With Samba 4.2.1 I'm unable to even initiate trust to AD with FreeIPA because Samba cannot start lsasd daemon with external LSARPC pipe (and other pipes too).

On FreeIPA's python code trying to establish connection to lsarpc pipe, I'm getting:

[2015/04/30 10:26:46.527638,  2, pid=1257, effective(1981400000, 1981400000), real(1981400000, 0), class=rpc_srv] ../source3/rpc_server/rpc_ncacn_np.c:773(make_external_rpc_pipe)
tstream_npa_connect_recv  to /run/samba/ncalrpc/np for pipe lsarpc and user F22\admin failed: No such file or directory
..
NT error packet at ../source3/smbd/nttrans.c(299) cmd=162 (SMBntcreateX) NT_STATUS_OBJECT_NAME_NOT_FOUND


When Samba starts, it forks out an lsasd daemon off smbd. That daemon creates TCP sockets and forks out handlers for specific pipes. However, it looks like the socket setup succeeds and then a failure is returned by dcerpc_binding_vector_add_port() which causes rpc_create_tcpip_sockets() to bail out and return to lsasd initialization code, causing it to exit.

Using a modified systemd unit for smb.service I was able to catch strace what is happening:
write(14, "  bind succeeded on port 1024\n", 30) = 30
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=127, ...}) = 0
getgid()                                = 0
getuid()                                = 0
getegid()                               = 0
geteuid()                               = 0
geteuid()                               = 0
write(14, "[2015/04/29 18:46:36.028567, 10, pid=1331, effective(0, 0), real(0, 0)] ../source3/rpc_server/rpc_server.c:628(create_tcpip_socket)\n", 132) = 132
geteuid()                               = 0
write(14, "  Opened tcpip socket fd 16 for port 1024\n", 42) = 42
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 38
setsockopt(38, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
setsockopt(38, SOL_SOCKET, SO_REUSEPORT, [1], 4) = 0
bind(38, {sa_family=AF_INET, sin_port=htons(1024), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=127, ...}) = 0
getgid()                                = 0
getuid()                                = 0
getegid()                               = 0
geteuid()                               = 0
geteuid()                               = 0
write(14, "[2015/04/29 18:46:36.029143, 10, pid=1331, effective(0, 0), real(0, 0)] ../source3/lib/util_sock.c:531(open_socket_in)\n", 119) = 119
geteuid()                               = 0
write(14, "  bind succeeded on port 1024\n", 30) = 30
stat("/etc/localtime", {st_mode=S_IFREG|0644, st_size=127, ...}) = 0
getgid()                                = 0
getuid()                                = 0
getegid()                               = 0
geteuid()                               = 0
geteuid()                               = 0
write(14, "[2015/04/29 18:46:36.029404, 10, pid=1331, effective(0, 0), real(0, 0)] ../source3/rpc_server/rpc_server.c:628(create_tcpip_socket)\n", 132) = 132
geteuid()                               = 0
write(14, "  Opened tcpip socket fd 38 for port 1024\n", 42) = 42
close(38)                               = 0
close(35)                               = 0
close(3)                                = 0
close(31)                               = 0
exit_group(1)                           = ?
+++ exited with 1 +++

As you can see, socket is created and then closed/exited.

The code is roughly here:
in start_lsasd():
        ok = lsasd_create_sockets(ev_ctx, msg_ctx, listen_fd, &listen_fd_size);
        if (!ok) {
                exit(1);
        }

in lsasd_create_sockets():
        /* Create only one tcpip listener for all services */
        status = rpc_create_tcpip_sockets(&ndr_table_lsarpc,
                                          v_orig,
                                          0,
                                          listen_fd,
                                          listen_fd_size);
        if (!NT_STATUS_IS_OK(status)) {
                goto done;
        }


in rpc_create_tcpip_sockets():
                for (sock_ptr = sock_addr;
                     next_token_talloc(talloc_tos(), &sock_ptr, &sock_tok, " \t,");
                    ) {
                        struct sockaddr_storage ss;
                        int fd;

                        /* open an incoming socket */
                        if (!interpret_string_addr(&ss,
                                                   sock_tok,
                                                   AI_NUMERICHOST|AI_PASSIVE)) {
                                continue;
                        }

                        fd = create_tcpip_socket(&ss, &p);
                        if (fd < 0 || p == 0) {
                                status = NT_STATUS_UNSUCCESSFUL;
                                if (fd != -1) {
                                        close(fd);
                                }
                                goto done;
                        }
                        listen_fd[*listen_fd_size] = fd;
                        (*listen_fd_size)++;

                        if (bvec != NULL) {
                                status = dcerpc_binding_vector_add_port(iface,
                                                                        bvec,
                                                                        sock_ptr,
                                                                        p);
                                if (!NT_STATUS_IS_OK(status)) {
                                        close(fd);
                                        goto done;
                                }
                        }
                }

the way I interpret the strace output is that dcerpc_binding_vector_add_port() fails. It looks like the code in commit 52ccd28ca75bef4f7ac2489389a5aebf5db2b34a may be responsible here.

Commit 52ccd28ca75bef4f7ac2489389a5aebf5db2b34a
s3:dcerpc_ep: make use of dcerpc_binding helper functions
We should not dereference 'struct dcerpc_binding'.

$ git tag --contains 52ccd28ca75bef4f7ac2489389a5aebf5db2b34a |grep samba
samba-4.2.0
samba-4.2.0rc1
samba-4.2.0rc2
samba-4.2.0rc3
samba-4.2.0rc4
samba-4.2.0rc5
samba-4.2.1