Ubuntu Manpage: cifs.upcall - Userspace upcall helper for Common Internet File System (CIFS)

NAME

       cifs.upcall - Userspace upcall helper for Common Internet File System (CIFS)

SYNOPSIS

          cifs.upcall [--trust-dns|-t] [--version|-v] [--legacy-uid|-l]
                 [--krb5conf=/path/to/krb5.conf|-k        /path/to/krb5.conf]       [--keytab=/path/to/keytab|-K
                 /path/to/keytab] [--expire|-e nsecs] {keyid}

DESCRIPTION

       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.

OPTIONS

-c This option is deprecated and is currently ignored.

--no-env-probe|-E
Normally, cifs.upcall will probe the environment variable space of the process that initiated the
upcall in order to fetch the value of $KRB5CCNAME. This can assist the program with finding
credential caches in non-default locations. If this option is set, then the program won't do this
and will rely on finding credcaches in the default locations specified in krb5.conf. Note that
this is never performed when the uid is 0. The default credcache location is always used when the
uid is 0, regardless of the environment variable setting in the process.

--krb5conf|-k=/path/to/krb5.conf
This option allows administrators to set an alternate location for the krb5.conf file that
cifs.upcall will use.

--keytab=|-K=/path/to/keytab
This option allows administrators to specify a keytab file to be used. When a user has no
credential cache already established, cifs.upcall will attempt to use this keytab to acquire them.
The default is the system-wide keytab /etc/krb5.keytab.

--trust-dns|-t
With krb5 upcalls, the name used as the host portion of the service principal defaults to the
hostname portion of the UNC. This option allows the upcall program to reverse resolve the network
address of the server in order to get the hostname.

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.

--legacy-uid|-l
Traditionally, the kernel has sent only a single uid= parameter to the upcall for the SPNEGO
upcall that's used to determine what user's credential cache to use. This parameter is affected
by the uid= mount option, which also governs the ownership of files on the mount.

Newer kernels send a creduid= option as well, which contains what uid it thinks actually owns the
credentials that it's looking for. At mount time, this is generally set to the real uid of the
user doing the mount. For multisession mounts, it's set to the fsuid of the mount user. Set this
option if you want cifs.upcall to use the older uid= parameter instead of the creduid= parameter.

--expire|-e
Override default timeout value (600 seconds) for dns_resolver key.

--version|-v
Print version number and exit.

ENVIRONMENT VARIABLES

       GSS_USE_PROXY="yes"
              Enable usage of gssproxy for credential retrieval. This includes keytab based client initiation as
              well as (Resource Based) Constrained Delegation.  See gssproxy-mech(8).

CONFIGURATION FOR KEYCTL

       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:

       cifs.spnego
              This keytype is for retrieving kerberos session keys

       dns_resolver
              This key type is for resolving hostnames  into  IP  addresses.  Support  for  this  key  type  may
              eventually be deprecated (see below).

              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/sbin/cifs.upcall %k
                 create      dns_resolver   * * /usr/sbin/cifs.upcall %k

              See request-key.conf(5) for more info on each field.

              The keyutils package has also started including a dns_resolver handling program as  well  that  is
              preferred  over  the  one  in cifs.upcall. If you are using a keyutils version equal to or greater
              than 1.5,  you  should  use  key.dns_resolver  to  handle  the  dns_resolver  keytype  instead  of
              cifs.upcall. See key.dns_resolver(8) for more info.

AUTHOR

       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.

                                                                                                  CIFS.UPCALL(8)