Reply on same network interface (UDP) - linux

Question

is it possible to reply all incoming packets/request on same network interface?
This is my setting: I have a headless Raspberry Pi (raspbian) with two network interfaces (eth0 and eth1). The fist interface (eth0) uses a public IP address, which is static. This interface is intended to provide access to the Pi (time- and web server, SSH) via the Internet. The second interface (eth1) uses the Raspberry Pi for general Internet connection (perform updates, sync own time or whatever) and uses a dynamic IP via DHCP. A general Internet connectivity over eth0 is not possible, so I have to use eth1 on the Pi.
My problem is that Internet (on the Pi) and the Internet access to the Pi are not working correctly.
first configuration (/etc/dhcpcd.conf):
interface eth0
static ip_address=141.41.241.68/28
static routers=141.41.241.65 192.168.0.1
after reboot 'ifconfig' shows the correct IP settings:
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 141.41.241.68 netmask 255.255.255.240 broadcast 141.41.241.79
...
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.0.101 netmask 255.255.255.0 broadcast 192.168.0.255
...
result:
Internet on the Raspberry Pi (eth1): doesn't work
access to Raspberry Pi over Internet (via eth0):
SSH (TCP): works fine
Apache web server (TCP): works fine
NTP time server (UDP): works fine
Now I tried to change the metric of the interfaces, hoping that the change of prioritization is successful
second configuration (/etc/dhcpcd.conf):
interface eth1
metric 200
interface eth0
static ip_address=141.41.241.68/28
static routers=141.41.241.65 192.168.0.1
metric 201
result:
Internet on the Raspberry Pi (eth1): works fine
access to Raspberry Pi over Internet (via eth0):
SSH (TCP): doesn't work
Apache web server (TCP): doesn't work
NTP time server (UDP): doesn't work
Ok. I think all traffic going out to eth1 at default. With the tool 'iptraf-ng' I was able to see the problem:
TCP Connections (Source Host:Port) Iface
--------------------------------------------------
80.187.108.126:53024 eth0
141.41.241.68:80 eth0
141.41.241.68:80 eth1
80.187.108.126:53024 eth1
80.187.108.126:53025 eth0
141.41.241.68:80 eth0
141.41.241.68:80 eth1
80.187.108.126:53025 eth1
UDP Connections
--------------------------------------------------
UDP (76 bytes) from 80.187.108.126:28599 to 141.41.241.68:123 on eth0
UDP (76 bytes) from 192.168.0.101:123 to 80.187.108.126:28599 on eth1
We see:
On TCP: some connections going out to the wrong interface (eth1).
On UDP: The request from 80.187.108.126 came over eth0 and the response was sent over eth1.
Next, I defined the routing table to reply incoming packets on same network interface...
echo 100 public >> /etc/iproute2/rt_tables
ip rule add from 141.41.241.68/32 table public
ip route add default via 141.41.241.65 dev eth0 table public
result:
Internet on the Raspberry Pi (eth1): works fine
access to Raspberry Pi over Internet (via eth0):
SSH (TCP): works fine
Apache web server (TCP): works fine
NTP time server (UDP): doesn't work
and 'iptraf-ng' shows:
TCP Connections (Source Host:Port) Iface
--------------------------------------------------
141.41.241.68:80 eth0
80.187.108.126:52083 eth0
141.41.241.68:80 eth0
80.187.108.126:52084 eth0
141.41.241.68:80 eth0
80.187.108.126:52085 eth0
141.41.241.68:80 eth0
80.187.108.126:52086 eth0
141.41.241.68:80 eth0
80.187.108.126:52087 eth0
UDP Connections
--------------------------------------------------
UDP (76 bytes) from 80.187.108.126:28599 to 141.41.241.68:123 on eth0
UDP (76 bytes) from 192.168.0.101:123 to 80.187.108.126:28599 on eth1
We see:
On TCP: now it works correctly
On UDP: same problem :(
What can I do to send UDP responses over the correct interface (eth0)? I have no idea why TCP works fine but UDP fails :(
Its very frustrating and I have no more ideas.
I hope someone can help.
best regards,
SBond