Following information might get handy if forwarding does not work:
DNS forwarding chapter in official IdM guide
Forwarding chapter in BIND ARM
If you suspect that there is a problem inside bind-dyndb-ldap, bump
debug log to say 5 (rndc debug 5) and read logs to see if zone
forwarding was configured properly.
To inspect forwarding table in GDB, find pointer to the forwarding
table in viewlist:
(gdb) p *ns_g_server->viewlist.head.fwdtable->table
Then you can inspect list of forwarders using the pointer from previous step:
(gdb) p *((dns_forwarders_t *)0xCHANGEME).addrs.head