network driver: Use a separate dhcp leases file for each network

By default, all dnsmasq processes share the same leases file. libvirt
also uses the --dhcp-lease-max option to control the maximum number of
leases allowed. The problem is that libvirt puts in a number equal to
the number of addresses in the range for the one network handled by a
single instance of dnsmasq, but dnsmasq checks the total number of
leases in the file (which could potentially contain many more).

The solution is to tell each instance of dnsmasq to create and use its
own leases file. (/var/lib/libvirt/network/<net-name>.leases).

This file is created by dnsmasq when it starts, but not deleted when
it exists. This is fine when the network is just being stopped, but if
the leases file was left around when a network was undefined, we could
end up with an ever-increasing number of dead files - instead, we
explicitly unlink the leases file when a network is undefined.

Note that Ubuntu carries a patch against an older version of libvirt for this:

hhttps://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/713071
ttp://bazaar.launchpad.net/~serge-hallyn/ubuntu/maverick/libvirt/bugall/revision/109

I was certain I'd also seen discussion of this on libvir-list or
libvirt-users, but couldn't find it.

diff --git a/src/network/bridge_driver.c b/src/network/bridge_driver.c
index 48f3e4da76fed83852a625ae91689944431e81ac..ca2ae8d366cac0a430217b1a94c42e056560e5a9 100644 (file)
--- a/src/network/bridge_driver.c
+++ b/src/network/bridge_driver.c
@@ -108,6 +108,16 @@ static void networkReloadIptablesRules(struct network_driver *driver);
 
 static struct network_driver *driverState = NULL;
 
+static char *
+networkDnsmasqLeaseFileName(const char *netname)
+{
+    char *leasefile;
+
+    virAsprintf(&leasefile, DNSMASQ_STATE_DIR "/%s.leases",
+                netname);
+    return leasefile;
+}
+
 static char *
 networkRadvdPidfileBasename(const char *netname)
 {
@@ -531,6 +541,11 @@ networkBuildDnsmasqArgv(virNetworkObjPtr network,
         }
 
         if (ipdef->nranges > 0) {
+            char *leasefile = networkDnsmasqLeaseFileName(network->def->name);
+            if (!leasefile)
+                goto cleanup;
+            virCommandAddArgFormat(cmd, "--dhcp-leasefile=%s", leasefile);
+            VIR_FREE(leasefile);
             virCommandAddArgFormat(cmd, "--dhcp-lease-max=%d", nbleases);
         }
 
@@ -2195,12 +2210,19 @@ static int networkUndefine(virNetworkPtr net) {
     }
 
     if (dhcp_present) {
+        char *leasefile;
         dnsmasqContext *dctx = dnsmasqContextNew(network->def->name, DNSMASQ_STATE_DIR);
         if (dctx == NULL)
             goto cleanup;
 
         dnsmasqDelete(dctx);
         dnsmasqContextFree(dctx);
+
+        leasefile = networkDnsmasqLeaseFileName(network->def->name);
+        if (!leasefile)
+            goto cleanup;
+        unlink(leasefile);
+        VIR_FREE(leasefile);
     }
 
     if (v6present) {