This tool is part of the cifs-utils suite.
cifs.upcall is a userspace helper program for the linux CIFS client filesystem. There are a number of activities that the kernel cannot easily do itself. This program is a callout program that does these things for the kernel and then returns the result.
cifs.upcall is generally intended to be run when the kernel calls request-key(8) for a particular key type. While it can be run directly from the command-line, it's not generally intended to be run that way.
This is less secure than not trusting DNS. When using this option, it's possible that an attacker could get control of DNS and trick the client into mounting a different server altogether. It's preferable to instead add server principals to the KDC for every possible hostname, but this option exists for cases where that isn't possible. The default is to not trust reverse hostname lookups in this fashion.
cifs.upcall is designed to be called from the kernel via the request-key callout program. This requires that request-key be told where and how to call this program. The current cifs.upcall program handles two different key types:
To make this program useful for CIFS, you'll need to set up entries for them in request-key.conf(5). Here's an example of an entry for each key type:
#OPERATION TYPE D C PROGRAM ARG1 ARG2... #========= ============= = = ================================ create cifs.spnego * * /usr/local/sbin/cifs.upcall %k create dns_resolver * * /usr/local/sbin/cifs.upcall %k
See request-key.conf5() for more info on each field.
Igor Mammedov wrote the cifs.upcall program.
Jeff Layton authored this manpage.
The maintainer of the Linux CIFS VFS is Steve French.
The Linux CIFS Mailing list is the preferred place to ask questions regarding these programs.