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)