[Dnsmasq-discuss] [PATCH] Change dhcp_release to use first address when no IP subnet matches

[Brian Haley]

Currently, dhcp_release will only send a 'fake' release
when the address given is in the same subnet as an IP
on the interface that was given.

This doesn't work in an environment where dnsmasq is
managing leases for remote subnets via a DHCP relay, as
running dhcp_release locally will just cause it to
silently exit without doing anything, leaving the lease
n the database.

Change it to save the first IP it finds on the interface,
and if no matching subnet IP is found, use it as a fall-back.
This fixes an issue we are seeing in certain Openstack
deployments where we are using dnsmasq to provision baremetal
systems in a datacenter.

While using Dbus might have seemed like an obvious solution,
because of our extensive use of network namespaces (which
Dbus doesn't support), this seemed like a better solution
than creating system.d policy files for each dnsmasq we
might spawn and using --enable-dbus=$id in order to isolate
messages to specific dnsmasq instances.

Signed-off-by: Brian Haley <haleyb.dev at gmail.com>
---
 contrib/lease-tools/dhcp_release.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/contrib/lease-tools/dhcp_release.c b/contrib/lease-tools/dhcp_release.c
index 59883d4..63a732f 100644
--- a/contrib/lease-tools/dhcp_release.c
+++ b/contrib/lease-tools/dhcp_release.c
@@ -180,6 +180,7 @@ static int is_same_net(struct in_addr a, struct in_addr b, struct in_addr mask)
 
 static struct in_addr find_interface(struct in_addr client, int fd, unsigned int index)
 {
+  struct in_addr first_addr = {};
   struct sockaddr_nl addr;
   struct nlmsghdr *h;
   ssize_t len;
@@ -218,7 +219,11 @@ static struct in_addr find_interface(struct in_addr client, int fd, unsigned int
 
       for (h = (struct nlmsghdr *)iov.iov_base; NLMSG_OK(h, (size_t)len); h = NLMSG_NEXT(h, len))
 	if (h->nlmsg_type == NLMSG_DONE)
-	  exit(0);
+          {
+	    if (first_addr.s_addr)
+	      return first_addr;
+	    exit(0);
+          }
 	else if (h->nlmsg_type == RTM_NEWADDR)
           {
             struct ifaddrmsg *ifa = NLMSG_DATA(h);  
@@ -234,7 +239,11 @@ static struct in_addr find_interface(struct in_addr client, int fd, unsigned int
                 
                 for (rta = IFA_RTA(ifa); RTA_OK(rta, len1); rta = RTA_NEXT(rta, len1))
 		  if (rta->rta_type == IFA_LOCAL)
-		    addr = *((struct in_addr *)(rta+1));
+		    {
+		      addr = *((struct in_addr *)(rta+1));
+		      if (!first_addr.s_addr)
+			first_addr = addr;
+		    }
 		
                 if (addr.s_addr && is_same_net(addr, client, netmask))
 		  return addr;
-- 
2.17.1