MEDIUM: proxy: support use_backend with dynamic names

We have a use case where we look up a customer ID in an HTTP header
and direct it to the corresponding server. This can easily be done
using ACLs and use_backend rules, but the configuration becomes
painful to maintain when the number of customers grows to a few
tens or even a several hundreds.

We realized it would be nice if we could make the use_backend
resolve its name at run time instead of config parsing time, and
use a similar expression as http-request add-header to decide on
the proper backend to use. This permits the use of prefixes or
even complex names in backend expressions. If no name matches,
then the default backend is used. Doing so allowed us to get rid
of all the use_backend rules.

Since there are some config checks on the use_backend rules to see
if the referenced backend exists, we want to keep them to detect
config errors in normal config. So this patch does not modify the
default behaviour and proceeds this way :

  - if the backend name in the use_backend directive parses as a log
    format rule, it's used as-is and is resolved at run time ;

  - otherwise it's a static name which must be valid at config time.

There was the possibility of doing this with the use-server directive
instead of use_backend, but it seems like use_backend is more suited
to this task, as it can be used for other purposes. For example, it
becomes easy to serve a customer-specific proxy.pac file based on the
customer ID by abusing the errorfile primitive :

     use_backend bk_cust_%[hdr(X-Cust-Id)] if { hdr(X-Cust-Id) -m found }
     default_backend bk_err_404

     backend bk_cust_1
         errorfile 200 /etc/haproxy/static/proxy.pac.cust1

Signed-off-by: Bertrand Jacquin <bjacquin@exosec.fr>

doc/configuration.txt

@@ -2578,7 +2578,9 @@ fullconn <conns>
 
   Since it's hard to get this value right, haproxy automatically sets it to
   10% of the sum of the maxconns of all frontends that may branch to this
-  backend. That way it's safe to leave it unset.
+  backend (based on "use_backend" and "default_backend" rules). That way it's
+  safe to leave it unset. However, "use_backend" involving dynamic names are
+  not counted since there is no way to know if they could match or not.
 
   Example :
      # The servers will accept between 100 and 1000 concurrent connections each
@@ -7711,7 +7713,8 @@ use_backend <backend> unless <condition>
   May be used in sections :   defaults | frontend | listen | backend
                                   no   |    yes   |   yes  |   no
   Arguments :
-    <backend>   is the name of a valid backend or "listen" section.
+    <backend>   is the name of a valid backend or "listen" section, or a
+                "log-format" string resolving to a backend name.
 
     <condition> is a condition composed of ACLs, as described in section 7.
 
@@ -7740,7 +7743,22 @@ use_backend <backend> unless <condition>
   a complete HTTP request to get in. This feature is useful when a frontend
   must decode several protocols on a unique port, one of them being HTTP.
 
-  See also: "default_backend", "tcp-request", and section 7 about ACLs.
+  When <backend> is a simple name, it is resolved at configuration time, and an
+  error is reported if the specified backend does not exist. If <backend> is
+  a log-format string instead, no check may be done at configuration time, so
+  the backend name is resolved dynamically at run time. If the resulting
+  backend name does not correspond to any valid backend, no other rule is
+  evaluated, and the default_backend directive is applied instead. Note that
+  when using dynamic backend names, it is highly recommended to use a prefix
+  that no other backend uses in order to ensure that an unauthorized backend
+  cannot be forced from the request.
+
+  It is worth mentionning that "use_backend" rules with an explicit name are
+  used to detect the association between frontends and backends to compute the
+  backend's "fullconn" setting. This cannot be done for dynamic names.
+
+  See also: "default_backend", "tcp-request", "fullconn", "log-format", and
+            section 7 about ACLs.
 
 
 use-server <server> if <condition>

include/types/proxy.h

@@ -379,9 +379,11 @@ struct proxy {
 struct switching_rule {
 	struct list list;			/* list linked to from the proxy */
 	struct acl_cond *cond;			/* acl condition to meet */
+	int dynamic;				/* this is a dynamic rule using the logformat expression */
 	union {
 		struct proxy *backend;		/* target backend */
 		char *name;			/* target backend name during config parsing */
+		struct list expr;		/* logformat expression to use for dynamic rules */
 	} be;
 };
 

src/cfgparse.c

@@ -6813,6 +6813,33 @@ int check_config_validity()
 		/* find the target proxy for 'use_backend' rules */
 		list_for_each_entry(rule, &curproxy->switching_rules, list) {
 			struct proxy *target;
+			struct logformat_node *node;
+			char *pxname;
+
+			/* Try to parse the string as a log format expression. If the result
+			 * of the parsing is only one entry containing a simple string, then
+			 * it's a standard string corresponding to a static rule, thus the
+			 * parsing is cancelled and be.name is restored to be resolved.
+			 */
+			pxname = rule->be.name;
+			LIST_INIT(&rule->be.expr);
+			parse_logformat_string(pxname, curproxy, &rule->be.expr, 0, SMP_VAL_FE_HRQ_HDR,
+			                       curproxy->conf.args.file, curproxy->conf.args.line);
+			node = LIST_NEXT(&rule->be.expr, struct logformat_node *, list);
+
+			if (!LIST_ISEMPTY(&rule->be.expr)) {
+				if (node->type != LOG_FMT_TEXT || node->list.n != &rule->be.expr) {
+					rule->dynamic = 1;
+					free(pxname);
+					continue;
+				}
+				/* simple string: free the expression and fall back to static rule */
+				free(node->arg);
+				free(node);
+			}
+
+			rule->dynamic = 0;
+			rule->be.name = pxname;
 
 			target = findproxy_mode(rule->be.name, curproxy->mode, PR_CAP_BE);
 
@@ -7722,7 +7749,7 @@ int check_config_validity()
 				/* check if a "use_backend" rule matches */
 				if (!found) {
 					list_for_each_entry(rule, &fe->switching_rules, list) {
-						if (rule->be.backend == curproxy) {
+						if (!rule->dynamic && rule->be.backend == curproxy) {
 							found = 1;
 							break;
 						}

src/session.c

@@ -1244,7 +1244,24 @@ static int process_switching_rules(struct session *s, struct channel *req, int a
 				ret = !ret;
 
 			if (ret) {
-				if (!session_set_backend(s, rule->be.backend))
+				/* If the backend name is dynamic, try to resolve the name.
+				 * If we can't resolve the name, or if any error occurs, break
+				 * the loop and fallback to the default backend.
+				 */
+				struct proxy *backend;
+
+				if (rule->dynamic) {
+					struct chunk *tmp = get_trash_chunk();
+					if (!build_logline(s, tmp->str, tmp->size, &rule->be.expr))
+						break;
+					backend = findproxy(tmp->str, PR_CAP_BE);
+					if (!backend)
+						break;
+				}
+				else
+					backend = rule->be.backend;
+
+				if (!session_set_backend(s, backend))
 					goto sw_failed;
 				break;
 			}