Description of problem:

I've tried to get nsupdate working in our Samba test environment. We use a tool called socket_wrapper [1] to build an isolated network. socket_wrapper is a library passing all socket communications through unix sockets.

socket_wrapper limits the max packet size to 1500 bytes to make it easier to write pcap files.

The function doio_send() uses sendmsg() to send the packet over the wire. nsupdate sends the tsig over a TCP connection. The packet size is 1505 bytes. socket_wrapper limits the packet size to 1500 and sends 1500 bytes, which is the return value. doio_send() detects it (see gdb output below) sets a soft error.

doio_send (sock=sock@entry=0x7ffff7f764b0, dev=0x7ffff7ed0678) at socket.c:1866
1866            if (cc < 0) {
(gdb)
1860            cc = sendmsg(sock->fd, &msghdr, 0);
(gdb) p msghdr
$6 = {msg_name = 0x0, msg_namelen = 0, msg_iov = 0x7ffff303ab70, msg_iovlen = 1, msg_control = 0x0, msg_controllen = 0, msg_flags = 0}
(gdb) n
1861            send_errno = errno;
(gdb) n
1866            if (cc < 0) {
(gdb) p cc
$7 = 1500
(gdb) n
1927            if (cc == 0) {
(gdb) n
1939            dev->n += cc;
(gdb) n
1940            if ((size_t)cc != write_count)
(gdb) p write_count
$8 = 1505


doio_send() should call sendmsg() again and just send the remaining bytes!



[1] https://cwrap.org/socket_wrapper.html