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, 53 insertions, 10 deletions

diff --git a/passt.1 b/passt.1
index 92681f6..7070a31 100644
--- a/passt.1
+++ b/passt.1
@@ -165,19 +165,62 @@ Default is to use the interface with the first default route.
 
 .TP
 .BR \-D ", " \-\-dns " " \fIaddr
-Assign IPv4 \fIaddr\fR via DHCP (option 23) or IPv6 \fIaddr\fR via NDP Router
-Advertisement (option type 25) and DHCPv6 (option 23) as DNS resolver.
+Use \fIaddr\fR (IPv4 or IPv6) for DHCP, DHCPv6, NDP or DNS forwarding, as
+configured (see options \fB--no-dhcp-dns\fR, \fB--dhcp-dns\fR,
+\fB--dns-forward\fR) instead of reading addresses from \fI/etc/resolv.conf\fR.
 This option can be specified multiple times, and a single, empty option disables
-DNS options altogether.
-In \fBpasst\fR mode, default is to use addresses from \fI/etc/resolv.conf\fR,
-and, in \fBpasta\fR mode, no addresses are sent by default.
+usage of DNS addresses altogether.
+
+.TP
+.BR \-D ", " \-\-dns " " \fIaddr
+Use \fIaddr\fR (IPv4 or IPv6) for DHCP, DHCPv6, NDP or DNS forwarding, as
+configured (see options \fB--no-dhcp-dns\fR, \fB--dhcp-dns\fR,
+\fB--dns-forward\fR) instead of reading addresses from \fI/etc/resolv.conf\fR.
+This option can be specified multiple times, and a single, empty option disables
+usage of DNS addresses altogether.
+
+.TP
+.BR \-\-dns-forward " " \fIaddr
+Map \fIaddr\fR (IPv4 or IPv6) as seen from guest or namespace to the first
+configured DNS resolver (with corresponding IP version). Mapping is limited to
+UDP traffic directed to port 53, and DNS answers are translated back with a
+reverse mapping.
+This option can be specified zero to two times (once for IPv4, once for IPv6).
+
 .TP
 .BR \-S ", " \-\-search " " \fIlist
-Assign space-separated \fIlist\fR via DHCP (option 119), via NDP Router
-Advertisement (option type 31) and DHCPv6 (option 24) as DNS domain search list.
-A single, empty option disables sending the DNS domain search list.
-In \fBpasst\fR mode, default is to use the search list from
-\fI/etc/resolv.conf\fR, and, in \fBpasta\fR mode, no list is sent by default.
+Use space-separated \fIlist\fR for DHCP, DHCPv6, and NDP purposes, instead of
+reading entries from \fI/etc/resolv.conf\fR. See options \fB--no-dhcp-search\fR
+and \fB--dhcp-search\fR. A single, empty option disables the DNS domain search
+list altogether.
+
+.TP
+.BR \-\-no-dhcp-dns " " \fIaddr
+In \fIpasst\fR mode, do not assign IPv4 addresses via DHCP (option 23) or IPv6
+addresses via NDP Router Advertisement (option type 25) and DHCPv6 (option 23)
+as DNS resolvers.
+By default, all the configured addresses are passed.
+
+.TP
+.BR \-\-dhcp-dns " " \fIaddr
+In \fIpasta\fR mode, assign IPv4 addresses via DHCP (option 23) or IPv6
+addresses via NDP Router Advertisement (option type 25) and DHCPv6 (option 23)
+as DNS resolvers.
+By default, configured addresses, if any, are not passed.
+
+.TP
+.BR \-\-no-dhcp-search " " \fIaddr
+In \fIpasst\fR mode, do not send the DNS domain search list addresses via DHCP
+(option 119), via NDP Router Advertisement (option type 31) and DHCPv6 (option
+24).
+By default, the DNS domain search list resulting from configuration is passed.
+
+.TP
+.BR \-\-dhcp-search " " \fIaddr
+In \fIpasta\fR mode, send the DNS domain search list addresses via DHCP (option
+119), via NDP Router Advertisement (option type 31) and DHCPv6 (option 24).
+By default, the DNS domain search list resulting from configuration is not
+passed.
 
 .TP
 .BR \-\-no-tcp