# cifs.upcall

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


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