From 268080fc19990711a1d1e1acd68a50aa2f6cb5fb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20Men=C5=A1=C3=ADk?= <pemensik@redhat.com>
Date: Fri, 17 Sep 2021 20:12:21 +0200
Subject: [PATCH] Offer alternative DHCPv6 address if requested is taken

In some cases multiple requests might arrive from single DUID. It may
happen just one address is offered to different IAID requests. When
the first request confirms lease, another would be offered alternative
address instead of address in use error.

Includes check on such Rapid commit equivalents and returns NotOnLink
error, required by RFC 8145, if requested address were not on any
supported prefix.
---
 src/rfc3315.c | 39 ++++++++++++++++++++++++++++-----------
 1 file changed, 28 insertions(+), 11 deletions(-)

diff --git a/src/rfc3315.c b/src/rfc3315.c
index 5c2ff97..d1534ad 100644
--- a/src/rfc3315.c
+++ b/src/rfc3315.c
@@ -614,7 +614,7 @@ static int dhcp6_no_relay(struct state *state, int msg_type, void *inbuff, size_
       
     case DHCP6SOLICIT:
       {
-      	int address_assigned = 0;
+	int address_assigned = 0, ia_invalid = 0;
 	/* tags without all prefix-class tags */
 	struct dhcp_netid *solicit_tags;
 	struct dhcp_context *c;
@@ -697,6 +697,8 @@ static int dhcp6_no_relay(struct state *state, int msg_type, void *inbuff, size_
 		    get_context_tag(state, c);
 		    address_assigned = 1;
 		  }
+		else
+		  ia_invalid++;
 	      }
 	    
 	    /* Suggest configured address(es) */
@@ -782,11 +784,26 @@ static int dhcp6_no_relay(struct state *state, int msg_type, void *inbuff, size_
 	    tagif = add_options(state, 0);
 	  }
 	else
-	  { 
+	  {
+	    char *errmsg;
 	    /* no address, return error */
 	    o1 = new_opt6(OPTION6_STATUS_CODE);
-	    put_opt6_short(DHCP6NOADDRS);
-	    put_opt6_string(_("no addresses available"));
+	    if (state->lease_allocate && ia_invalid)
+	      {
+		/* RFC 8415, Section 18.3.2:
+		   If any of the prefixes of the included addresses are not
+		   appropriate for the link to which the client is connected,
+		   the server MUST return the IA to the client with a Status
+		   Code option with the value NotOnLink. */
+		put_opt6_short(DHCP6NOTONLINK);
+		errmsg = _("not on link");
+	      }
+	    else
+	      {
+		put_opt6_short(DHCP6NOADDRS);
+		errmsg = _("no addresses available");
+	      }
+	    put_opt6_string(errmsg);
 	    end_opt6(o1);
 
 	    /* Some clients will ask repeatedly when we're not giving
@@ -795,7 +812,7 @@ static int dhcp6_no_relay(struct state *state, int msg_type, void *inbuff, size_
 	    for (c = state->context; c; c = c->current)
 	      if (!(c->flags & CONTEXT_RA_STATELESS))
 		{
-		  log6_packet(state, state->lease_allocate ? "DHCPREPLY" : "DHCPADVERTISE", NULL, _("no addresses available"));
+		  log6_packet(state, state->lease_allocate ? "DHCPREPLY" : "DHCPADVERTISE", NULL, errmsg);
 		  break;
 		}
 	  }
@@ -831,7 +848,7 @@ static int dhcp6_no_relay(struct state *state, int msg_type, void *inbuff, size_
 		 /* If we get a request with an IA_*A without addresses, treat it exactly like
 		    a SOLICT with rapid commit set. */
 		 save_counter(start);
-		 goto request_no_address; 
+		 goto request_no_address;
 	       }
 
 	    o = build_ia(state, &t1cntr);
@@ -861,11 +878,11 @@ static int dhcp6_no_relay(struct state *state, int msg_type, void *inbuff, size_
 		      }
 		    else if (!check_address(state, &req_addr))
 		      {
-			/* Address leased to another DUID/IAID */
-			o1 = new_opt6(OPTION6_STATUS_CODE);
-			put_opt6_short(DHCP6UNSPEC);
-			put_opt6_string(_("address in use"));
-			end_opt6(o1);
+			/* Address leased to another DUID/IAID.
+			   Find another address for the client, treat it exactly like
+			   a SOLICT with rapid commit set. */
+			save_counter(start);
+			goto request_no_address;
 		      } 
 		    else 
 		      {
-- 
2.31.1