How to determine the external IP of a user working through a NAT pool?

Question

iptables

Good afternoon!
Please write how you can determine the external IP of the user. The scheme of work is as follows: there is an rp-pppoe server, users connect to it and they are given an internal private address, general rules are written for users
iptables -t nat -A POSTROUTING -o eth2 -j SNAT --to-source ххх.ххх.xxxx. 128-xxx.xxx.xxx.250 --persistent
There is a need to define the external IP address with which the user exits through NAT.
Tried the conntrack option during interface raising in the rule for ppp auth-up to start listening for a connection for the internal IP, but it doesn't work. Parsing /proc/net/ip_conntrack is not an option - it's just huge (because there are a lot of users).
I would be grateful for any advice.