[Qemu-devel] [PATCH] slirp: Fix assertion failure on rejected DHCP requests

[David Gibson <david@gibson.dropbear.id.au>]

The guest network stack might DHCPREQUEST an address that the slirp built
in dhcp server can't let it have - for example if the guest has an old
leases file from another network configuration.  In this case the dhcp
server should and does reject the request and prepares to send a DHCPNAK
to the client.

However, in this case the daddr variable in bootp_reply() is set to
0.0.0.0.  Shortly afterwards, it unconditionally attempts to pre-insert the
new client address into the ARP table.  This causes an assertion failure in
arp_address_add() because of the 0.0.0.0 address.

According to RFC2131, DHCPNAK messages for clients on the same subnet
must be sent to the broadcast address (S3.2, subpoint 2).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 slirp/bootp.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/slirp/bootp.c b/slirp/bootp.c
index efd1fe7..64eac7d 100644
--- a/slirp/bootp.c
+++ b/slirp/bootp.c
@@ -200,7 +200,8 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)
             daddr.sin_addr = preq_addr;
             memcpy(bc->macaddr, client_ethaddr, ETH_ALEN);
         } else {
-            daddr.sin_addr.s_addr = 0;
+            /* DHCPNAKs should be sent to broadcast */
+            daddr.sin_addr.s_addr = 0xffffffff;
         }
     } else {
         bc = find_addr(slirp, &daddr.sin_addr, bp->bp_hwaddr);
-- 
1.7.9