conf: Treat --dns addresses as guest visible addresses

Although it's not 100% explicit in the man page, addresses given to
the --dns option are intended to be addresses as seen by the guest.
This differs from addresses taken from the host's /etc/resolv.conf,
which must be translated to guest accessible versions in some cases.

Our implementation is currently inconsistent on this: when using
--dns-forward, you must usually also give --dns with the matching address,
which is meaningful only in the guest's address view.  However if you give
--dns with a loopback addres, it will be translated like a host view
address.

Move the remapping logic for DNS addresses out of add_dns4() and add_dns6()
into add_dns_resolv() so that it is only applied for host nameserver
addresses, not for nameservers given explicitly with --dns.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

conf.c

@@ -353,7 +353,7 @@ bind_all_fail:
 /**
  * add_dns4() - Possibly add the IPv4 address of a DNS resolver to configuration
  * @c:		Execution context
- * @addr:	Address found in /etc/resolv.conf
+ * @addr:	Guest nameserver IPv4 address
  * @idx:	Index of free entry in array of IPv4 resolvers
  *
  * Return: Number of entries added (0 or 1)
@@ -361,64 +361,29 @@ bind_all_fail:
 static unsigned add_dns4(struct ctx *c, const struct in_addr *addr,
 			 unsigned idx)
 {
-	unsigned added = 0;
-
 	if (idx >= ARRAY_SIZE(c->ip4.dns))
 		return 0;
 
-	/* Guest or container can only access local addresses via redirect */
-	if (IN4_IS_ADDR_LOOPBACK(addr)) {
-		if (!c->no_map_gw) {
-			c->ip4.dns[idx] = c->ip4.gw;
-			added++;
-
-			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.dns_match))
-				c->ip4.dns_match = c->ip4.gw;
-		}
-	} else {
 	c->ip4.dns[idx] = *addr;
-		added++;
+	return 1;
-	}
-
-	if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.dns_host))
-		c->ip4.dns_host = *addr;
-
-	return added;
 }
 
 /**
  * add_dns6() - Possibly add the IPv6 address of a DNS resolver to configuration
  * @c:		Execution context
- * @addr:	Address found in /etc/resolv.conf
+ * @addr:	Guest nameserver IPv6 address
  * @idx:	Index of free entry in array of IPv6 resolvers
  *
  * Return: Number of entries added (0 or 1)
  */
-static unsigned add_dns6(struct ctx *c, struct in6_addr *addr, unsigned idx)
+static unsigned add_dns6(struct ctx *c, const struct in6_addr *addr,
+			 unsigned idx)
 {
-	unsigned added = 0;
-
 	if (idx >= ARRAY_SIZE(c->ip6.dns))
 		return 0;
 
-	/* Guest or container can only access local addresses via redirect */
-	if (IN6_IS_ADDR_LOOPBACK(addr)) {
-		if (!c->no_map_gw) {
-			c->ip6.dns[idx] = c->ip6.gw;
-			added++;
-
-			if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_match))
-				c->ip6.dns_match = c->ip6.gw;
-		}
-	} else {
 	c->ip6.dns[idx] = *addr;
-		added++;
+	return 1;
-	}
-
-	if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_host))
-		c->ip6.dns_host = *addr;
-
-	return added;
 }
 
 /**
@@ -437,11 +402,44 @@ static void add_dns_resolv(struct ctx *c, const char *nameserver,
 	struct in6_addr ns6;
 	struct in_addr ns4;
 
-	if (idx4 && inet_pton(AF_INET, nameserver, &ns4))
+	if (idx4 && inet_pton(AF_INET, nameserver, &ns4)) {
-		*idx4 += add_dns4(c, &ns4, *idx4);
+		if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.dns_host))
+			c->ip4.dns_host = ns4;
+
+		/* Guest or container can only access local addresses via
+		 * redirect
+		 */
+		if (IN4_IS_ADDR_LOOPBACK(&ns4)) {
+			if (c->no_map_gw)
+				return;
+
+			ns4 = c->ip4.gw;
+			if (IN4_IS_ADDR_UNSPECIFIED(&c->ip4.dns_match))
+				c->ip4.dns_match = c->ip4.gw;
+		}
+
+		*idx4 += add_dns4(c, &ns4, *idx4);
+	}
+
+	if (idx6 && inet_pton(AF_INET6, nameserver, &ns6)) {
+		if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_host))
+			c->ip6.dns_host = ns6;
+
+		/* Guest or container can only access local addresses via
+		 * redirect
+		 */
+		if (IN6_IS_ADDR_LOOPBACK(&ns6)) {
+			if (c->no_map_gw)
+				return;
+
+			ns6 = c->ip6.gw;
+
+			if (IN6_IS_ADDR_UNSPECIFIED(&c->ip6.dns_match))
+				c->ip6.dns_match = c->ip6.gw;
+		}
 
-	if (idx6 && inet_pton(AF_INET6, nameserver, &ns6))
 		*idx6 += add_dns6(c, &ns6, *idx6);
+	}
 }
 
 /**

passt.1

@@ -236,11 +236,15 @@ interface will be chosen instead.
 
 .TP
 .BR \-D ", " \-\-dns " " \fIaddr
-Use \fIaddr\fR (IPv4 or IPv6) for DHCP, DHCPv6, NDP or DNS forwarding, as
+Instruct the guest (via DHCP, DHVPv6 or NDP) to use \fIaddr\fR (IPv4
-configured (see options \fB--no-dhcp-dns\fR, \fB--dhcp-dns\fR,
+or IPv6) as a nameserver, as configured (see options
-\fB--dns-forward\fR) instead of reading addresses from \fI/etc/resolv.conf\fR.
+\fB--no-dhcp-dns\fR, \fB--dhcp-dns\fR) instead of reading addresses
-This option can be specified multiple times.  Specifying \fB-D none\fR disables
+from \fI/etc/resolv.conf\fR.  This option can be specified multiple
-usage of DNS addresses altogether.
+times.  Specifying \fB-D none\fR disables usage of DNS addresses
+altogether.  Unlike addresses from \fI/etc/resolv.conf\fR, \fIaddr\fR
+is given to the guest without remapping.  For example \fB--dns
+127.0.0.1\fR will instruct the guest to use itself as nameserver, not
+the host.
 
 .TP
 .BR \-\-dns-forward " " \fIaddr