From 215937ff0f37efcd18def54ed4f0a22a5c29e493 Mon Sep 17 00:00:00 2001
From: Martin Stein <martin.stein@genode-labs.com>
Date: Mon, 25 Sep 2017 17:40:35 +0200
Subject: [PATCH] nic_router: handle ARP requests for foreign IPs

If the router has no gateway attribute for a domain (means that the router
itself is the gateway), and it gets an ARP request for a foreign IP, it shall
answer with its own IP.

Ref #2490
---
 repos/os/src/server/nic_router/domain.h     | 23 +++++++++++----------
 repos/os/src/server/nic_router/interface.cc | 18 ++++++++++++----
 2 files changed, 26 insertions(+), 15 deletions(-)

diff --git a/repos/os/src/server/nic_router/domain.h b/repos/os/src/server/nic_router/domain.h
index 93eba51a3e..c6875bad71 100644
--- a/repos/os/src/server/nic_router/domain.h
+++ b/repos/os/src/server/nic_router/domain.h
@@ -128,17 +128,18 @@ class Net::Domain : public Domain_base
 		 ** Accessors **
 		 ***************/
 
-		Domain_name const   &name()              { return _name; }
-		Ip_rule_list        &ip_rules()          { return _ip_rules; }
-		Forward_rule_tree   &tcp_forward_rules() { return _tcp_forward_rules; }
-		Forward_rule_tree   &udp_forward_rules() { return _udp_forward_rules; }
-		Transport_rule_list &tcp_rules()         { return _tcp_rules; }
-		Transport_rule_list &udp_rules()         { return _udp_rules; }
-		Nat_rule_tree       &nat_rules()         { return _nat_rules; }
-		Ipv4_address_prefix &interface_attr()    { return _interface_attr; }
-		Pointer<Interface>  &interface()         { return _interface; }
-		Configuration       &config() const      { return _config; }
-		Domain_avl_member   &avl_member()        { return _avl_member; }
+		bool                 gateway_valid() const { return _gateway_valid; }
+		Domain_name const   &name()                { return _name; }
+		Ip_rule_list        &ip_rules()            { return _ip_rules; }
+		Forward_rule_tree   &tcp_forward_rules()   { return _tcp_forward_rules; }
+		Forward_rule_tree   &udp_forward_rules()   { return _udp_forward_rules; }
+		Transport_rule_list &tcp_rules()           { return _tcp_rules; }
+		Transport_rule_list &udp_rules()           { return _udp_rules; }
+		Nat_rule_tree       &nat_rules()           { return _nat_rules; }
+		Ipv4_address_prefix &interface_attr()      { return _interface_attr; }
+		Pointer<Interface>  &interface()           { return _interface; }
+		Configuration       &config() const        { return _config; }
+		Domain_avl_member   &avl_member()          { return _avl_member; }
 };
 
 
diff --git a/repos/os/src/server/nic_router/interface.cc b/repos/os/src/server/nic_router/interface.cc
index 3040e35264..629c94fef3 100644
--- a/repos/os/src/server/nic_router/interface.cc
+++ b/repos/os/src/server/nic_router/interface.cc
@@ -462,18 +462,28 @@ void Interface::_handle_arp_request(Ethernet_frame &eth,
                                     size_t const    eth_size,
                                     Arp_packet     &arp)
 {
-	/* ignore packets that do not target the router */
-	if (arp.dst_ip() != _router_ip()) {
+	/*
+	 * We handle ARP only if it asks for the routers IP or if the router
+	 * shall forward an ARP to a foreign address as gateway. The second
+	 * is the case if no gateway attribute is specified (so we're the
+	 * gateway) and the address is not of the same subnet like the interface
+	 * attribute.
+	 */
+	if (arp.dst_ip() != _router_ip() &&
+	    (_domain.gateway_valid() ||
+	     _domain.interface_attr().prefix_matches(arp.dst_ip())))
+	{
 		if (_config().verbose()) {
-			log("ARP request for unknown IP"); }
+			log("Ignore ARP request"); }
 
 		return;
 	}
 	/* interchange source and destination MAC and IP addresses */
+	Ipv4_address dst_ip = arp.dst_ip();
 	arp.dst_ip(arp.src_ip());
 	arp.dst_mac(arp.src_mac());
 	eth.dst(eth.src());
-	arp.src_ip(_router_ip());
+	arp.src_ip(dst_ip);
 	arp.src_mac(_router_mac);
 	eth.src(_router_mac);