CIFS.UPCALL(8) System Manager's Manual CIFS.UPCALL(8)
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 re-
quest-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 ser-
vice 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 op-
tion, it's possible that an attacker could get control of DNS
and trick the client into mounting a different server alto-
gether. 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 re-
verse 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 look-
ing 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 cre-
duid= 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) Con-
strained Delegation. See gssproxy-mech(8).
CONFIGURATION FOR KEYCTL
cifs.upcall is designed to be called from the kernel via the re-
quest-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. Sup-
port for this key type may eventually be deprecated (see below).
To make this program useful for CIFS, you'll need to set up en-
tries 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_re-
solver(8) for more info.
SEE ALSO
request-key.conf(5), mount.cifs(8), key.dns_resolver(8)
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 re-
garding these programs.
CIFS.UPCALL(8)
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