docs: add section on config language

docs/content/cfg/_index.md

@@ -0,0 +1,151 @@
+---
+title: "Config Language"
+weight: 550
+---
+
+The routing table is configured with commands in the language specified below.
+Rules are automatically generated by the configured backend. Additional rules
+can be stored in the Consul KV store configured by
+[`registry.consul.kvpath`](/ref/registry.consul.kvpath/) which by default is
+`/fabio/config`. 
+
+As of fabio 1.5.7 the path is interpreted as a prefix and the values of all sub
+keys are appended in alphabetical order. When the
+[`log.routes.format`](/ref/log.routes.format/) is set to `all` then the routing
+table contains comments on the source of the fragment.
+
+### Comments
+
+Blank lines and lines starting with `#` or `//` are ignored.
+
+### `route add`
+
+Add a route for a service `svc` for the `src` (e.g. `/path` or `:port`) to a `dst` (e.g. `URL` or `host:port`).
+
+`route add <svc> <src> <dst>[ weight <w>][ tags "<t1>,<t2>,..."][ opts "k1=v1 k2=v2 ..."]`
+
+Option               | Description
+-------------------- | -----------
+`strip=/path`        | Forward `/path/to/file` as `/to/file`
+`proto=tcp`          | Upstream service is TCP, `dst` must be `:port`
+`proto=https`        | Upstream service is HTTPS
+`tlsskipverify=true` | Disable TLS cert validation for HTTPS upstream
+`host=name`          | Set the `Host` header to `name`. If `name == 'dst'` then the `Host` header will be set to the registered upstream host name
+
+##### Example
+
+```
+# route traffic for product-svc to 1.2.3.4:8000 and :9000
+route add product-svc /product http://1.2.3.4:8000
+route add product-svc /product http://1.2.3.4:9000
+```
+
+### `route del`
+
+Remove one or more routes which match the given criteria.
+
+```
+route del <svc>[ <src>[ <dst>]]
+route del <svc> tags "<t1>,<t2>,..."
+route del tags "<t1>,<t2>,..."
+```
+
+##### Example
+
+```
+# remove all routes for 'product-svc'
+route del product-svc
+
+# remove all routes for 'product-svc' and /path
+route del product-svc /path
+
+# remove single route
+route del product-svc /path http://1.2.3.4:8000
+
+# remove all routes for 'product-svc' matching a tag
+route del product-svc tags "green"
+
+# remove all routes matching a tag
+route del tags "yesterday"
+```
+
+### `route weight`
+
+Directs a certain amount of traffic to instances matching certain criteria.
+
+The weight `w` is expressed as follows:
+
+ * `w` is a float > 0 describing a percentage, e.g. 0.5 == 50%
+ * `w <= 0`: means no fixed weighting. Traffic is evenly distributed
+ * `w > 0`: route will receive n% of traffic. If sum(w) > 1 then w is normalized.
+ * `sum(w) >= 1`: only matching services will receive traffic
+
+Note that the total sum of traffic sent to all matching routes is `w`%.
+
+The order of commands matters but routes are always ordered from most to least
+specific by prefix length.
+
+```
+route weight <svc> <src> weight <w> tags "<t1>,<t2>,..."
+route weight <src> weight <w> tags "<t1>,<t2>,..."
+route weight <svc> <src> weight <w>
+route weight service host/path weight w tags "tag1,tag2"
+```
+
+##### Example
+
+```
+# Route 5% of the traffic to product-svc:/path with tags "green"
+route weight product-svc /path weight .05 tags "green"
+
+# Route 5% of the traffic to '/path' to the Go implementation.
+# Route the rest (95%) to the other implementation
+route weight /path weight .05 tags "lang=go"
+
+# Route 5% of the traffice to '/path' on the product-svc-go
+route weight product-svc-go /path weight .05
+```
+
+### Routing rules
+
+The routing table contains first all routes with a host sorted by prefix
+length in descending order and then all routes without a host again sorted by
+prefix length in descending order.
+
+For each incoming request the routing table is searched top to bottom for a
+matching route. A route matches if either `host/path` or - if there was no
+match - just `/path` matches.
+
+The matching route determines the target URL depending on the configured
+strategy. `rnd` and `rr` are available with `rnd` being the default.
+
+##### Example
+
+The auto-generated routing table is
+
+```
+route add service-a www.mp.dev/accounts/ http://host-a:11050/ tags "a,b"
+route add service-a www.kjca.dev/accounts/ http://host-a:11050/ tags "a,b"
+route add service-a www.dba.dev/accounts/ http://host-a:11050/ tags "a,b"
+route add service-b www.mp.dev/auth/ http://host-b:11080/ tags "a,b"
+route add service-b www.kjca.dev/auth/ http://host-b:11080/ tags "a,b"
+route add service-b www.dba.dev/auth/ http://host-b:11080/ tags "a,b"
+```
+
+The manual configuration under `/fabio/config` is
+
+```
+route del service-b www.dba.dev/auth/
+route add service-c www.somedomain.com/ http://host-z:12345/
+```
+
+The complete routing table then is
+
+```
+route add service-a www.mp.dev/accounts/ http://host-a:11050/ tags "a,b"
+route add service-a www.kjca.dev/accounts/ http://host-a:11050/ tags "a,b"
+route add service-a www.dba.dev/accounts/ http://host-a:11050/ tags "a,b"
+route add service-b www.mp.dev/auth/ http://host-b:11080/ tags "a,b"
+route add service-b www.kjca.dev/auth/ http://host-b:11080/ tags "a,b"
+route add service-c www.somedomain.com/ http://host-z:12345/ tags "a,b"
+```