OpenVPN: tun tap invalid argument (code=22)

Fri, 11 May 2018 12:29:10 +0000

After upgrading my OpenVPN server to CentOS 7.5 I had trouble connecting to it.
Specifically, I had two different issues:
** the laptop, which is running Fedora 28, was able to connect just fine but DNS resolution was broken.
** OpenVPN for Android was also connecting just fine but reporting a weird error: OpenVPN: tun tap invalid argument (code=22).
The first one was caused by me because after the CentOS upgrade procedure was completed I also run yum autoremove which deleted dnsmasq; the solution was fairly simple, reinstall and reconfigure dnsmasq.
For the second issue the solution was to enable comp-lzo and voilà, everything started working again.

OpenVPN server and CentOS

Sun, 19 May 2013 17:52:52 +0000

OpenVPN is the de facto standard VPN free open source software; it is widely used, tested, well documented and also included in the CentOS repos (EPEL).

.:. Server side configuration

yum install openvpn easy-rsa dnsmasq

When yum is done installing the required packages, copy the sample config file.

cp /usr/share/doc/openvpn-*/sample/sample-config-files/server.conf /etc/openvpn

Uncomment/edit the following lines in /etc/openvpn/server.conf:

port 1194
proto udp
dev tun
ca ca.crt
cert server.crt
key server.key
dh dh4096.pem
server 10.8.0.0 255.255.255.0
ifconfig-pool-persist ipp.txt
push "redirect-gateway def1"
push "dhcp-option DNS 10.8.0.1"
keepalive 10 120
tls-auth ta.key 0 # This file is secret
key-direction 0
tls-version-min 1.2
tls-cipher TLS-DHE-RSA-WITH-AES-256-CBC-SHA256
auth SHA512
cipher AES-256-GCM
#comp-lzo # Disable LZO compression
persist-key
persist-tun
status openvpn-status.log
;log openvpn.log # disable log, optional
;log-append openvpn.log # disable log, optional
user nobody
group nobody

Now, create two folders easy-rsa/keys in /etc/openvpn and copy some files into them:

Openvpn on nagg.eu

OpenVPN: tun tap invalid argument (code=22)

OpenVPN server and CentOS

.:. Server side configuration