Use separate gateway based on iptables owner (UID)

route_based_on_uid.sh

#!/bin/bash

# (In this case) default gateway is 192.168.1.3, alternate is 192.168.1.1.
# Usually default is either .1 or .254 - pay attention.
# If anything involving CDNs is involved, use https://github.com/hadess/resolvconf-override .

# Add new routing table, which uses alternate gateway by default
echo 100 altgateway >> /etc/iproute2/rt_tables

# Mark traffic sourced from UID as needing to go via other gateway (4 is arbitrary)
iptables -t mangle -A OUTPUT -m owner --uid-owner alt_gateway_user -j MARK --set-mark 4

# Use altgateway table for marked traffic (4 is arbitrary)
ip rule add fwmark 4 table altgateway

# altgateway table routes
ip -4 r a default via 192.168.1.1 dev br0 table altgateway
ip -4 r a 192.168.1.0/24 dev br0 table altgateway

# https://tools.ietf.org/html/rfc1166
whoami
traceroute -n -m 1 192.0.2.0
echo "alt_gateway_user"
su - alt_gateway_user -s "/bin/bash" -c "traceroute -n -m 1 192.0.2.0"