> We have a server (real name substituted by mapserver.example.com) running nginx 1.18.0 on CentOS 7 with php-fpm listening on port 9001.

Does the fpm listen also on ipv6 interface?
Check: ss -ntlr | grep 9001
If you see [::]:9001

Since you have fastcgi_pass localhost:9001; I assume at some points the localhost resolves to an ipv6


> one with an IPv6 client and one with an IPv4 client):

All your logs show ipv6 clients only - 2a02:587:da23:700:[last_4_parts_removed], and ::ffff:193.140.[last_2_octets_removed] are both ipv6 ips (the second is so called ipv4-mapped ipv6 address)


Either make fpm listen to all interfaces or change the nginx config to: fastcgi_pass 127.0.0.1:9001;

Or even better is to use unix sockets so you can avoid the tcp stack between nginx and php-fpm at all.


rr


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