"No Protocol Specified" even when I can telnet into the X server port?

Question

I have a Win 11 box running Xming, a binary I've used on Win 7 without issue.

Xming is version 6.9, a 32-bit binary that nonetheless seems to run on 64-bit Linux. However I wonder if its not able to understand incoming protocol due to the bit width being wrong? netstat -an on the Win11 box reports no use of port 6000 (the default X server listening port). I start Xming, and netstat -an now reports port 6000:

TCP    0.0.0.0:6000           0.0.0.0:0              LISTENING

I have a Fedora 31 box. I can run xeyes with DISPLAY=:0 and get a display on the Linux machine, whether the terminal is an xterm on the Linux machine or a Putty window on Win11.

strace xeyes |& less shows the connection is working. Connect() returns 0 (success). It's writing to the socket. File descriptor 3 is the TCP connection, returned by socket(), and it must have been opened as we see it being written to and receiving a little data: an 8-byte header and the string "No protocol specified."

socket(AF_INET, SOCK_STREAM|SOCK_CLOEXEC, IPPROTO_TCP) = 3
setsockopt(3, SOL_TCP, TCP_NODELAY, [1], 4) = 0
setsockopt(3, SOL_SOCKET, SO_KEEPALIVE, [1], 4) = 0
connect(3, {sa_family=AF_INET, sin_port=htons(6000), 
sin_addr=inet_addr("192.168.11.17")}, 16) = 0
getpeername(3, {sa_family=AF_INET, sin_port=htons(6000), 
sin_addr=inet_addr("192.168.11.17")}, [124->16]) = 0
access("/home/fs/.Xauthority", R_OK)    = 0
openat(AT_FDCWD, "/home/fs/.Xauthority", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0600, st_size=702, ...}) = 0
read(4, "\0\0\0\4\300\250\10\6\0\0010\0\22MIT-MAGIC-COOKIE-1\0"..., 4096) = 702
read(4, "", 4096)                       = 0
close(4)                                = 0
fcntl(3, F_GETFL)                       = 0x2 (flags O_RDWR)
fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
poll([{fd=3, events=POLLIN|POLLOUT}], 1, -1) = 1 ([{fd=3, revents=POLLOUT}])
writev(3, [{iov_base="l\0\v\0\0\0\0\0\0\0\0\0", iov_len=12}, {iov_base="", iov_len=0}],2) = 12
recvfrom(3, 0x562bca54c080, 8, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
poll([{fd=3, events=POLLIN}], 1, -1)    = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "\0\26\v\0\0\0\6\0", 8, 0, NULL, NULL) = 8
recvfrom(3, "No protocol specified\n\0\0", 24, 0, NULL, NULL) = 24
write(2, "No protocol specified\n", 22No protocol specified) = 22
shutdown(3, SHUT_RDWR)                  = 0
close(3)                                = 0

Further: clicking the Xming icon in the icon tray (lower right corner) and opening log file after such a failed connection reports:

AUDIT: client 4 rejected from IP 

So why can't I connect?

Answer 1

I don't see this mentioned in the documentation but editing C:\Program Files (x86)\Xming\X0.hosts to include the Linux box's address now allows connections.

Command line option -ac will simply ignore the X0.hosts file. This will, however, allow connections from any machine so is not suitable for public networks.