conf, udp: Introduce basic DNS forwarding

For compatibility with libslirp/slirp4netns users: introduce a
mechanism to map, in the UDP routines, an address facing guest or
namespace to the first IPv4 or IPv6 address resulting from
configuration as resolver. This can be enabled with the new
--dns-forward option.

This implies that sourcing and using DNS addresses and search lists,
passed via command line or read from /etc/resolv.conf, is not bound
anymore to DHCP/DHCPv6/NDP usage: for example, pasta users might just
want to use addresses from /etc/resolv.conf as mapping target, while
not passing DNS options via DHCP.

Reflect this in all the involved code paths by differentiating
DHCP/DHCPv6/NDP usage from DNS configuration per se, and in the new
options --dhcp-dns, --dhcp-search for pasta, and --no-dhcp-dns,
--no-dhcp-search for passt.

This should be the last bit to enable substantial compatibility
between slirp4netns.sh and slirp4netns(1): pass the --dns-forward
option from the script too.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 11 insertions, 3 deletions

diff --git a/passt.h b/passt.h
index d7011da..2589ee7 100644
--- a/passt.h
+++ b/passt.h
@@ -114,6 +114,7 @@ enum passt_modes {
  * @mask4:		IPv4 netmask, network order
  * @gw4:		Default IPv4 gateway, network order
  * @dns4:		IPv4 DNS addresses, zero-terminated, network order
+ * @dns4_fwd:		Address forwarded (UDP) to first IPv4 DNS, network order
  * @dns_search:		DNS search list
  * @v6:			Enable IPv6 transport
  * @addr6:		IPv6 address for external, routable interface
@@ -121,7 +122,8 @@ enum passt_modes {
  * @addr6_seen:		Latest IPv6 global/site address seen as source from tap
  * @addr6_ll_seen:	Latest IPv6 link-local address seen as source from tap
  * @gw6:		Default IPv6 gateway
- * @dns4:		IPv4 DNS addresses, zero-terminated
+ * @dns6:		IPv6 DNS addresses, zero-terminated
+ * @dns6_fwd:		Address forwarded (UDP) to first IPv6 DNS, network order
  * @ifi:		Index of routable interface
  * @pasta_ifn:		Name of namespace interface for pasta
  * @pasta_ifn:		Index of namespace interface for pasta
@@ -133,8 +135,10 @@ enum passt_modes {
  * @no_icmp:		Disable ICMP operation
  * @icmp:		Context for ICMP protocol handler
  * @mtu:		MTU passed via DHCP/NDP
- * @no_dns:		Do not assign any DNS server via DHCP/DHCPv6/NDP
- * @no_dns_search:	Do not assign any DNS domain search via DHCP/DHCPv6/NDP
+ * @no_dns:		Do not source/use DNS servers for any purpose
+ * @no_dns_search:	Do not source/use domain search lists for any purpose
+ * @no_dhcp_dns:	Do not assign any DNS server via DHCP/DHCPv6/NDP
+ * @no_dhcp_dns_search:	Do not assign any DNS domain search via DHCP/DHCPv6/NDP
  * @no_dhcp:		Disable DHCP server
  * @no_dhcpv6:		Disable DHCPv6 server
  * @no_ndp:		Disable NDP handler altogether
@@ -172,6 +176,7 @@ struct ctx {
 	uint32_t mask4;
 	uint32_t gw4;
 	uint32_t dns4[MAXNS + 1];
+	uint32_t dns4_fwd;
 
 	struct fqdn dns_search[MAXDNSRCH];
 
@@ -182,6 +187,7 @@ struct ctx {
 	struct in6_addr addr6_ll_seen;
 	struct in6_addr gw6;
 	struct in6_addr dns6[MAXNS + 1];
+	struct in6_addr dns6_fwd;
 
 	unsigned int ifi;
 	char pasta_ifn[IF_NAMESIZE];
@@ -198,6 +204,8 @@ struct ctx {
 	int mtu;
 	int no_dns;
 	int no_dns_search;
+	int no_dhcp_dns;
+	int no_dhcp_dns_search;
 	int no_dhcp;
 	int no_dhcpv6;
 	int no_ndp;