This question was off-topic on Networkengineering and the recommendation was to post it here. So, here we go...
When connected to a network via a VPN tunnel (IPsec/L2TP), I can normally access a MySQL server host via SSH on port 22, but have critical connection problems when building a MySQL connection to the server on the default port 3306. I realised that only a very, very few number of packets get through on port 3306, and when they do get through I can even build a successful MySQL connection. After many tests, I could reduce the problem to a network issue. Plese find below the respective configuration and the latest tests I performed to isolate the issue. For the sake of completeness: When locally connected to the target network (e.g. on subnet 172.29.1.0/24) it works like a charm.
System configuration on target network:
- VPN Server: Ubiquiti UDM Pro | Protocol: L2TP | IP: 10.1.1.1 | Subnet of VPN clients: 10.4.4.0/24
- MySQL Server: Running on Photon4 on VMWare ESXi | IP: 10.1.1.82
System configuration on source network:
- Host: Windows 10 | Subnet: 172.27.0.0/16
- VPN Connection: Windows WAN Miniport Adapeter | Type: L2TP/IPsec | IP: 10.4.4.1
Network tests and results:
- Test of TCP connection to target host on port 22: always works
$ telnet 10.1.1.82 22
Trying 10.1.1.82...
Connected to 10.1.1.82.
Escape character is '^]'.
SSH-2.0-OpenSSH_8.8
- Test of TCP connection to target host on port 3306: almost never works
# 99% of the cases
$ telnet 10.1.1.82 3306
Trying 10.1.1.82...
telnet: Unable to connect to remote host: Resource temporarily unavailable
# 1% of the cases
$ telnet 10.1.1.82 3306
Trying 10.1.1.82...
Connected to 10.1.1.82.
Escape character is '^]'.
5.5.5-10.7.3-MariaDBOoaxW+m9'V]R`k{J=2#mysql_native_password
- It is even possible to build a successful MySQL connection after several attempts, but just after that no further packets get through:
$ while true; do mysql -h 10.1.1.82 -u <USER> -p<PASSWORD>; sleep 1; done
ERROR 2002 (HY000): Can't connect to MySQL server on '10.1.1.82' (115)
ERROR 2002 (HY000): Can't connect to MySQL server on '10.1.1.82' (115)
ERROR 2002 (HY000): Can't connect to MySQL server on '10.1.1.82' (115)
(... Meanwhile, at the Hall of Justice...)
ERROR 2002 (HY000): Can't connect to MySQL server on '10.1.1.82' (115)
ERROR 2002 (HY000): Can't connect to MySQL server on '10.1.1.82' (115)
ERROR 2002 (HY000): Can't connect to MySQL server on '10.1.1.82' (115)
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 6
Server version: 10.7.3-MariaDB Source distribution
Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]> select 1;
ERROR 2013 (HY000): Lost connection to MySQL server during query
MariaDB [(none)]> select 1;
ERROR 2006 (HY000): MySQL server has gone away
ERROR 2002 (HY000): Can't connect to MySQL server on '10.1.1.82' (115)
ERROR: Can't connect to the server
unknown [(none)]>
- I just realised that the tests above may be more likely to succeed after some period of rest or a reboot of the target host.
Now I need your help. What can be causing this issue and how can I further investigate it? It seems to occur independent of the local network infrastructure, since it happens even when connected to a mobile hotspot. If you need any further details, just let me know. Many thanks in advance!
