Short answer: you can't. Ports below 1024 can be opened only by root. As per comment - well, you can, using CAP_NET_BIND_SERVICE, but that approach, applied to java bin will make any java program to be run with this setting, which is undesirable, if not a security risk.
The long answer: you can redirect connections on port 80 to some other port you can open as normal user.
Run as root:
# iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080
As loopback devices (like localhost) do not use the prerouting rules, if you need to use localhost, etc., add this rule as well (thanks @Francesco):
# iptables -t nat -I OUTPUT -p tcp -d 127.0.0.1 --dport 80 -j REDIRECT --to-ports 8080
NOTE: The above solution is not well suited for multi-user systems, as any user can open port 8080 (or any other high port you decide to use), thus intercepting the traffic. (Credits to CesarB).
EDIT: as per comment question - to delete the above rule:
# iptables -t nat --line-numbers -n -L
This will output something like:
Chain PREROUTING (policy ACCEPT)
num target prot opt source destination
1 REDIRECT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:8080 redir ports 8088
2 REDIRECT tcp -- 0.0.0.0/0 0.0.0.0/0 tcp dpt:80 redir ports 8080
The rule you are interested in is nr. 2, so to delete it:
# iptables -t nat -D PREROUTING 2
No, there isn't (I just checked 1 hour ago). You can comment the bind-address in my.cnf:
Note: « 1 hour ago » is now more than 10 years ago.
#skip-networking
#bind-address = 127.0.0.1
If you want only 2 IPs, you will then have to use a firewall.
For MySql version 8.0.13 and above, you can specify a list of comma-separated IP addresses.
bind-address = 10.0.0.1,10.0.1.1,10.0.2.1
Relevant MySql documentation.
Remember to restart your MySQL instance after changing the config file.
Best Answer
You can use UnionFS that seems to be actively developed or in case of the directories being separated hard drives, MHDDFS, but I see less references to that last one.
EDIT: There's an wikipedia article with more palatable info on UnionFS