READERS.CONF(5)

NAME
readers.conf - Access control and configuration for nnrpd

IN A NUTSHELL
The readers.conf file parameters who is allowed to connect as a news reader and what they're allowed to do after they connect. Bear in mind that in readers.conf, authentication and authorization are configured in different blocks. First, a user is authenticated, and assigned an identity (in an "auth" block). Then this identity is authorized to access certain newsgroups with certain rights (in an "access" block).

As for authentication, your "auth" block for password users could look like this:

    auth "foreignokay" {
        auth: "ckpasswd -f <pathdb in inn.conf>/newsusers"
        default: "<unauthenticated>"
    }

See the documentation of the -f flag in the ckpasswd(8) man page for how to generate passwords and make use of this newsusers file.

This way, with the "foreignokay" authentication block, a user successfully authenticated as user "myusername" will be assigned the identity "myusername". If authentication fails, it will be assigned the default identity "<unauthenticated>" that will later be checked in "access" blocks.

Authentication blocks are checked from the last one in the readers.conf file to the first one (bottom up). As soon as one matches, the corresponding identity is assigned to the user.

As for authorization, let's do something in an "access" block for people successfully authenticated with passwords:

    access "authenticatedpeople" {
        users: "*"
        newsgroups: "*,!junk,!control,!control.*"
    }

And then something like one of the following two, depending on whether unauthenticated users get any access:

    access "restrictive" {
        users: "<unauthenticated>"
        newsgroups: "!*"
    }

    access "readonly" {
        users: "<unauthenticated>"
        read: "local.*"
        post: "!*"
    }

Please note that the "authenticatedpeople" block must appear in readers.conf before "restrictive" or "readonly" blocks because access blocks are checked from the last one in the readers.conf file to the first one (bottom up). As soon as one matches the identity previously assigned by an authentication block, it is chosen. The "authenticatedpeople" access block matches every user that has not been assigned "<unauthenticated>" as identity.

More examples and features are detailed below in this man page (notably without any password file, with PAM, with Perl or Python hooks).

You don't need to reload anything after modifying readers.conf; every time a news client connects to the server, a new nnrpd process is spawned and reads its configuration from disk. Nonetheless, after any changes, you can run "inncheck" to perform basic syntax checks against the modified readers.conf file.

DESCRIPTION
readers.conf in pathetc specifies access control for nnrpd(8). It controls who is allowed to connect as a news reader and what they're allowed to do after they connect. nnrpd reads this file every time a news client connects to the server, even when running in daemon mode with -D, which means that any changes on this file take effect immediately on all subsequent connections. (The location of readers.conf in pathetc is only the default; the same format applies to any file specified with "nnrpd -c".)

There are two types of entries in readers.conf: parameter/value pairs and configuration groups. Blank lines and anything after a number sign ("#") are ignored, unless the character "#" is escaped with "\". The maximum number of characters on each line is 8,191.

Parameter/value pairs consist of a keyword immediately followed by a colon, at least one whitespace character, and a value. The case of the parameter is significant (parameter should generally be in all lowercase), and a parameter may contain any characters except colon, "#", and whitespace. An example:

    hosts: *.example.com

Values that contain whitespace should be quoted with double quotes, as in:

    hosts: "*.example.com, *.example.net"

If the parameter does not contain whitespace, such as:

    hosts: *.example.com,*.example.net

it's not necessary to quote it, although you may wish to anyway for clarity.

There is no way to continue a line on the next line, and therefore no way to have a single parameter with a value longer than about 8,180 characters.

Many parameters take a boolean value. For all such parameters, the value may be specified as "true", "yes", or "on" to turn it on and may be any of "false", "no", or "off" to turn it off. The case of these values is not significant.

There are two basic types of configuration groups: auth and access. The auth group provides mechanisms to establish the identity of the user, who they are. The access group determines, given the user's identity, what that user is permitted to do. Writing a readers.conf file for your setup is a two-step process: first assigning an identity to each incoming connection using auth groups, and then giving each identity appropriate privileges with access group. We recommend not intermingling auth groups and access groups in the config file; it is often more sensible (in the absence of the key parameter) to put all of the auth groups first, and all of the access groups below.

A user identity, as established by an auth group, looks like an e-mail address if a domain is specified (like "<username>@<domain>") or otherwise just "<username>".

If nnrpdauthsender is set in inn.conf, the user identity is also put into the Sender header field of posts made by that user. See the documentation of that option in inn.conf(5) for more details.

An auth group definition looks like:

    auth <name> {
        hosts: <host-wildmat>
        auth: <auth-program>
        res: <res-program>
        default: <defuser>
        default-domain: <defdomain>
        # ...possibly other settings
    }

The <name> is used as a label for the group and is only for documentation purposes. (If your syslog configuration records the "news.debug" facility, the <name> will appear in the debugging output of nnrpd. Examining that output can be very helpful in understanding why your configuration doesn't do what you expect it to.)

A given auth group applies only to hosts whose name or IP address matches the wildmat expression given with the hosts parameter (comma-separated wildmat expressions are allowed, but "@" is not supported). Rather than wildmat expressions, you may also use CIDR notation to match any IP address in a netblock; for example, "10.10.10.0/24" will match any IP address between 10.10.10.0 and 10.10.10.255 inclusive.

If compiled against the TLS/SSL or SASL libraries, an auth group with the require_encryption parameter set to true only applies if the incoming connection is using an encryption layer, either from the beginning if the -S flag was passed to nnrpd, or after a successful use of STARTTLS, or after a successful authentication using an SASL mechanism which negotiates an encryption layer.

For any connection from a host that matches that wildmat expression or netblock, each <res-program> (multiple res lines may be present in a block; they are run in sequence until one succeeds), if any, is run to determine the identity of the user just from the connection information. If all the resolvers fail, or if the res parameter isn't present, the user is assigned an identity of "<defuser>@<defdomain>"; in other words, the values of the default and default-domain parameters are used. If <res-program> only returns a username, <defdomain> is used as the domain.

If the user later authenticates via the AUTHINFO USER/PASS commands, the provided username and password are passed to each <auth-program> (multiple auth, perl_auth, or python_auth lines may be present in a block; they are run in sequence until one succeeds), if any. If one succeeds and returns a different identity than the one assigned at the time of the connection, it is matched against the available access groups again and the actions the user is authorized to do may change. The most common <auth-program> to use is ckpasswd, which supports several ways of checking passwords including using PAM. See the ckpasswd(8) man page for more details.

When matching auth groups, the last auth group in the file that matches a given connection and username/password combination is used.

An access group definition usually looks like:

    access <name> {
        users: <identity-wildmat>
        newsgroups: <group-wildmat>
        # ...possibly other settings
    }

Again, <name> is just for documentation purposes. This says that all users whose identity matches <identity-wildmat> can read and post to all newsgroups matching <group-wildmat> (as before, comma-separated wildmat expressions are allowed, but "@" is not supported). Alternately, you can use the form:

    access <name> {
        users: <identity-wildmat>
        read: <read-wildmat>
        post: <post-wildmat>
    }

and matching users will be able to read any group that matches <read-wildmat> and post to any group that matches <post-wildmat>. You can also set several other things in the access group as well as override various inn.conf(5) parameters for just a particular group of users.

Just like with auth groups, when matching access groups the last matching one in the file is used to determine the user's permissions. There is an exception to this rule: if the auth group which matched the client contains a perl_access or python_access parameter, then the script given as argument is used to dynamically generate an access group. This new access group is then used to determine the access rights of the client; the access groups in the file are ignored.

There is one additional special case to be aware of. When forming particularly complex authentication and authorization rules, it is sometimes useful for the identities provided by a given auth group to only apply to particular access groups; in other words, rather than checking the identity against the users parameter of every access group, it's checked against the users parameter of only some specific access groups. This is done with the key parameter. For example:

    auth example {
        key: special
        hosts: *.example.com
        default: <SPECIAL>
    }

    access example {
        key: special
        users: <SPECIAL>
        newsgroups: *
        addcanlockuser: none
    }

In this case, the two key parameters bind this auth group with this access group. For any incoming connection matching "*.example.com" (assuming there isn't any later auth group that also matches such hosts), no access group that doesn't have "key: special" will even be considered. Similarly, the above access group will only be checked if the user was authenticated with an auth group containing "key: special". This mechanism normally isn't useful; there is almost always a better way to achieve the same result.

Also note in the example that there's no default-domain parameter, which means that no domain is appended to the default username and the identity for such connections is just "<SPECIAL>". Note that some additional add-ons to INN may prefer that authenticated identities always return a full e-mail address (including a domain), so you may want to set up your system that way if you are using add-ons. As several different persons can be assigned the same "<SPECIAL>" identity, no user-specific Cancel-Lock hashes should be generated.

Configuration files can be included in other configuration files via the syntax:

    include <filename>

The file named <filename> is then included. This syntax is allowed only at top-level.

Below is the full list of allowable parameters for auth groups and access groups, and after that are some examples that may make this somewhat clearer.

AUTH GROUP PARAMETERS
An auth group without at least one of the res, auth, perl_auth, python_auth, or default parameters makes no sense (and in practice will just be ignored).

auth
    A simple command line for a user authenticator (shell metacharacters are not supported). If a full path is not given, the program executed must be located in the pathbin/auth/passwd directory. An authenticator is a program used to handle a user-supplied username and password, via a mechanism such as AUTHINFO USER/PASS. Like with res, one auth group can have multiple auth parameters; they will be tried in order and the results of the first successful one will be used. See also perl_auth.

    The most common authenticator to use is ckpasswd(8); see its man page for more information.

default
    The default username for connections matching this auth group. This is the username assigned to the user at connection time if all resolvers fail or if there are no res parameters. Note that it can be either a bare username, in which case default-domain (if present) is appended after an "@", or a full identity string containing an "@", in which case it will be used verbatim.

default-domain
    The default domain string for this auth group. If a user resolver or authenticator doesn't provide a domain, or if the default username is used and it doesn't contain an "@", this domain is used to form the user identity. (Note that for a lot of setups, it's not really necessary for user identities to be qualified with a domain name, in which case there's no need to use this parameter.)

hosts
    A comma-separated list of remote hosts, wildmat patterns matching either hostnames or IP addresses, or IP netblocks specified in CIDR notation. If a user connects from a host that doesn't match this parameter, this auth group will not match the connection and is ignored.

    Note that if you have a large number of patterns that can't be merged into broader patterns (such as a large number of individual systems scattered around the net that should have access), the hosts parameter may exceed the maximum line length of 8,192 characters. In that case, you'll need to break that auth group into multiple auth groups, each with a portion of the hosts listed in its hosts parameter, and each assigning the same user identity.

    All hosts match if this parameter is not given.

key If this parameter is present, any connection matching this auth group will have its privileges determined only by the subset of access groups containing a matching key parameter.

localaddress
    A comma-separated list of local host or address patterns with the same syntax as the hosts parameter. If this parameter is specified, its auth group will only match connections made to a matching local interface. (Obviously, this is only useful for servers with multiple interfaces.)

    All local addresses match if this parameter is not given.

perl_access
    A path to a Perl script for dynamically generating an access group. If an auth group matches successfully and contains a perl_access parameter, then the argument Perl script will be used to create an access group. This group will then determine the access rights of the client, overriding any access groups in readers.conf. If and only if a successful auth group contains the perl_access parameter, readers.conf access groups are ignored and the client's rights are instead determined dynamically. This parameter is only valid if INN is compiled with Perl support (--with-perl passed to "configure"). More information may be found in the file doc/hook-perl.

perl_auth
    A path to a Perl script for authentication. The perl_auth parameter works exactly like auth, except that it calls the named script using the Perl hook rather than an external program. Multiple/mixed use of the auth, perl_auth, and python_auth parameters is permitted within any auth group; each line is tried in the order it appears. perl_auth has more power than auth in that it provides the authentication program with additional information about the client and the ability to return an error string and a username. This parameter is only valid if INN is compiled with Perl support (--with-perl passed to "configure"). More information may be found in doc/hook-perl.

python_access
    A Python script for dynamically generating an access group. If an auth group matches successfully and contains a python_access parameter, then the argument script (without its ".py" extension) located in the pathfilter directory will be used to create an access group. This group will then determine the access rights of the client, overriding any access groups in readers.conf.

    If and only if a successful auth group contains the python_access parameter, readers.conf access groups are ignored and the client's rights are instead determined dynamically.

    This parameter is only valid if INN is compiled with Python support (--with-python passed to "configure"). More information may be found in the file doc/hook-python.

python_auth
    A Python script for authentication. The python_auth parameter works exactly like auth, except that it calls the named script (without its ".py" extension) located in the pathfilter directory using the Python hook rather than an external program. Multiple/mixed use of the auth, perl_auth, and python_auth parameters is permitted within any auth group; each line is tried in the order it appears. python_auth has more power than auth in that it provides the authentication program with additional information about the client and the ability to return an error string and a username.

    This parameter is only valid if INN is compiled with Python support (--with-python passed to "configure"). More information may be found in doc/hook-python.

python_dynamic
    A Python script for applying access control dynamically on a per newsgroup basis. If an auth group matches successfully and contains a python_dynamic parameter, then the argument script (without its ".py" extension) located in the pathfilter directory will be used to determine the clients rights each time the user attempts to view a newsgroup, or read or post an article. Access rights as determined by python_dynamic override the values of access group parameters such as newsgroups, read and post.

    This parameter is only valid if INN is compiled with Python support (--with-python passed to "configure"). More information may be found in the file doc/hook-python.

require_encryption
    If set to true, an incoming connection only matches this auth group if it is encrypted, either from the beginning if the -S flag was passed to nnrpd, or after a successful use of STARTTLS, or after a successful authentication using an SASL mechanism which negotiates an encrypted layer. This parameter is only valid if INN is compiled with TLS/SSL or SASL support (by default if the OpenSSL SSL and crypto libraries or the Cyrus SASL library are found at configure time, otherwise see the --with-openssl and --with-sasl flags passed to "configure").

res A simple command line for a user resolver (shell metacharacters are not supported). If a full path is not given, the program executed must be in the pathbin/auth/resolv directory. A resolver is an authentication program which attempts to figure out the identity of the connecting user using nothing but the connection information (in other words, the user has not provided a username and password). An example of a resolver would be a program that assigns an identity from an ident callback or from the user's hostname.

    One auth group can have multiple res parameters, and they will be tried in the order they're listed. The results of the first successful one will be used.

    Alternatively, a res block can be used instead of a res parameter. The recognized parameters in such res blocks are:

    log A string to log in pathlog/news.notice (with "res also-log:" prepended) before the resolver is tried. One res group can have multiple log parameters, and they will be logged in the order they're listed.

    program
        This parameter is mandatory in a res block. Its meaning is the same as the res parameter used directly in an auth block.

            auth <auth-name> {
                res: <res-program>
            }

        is therefore equivalent to:

            auth <auth-name> {
                res <res-name> {
                    program: <res-program>
                }
            }

ACCESS GROUP PARAMETERS

access
    A set of letters specifying the permissions granted to the client. The letters are chosen from the following set:

    A   The client may post articles with Approved header fields (in other words, may approve articles for moderated newsgroups). By default, this is not allowed.

    I   The client may inject articles with IHAVE. Note that in order to inject articles with the IHAVE command, the user must also have POST permission (the "P" option). Articles injected with IHAVE are treated as though they were injected with POST, that is to say such articles must not have been previously injected (they must not contain header fields like Injection-Info).

    L   The client may post to newsgroups that are set to disallow local posting (status fields "j", "n" and "x" in the active(5) file).

    N   The client may use the NEWNEWS command, overriding the global setting.

    P   The client may post articles.

    R   The client may read articles.

    Note that if this parameter is given, allownewnews in inn.conf is ignored for connections matching this access group and the ability of the client to use NEWNEWS is entirely determined by the presence of "N" in the access string. If you want to support NEWNEWS, make sure to include "N" in the access string when you use this parameter.

    Note that if this parameter is given and "R" isn't present in the access string, the client cannot read regardless of newsgroups or read parameters. Similarly, if this parameter is given and "P" isn't present, the client cannot post. This use of access is deprecated and confusing; it's strongly recommended that if the access parameter is used, "R" and "P" always be included in the access string and newsgroups, read, and post be used to control access. (To grant read access but no posting access, one can have just a read parameter and no post parameter.)

addcanlockuser
    If set to "none", posts made by these users will not have a user-specific hash in the Cancel-Lock header field (only the admin hash will be generated). No Cancel-Key header field will be present in cancel or supersede requests. This is a string value and the default is "username", that is to say a user-specific hash based on the identity assigned to the connection is used.

    Another possible value to this parameter is "ip", in which case a user-specific hash based on the IP of the connection is used (which should be a static IP so as to actually identify users).

    See inn-secrets.conf(5) for more information about Cancel-Lock. The main reason for this parameter is to deactivate this authentication mechanism when the same identity or IP can be assigned by an access group to different people. Otherwise, any of these people would be able to send an authenticated withdrawal of an article originally sent by another person with the same identity or IP.

domain
    When set, the value of this parameter is used for the domain part (the right-hand side after "@") of autogenerated Message-IDs, as well as for the server name indicated in Injection-Info header fields.

    By default, the value of this parameter is inherited from the eponym parameter in inn.conf. However, if domain is not explicitly set in readers.conf, the specific behaviour described above won't be triggered unless virtualhost is set to true.

    You may also want to set the fromhost parameter (inherited from inn.conf) for the complaints address in the Injection-Info header field, if its default value should be changed, and the complaints parameter is not already set or newsmaster does not already contain an "@".

groupexactcount
    This parameter controls the threshold below with nnrpd should give the exact number of still existing articles present in newsgroups instead of an estimate. The exact number is usually updated once a day during the expiry process by expirover (run by news.daily), and is updated when new articles arrive but not necessarily when articles are cancelled or overwritten in self-expiring storage methods like CNFS.

    When the estimated count of articles is strictly smaller than groupexactcount, nnrpd will recount the number of still existing articles, and report the exact value. If groupexactcount is set to 0, nnrpd will always recount. If set to 1, it will never recount.

    Though exact article counts are not required, they may be useful to distinguish empty newsgroups by reporting an exact count of 0 instead of an estimate of 1 (when it happens, the news client may show that 1 article is present in the newsgroup, then it tries to retrieve the article, and finally does not show anything to the user).

    It is not recommended to set a high value to this parameter because it will slow the responses to news clients (recounting existing articles takes time, especially on newsgroups with a lot of articles).

    This is an integer value, and the default is 5.

key If this parameter is present, this access group is only considered when finding privileges for users matching auth groups with this same key parameter.

localtime
    If a Date or an Injection-Date header field is not included in a posted article, nnrpd normally adds these header fields in UTC. If this is set to true, the Date header field will be formatted in local time instead. (The Injection-Date header field is added according to the behaviour of the addinjectiondate parameter in inn.conf, and will remain in UTC, though.) This is a boolean value and the default is false.

    This parameter permits handling a relatively unusual corner case. It is mostly a tool for people who want to disclose their local time zone (it can be useful information in certain types of discussions), but whose clients don't for some reason, and who can arrange for the server to be in the same time zone as the client.

max_rate
    If this parameter is present (and non-zero), it is used for nnrpd's rate-limiting code. The client will only be able to download at this speed (in bytes/second). Note that if an encryption layer is being used, limiting is applied to the pre-encryption datastream.

newsgroups
    Users that match this access group are allowed to read and post to all newsgroups matching this comma-separated list of wildmat patterns. The empty string is equivalent to "newsgroups: *"; if this parameter is missing, the connection will be rejected (unless read and/or post are used instead).

newsmaster
    Used as the contact address in the help message returned by nnrpd(8), if the virtualhost: parameter is set to true.

perlfilter
    If set to false, posts made by these users do not pass through the Perl filter even if it is otherwise enabled. This is a boolean value and the default is true.

post
    Like the newsgroups parameter, but the client is only given permission to post to the matching newsgroups. This parameter is often used with read to define the patterns for reading and posting separately (usually to give the user permission to read more newsgroups than they're permitted to post to). It cannot be used in the same access group with a newsgroups parameter.

pythonfilter
    If set to false, posts made by these users do not pass through the Python filter even if it is otherwise enabled. This is a boolean value and the default is true.

    This parameter currently has no effect as Python filtering is not implemented yet in nnrpd. If you want to do Python filtering, you have to use the one implemented in innd.

read
    Like the newsgroups parameter, but the client is only given permission to read the matching newsgroups. This parameter is often used with post to specify some read-only groups; it cannot be used in the same access group with a newsgroups parameter. (If read is used and post is missing, the client will have only read-only access.)

reject_with
    If this parameter is present, a client matching this block will be disconnected with a "Permission denied" message containing the contents (a "reason" string) of this parameter. Some newsreaders will then display the reason to the user.

strippath
    If set to true, any Path header field provided by a user in a post is stripped rather than used as the beginning of the Path header field body of the article. This is a boolean value and the default is false.

users
    The privileges given by this access group apply to any user identity which matches this comma-separated list of wildmat patterns. If this parameter isn't given, the access group applies to all users (and is essentially equivalent to "users: *").

virtualhost
    Set this parameter to true in order to make nnrpd behave as if it is running on a server with a different name than it actually has. The domain parameter must then be set, either in inn.conf or in the same access group. All articles and overview data displayed to clients will have their Xref header field bodies altered to appear to be from the server named in domain, and posted articles will use that server name in the Message-ID and Injection-Info header field bodies. (Moreover, if the -I flag was given to nnrpd, posting agents will be proposed to use Message-IDs containing the string specified with -I.)

    Similarly, the Path header field bodies displayed to clients or generated in posted articles will use the value of domain (if pathhost is not set in the access group, or has the same value as in inn.conf) or pathhost (if pathhost is set in the access group to something different than is set in inn.conf).

    Also, the domain part (the right-hand side after "@") of the newsmaster e-mail mentioned in the help message returned by nnrpd will use the value of domain if the newsmaster parameter does not already have a domain part.

    At least one of the domain or pathhost parameters must be set in the access group to something different than in inn.conf, otherwise nnrpd will fail to start.

    Note that setting this parameter requires the server modify all posts before presenting them to the client and therefore may decrease performance slightly.

In addition, all of the following parameters are valid in access groups and override the global setting in inn.conf. See inn.conf(5) for the descriptions of these parameters:

    addinjectiondate, addinjectionpostingaccount,
    addinjectionpostinghost, backoff_auth, backoff_db, backoff_k,
    backoff_postfast, backoff_postslow, backoff_trigger,
    checkincludedtext, clienttimeout, complaints, fromhost,
    localmaxartsize, moderatormailer, nnrpdauthsender, nnrpdcheckart,
    nnrpdoverstats, nnrpdposthost, nnrpdpostport, organization,
    pathhost, readertrack, spoolfirst, strippostcc.

SUMMARY
Here's a basic summary of what happens when a client connects:

•   All auth groups are scanned and the ones that don't match the client (due to hosts, localaddress, require_encryption, etc.) are eliminated.

•   The remaining auth groups are scanned from the last to the first, and an attempt is made to apply it to the current connection. This means running res programs, if any, and otherwise applying default. The first auth group (starting from the bottom) to return a valid user is kept as the active auth group.

•   If no auth groups yield a valid user (none have default parameters or successful res programs) but some of the auth groups have auth lines (indicating a possibility that the user can authenticate and then obtain permissions), the connection is considered to have no valid auth group (which means that the access groups are ignored completely) but the connection isn't closed. Instead, a 480 response code (meaning that authentication is required) is returned for everything until the user authenticates.

•   When the user authenticates, the auth groups are rescanned, and only the matching ones which contain at least one auth, perl_auth, or python_auth line are considered. These auth groups are scanned from the last to the first, running auth programs and perl_auth or python_auth scripts. The first auth group (starting from the bottom) to return a valid user is kept as the active auth group.

•   Regardless of how an auth group is established, as soon as one is, that auth group is used to assign a user identity by taking the result of the successful res, auth, perl_auth, or python_auth line (or the default if necessary), and appending default-domain if necessary. (If the perl_access or python_access parameter is present.)

•   Finally, an access group is selected by scanning the access groups from bottom up and finding the first match. (If the established auth group contained a perl_access or python_access line, the dynamically generated access group returned by the script is used instead.) User permissions are granted based on the established access group.

EXAMPLES
Probably the simplest useful example of a complete readers.conf, this gives permissions to read and post to all groups to any connections from the "example.com" domain, except for Bob's machine, and no privileges for anyone connecting elsewhere:

    auth example.com {
        hosts: "*.example.com, example.com, !bob.example.com"
        default: "<EXAMPLE>"
    }

    access full {
        users: "<EXAMPLE>"
        newsgroups: *
        addcanlockuser: none
    }

Note that the above access realm could also be written without the users key, in which case it applies to any user identity (though in this example, the user identity that will be assigned to all matching connections is "<EXAMPLE>"). It is however recommended to keep an explicit users key so as to better view to whom the access block applies.

As the only available auth realm only matches hosts in the "example.com" domain, any connections from other hosts will be rejected immediately.

If you have some systems that should only have read-only access to the server, you can modify the example above slightly by adding an additional auth and access group:

    auth lab {
        hosts: "*.lab.example.com"
        default: <LAB>
    }

    access lab {
        users: <LAB>
        read: *
    }

If those are put in the file after the above example, they'll take precedence (because they're later in the file) for any user coming from a machine in the lab.example.com domain, everyone will only have read access, not posting access.

Here's a similar example for a news server that accepts connections from anywhere but requires the user to specify a username and password. The username and password are first checked against an external database of usernames and passwords, and then make use of PAM:

    auth all {
        auth: "ckpasswd -f <pathdb in inn.conf>/newsusers"
        auth: ckpasswd
    }

    access full {
        users: *
        newsgroups: *
    }

When the user first connects, there are no res keys and no default, so they don't receive any valid identity and the connection won't match any access groups (even ones with "users: *"). Such users receive nothing but authentication-required responses from nnrpd until they authenticate.

If they then later authenticate, the username and password are checked first by running ckpasswd with the -f option for an external file of encrypted passwords, and then uses PAM (if INN was built with PAM support) to check the password (and if that fails, it tries to check the password against the password field returned by getpwnam(3)). If both of those fail, the user will continue to have no identity; otherwise, an identity will be assigned (usually the supplied username, perhaps with a domain appended, although an authenticator technically can provide a completely different username for the identity), and the access group will match, giving full access.

It may be educational to consider how to combine the above examples; general groups always go first. The order of the auth groups actually doesn't matter, since the "hosts: example.com" one only matches connections before username/password is sent, and the "auth: ckpasswd" one only matches after; order would matter if either group applied to both cases. The order of the access groups in this case does matter, provided the newsgroups lines differ; the access group with no users line needs to be first, with the "users: <LOCAL>" group after.

Here's an example of another common case: a server that only allows connections from a local domain and has an additional hierarchy that's password-restricted.

    auth "example.com" {
        hosts: "*.example.com"
        auth: "ckpasswd -f <pathdb in inn.conf>/newsusers"
        default: "anonymous"
    }

    access regular {
        newsgroups: "*,!example.restricted.*"
        addcanlockuser: none
    }

    access full {
        users: "*,!anonymous"
        newsgroups: *
    }

In this example, unauthenticated users get the identity "anonymous", which matches only the first access group and hence doesn't get access to the "example.restricted.*" hierarchy. No user-specific Cancel-Lock elements should be generated for anonymous users. Anyone who authenticates using a password in the newsusers file gets full access to all groups. However, note that the only authentication block is limited to hostnames in the example.com domain; connections outside of that domain will never be allowed access nor have an opportunity to authenticate.

Here's a very complicated example. This is for an organization that has an internal hierarchy "example.*" only available to local shell users, who are on machines where identd(8) can be trusted. Dialup users must provide a username and password, which is then checked against RADIUS. Remote users have to use a username and password that's checked against a database on the news server. Finally, the admin staff (users "joe" and "jane") can post anywhere (including the "example.admin.*" groups that are read-only for everyone else), and are exempted from the Perl filter. For an additional twist, posts from dialup users have their Sender header field replaced by their authenticated identity.

Since this organization has some internal moderated newsgroups, the admin staff can also post messages with Approved header fields, but other users cannot.

    auth default {
        auth: "ckpasswd -f <pathdb in inn.conf>/newsusers"
        default: <FAIL>
        default-domain: example.com
    }

    auth shell {
        hosts: *.shell.example.com
        res: ident
        auth: ckpasswd
        default: <FAIL>
        default-domain: shell.example.com
    }

    auth dialup {
        hosts: *.dialup.example.com
        auth: radius
        default: <FAIL>
        default-domain: dialup.example.com
    }

    access shell {
        users: *@shell.example.com
        read: *
        post: "*, !example.admin.*"
    }

    access dialup {
        users: *@dialup.example.com
        newsgroups: *,!example.*
        nnrpdauthsender: true
    }

    access other {
        users: "*@example.com, !<FAIL>@example.com"
        newsgroups: *,!example.*
    }

    access fail {
        users: "<FAIL>@*"
        newsgroups: !*
    }

    access admin {
        users: "joe@*,jane@*"
        newsgroups: *
        access: "RPA"
        perlfilter: false
    }

Note the use of different domains to separate dialup from shell users easily. Another way to do that would be with key parameters, but this way provides slightly more intuitive identity strings. Note also that the fail access group catches not only failing connections from external users but also failed authentication of shell and dialup users before they've authenticated. The identity string given for, say, dialup users before RADIUS authentication has been attempted matches both the dialup access group and the fail access group, since it's "<FAIL>@dialup.example.com", but the fail group is last, so it takes precedence.

The shell auth group has an auth parameter so that users "joe" and "jane" can, if they choose, use username and password authentication to gain their special privileges even if they're logged on as a different user on the shell machines (or if ident isn't working). When they first connect, they'd have the default access for that user, but they could then authenticate in order to get their extended access.

Also note that if the users "joe" and "jane" are using their own accounts, they get their special privileges regardless of how they connect, whether the dialups, the shell machines, or even externally with a username and password.

Finally, here's a very simple example of a configuration for a public server for a particular hierarchy.

    auth default {
        hosts: *
        default: <PUBLIC>
    }

    access default {
        users: <PUBLIC>
        newsgroups: example.*
        addcanlockuser: none
    }

Notice that clients aren't allowed to read any other groups; this keeps them from getting access to administrative groups or reading control messages, just as a precaution. When running a public server like this, be aware that many public hierarchies will later be pulled down and reinjected into the main Usenet, so it's highly recommended that you also run a Perl or Python filter to reject any messages crossposted out of your local hierarchy, and any messages containing a Supersedes header field. This will keep messages posted to your public hierarchy from hurting any of the rest of Usenet if they leak out.

For more complex cases, or if you just prefer to use a database to store authentication and/or authorization information, you can use Perl or Python scripts to dynamically authenticate the users and/or generate their access blocks. You may then put all the related information (user, password, newsgroups for reading, newsgroups for posting, etc.) for instance in an SQLite database, and use an nnrpd_auth.pl or nnrpd_auth.py script to authenticate the users, and/or an nnrpd_access.pl or nnrpd_access.py script to grant the access. Then readers.conf only needs defining:

    auth checkcredentials {
        perl_auth: "/path/to/script/nnrpd_auth.pl"
        perl_access: "/path/to/script/nnrpd_access.pl"
    }

or:

    auth checkcredentials {
        python_auth: "nnrpd_auth"
        python_access: "nnrpd_access"
    }

The advantage of these external scripts is that you can code whatever rules you want, possibly based on a database shared between your news service and other services you may provide to your users. You can more easily handle hundreds or thousands of users with external scripts than with the mere use of readers.conf stock blocks and a flat file for the credentials of each listed user.

SECURITY CONSIDERATIONS
In general, separate passwords should be used for NNTP wherever possible; the NNTP protocol itself does not protect passwords from casual interception, and many implementations (including this one) do not "lock out" accounts or otherwise discourage password-guessing attacks. So it is best to ensure that a compromised password has minimal effects.

Authentication using the AUTHINFO USER/PASS commands passes unencrypted over the network. Extreme caution should therefore be used especially with system passwords (e.g. "auth: ckpasswd -s"). Passwords can be protected by using NNTP over TLS/SSL or through ssh tunnels, and this usage can be enforced by a well-considered server configuration that only permits certain auth groups to be applied in certain cases. One can also authenticate using a strong SASL mechanism. Here are some ideas:

•   To restrict connections on the standard NNTP port (119) to use an encryption layer for some (or all) of the auth groups to match, use the require_encryption parameter. Note that a client can use STARTTLS to negotiate an encrypted TLS connection. A secure layer can also be negotiated during authentication via AUTHINFO SASL.

•   If you consider your local network (but not the internet) secure, have some auth groups with a restrictive hosts parameter; they would go above, with ones having global applicability below.

•   Consider running nnrpd with the -S flag (either also with -D, or out of "super-server" like inetd) on the NNTPS port (563) for clients that support TLS/SSL. See nnrpd(8) for more details about how to configure that. You can use the require_encryption parameter or the -c flag to specify an alternate readers.conf file if you want a substantially different configuration for this case.

•   If you want to restrict an auth group to only match loopback connections (for users running newsreaders on localhost or connecting via an ssh tunnel), use the localaddress parameter.

HISTORY
Written by Aidan Cully <aidan@panix.com> for InterNetNews. Substantially expanded by Russ Allbery <eagle@eyrie.org>.

SEE ALSO
auth_krb5(8), ckpasswd(8), inn.conf(5), innd(8), inn-secrets.conf(5), libinn_uwildmat(3), newsfeeds(5), nnrpd(8).