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In os/xdmcp.c the funktion static void get_xdmcp_sock(void) uses xdmcpSocket6 = socket(AF_INET6, SOCK_DGRAM, 0) for IPv6 and xdmcpSocket = socket(AF_INET, SOCK_DGRAM, 0) for IPv4. When using the "-from" option with an Ipv6 address - the manpage requires only, that it can be resolved with gethostbyname(3) - a bind(2) is attempted using xdmcpSocket instead of xdmcpSocket6 which must fail.

I suggest either adding a "-from6" option or selecting the appropriate socket filedescriptor based on the address family of the argument to "-from".

If you can tell me which one you'd prefer I'll prepare a patch.