After applying these changes, verify with:

sysctl -a | grep \\.rp_filter
ip route get $(ip addr show ens20.4 | grep inet | awk '{print $2}' | cut -d/ -f1)

For testing packet flow, use either tcpdump or a simple ping test:

tcpdump -i ens20.4 icmp
ping -I ens20.4 8.8.8.8

For more complex scenarios, consider using network namespaces to completely isolate the interface requiring relaxed rp_filter:

# Create new namespace
ip netns add special_vlan

# Move interface to namespace
ip link set ens20.4 netns special_vlan

# Configure routing independently
ip netns exec special_vlan ip route add default via 192.168.4.1

When disabling rp_filter on any interface:

• Implement complementary security measures like eBPF filters
• Consider using firewalls (iptables/nftables) to restrict allowed source IPs
• Monitor interface for suspicious traffic patterns

When working with VLAN interfaces on Linux routers, we often encounter a tricky scenario where we need to disable reverse path filtering (rp_filter) for specific interfaces while keeping it active system-wide. The challenge becomes particularly evident when dealing with VLAN sub-interfaces.

The common approach of modifying rp_filter for a single interface often doesn't work because of Linux's network stack behavior with VLAN devices. Here's what's happening under the hood:

# This typically won't work alone for VLAN interfaces
echo 0 > /proc/sys/net/ipv4/conf/ens20.4/rp_filter

The kernel requires additional configuration due to the relationship between VLAN interfaces and their parent devices.

To properly disable rp_filter for a specific VLAN interface while maintaining system security, follow this comprehensive approach:

# Step 1: Disable for the specific VLAN interface
echo 0 > /proc/sys/net/ipv4/conf/ens20.4/rp_filter

# Step 2: Configure the parent physical interface
echo 2 > /proc/sys/net/ipv4/conf/ens20/rp_filter  # Loose mode

# Step 3: Ensure all other interfaces maintain strict filtering
for i in $(ls /proc/sys/net/ipv4/conf/ | grep -v ens20.4); do
    echo 1 > /proc/sys/net/ipv4/conf/$i/rp_filter
done

# Step 4: Keep global all/rp_filter setting (optional but recommended)
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter

To ensure these settings survive reboots, add them to /etc/sysctl.conf or create a custom config file in /etc/sysctl.d/:

# /etc/sysctl.d/10-vlan-rpfilter.conf
net.ipv4.conf.ens20.4.rp_filter = 0
net.ipv4.conf.ens20.rp_filter = 2
net.ipv4.conf.all.rp_filter = 1

After applying changes, verify with:

sysctl -a | grep rp_filter

Test packet flow using tools like hping3 or scapy to confirm the desired behavior.

While relaxing rp_filter on specific interfaces, consider implementing additional security measures:

# Example: Enable martian log for the interface
echo 1 > /proc/sys/net/ipv4/conf/ens20.4/log_martians

# Implement eBPF filtering for advanced control
bpftool prog load filter_spoofed_packets.o /sys/fs/bpf/filter_spoofed
bpftool net attach xdp /sys/fs/bpf/filter_spoofed dev ens20.4

Remember that disabling rp_filter on any interface should be accompanied by proper firewall rules and network segmentation to prevent potential security issues.