I’m running my containers with Podman in Rootless Mode on Alpine for about four months now. However, an annoying problem has haunted me ever since:

When a container was connected to more than one network, outgoing connections were not working correctly.

Consider a container connected to two bridge networks:

$ podman run --rm -it \
      --network net1 \
      --network net2 \
      alpine /bin/ash

Inside the container, the two networks are connected correctly:

# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000
    inet scope host lo
       valid_lft forever preferred_lft forever
2: eth1@if17: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state UP qlen 1000
    inet brd scope global eth1
4: eth0@if18: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state UP qlen 1000
    inet brd scope global eth0

However, pinging a host on the internet only works using one of the two network interfaces:

# ping -I eth0
PING ( 56 data bytes
64 bytes from seq=0 ttl=42 time=4.075 ms
# ping -I eth1
PING ( 56 data bytes
2 packets transmitted, 0 packets received, 100% packet loss

The solution

The solution is quite simple: You will need to set net.ipv4.conf.all.rp_filter to 2.

On my Alpine system, rp_filter was set to 1 by default. The setting controls the source path validation within the kernel’s IPv4 network stack. 1 means “strict”, whereas 2 means “loose”.

You can try the solution temporarily by running:

# sysctl -w net.ipv4.conf.all.rp_filter=2

To survive the next reboot, persist the setting by adding it to /etc/sysctl.conf:

# echo "net.ipv4.conf.all.rp_filter=2" >> /etc/sysctl.conf

For more information, you can take a look at this article.