Document log replacements

Rename latency to duration.

middleware/log/README.md

@@ -33,7 +33,26 @@ CoreDNS will create it before appending to it.
 
 ## Log Format
 
-You can specify a custom log format with any placeholder values. Log supports both request and response placeholders.
+You can specify a custom log format with any placeholder values. Log supports both request and
+response placeholders.
+
+The following place holders are supported:
+
+* `{type}`: qtype of the request.
+* `{name}`: qname of the request.
+* `{class}`: class of the request.
+* `{proto}`: protocol used (tcp or udp).
+* `{when}`: time of the query.
+* `{remote}`: client's IP address.
+* `{port}`: client's port.
+* `{rcode}`: response RCODE.
+* `{size}`: response size.
+* `{duration}`: response duration (in seconds).
+* `{>bufsize}`: the EDNS0 buffer size advertized by the client.
+* `{>do}`: is the EDNS0 DO (DNSSEC OK) bit set.
+* `{>id}`: query ID
+* `{>opcode}`: query OPCODE
+
 
 ## Log Rotation
 

middleware/replacer.go

 package middleware
 
 import (
+	"fmt"
 	"strconv"
 	"strings"
 	"time"
@@ -54,13 +55,15 @@ func NewReplacer(r *dns.Msg, rr *ResponseRecorder, emptyValue string) Replacer {
 		}
 		rep.replacements["{rcode}"] = rcode
 		rep.replacements["{size}"] = strconv.Itoa(rr.size)
-		rep.replacements["{latency}"] = time.Since(rr.start).String()
+		rep.replacements["{duration}"] = time.Since(rr.start).String()
 	}
 
 	// Header placeholders (case-insensitive)
 	// TODO(miek): syntax for flags and document it
 	rep.replacements[headerReplacer+"id}"] = strconv.Itoa(int(r.Id))
 	rep.replacements[headerReplacer+"opcode}"] = strconv.Itoa(int(r.Opcode))
+	rep.replacements[headerReplacer+"do}"] = fmt.Sprintf("%b", state.Do())
+	rep.replacements[headerReplacer+"bufsize}"] = strconv.Itoa(state.Size())
 
 	return rep
 }