To avoid below error messages like

  [23109:0] error: Could not open autotrust file for writing, /etc/dnssec/root-anchors.txt: Permission denied

set 'trust-anchor-file' to same value in 'auto-trust-anchor-file'.

diff -ur unbound-1.5.7.orig/doc/example.conf.in unbound-1.5.7/doc/example.conf.in
--- unbound-1.5.7.orig/doc/example.conf.in	2015-12-10 08:59:18.000000000 +0100
+++ unbound-1.5.7/doc/example.conf.in	2016-01-05 04:08:01.666760015 +0100
@@ -378,7 +378,7 @@
 	# with several entries, one file per entry.
 	# Zone file format, with DS and DNSKEY entries.
 	# Note this gets out of date, use auto-trust-anchor-file please.
-	# trust-anchor-file: ""
+	# trust-anchor-file: "@UNBOUND_ROOTKEY_FILE@"
 
 	# Trusted key for validation. DS or DNSKEY. specify the RR on a
 	# single line, surrounded by "". TTL is ignored. class is IN default.