[Dnsmasq-discuss] [PATCH v2] Change dhcp_release to use default address when no IP subnet matches

haleyb.dev at gmail.com haleyb.dev at gmail.com
Wed Aug 28 21:13:23 BST 2019

From: Brian Haley <haleyb.dev at gmail.com>

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
in the database.

Change it to use the default IP on the interface, as the
dnsmasq source code at src/dhcp.c does, if no matching subnet
IP is found, 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 | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/contrib/lease-tools/dhcp_release.c b/contrib/lease-tools/dhcp_release.c
index 59883d4..c866cd9 100644
--- a/contrib/lease-tools/dhcp_release.c
+++ b/contrib/lease-tools/dhcp_release.c
@@ -178,7 +178,7 @@ static int is_same_net(struct in_addr a, struct in_addr b, struct in_addr mask)
   return (a.s_addr & mask.s_addr) == (b.s_addr & mask.s_addr);
-static struct in_addr find_interface(struct in_addr client, int fd, unsigned int index)
+static struct in_addr find_interface(struct in_addr client, int fd, unsigned int index, int ifrfd, struct ifreq *ifr)
   struct sockaddr_nl addr;
   struct nlmsghdr *h;
@@ -218,7 +218,13 @@ 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);
+          {
+	    /* No match found, return first address as src/dhcp.c code does */
+	    ifr->ifr_addr.sa_family = AF_INET;
+	    if (ioctl(ifrfd, SIOCGIFADDR, ifr) != -1)
+	      return ((struct sockaddr_in *)&ifr->ifr_addr)->sin_addr;
+	    exit(0);
+          }
 	else if (h->nlmsg_type == RTM_NEWADDR)
             struct ifaddrmsg *ifa = NLMSG_DATA(h);  
@@ -285,7 +291,7 @@ int main(int argc, char **argv)
   lease.s_addr = inet_addr(argv[2]);
-  server = find_interface(lease, nl, if_nametoindex(argv[1]));
+  server = find_interface(lease, nl, if_nametoindex(argv[1]), fd, &ifr);
   memset(&packet, 0, sizeof(packet));

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