man iptables:
owner
This module attempts to match various characteristics of the packet creator, for locally generated packets. This
match is only valid in the OUTPUT and POSTROUTING chains. Forwarded packets do not have any socket associated with
them. Packets from kernel threads do have a socket, but usually no owner.
[!] --uid-owner username
[!] --uid-owner userid[-userid]
Matches if the packet socket's file structure (if it has one) is owned by the given user. You may also
specify a numerical UID, or an UID range.
[!] --gid-owner groupname
[!] --gid-owner groupid[-groupid]
Matches if the packet socket's file structure is owned by the given group. You may also specify a numeri-
cal GID, or a GID range.
[!] --socket-exists
Matches if the packet is associated with a socket.
