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dacsrlink - create and administer rule links


dacsrlink [dacsoptions[1]] op [arg...]


This program is part of the DACS suite.

The dacsrlink command is used to create and manage special URLs called Rlinks (rule
links). Basically, an Rlink is an ordinary URL that also includes a special component
called an Rname that indirectly specifies a DACS access control rule that applies to the
Rlink. Depending on the application, the creator of an Rlink may expect it to be kept
secret by everyone he distributes it to. A given resource may have Rlinks with different
Rnames "pointing to it". Rlinks are processed by dacs_acs[2] during authorization

A DACS identity may be attached to an Rlink through the rlink and rname operations. When
an Rlink with an attached identity is used, that identity is available to dacs_acs[3] for
access control purposes. There are two modes of attachment: direct and indirect.
Identities for use with the direct mode are encrypted using the jurisdiction_keys item
type (see dacskey(1)[4]); the program's user must therefore be able to read these keys.
Changing these keys will invalidate all existing encrypted identities.

The special, temporary credentials associated with an Rlink have the authentication style
"rlink" (refer to user()[5] with the style keyword), but not passwd, even if a password is
required to gain access to a resource.

There are many applications of Rlinks. Perhaps their main application is to provide
identity-restricted access to a resource without having to create per-identity accounts.
The identity associated with an Rlink need not exist outside of its use by the Rlink. When
the Rlink is invoked (possibly accompanied by a password bound to the URL), the identity
is available to the access control rule and an invoked web service just as if "real" DACS
credentials had been used.

dacsrlink can also be used as a simple front end for creating ordinary access control


dacsrlink recognizes the standard dacsoptions[1], which are followed by an operation name
(op), various operation-dependent flags, and finally non-flag arguments. The -- flag can
be used to terminate the operation-dependent list of flags. Flags that are not recognized
by the selected operation are ignored. A rule is always syntax checked (as by
dacsacl(1)[6]) before being written; if an error is found, the operation is aborted.
Several flags are recognized by more than one operation.

By default, the virtual filestore item type rlinks specifies where Rlinks are stored. This
can be overridden for most operations by giving the -vfs flag, which can specify a DACS
URI, alternate item type, or absolute pathname.

Access to the rules and to listings of their names must be restricted, otherwise
Rnames could be revealed. Only a DACS administer should be permitted to create, edit,
delete, etc. rules. dacs_acs must be able to access the rules if Rlinks are enabled.
Ensure that file permissions are set appropriately.

The optional -out flag is followed by a filename to which the rule should be written
instead of a filestore; if - is given, the standard output is used.

The default alphabet used to generate Rnames can be overridden using the -ralpha flag;
alpha is a character specification in the syntax of strtr()[7] (e.g., "a-zA-Z0-9", which
is the default). The default length of an Rname can be overridden using the -rlen flag.
Alternatively, some operations take a -rname flag that specifies the Rname to use.

The following op arguments are understood:

Perform a syntax check on the rule identified by rname to the standard output. If no
error is found, an exit status of 0 is returned, otherwise an error message is
produced and 1 is returned.

Create a new link identical to rname but with a new Rname. If the -rname flag is
given, use rname as the Rname instead of generating one.

[{-a | -allow}name] [{-p password} | {-pf file}]...
[-palg alg-name] [-r redirect-URL] [-rname rname] [-ralpha alpha] [-rlen len]
[-expires {seconds | date}] path...
Create a new Rlink and either write it to the filestore, a specified file, or the
standard output. The optional -a (or -allow) flag is followed by name, which is a
string that will become the argument to the user()[8] function that will be called
from the allow clause of the ACL that is created. Each name will therefore be granted
access to each of the named path arguments, which are URI path components relative to
the current jurisdiction.

A password that applies only to this user can optionally follow as the next argument
using a -p or -pf flag; its hashed value will be embedded in the Rlink and compared
against a hash of an argument named PASSWORD that must be submitted with the Rlink. If
a -p or -pf flag precedes any -a (-allow) flag, however, it establishes a default
password for all users specified later on the command line. The -pf flag is followed
by a filename from which the password is read; if file is "-", then the password is
read from the standard input. A password may be specified even if no -a flag is
present; the request will not have an identity bound to it but a valid PASSWORD
argument must be provided. The -palg flag overrides the default password hashing
algorithm (see password()[9]).

If the -rname flag is given, rname is used as the Rname instead of generating one. The
-expires assigns an expires_expr attribute to the Rlink, which will render the Rlink
invalid after the specified date. The flag is followed either by an unsigned integer,
which is interpreted as a number of seconds in the future, or a date in one of the
recognized formats[10].

If the -r flag appears, no usernames can be specified. An attempt to access any of the
resources associated with the Rlink will cause the client to be redirected to
redirect-URL, which may contain a properly encoded query component. This lets an Rlink
serve as a "short link", akin to the services provided by bit.ly[11], TinyURL.com[12],
Metamark Shorten Service[13], and many others.

Administrators should review the rule that is created. The show[14] operation can
be used to display the rule and the edit[15] operation can be used to modify it.

Delete the Rlink named rname in the selected filestore.

Interactively edit a copy of the Rlink named rname in the selected filestore. If the
environment variable EDITOR is set, it is used as the name of the editor to use,
otherwise the compile time symbol DEFAULT_EDITOR is used. When editing is completed,
the Rlink is replaced with the edited copy, provided the new version is syntactically

Decode and print rname-ident, an Rname with an identity component produced by the
rlink or rname operations.

Print a listing of all Rnames in the selected filestore.

Emit an Rlink to the standard output that integrates rname into the uri according to
link-mode. The link-mode is one of dacs_acs (or just acs), query, or path,
representing the three general forms of an Rlink. If ident is specified, it describes
a user in the concise user syntax[16] that is associated with the link. The ident may
include an expiry date.

The -imode specifies whether a direct or indirect identity should be associated with
the Rname, or whether there is none (the default). For direct, ident (specified by -i
or -ident) is used; it describes an identity in the concise user syntax[16] that is
associated with the link. For the indirect mode, a random identifier is generated
(using the same algorithm selected for Rnames); if the -iptr flag is given, however,
iptr is used as the identifier string.

If uri is a URI path component (i.e., it begins with a '/'), the configuration
variable rlink_base_prefix must be defined; its value is prepended to the URI path.

Additional query arguments can be attached to the emitted link. If a password is
required by the ACL for the resource, for example, a PASSWORD argument is required.

Implementation of query and path modes is incomplete, so URLs for those Rlinks must be
generated manually.

[-rname rname]
This operation emits an Rname that satisfies the given constraints and prints it to
the standard output. The Rname is suitable for use with the -rname flag. It does not
create an ACL. This operation might be useful when Rlinks are created manually or
using another program.

The -imode, -i, and -iptr flags are as described for the rlink operation.

Display the rule identified by rname to the standard output.


The following examples assume that the jurisdiction EXAMPLE includes the following

RLINK '"${Args::RNAME:?}" /usr/local/dacs/rlinks'
EVAL ${Conf::rlink_base_prefix} = "https://www.example.com"
VFS "[rlinks]file:///usr/local/dacs/rlinks"

These directives enable Rlink processing by dacs_acs, and cause URLs generated by
dacsrlink to be prefixed by https://www.example.com and ACLs that it creates to be stored
as files in the /usr/local/dacs/rlinks directory.

This command creates an Rname called IRCl7p4Q, and associates it with the relative URL
/cgi-bin/dacs/dacs_prenv; the Rname will expire in 300 seconds (relative to this
jurisdiction's clock):

% dacsrlink -uj EXAMPLE create -expires 300 /cgi-bin/dacs/dacs_prenv

Once an Rname has been created, a URL can be generated that incorporates the Rname:

% dacsrlink -uj EXAMPLE rlink -lmode acs IRCl7p4Q /cgi-bin/dacs/dacs_prenv

In this example, the Rname has been incorporated into the URL through the DACS_ACS

To display the ACL for Rname IRCl7p4Q:

% dacsrlink -uj EXAMPLE show IRCl7p4Q
<acl_rule status="enabled" name="IRCl7p4Q" expires_expr='time(now) ge 1178917167'>
<service url_pattern="/cgi-bin/dacs/dacs_prenv"/>

<rule order="allow,deny">

Or, since the access control rule created by dacsrlink can be found in

% cat /usr/local/dacs/rlinks/IRCl7p4Q

The default rule for dacs_prenv restricts access to a DACS administrator, but anyone who
uses this Rlink before it expires will be granted access to dacs_prenv. This rule can be
manually customized at anytime. Note that unlike ordinary access control rules, there is
no index file for Rlinks.

This command creates a rule that applies to two resources and grants access to two users:

% dacsrlink -uj EXAMPLE create -a :auggie -a :harley /private/a.html /private/b.html
% dacsrlink -uj EXAMPLE show 7tW3SJou
<acl_rule status="enabled" name="7tW3SJou">
<service url_pattern="/private/a.html"/>
<service url_pattern="/private/b.html"/>
<rule order="allow,deny">

To generate URLs to give to these two users so that they can access these resource,
commands like the following would be used:

% dacsrlink -uj EXAMPLE rlink -imode direct -i ":auggie" -lmode acs 7tW3SJou /private/a.html
% dacsrlink -uj EXAMPLE rlink -imode direct -i ":harley" -lmode acs 7tW3SJou /private/b.html

When the first of these links is invoked, it will appear as if EXAMPLE:auggie is accessing
a.html. Since no expiration was specified for the identities or the resources, the two
links will be valid indefinitely. The rule can be deleted at any time:

% dacsrlink -uj EXAMPLE delete 7tW3SJou

This demonstrates how to create a password-controlled link:

% dacsrlink -uj EXAMPLE create -a :auggie -p abracadabra /private/c.txt
% dacsrlink -uj EXAMPLE show rIPZaJeN
<acl_rule status="enabled" name="rIPZaJeN">
<service url_pattern="/private/c.html"/>
<rule order="allow,deny">
and password(check, ${Args::PASSWORD}, "2|XYZZYnahdnl3VtLqGtpbW|2GoDncq34p2EMO4PA5Uj6iWkFb9")
% dacsrlink -uj EXAMPLE rlink -imode direct -i :auggie -lmode acs rIPZaJeN /private/c.txt
% http "https://www.example.com/private/c.txt?DACS_ACS=-rname+rIPZaJeN:r6RdcTcmUyhTtgbtJg&PASSWORD=abracadabra"
Hello, world


The program exits 0 if everything was fine, 1 if an error occurred.

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