Hello all,

According to the document, I built memcached as a back end, so I built it from the source, but the following error message is output and can not be used.  

https://knot-resolver.readthedocs.io/en/stable/build.html#building-from-sources

$ sudo kresd
[system] interactive mode
> modules.load 'memcached'
error: error loading module 'memcached' from file '/usr/local/lib/kdns_modules/memcached.so':
       /usr/local/lib/kdns_modules/memcached.so: undefined symbol: luaopen_memcached
error: No such file or directory
> 

$ ll /usr/local/lib/kdns_modules/
total 404
drwxr-xr-x 4 root root  4096 Dec 13 21:59 ./
drwxr-xr-x 7 root root  4096 Dec 13 21:59 ../
-rwxr-xr-x 1 root root 35904 Dec 13 21:59 ahocorasick.so*
-rwxr-xr-x 1 root root 38848 Dec 13 21:59 cookies.so*
drwxr-xr-x 2 root root  4096 Dec 13 21:59 daf/
-rw-r--r-- 1 root root  8664 Dec 13 21:59 daf.lua
-rw-r--r-- 1 root root  1098 Dec 13 21:59 detect_time_jump.lua
-rw-r--r-- 1 root root  2329 Dec 13 21:59 detect_time_skew.lua
-rw-r--r-- 1 root root  2218 Dec 13 21:59 dns64.lua
-rwxr-xr-x 1 root root 45016 Dec 13 21:59 dnstap.so*
-rw-r--r-- 1 root root  1204 Dec 13 21:59 etcd.lua
-rw-r--r-- 1 root root  3199 Dec 13 21:59 graphite.lua
-rwxr-xr-x 1 root root 44376 Dec 13 21:59 hints.so*
drwxr-xr-x 2 root root  4096 Dec 13 21:59 http/
-rw-r--r-- 1 root root 11077 Dec 13 21:59 http.lua
-rw-r--r-- 1 root root  1263 Dec  2 00:16 ketcd.lua
-rw-r--r-- 1 root root  7967 Dec 13 21:59 kres-gen.lua
-rw-r--r-- 1 root root 10178 Dec 13 21:59 kres.lua
-rwxr-xr-x 1 root root 14216 Dec 13 21:59 memcached.so*
-rw-r--r-- 1 root root 15867 Dec 13 21:59 policy.lua
-rw-r--r-- 1 root root  5346 Dec 13 21:59 predict.lua
-rw-r--r-- 1 root root  4267 Dec 13 21:59 priming.lua
-rw-r--r-- 1 root root  4313 Dec 13 21:59 prometheus.lua
-rwxr-xr-x 1 root root 18336 Dec 13 21:59 redis.so*
-rw-r--r-- 1 root root  3639 Dec 13 21:59 renumber.lua
-rwxr-xr-x 1 root root 34048 Dec 13 21:59 stats.so*
-rw-r--r-- 1 root root  1818 Dec 13 21:59 ta_signal_query.lua
-rw-r--r-- 1 root root 15314 Dec 13 21:59 trust_anchors.lua
-rw-r--r-- 1 root root  2068 Dec 13 21:59 version.lua
-rw-r--r-- 1 root root  2656 Dec 13 21:59 view.lua
-rw-r--r-- 1 root root  1658 Dec 13 21:59 workarounds.lua
-rw-r--r-- 1 root root  5307 Dec 13 21:59 zonefile.lua

Is it something you need to do with a description other than documentation? 
The source file is working with knot-resolver-1.5.1.  

env:

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 16.04.3 LTS
Release:        16.04
Codename:       xenial

$ sudo make info PREFIX="/usr/local" ETCDIR="/etc/knot-resolver/" CFLAGS="-fstack-protector"
fatal: Not a git repository (or any of the parent directories): .git
Target:     Knot DNS Resolver 1.5.1-POSIX
Compiler:   cc -fstack-protector -std=c99 -D_GNU_SOURCE -Wno-unused -Wtype-limits -Wformat -Wformat-security -Wall -I/home/kometch/knot-resolver-1.5.1 -I/home/kometch/knot-resolver-1.5.1/lib/generic -I/home/kometch/knot-resolver-1.5.1/contrib -I/home/kometch/knot-resolver-1.5.1/contrib/lmdb -DPACKAGE_VERSION="\"1.5.1\"" -DPREFIX="\"/usr/local\"" -DMODULEDIR="\"/usr/local/lib/kdns_modules\"" -O2 -D_FORTIFY_SOURCE=2 -I/usr/include/p11-kit-1    -I/usr/include/luajit-2.1 -I/usr/include/p11-kit-1  -Icontrib/ccan/compiler -Icontrib/ccan/ilog -Icontrib/ccan/isaac -Icontrib/ccan/json -Icontrib/ccan/asprintf -Icontrib/murmurhash3 -DENABLE_COOKIES

Variables
---------
HARDENING:  yes
BUILDMODE:  dynamic
PREFIX:     /usr/local
PREFIX:     /usr/local
DESTDIR:    
BINDIR:     /usr/local/bin
SBINDIR:    /usr/local/sbin
LIBDIR:     /usr/local/lib
ETCDIR:     /etc/knot-resolver/
INCLUDEDIR: /usr/local/include
MODULEDIR:  /usr/local/lib/kdns_modules

Core Dependencies
------------
[yes] libknot (lib)
[yes] embedded lmdb (lib)
[yes] luajit (daemon)
[yes] libuv (daemon)
[yes] libgnutls (daemon)

Optional
--------
[no] doxygen (doc)
[no] sphinx-build (doc)
[no] python-breathe (doc)
[yes] go (modules/go, Go buildmode=c-shared support)
[yes] libmemcached (modules/memcached)
[yes] hiredis (modules/redis)
[yes] cmocka (tests/unit)
[yes] systemd (daemon)
[yes] nettle (modules/cookies)
[yes] Lua socket ltn12 (trust anchor bootstrapping)
[yes] Lua ssl.https (trust anchor bootstrapping)
[yes] libedit (client)
[yes] libfstrm (modules/dnstap)
[yes] libprotobuf-c (modules/dnstap)
[yes] proto-c (modules/dnstap)
[yes] lcov (code coverage)
[yes] luacov (code coverage)

make: Nothing to be done for 'info'.

ii  libmemcached-dev               1.0.18-4.1                                 arm64        C and C++ client library to the memcached server (development files)
ii  libmemcached-tools             1.0.18-4.1                                 arm64        Commandline tools for talking to memcached via libmemcached
ii  libmemcached11:arm64           1.0.18-4.1                                 arm64        C and C++ client library to the memcached server
ii  libmemcachedutil2:arm64        1.0.18-4.1                                 arm64        library implementing connection pooling for libmemcached

$ sudo memcached -V
memcached 1.5.3

Best regards.