12 @@ DEFINE_STRING_TABLE_LOOKUP_WITH_BOOLEAN(dns_over_tls_mode, DnsOverTlsMode, DNS_O
 
 bool dns_server_address_valid(int family, const union in_addr_union *sa) {
 
-        /* Refuses the 0 IP addresses as well as 127.0.0.53 (which is our own DNS stub) */
+        /* Refuses the 0 IP addresses as well as 127.0.0.53/127.0.0.54 (which is our own DNS stub) */
 
         if (!in_addr_is_set(family, sa))
                 return false;
 
-        if (family == AF_INET && sa->in.s_addr == htobe32(INADDR_DNS_STUB))
+        if (family == AF_INET && IN_SET(be32toh(sa->in.s_addr), INADDR_DNS_STUB, INADDR_DNS_PROXY_STUB))
                 return false;
 
         return true;
diff --git a/src/shared/resolve-util.h b/src/shared/resolve-util.h
index c4d9275368..d9ab387301 100644
--- a/src/shared/resolve-util.h
+++ b/src/shared/resolve-util.h
@@ -5,9 +5,12 @@
 #include "in-addr-util.h"
 #include "macro.h"
 
-/* 127.0.0.53 in native endian */
+/* 127.0.0.53 in native endian (The IP address we listen on with the full DNS stub, i.e. that does LLMNR/mDNS, and stuff) */
 #define INADDR_DNS_STUB ((in_addr_t) 0x7f000035U)
 
+/* 127.0.0.54 in native endian (The IP address we listen on we only implement "proxy" mode) */
+#define INADDR_DNS_PROXY_STUB ((in_addr_t) 0x7f000036U)
+
 typedef enum DnsCacheMode DnsCacheMode;
 
 enum DnsCacheMode {