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cifs.upcall

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Userspace upcall helper for Common Internet File System (CIFS)

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:Manual section: 8

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SYNOPSIS

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  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}

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DESCRIPTION

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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.

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OPTIONS

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-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.

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CONFIGURATION FOR KEYCTL

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``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    * * @sbindir@/cifs.upcall %k

      create      dns_resolver   * * @sbindir@/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.

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SEE ALSO

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request-key.conf(5), mount.cifs(8), key.dns_resolver(8)

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AUTHOR

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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.