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 We encountered a really strange behaviour with OpenVPN 2.3.6 on CentOS 5. A device of ours that is behind a SAT Link is having troubles getting through the buildup phase.
 For some strange reason it sends the packages not in the right order.
 Sometimes it works. But its pure chance and chance varies over the day.
 We can rule out firewall or connection issues, as this wrong sent packet order (from client to server) (...15, 17, 16, 17...) is already present in the logs on client side.
 If we switch to fiber connection everything is fine.
 So for us it seems that some strange circumstance with the SAT Link connection triggers a Bug in OpenVpn and it sends one packge early.
 
 So two questions:
 
 1) Why does the client send packet 17 two times and for the first time, too early?
 
 2) Why does the server not wait for the missing package and instead fails instantly with: "Authenticate/Decrypt packet error: packet HMAC authentication failed"?
 
 Client Side: Notice PID series: 14, 15, 17, 16, 17, 18
 
 ...
 Apr 10 17:39:08 client openvpn[1300]: TCPv4_CLIENT WRITE [154] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #41 ] [ 39 ] pid=2 DATA len=100
 Apr 10 17:39:08 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #42 ] [ ] pid=3 DATA len=100
 Apr 10 17:39:08 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #43 ] [ ] pid=4 DATA len=100
 Apr 10 17:39:08 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #44 ] [ ] pid=5 DATA len=100
 Apr 10 17:39:09 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #45 ] [ ] pid=6 DATA len=100
 Apr 10 17:39:10 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #46 ] [ ] pid=7 DATA len=100
 Apr 10 17:39:10 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #47 ] [ ] pid=8 DATA len=100
 Apr 10 17:39:10 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #48 ] [ ] pid=9 DATA len=100
 Apr 10 17:39:10 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #49 ] [ ] pid=10 DATA len=100
 Apr 10 17:39:11 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #50 ] [ ] pid=11 DATA len=100
 Apr 10 17:39:11 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #51 ] [ ] pid=12 DATA len=100
 Apr 10 17:39:11 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #52 ] [ ] pid=13 DATA len=100
 Apr 10 17:39:12 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #53 ] [ ] pid=14 DATA len=100
 Apr 10 17:39:12 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #54 ] [ ] pid=15 DATA len=100
 Apr 10 17:39:13 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #55 ] [ ] pid=17 DATA len=100
 Apr 10 17:39:13 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #56 ] [ ] pid=16 DATA len=100
 Apr 10 17:39:13 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #57 ] [ ] pid=17 DATA len=100
 Apr 10 17:39:13 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #58 ] [ ] pid=18 DATA len=100
 Apr 10 17:39:14 client openvpn[1300]: TCPv4_CLIENT WRITE [142] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_V1 kid=0 pid=[ #59 ] [ ] pid=19 DATA len=100
 Apr 10 17:39:20 client openvpn[1300]: TCPv4_CLIENT WRITE [42] to xxx.xxx.xxx.xxx:yyyy: P_CONTROL_HARD_RESET_CLIENT_V2 kid=0 pid=[ #1 ] [ ] pid=0 DATA len=0
 Apr 10 17:39:20 client openvpn[1300]: TCPv4_CLIENT READ [54] from xxx.xxx.xxx.xxx:yyyy: P_CONTROL_HARD_RESET_SERVER_V2 kid=0 pid=[ #1 ] [ 0 ] pid=0 DATA len=0
 
 
 Server side: Cancels on jump from PID=15 to 17
 
 ...
 Apr 10 19:39:09 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [154] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #41 ] [ 39 ] pid=2 DATA len=100
 Apr 10 19:39:09 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #42 ] [ ] pid=3 DATA len=100
 Apr 10 19:39:09 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #43 ] [ ] pid=4 DATA len=100
 Apr 10 19:39:09 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #44 ] [ ] pid=5 DATA len=100
 Apr 10 19:39:10 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #45 ] [ ] pid=6 DATA len=100
 Apr 10 19:39:10 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #46 ] [ ] pid=7 DATA len=100
 Apr 10 19:39:11 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #47 ] [ ] pid=8 DATA len=100
 Apr 10 19:39:11 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #48 ] [ ] pid=9 DATA len=100
 Apr 10 19:39:11 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #49 ] [ ] pid=10 DATA len=100
 Apr 10 19:39:12 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #50 ] [ ] pid=11 DATA len=100
 Apr 10 19:39:12 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #51 ] [ ] pid=12 DATA len=100
 Apr 10 19:39:12 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #52 ] [ ] pid=13 DATA len=100
 Apr 10 19:39:13 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #53 ] [ ] pid=14 DATA len=100
 Apr 10 19:39:13 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #54 ] [ ] pid=15 DATA len=100
 Apr 10 19:39:14 server openvpn[32482]: 192.168.19.253:48396 TCPv4_SERVER READ [142] from 192.168.19.253:48396: P_CONTROL_V1 kid=0 pid=[ #55 ] [ ] pid=17 DATA len=100
 Apr 10 19:39:14 server openvpn[32482]: 192.168.19.253:48396 Authenticate/Decrypt packet error: packet HMAC authentication failed
 
 
 Server Conf:
 
 port yyyy
 proto tcp
 dev tun
 ca ca.crt
 tls-auth ta.key 0
 cert ______________.com.crt
 key ______________.com.key
 dh dh2048.pem
 server 172.20.0.0 255.255.0.0
 client-config-dir clients
 ifconfig-pool-persist ipp_server.txt
 push "route 172.18.0.0 255.255.0.0"
 keepalive 10 60
 cipher AES-256-CBC
 comp-lzo
 max-clients 5000
 user nobody
 group nobody
 persist-key
 persist-tun
 status status.log
 verb 1
 
 
 Client COnf:
 
 client
 dev tun0
 proto tcp
 remote xxx.xxx.xxx.xxx yyyy
 resolv-retry infinite
 persist-key
 persist-tun
 ca ca.crt
 cert _________.crt
 key _________.key
 tls-auth ta.key 1
 cipher AES-256-CBC
 ns-cert-type server
 verb 8