Mercurial > prosody-modules
view mod_firewall/README.markdown @ 4270:243f7b0dbf35
mod_http_oauth2: Reduce authorization code validity time to 2 minutes
RFC 6749 states
> A maximum authorization code lifetime of 10 minutes is RECOMMENDED.
So 15 minutes was way too long. I was thinking 5 minutes at first but
since this should generally be instant, I settled on 2 minutes as a
large guesstimate on how slow it might be on slow links.
| author | Kim Alvefur <zash@zash.se> |
|---|---|
| date | Sun, 22 Nov 2020 18:46:25 +0100 |
| parents | c316ad1087d4 |
| children | bb8459c220c9 |
line wrap: on
line source
--- labels: - 'Stage-Alpha' summary: 'A rule-based stanza filtering module' rockspec: build: modules: mod_firewall.actions: actions.lib.lua mod_firewall.conditions: conditions.lib.lua mod_firewall.definitions: definitions.lib.lua mod_firewall.marks: marks.lib.lua mod_firewall.test: test.lib.lua --- ------------------------------------------------------------------------ **Note:** mod\_firewall is in its very early stages. This documentation is liable to change, and some described functionality may be missing, incomplete or contain bugs. ------------------------------------------------------------------------ Introduction ============ A firewall is an invaluable tool in the sysadmin's toolbox. However while low-level firewalls such as iptables and pf are incredibly good at what they do, they are generally not able to handle application-layer rules. The goal of mod\_firewall is to provide similar services at the XMPP layer. Based on rule scripts it can efficiently block, bounce, drop, forward, copy, redirect stanzas and more! Furthermore all rules can be applied and updated dynamically at runtime without restarting the server. Details ======= mod\_firewall loads one or more scripts, and compiles these to Lua code that reacts to stanzas flowing through Prosody. The firewall script syntax is unusual, but straightforward. A firewall script is dominated by rules. Each rule has two parts: conditions, and actions. When a stanza matches all of the conditions, all of the actions are executed in order. Here is a simple example to block stanzas from spammer@example.com: FROM: spammer@example.com DROP. FROM is a condition, and DROP is an action. This is about as simple as it gets. How about heading to the other extreme? Let's demonstrate something more complex that mod\_firewall can do for you: %ZONE myorganisation: staff.myorg.example, support.myorg.example ENTERING: myorganisation KIND: message TIME: 12am-9am, 5pm-12am, Saturday, Sunday REPLY=Sorry, I am afraid our office is closed at the moment. If you need assistance, please call our 24-hour support line on 123-456-789. This rule will reply with a short message whenever someone tries to send a message to someone at any of the hosts defined in the 'myorganisation' outside of office hours. Specifying rule sets -------------------- Firewall rules should be written into text files, e.g. `ruleset.pfw` file. One or more rule files can be specified in the configuration using: firewall_scripts = { "path/to/ruleset.pfw", "path/to/ruleset2.pfw" } If multiple files are specified and they both add rules to the same [chains](#chains), each file's rules will be processed in order, but the order of files is undefined. Reloading Prosody's configuration also reloads firewall rules. Make sure that `firewall_scripts` is in the global section of the configuration file and not below a virtual host or a component - unless you want per-vhost firewall rules. Conditions ---------- All conditions must come before any action in a rule block. The condition name is followed by a colon (':'), and the value to test for. A condition can be preceded or followed by `NOT` to negate its match. For example: NOT FROM: user@example.com KIND NOT: message Some conditions do not take parameters, and these should end with just a question mark, like: IN ROSTER? ### Zones A 'zone' is one or more hosts or JIDs. It is possible to match when a stanza is entering or leaving a zone, while at the same time not matching traffic passing between JIDs in the same zone. Zones are defined at the top of a script with the following syntax (they are not part of a rule block): %ZONE myzone: host1, host2, user@host3, foo.bar.example There is an automatic zone named `$local`, which automatically includes all of the current server's active hosts (including components). It can be used to match stanzas entering or leaving the current server. A host listed in a zone also matches all users on that host (but not subdomains). The following zone-matching conditions are supported: Condition Matches ------------ ------------------------------------------ `ENTERING` When a stanza is entering the named zone `LEAVING` When a stanza is leaving the named zone ### Lists It is possible to create or load lists of strings for use in scripts. For example, you might load a JID blacklist, a list of malware URLs or simple words that you want to filter messages on. List type Example ----------- ----------------------- memory %LIST spammers: memory file %LIST spammers: file:/etc/spammers.txt http %LIST spammers: http://example.com/spammers.txt #### List types ##### memory ``` %LIST name: memory (limit: number) ``` A memory-only list, with an optional limit. Supports addition and removal of items by scripts. If a limit is provided, the oldest item will be discarded to make room for a new item if the list is full. The limit is useful to prevent infinite memory growth on busy servers. ##### file ``` %LIST name: file:/path/to/file (missing: string) ``` Reads a list from a file. The list can be added to and removed from by scripts, but these changes do not persist between restarts. If the file is missing, an error will be raised. The optional 'missing' parameter can be set to 'ignore' (e.g. `(missing: ignore)`) to ignore a missing file. ##### http ``` %LIST name: http://example.com/ (ttl: number, pattern: pat, hash: sha1, checkcerts: when-sni) ``` Fetches a list from a HTTP or HTTPS URL. The following options are accepted: Option Description ------- ----------- ttl Seconds to cache the list for. After expiry, it will be refetched. Default 3600 (1 hour). pattern Optional pattern used to extract list entries from the response. Default is to treat each line as a single item. hash Optional hash to be applied to items before looking them up in the list, e.g. sha1 or sha256. checkcert Whether to verify HTTPS certificates. May be "always", "never" or "when-sni". Default "when-sni". The "when-sni" default disables certificate verification when Prosody's HTTP client API doesn't support SNI, as in Prosody 0.11.6 and earlier. #### CHECK LIST Checks whether a simple [expression](#expressions) is found in a given list. Example: %LIST blacklist: file:/etc/prosody/blacklist.txt # Rule to block presence subscription requests from blacklisted JIDs KIND: presence TYPE: subscribe CHECK LIST: blacklist contains $<@from> BOUNCE=policy-violation (Your JID is blacklisted) #### SCAN SCAN allows you to search inside a stanza for a given pattern, and check each result against a list. For example, you could scan a message body for words and check if any of the words are found in a given list. Before using SCAN, you need to define a search location and a pattern. The search location uses the same 'path' format as documented under the 'INSPECT' condition. Patterns can be any valid Lua pattern. To use the above example: # Define a search location called 'body' which fetches the text of the 'body' element %SEARCH body: body# # Define a pattern called 'word' which matches any sequence of letters %PATTERN word: [A-Za-z]+ # Finally, we also need our list of "bad" words: %LIST badwords: file:/etc/prosody/bad_words.txt # Now we can use these to SCAN incoming stanzas # If it finds a match, bounce the stanza SCAN: body for word in badwords BOUNCE=policy-violation (This word is not allowed!) #### COUNT COUNT is similar to SCAN, in that it uses a defined SEARCH and breaks it up according to a PATTERN. Then it counts the number of results. For example, to block every message with more than one URL: # Define a search location called 'body' which fetches the text of the 'body' element %SEARCH body: body# # Define a pattern called 'url' which matches HTTP links %PATTERN url: https?://%S+ COUNT: url in body > 1 BOUNCE=policy-violation (Up to one HTTP URL is allowed in messages) ### Stanza matching Condition Matches ----------- ------------------------------------------------------------------------------------------------------------------------------------------------------------ `KIND` The kind of stanza. May be 'message', 'presence' or 'iq' `TYPE` The type of stanza. This varies depending on the kind of stanza. See 'Stanza types' below for more information. `PAYLOAD` The stanza contains a child with the given namespace. Useful for determining the type of an iq request, or whether a message contains a certain extension. `INSPECT` The node at the specified path exists or matches a given string. This allows you to look anywhere inside a stanza. See below for examples and more. #### Stanza types Stanza Valid types ---------- ------------------------------------------------------------------------------------------ iq get, set, result, error presence *available*, unavailable, probe, subscribe, subscribed, unsubscribe, unsubscribed, error message normal, chat, groupchat, headline, error **Note:** The type 'available' for presence does not actually appear in the protocol. Available presence is signalled by the omission of a type. Similarly, a message stanza with no type is equivalent to one of type 'normal'. mod\_firewall handles these cases for you automatically. ### Sender/recipient matching Condition Matches ------------- ------------------------------------------------------- `FROM` The JID in the 'from' attribute matches the given JID. `TO` The JID in the 'to' attribute matches the given JID. `TO SELF` The stanza is sent by any of a user's resources to their own bare JID. `TO FULL JID` The stanza is addressed to a valid full JID on the local server (full JIDs include a resource at the end, and only exist for the lifetime of a single session, therefore the recipient must be online, or this check will not match). The TO and FROM conditions both accept wildcards in the JID when it is enclosed in angle brackets ('\<...\>'). For example: # All users at example.com FROM: <*>@example.com # The user 'admin' on any subdomain of example.com FROM: admin@<*.example.com> You can also use [Lua's pattern matching](http://www.lua.org/manual/5.1/manual.html#5.4.1) for more powerful matching abilities. Patterns are a lightweight regular-expression alternative. Simply contain the pattern in double angle brackets. The pattern is automatically anchored at the start and end (so it must match the entire portion of the JID). # Match admin@example.com, and admin1@example.com, etc. FROM: <<admin%d*>>@example.com **Note:** It is important to know that 'example.com' is a valid JID on its own, and does **not** match 'user@example.com'. To perform domain whitelists or blacklists, use [Zones](#zones). Condition Matches ---------------- --------------------------------------------------------------- `FROM_EXACTLY` The JID in the 'from' attribute exactly matches the given JID `TO_EXACTLY` The JID in the 'to' attribute exactly matches the given JID These additional conditions do not support pattern matching, but are useful to match the exact to/from address on a stanza. For example, if no resource is specified then only bare JIDs will be matched. TO and FROM match all resources if no resource is specified to match. **Note:** Some chains execute before Prosody has performed any normalisation or validity checks on the to/from JIDs on an incoming stanza. It is not advisable to perform access control or similar rules on JIDs in these chains (see the [chain documentation](#chains) for more info). #### INSPECT INSPECT takes a 'path' through the stanza to get a string (an attribute value or text content). An example is the best way to explain. Let's check that a user is not trying to register an account with the username 'admin'. This stanza comes from [XEP-0077: In-band Registration](http://xmpp.org/extensions/xep-0077.html#example-4): ``` xml <iq type='set' id='reg2'> <query xmlns='jabber:iq:register'> <username>bill</username> <password>Calliope</password> <email>bard@shakespeare.lit</email> </query> </iq> ``` KIND: iq TYPE: set PAYLOAD: jabber:iq:register INSPECT: {jabber:iq:register}query/username#=admin BOUNCE=not-allowed (The username 'admin' is reserved.) That weird string deserves some explanation. It is a path, divided into segments by '/'. Each segment describes an element by its name, optionally prefixed by its namespace in curly braces ('{...}'). If the path ends with a '\#' then the text content of the last element will be returned. If the path ends with '@name' then the value of the attribute 'name' will be returned. You can use INSPECT to test for the existence of an element or attribute, or you can check if it matches a specific value, e.g. by appending `=VALUE` (like in the example above, that checks if the content of username is 'admin'). #### INSPECT comparison operators As well as checking for an exact string match, there are some other modifiers you can apply to the comparison: Comparison Matches when ------------- ------------------------------------------------------- `=` The value is exactly the given string. `/=` The value is or *contains* the given string (e.g. `/=admin` would match `administrator` or `myadmin`). `~=` The value matches the given [Lua pattern](https://www.lua.org/manual/5.2/manual.html#6.4.1). Finally, if the comparison operator is preceded by a `$` character, [expressions](#expressions) will be interpreted in the string following the comparison operator. e.g. `INSPECT: {jabber:iq:register}query/username}$/=$(session.host)` would match if the username of an account registration contained the session's current hostname somewhere in it. #### INSPECT performance INSPECT can be somewhat slower than the other stanza matching conditions. To minimise performance impact, always place it below other faster condition checks where possible (e.g. in the example above we first checked KIND, TYPE and PAYLOAD matched what we wanted before reaching the INSPECT rule). ### Roster These functions access the roster of the recipient (only). Therefore they cannot (currently) be used in some [chains](#chains), such as for outgoing messages (the recipient may be on another server). Performance note: this check can potentially cause storage access (especially if the recipient is currently offline), so you may want to limit its use in high-traffic situations, and place it below other checks (such as a rate limiter). #### IN ROSTER Tests whether the sender is in the recipient's roster. IN ROSTER? #### IN ROSTER GROUP Tests whether the sender is in the recipient's roster, and in the named group. IN ROSTER GROUP: Friends #### SUBSCRIBED Tests whether the recipient is subscribed to the sender, ie will receive presence updates from them. Note that this *does* work, regardless of direction and which [chain](#chain) is used, since both the sender and the recipient will have mirrored roster entries. ### Groups Using Prosody's mod\_groups it is possible to define groups of users on the server. You can match based on these groups in firewall rules. Condition Matches ----------------- ---------------------------- `FROM GROUP` When the stanza is being sent from a member of the named group `TO GROUP` When the stanza is being sent to a member of the named group `CROSSING GROUPS` When the stanza is being sent between users of different named groups #### CROSSING GROUPS The `CROSSING GROUPS` condition takes a comma-separated list of groups to check. If the sender and recipient are not in the same group (only the listed groups are checked), then the this condition matches and the stanza is deemed to be crossing between groups. For example, if you had three groups: Engineering, Marketing and Employees. All users are members of the 'Employees' group, and the others are for employees of the named department only. To prevent employees in the marketing department from communicating with engineers, you could use the following rule: ``` CROSSING GROUPS: Marketing, Engineering BOUNCE=policy-violation (no communication between these groups is allowed!) ``` This works, even though both the users are in the 'Employees' group, because that group is not listed in the condition. In the above example, a user who is member of both groups is not restricted. #### SENT DIRECTED PRESENCE TO SENDER This condition matches if the recipient of a stanza has previously sent directed presence to the sender of the stanza. This is often done in XMPP to exchange presence information with JIDs that are not on your roster, such as MUC rooms. This condition does not take a parameter - end the condition name with a question mark: # Rule to bounce messages from senders not in the roster who haven't been sent directed presence NOT IN ROSTER? NOT SENT DIRECTED PRESENCE TO SENDER? BOUNCE=service-unavailable ### Admins Prosody allows certain JIDs to be declared as administrators of a host, component or the whole server. Condition Matches ---------------- ------------------------------------------------------------------------------------- `TO ADMIN` When the recipient of the stanza is admin of the current host `FROM ADMIN` When the sender of the stanza is admin of the current host `FROM ADMIN OF` When the sender of the stanza is an admin of the named host on the current server `TO ADMIN OF` When the recipient of the stanza is an admin of the named host on the current server ### Time and date #### TIME Matches stanzas sent during certain time periods. Condition Matches ----------- ------------------------------------------------------------------------------------------- TIME When the current server local time is within one of the comma-separated time ranges given TIME: 10pm-6am, 14:00-15:00 REPLY=Zzzz. #### DAY It is also possible to match only on certain days of the week. Condition Matches ----------- ----------------------------------------------------------------------------------------------------- DAY When the current day matches one, or falls within a rage, in the given comma-separated list of days Example: DAY: Sat-Sun, Wednesday REPLY=Sorry, I'm out enjoying life! All times and dates are handled in the server's local time. ### Rate-limiting It is possible to selectively rate-limit stanzas, and use rules to decide what to do with stanzas when over the limit. First, you must define any rate limits that you are going to use in your script. Here we create a limiter called 'normal' that will allow 2 stanzas per second, and then we define a rule to bounce messages when over this limit. Note that the `RATE` definition is not part of a rule (multiple rules can share the same limiter). %RATE normal: 2 (burst 3) KIND: message LIMIT: normal BOUNCE=policy-violation (Sending too fast!) The 'burst' parameter on the rate limit allows you to spread the limit check over a given time period. For example the definition shown above will allow the limit to be temporarily surpassed, as long as it is within the limit after 3 seconds. You will almost always want to specify a burst factor. Both the rate and the burst can be fractional values. For example a rate of 0.1 means only one event is allowed every 10 seconds. The LIMIT condition actually does two things; first it counts against the given limiter, and then it checks to see if the limiter over its limit yet. If it is, the condition matches, otherwise it will not. Condition Matches ----------- -------------------------------------------------------------------------------------------------- `LIMIT` When the named limit is 'used up'. Using this condition automatically counts against that limit. **Note:** Reloading mod\_firewall resets the current state of any limiters. #### Dynamic limits Sometimes you may want to have multiple throttles in a single condition, using some property of the session or stanza to determine which throttle to use. For example, you might have a limit for incoming stanzas, but you want to limit by sending JID, instead of all incoming stanzas sharing the same limit. You can use the 'on' keyword for this, like so: LIMIT: normal on EXPRESSION For more information on [expressions](#expressions), see the section later in this document. Each value of 'EXPRESSION' has to be tracked individually in a table, which uses a small amount of memory. To prevent memory exhaustion, the number of tracked values is limited to 1000 by default. You can override this by setting the maximum number of table entries when you define the rate: %RATE normal: 2 (burst 3) (entries 4096) Old values are automatically removed from the tracking table. However if the tracking table becomes full, new entries will be rejected - it will behave as if the rate limit was reached, even for values that have not been seen before. Since this opens up a potential denial of service (innocent users may be affected if malicious users can fill up the tracking table within the limit period). You can choose to instead "fail open", and allow the rate limit to be temporarily bypassed when the table is full. To choose this behaviour, add `(allow overflow)` to the RATE definition. ### Session marking It is possible to 'mark' sessions (see the MARK_ORIGIN action below). To match stanzas from marked sessions, use the `ORIGIN_MARKED` condition. Condition Description ------------------------------- --------------------------------------------------------------- ORIGIN MARKED: markname Matches if the origin has been marked with 'markname'. ORIGIN MARKED: markname (Xs) Matches if the origin has been marked with 'markname' within the past X seconds. Example usage: # This rule drops messages from sessions that have been marked as spammers in the past hour ORIGIN MARKED: spammer (3600s) DROP. # This rule marks the origin session as a spammer if they send a message to a honeypot JID KIND: message TO: honeypot@example.com MARK ORIGIN=spammer Actions ------- Actions come after all conditions in a rule block. There must be at least one action, though conditions are optional. An action without parameters ends with a full-stop/period ('.'), and one with parameters uses an equals sign ('='): # An action with no parameters: DROP. # An action with a parameter: REPLY=Hello, this is a reply. ### Route modification The most common actions modify the stanza's route in some way. Currently the first matching rule to do so will halt further processing of actions and rules (this may change in the future). Action Description ----------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------- `PASS.` Stop executing actions and rules on this stanza, and let it through this chain and any calling chains. `DROP.` Stop executing actions and rules on this stanza, and discard it. `DEFAULT.` Stop executing actions and rules on this stanza, prevent any other scripts/modules from handling it, to trigger the appropriate default "unhandled stanza" behaviour. Do not use in custom chains (it is treated as PASS). `REDIRECT=jid` Redirect the stanza to the given JID. `REPLY=text` Reply to the stanza (assumed to be a message) with the given text. `BOUNCE.` Bounce the stanza with the default error (usually service-unavailable) `BOUNCE=error` Bounce the stanza with the given error (MUST be a defined XMPP stanza error, see [RFC6120](http://xmpp.org/rfcs/rfc6120.html#stanzas-error-conditions). `BOUNCE=error (text)` As above, but include the supplied human-readable text with a description of the error `COPY=jid` Make a copy of the stanza and send the copy to the specified JID. The copied stanza flows through Prosody's routing code, and as such is affected by firewall rules. Be careful to avoid loops. `FORWARD=jid` Forward a copy of the stanza to the given JID (using XEP-0297). The stanza will be sent from the current host's JID. **Note:** It is incorrect behaviour to reply to an 'error' stanza with another error, so BOUNCE will simply act the same as 'DROP' for stanzas that should not be bounced (error stanzas and iq results). ### Stanza modification These actions make it possible to modify the content and structure of a stanza. Action Description ------------------------ ------------------------------------------------------------------------ `STRIP=name` Remove any child elements with the given name in the default namespace `STRIP=name namespace` Remove any child elements with the given name and the given namespace `INJECT=xml` Inject the given XML into the stanza as a child element ### Sessions It is possible to mark sessions, and then use these marks to match rules later on. Action Description ------------------------ -------------------------------------------------------------------------- `MARK ORIGIN=mark` Marks the originating session with the given flag. `UNMARK ORIGIN=mark` Removes the given mark from the origin session (if it is set). **Note:** Marks apply to sessions, not JIDs. E.g. if marking in a rule that matches a stanza received over s2s, it is the s2s session that is marked. It is possible to have multiple marks on an origin at any given time. ### Informational Action Description --------------- ------------------------------------------------------------------------------------------------------------------------ `LOG=message` Logs the given message to Prosody's log file. Optionally prefix it with a log level in square brackets, e.g. `[debug]` You can include [expressions](#expressions) in log messages, using `$(...)` syntax. For example, to log the stanza that matched the rule, you can use `$(stanza)`, or to log just the top tag of the stanza, use `$(stanza:top_tag())`. To fetch the sender JID, use `$(stanza.attr.from)`. Example: # Log all stanzas to user@example.com: TO: user@example.com LOG=[debug] User received: $(stanza) More info about expressions can be found below. Chains ------ Rules are grouped into "chains", which are injected at particular points in Prosody's routing code. Available built-in chains are: Chain Description -------------- ------------------------------------------------------------------------------------------- deliver Applies to stanzas delivered to local recipients (regardless of the stanza's origin) deliver_remote Applies to stanzas delivered to remote recipients (just before they leave the local server) preroute Applies to incoming stanzas from local users, before any routing rules are applied A chain is begun by a line `::name` where 'name' is the name of the chain you want the following rules to be inserted into. If no chain is specified, rules are put into the 'deliver' chain. It is possible to create custom chains (useful with the JUMP_CHAIN action described below). User-created chains must begin with "user/", e.g. "user/spam_filtering". Example of chain use: # example.com's firewall script # This line is optional, because 'deliver' is the default chain anyway: ::deliver # This rule matches any stanzas delivered to our local user bob: TO: bob@example.com DROP. # Oops! This rule will never match, because alice is not a local user, # and only stanzas to local users go through the 'deliver' chain: TO: alice@remote.example.com DROP. # Create a 'preroute' chain of rules (matched for incoming stanzas from local clients): ::preroute # These rules are matched for outgoing stanzas from local clients # This will match any stanzas sent to alice from a local user: TO: alice@remote.example.com DROP. Action Description ------------------------ ------------------------------------------------------------------------ `JUMP CHAIN=name` Switches chains, and passes the stanza through the rules in chain 'name'. If the new chain causes the stanza to be dropped/redirected, the current chain halts further processing. `RETURN.` Stops executing the current chain and returns to the parent chain. For built-in chains, equivalent to PASS. RETURN is implicit at the end of every chain. It is possible to jump to chains defined by other scripts and modules. Expressions ----------- Some conditions and actions in rules support "expressions" in their parameters (their documentation will indicate if this is the case). Most parameters are static once the firewall script is loaded and compiled internally, however parameters that allow expressions can be dynamically calculated when a rule is being run. There are two kinds of expression that you can use: stanza expressions, and code expressions. ### Stanza expressions Stanza expressions are of the form `$<...>`, where `...` is a stanza path. For syntax of stanza paths, see the documentation for the 'INSPECT' condition above. Example: LOG=Matched a stanza from $<@from> to $<@to> There are built in functions which can be applied to the output of a stanza expression, by appending the pipe ('|') operator, followed by the function name. These functions are: Function Description ------------ --------------------------------------- bare Given a JID, strip any resource node Return the node ('user part') of a JID host Return the host ('domain') part of a JID resource Return the resource part of a JID For example, to apply a rate limit to stanzas per sender domain: LIMIT normal on $<@from|host> If the path does not match (e.g. the element isn't found, or the attribute doesn't exist) or any of the functions fail to produce an output (e.g. an invalid JID was passed to a function that only handles valid JIDs) the expression will return the text `<undefined>`. You can override this by ending the expression with a double pipe ('||') followed by a quoted string to use as a default instead. E.g. to default to the string "normal" when there is no 'type' attribute: LOG=Stanza type is $<@type||"normal"> ### Code expressions Code expressions use `$(...)` syntax. Code expressions are powerful, and allow unconstrained access to Prosody's internal environment. Therefore code expressions are typically for advanced use-cases only. You may want to refer to Prosody's [developer documentation](https://prosody.im/doc/developers) for more information. In particular, within code expressions you may access the 'session' object, which is the session object of the origin of the stanza, and the 'stanza' object, which is the stanza being considered within the current rule. Whatever value the expression returns will be converted to a string. Example to limit stanzas per session type: LIMIT: normal on $(session.type)