The following python script

----------------------------------------------------------------
from samba.dcerpc import lsa

try:
    result = lsa.lsarpc('ncacn_np:fqdn.of.ad.dc[,]', None, None)
except RuntimeError, (num, message):
    print '%d %s\n' % (num, message)
----------------------------------------------------------------

or "smbtorture 'ncacn_np:qdn.of.ad.dc' rpc.spoolss.access" fail in dcerpc_pipe_connect() if both the IPv4 (A record) and IPv6 (AAAA record) addresses are returned by the DNS server for fqdn.of.ad.dc but the host is not reachable via IPv6, e.g. because of some network components not able to handle IPv6 in the path.

With strace I see

-----------------------------------------------------
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 4
fcntl64(4, F_GETFD)                     = 0
fcntl64(4, F_SETFD, FD_CLOEXEC)         = 0
fcntl64(4, F_GETFL)                     = 0x2 (flags O_RDWR)
fcntl64(4, F_SETFL, O_RDWR|O_NONBLOCK)  = 0
connect(4, {sa_family=AF_INET, sin_port=htons(445), sin_addr=inet_addr("192.168.201.118")}, 16) = -1 EINPROGRESS (Operation now in progress)
epoll_ctl(3, EPOLL_CTL_ADD, 4, {...})   = 0
gettimeofday({1359370069, 674788}, NULL) = 0
gettimeofday({1359370069, 674946}, NULL) = 0
epoll_wait(3, {}, 1, 2)                 = 0
gettimeofday({1359370069, 677161}, NULL) = 0
socket(PF_INET6, SOCK_STREAM, IPPROTO_IP) = 5
fcntl64(5, F_GETFD)                     = 0
fcntl64(5, F_SETFD, FD_CLOEXEC)         = 0
fcntl64(5, F_GETFL)                     = 0x2 (flags O_RDWR)
fcntl64(5, F_SETFL, O_RDWR|O_NONBLOCK)  = 0
connect(5, {sa_family=AF_INET6, sin6_port=htons(445), inet_pton(AF_INET6, "2430:54:0:4359:c581:513b:d49c:c4bf", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = -1 ENETUNREACH (Network is unreachable)
gettimeofday({1359370069, 678233}, NULL) = 0
epoll_ctl(3, EPOLL_CTL_DEL, 4, {...})   = 0
close(4)                                = 0
setsockopt(5, SOL_TCP, TCP_NODELAY, [1], 4) = 0
dup(5)                                  = 4
getsockname(5, {sa_family=AF_INET6, sin6_port=htons(0), inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, [28]) = 0
getpeername(5, 0x8808e30, [128])        = -1 ENOTCONN (Transport endpoint is not connected)
close(4)                                = 0
close(5)                                = 0
close(3)                                = 0
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb771e000
write(1, "-1073741801 Memory allocation er"..., 36-1073741801 Memory allocation error
) = 36
write(1, "\n", 1
)                       = 1
rt_sigaction(SIGINT, {SIG_DFL, [], 0}, {0x4effd570, [], 0}, 8) = 0
exit_group(0)                           = ?
+++ exited with 0 +++
-----------------------------------------------------

I guess dcerpc_pipe_connect() tries to open connections to all available addresses to avoid timeouts trying one after the other. But instead of failing until all connection attempts fail, it fails on the first failure.

Additionally, although connect() returned ENETUNREACH the related file descriptor is still used later on.