nic_router: DHCP server considers dns_server_from

The dns_server_from attribute of the dhcp-server tag has effect only if
the dns_server attribute of the same tag is not set. If this is the
case, the dns_server_from attribute states the domain from whose IP
config to take the DNS server address. This is useful, for instance, if
the stated domain receives the address of a local DNS server via DHCP.
Whenever the IP config of the stated domain becomes invalid, the DHCP
server switches to a mode where it drops all requests unanswered until
the IP config becomes valid again.

Issue #2730

repos/os/src/server/nic_router/README

@@ -292,18 +292,26 @@ this:
     <dhcp-server ip_first="10.0.1.80"
                  ip_last="10.0.1.100"
                  ip_lease_time_sec="3600"
-                 dns_server="10.0.0.2"/>
+                 dns_server="10.0.0.2"
+                 dns_server_from="uplink" />
     ...
 </domain>
 
-The attributes ip_first and ip_last define the available IPv4 address
+The attributes ip_first and ip_last define the available IPv4 address range
-range while ip_lease_time_sec defines the lifetime of an IPv4 address
+while ip_lease_time_sec defines the lifetime of an IPv4 address assignment in
-assignment in seconds. The IPv4 address range must be in the subnet
+seconds. The IPv4 address range must be in the subnet defined by the interface
-defined by the interface attribute of the domain tag and must not cover
+attribute of the domain tag and must not cover the IPv4 address in this
-the IPv4 address in this attribute. The dns_server attribute gives the
+attribute. The dns_server attribute gives the IPv4 address of the DNS server
-IPv4 address of the DNS server that might also be in another subnet.
+that might also be in another subnet. The dns_server_from attribute has effect
-The lifetime of an assignment that was yet only offered to the client
+only if the dns_server attribute is not set. If this is the case, the
-can be configured for all domains in the <config> tag of the router:
+dns_server_from attribute states the domain from whose IP config to take the
+DNS server address. This is useful, for instance, if the stated domain
+receives the address of a local DNS server via DHCP. Whenever the IP config
+of the stated domain becomes invalid, the DHCP server switches to a mode where
+it drops all requests unanswered until the IP config becomes valid again.
+
+The lifetime of an assignment that was yet only offered to the client can be
+configured for all domains in the <config> tag of the router:
 
 ! <config dhcp_offer_timeout_sec="6">
 

repos/os/src/server/nic_router/config.xsd

@@ -131,6 +131,7 @@
 									<xs:attribute name="ip_last"           type="Ipv4_address" />
 									<xs:attribute name="ip_lease_time_sec" type="Seconds" />
 									<xs:attribute name="dns_server"        type="Ipv4_address" />
+									<xs:attribute name="dns_server_from"   type="Domain_name" />
 								</xs:complexType>
 							</xs:element><!-- dhcp-server -->
 

repos/os/src/server/nic_router/dhcp_server.cc

@@ -14,6 +14,7 @@
 /* local includes */
 #include <dhcp_server.h>
 #include <interface.h>
+#include <domain.h>
 
 using namespace Net;
 using namespace Genode;
@@ -25,9 +26,11 @@ using namespace Genode;
 
 Dhcp_server::Dhcp_server(Xml_node            const  node,
                          Allocator                 &alloc,
-                         Ipv4_address_prefix const &interface)
+                         Ipv4_address_prefix const &interface,
+                         Domain_tree               &domains)
 :
 	_dns_server(node.attribute_value("dns_server", Ipv4_address())),
+	_dns_server_from(_init_dns_server_from(node, domains)),
 	_ip_lease_time  (_init_ip_lease_time(node)),
 	_ip_first(node.attribute_value("ip_first", Ipv4_address())),
 	_ip_last(node.attribute_value("ip_last", Ipv4_address())),
@@ -59,8 +62,11 @@ Microseconds Dhcp_server::_init_ip_lease_time(Xml_node const node)
 void Dhcp_server::print(Output &output) const
 {
 	if (_dns_server.valid()) {
-		Genode::print(output, "DNS server ", _dns_server, " ");
+		Genode::print(output, "DNS server ", _dns_server, ", ");
 	}
+	try { Genode::print(output, "DNS server from ", _dns_server_from(), ", "); }
+	catch (Pointer<Domain>::Invalid) { }
+
 	Genode::print(output, "IP first ", _ip_first,
 	                        ", last ", _ip_last,
 	                       ", count ", _ip_count,
@@ -94,6 +100,42 @@ void Dhcp_server::free_ip(Ipv4_address const &ip)
 }
 
 
+Pointer<Domain> Dhcp_server::_init_dns_server_from(Genode::Xml_node const  node,
+                                                   Domain_tree            &domains)
+{
+	if (_dns_server.valid()) {
+		return Pointer<Domain>();
+	}
+	Domain_name dns_server_from =
+		node.attribute_value("dns_server_from", Domain_name());
+
+	if (dns_server_from == Domain_name()) {
+		return Pointer<Domain>();
+	}
+	try { return domains.find_by_name(dns_server_from); }
+	catch (Domain_tree::No_match) { throw Invalid(); }
+}
+
+
+Ipv4_address const &Dhcp_server::dns_server() const
+{
+	try { return _dns_server_from().ip_config().dns_server; }
+	catch (Pointer<Domain>::Invalid) { }
+	return _dns_server;
+}
+
+
+bool Dhcp_server::ready() const
+{
+	if (_dns_server.valid()) {
+		return true;
+	}
+	try { return _dns_server_from().ip_config().valid; }
+	catch (Pointer<Domain>::Invalid) { }
+	return true;
+}
+
+
 /*********************
  ** Dhcp_allocation **
  *********************/

repos/os/src/server/nic_router/dhcp_server.h

@@ -18,6 +18,7 @@
 #include <ipv4_address_prefix.h>
 #include <bit_allocator_dynamic.h>
 #include <list.h>
+#include <pointer.h>
 
 /* Genode includes */
 #include <net/mac_address.h>
@@ -35,6 +36,8 @@ namespace Net {
 
 	/* forward declarations */
 	class Interface;
+	class Domain;
+	class Domain_tree;
 }
 
 
@@ -43,6 +46,7 @@ class Net::Dhcp_server : private Genode::Noncopyable
 	private:
 
 		Ipv4_address         const    _dns_server;
+		Pointer<Domain>      const    _dns_server_from;
 		Genode::Microseconds const    _ip_lease_time;
 		Ipv4_address         const    _ip_first;
 		Ipv4_address         const    _ip_last;
@@ -52,6 +56,9 @@ class Net::Dhcp_server : private Genode::Noncopyable
 
 		Genode::Microseconds _init_ip_lease_time(Genode::Xml_node const node);
 
+		Pointer<Domain> _init_dns_server_from(Genode::Xml_node const  node,
+		                                      Domain_tree            &domains);
+
 	public:
 
 		enum { DEFAULT_IP_LEASE_TIME_SEC = 3600 };
@@ -61,7 +68,8 @@ class Net::Dhcp_server : private Genode::Noncopyable
 
 		Dhcp_server(Genode::Xml_node    const  node,
 		            Genode::Allocator         &alloc,
-		            Ipv4_address_prefix const &interface);
+		            Ipv4_address_prefix const &interface,
+		            Domain_tree               &domains);
 
 		Ipv4_address alloc_ip();
 
@@ -69,6 +77,8 @@ class Net::Dhcp_server : private Genode::Noncopyable
 
 		void free_ip(Ipv4_address const &ip);
 
+		bool ready() const;
+
 
 		/*********
 		 ** log **
@@ -81,7 +91,7 @@ class Net::Dhcp_server : private Genode::Noncopyable
 		 ** Accessors **
 		 ***************/
 
-		Ipv4_address   const &dns_server()    const { return _dns_server; }
+		Ipv4_address   const &dns_server()    const;
 		Genode::Microseconds  ip_lease_time() const { return _ip_lease_time; }
 };
 

repos/os/src/server/nic_router/domain.cc

@@ -128,7 +128,6 @@ Domain::Domain(Configuration &config, Xml_node const node, Allocator &alloc)
 	if (_config.verbose_domain_state()) {
 		log("[", *this, "] NIC sessions: ", _interface_cnt);
 	}
-	_ip_config_changed();
 }
 
 
@@ -188,6 +187,16 @@ void Domain::__FIXME__dissolve_foreign_arp_waiters()
 }
 
 
+Dhcp_server &Domain::dhcp_server()
+{
+	Dhcp_server &dhcp_server = _dhcp_server();
+	if (!dhcp_server.ready()) {
+		throw Pointer<Dhcp_server>::Invalid();
+	}
+	return dhcp_server;
+}
+
+
 void Domain::init(Domain_tree &domains)
 {
 	/* read DHCP server configuration */
@@ -198,7 +207,8 @@ void Domain::init(Domain_tree &domains)
 			throw Invalid();
 		}
 		_dhcp_server = *new (_alloc)
-			Dhcp_server(dhcp_server_node, _alloc, ip_config().interface);
+			Dhcp_server(dhcp_server_node, _alloc, ip_config().interface,
+			            domains);
 
 		if (_config.verbose()) {
 			log("[", *this, "] DHCP server: ", _dhcp_server()); }

repos/os/src/server/nic_router/domain.h

@@ -180,7 +180,7 @@ class Net::Domain : public Domain_base
 		Interface_list      &interfaces()            { return _interfaces; }
 		Configuration       &config()          const { return _config; }
 		Domain_avl_member   &avl_member()            { return _avl_member; }
-		Dhcp_server         &dhcp_server()           { return _dhcp_server(); }
+		Dhcp_server         &dhcp_server();
 		Arp_cache           &arp_cache()             { return _arp_cache; }
 		Arp_waiter_list     &foreign_arp_waiters()   { return _foreign_arp_waiters; }
 		Link_side_tree      &tcp_links()             { return _tcp_links; }