CNF Networking Skill

CNF Networking Skill

This skill helps configure networking infrastructure for eBPF-based Cloud Native Network Functions.

What This Skill Does

Sets up and manages:

1. netkit devices - BPF-programmable network device pairs (kernel 6.6+)
2. tcx (Traffic Control eXpress) - Modern TC with eBPF (kernel 6.6+)
3. Network namespaces - Isolated network stacks for testing
4. Virtual interfaces - veth pairs, bridges
5. Routing configuration - Routes, rules, forwarding
6. eBPF program attachment - Attach to netkit, tcx, XDP

When to Use

• Setting up CNF testing environments
• Creating network topologies for packet processing
• Attaching eBPF programs to network interfaces
• Configuring traffic control with tcx
• Building network function chains
• Testing multi-namespace scenarios

Key Technologies

netkit - BPF-Programmable Network Device

netkit is a modern alternative to veth pairs that allows eBPF programs to be attached to both ends (primary and peer).

Advantages over veth:

• Native eBPF attachment points (primary/​peer)
• Better performance for BPF processing
• Cleaner API via link.AttachNetkit()
• Designed for cloud-native workloads

Creating netkit pairs:

# Create netkit pair
ip link add dev veth0 type netkit mode l3 peer name veth1 peer_mode l3

# Modes:
# - l2: Layer 2 mode (Ethernet)
# - l3: Layer 3 mode (no Ethernet header)

# Move peer to namespace
ip link set veth1 netns test-ns

# Configure addresses
ip addr add 10.0.0.1/​24 dev veth0
ip netns exec test-ns ip addr add 10.0.0.2/​24 dev veth1

# Bring up interfaces
ip link set veth0 up
ip netns exec test-ns ip link set veth1 up

Programmatic creation with Go:

For programmatic netkit creation using vishvananda/​netlink (with proper error handling, cleanup, and functional options), see EXAMPLES.md. The Go approach is preferred for production code and testing.

Attaching eBPF to netkit (Go):

import (
    "github.com/​cilium/​ebpf"
    "github.com/​cilium/​ebpf/​link"
)

// Get interface index
iface, err := net.InterfaceByName("veth0")
if err != nil {
    return err
}

// Attach to primary interface
primaryLink, err := link.AttachNetkit(link.NetkitOptions{
    Program:   prog,  // *ebpf.Program
    Interface: iface.Index,
    Attach:    ebpf.AttachNetkitPrimary,
})
if err != nil {
    return err
}
defer primaryLink.Close()

// Attach to peer (from primary side)
peerLink, err := link.AttachNetkit(link.NetkitOptions{
    Program:   prog,
    Interface: iface.Index,
    Attach:    ebpf.AttachNetkitPeer,
})
if err != nil {
    return err
}
defer peerLink.Close()

eBPF program for netkit:

#include <linux/​bpf.h>
#include <linux/​if_link.h>
#include <bpf/​bpf_helpers.h>

SEC("netkit/​primary")
int netkit_primary(struct __sk_buff *skb) {
    // Process packets on primary interface
    bpf_printk("Primary: packet length %d", skb->len);
    return NETKIT_PASS;  // or NETKIT_DROP, NETKIT_REDIRECT
}

SEC("netkit/​peer")
int netkit_peer(struct __sk_buff *skb) {
    // Process packets on peer interface
    bpf_printk("Peer: packet length %d", skb->len);
    return NETKIT_PASS;
}

char _license[] SEC("license") = "GPL";

tcx - Traffic Control eXpress (Kernel 6.6+)

tcx is the modern replacement for TC (Traffic Control) with native eBPF support.

Advantages over legacy TC:

• No qdisc required (cleaner setup)
• Better performance
• Simpler API via link.AttachTCX()
• Multi-program attachment with priorities
• No dependencies on tc-bpf tooling

Attaching eBPF to tcx (Go):

import (
    "github.com/​cilium/​ebpf"
    "github.com/​cilium/​ebpf/​link"
)

// Get interface index
iface, err := net.InterfaceByName("eth0")
if err != nil {
    return err
}

// Attach to ingress
ingressLink, err := link.AttachTCX(link.TCXOptions{
    Program:   prog,
    Attach:    ebpf.AttachTCXIngress,
    Interface: iface.Index,
})
if err != nil {
    return err
}
defer ingressLink.Close()

// Attach to egress
egressLink, err := link.AttachTCX(link.TCXOptions{
    Program:   prog,
    Attach:    ebpf.AttachTCXEgress,
    Interface: iface.Index,
})
if err != nil {
    return err
}
defer egressLink.Close()

eBPF program for tcx:

#include <linux/​bpf.h>
#include <linux/​pkt_cls.h>
#include <bpf/​bpf_helpers.h>

// Can use either "tc" or "tcx/​ingress" / "tcx/​egress"
SEC("tcx/​ingress")
int tcx_ingress(struct __sk_buff *skb) {
    bpf_printk("Ingress: packet from interface %d", skb->ifindex);
    return TC_ACT_OK;  // or TC_ACT_SHOT, TC_ACT_REDIRECT, TC_ACT_PIPE
}

SEC("tcx/​egress")
int tcx_egress(struct __sk_buff *skb) {
    bpf_printk("Egress: packet to interface %d", skb->ifindex);
    return TC_ACT_OK;
}

char _license[] SEC("license") = "GPL";

Legacy TC (for kernels < 6.6):

# Add clsact qdisc (required for legacy TC)
sudo tc qdisc add dev eth0 clsact

# Attach BPF program
sudo tc filter add dev eth0 ingress bpf direct-action obj prog.o sec tc

# Remove
sudo tc qdisc del dev eth0 clsact

Legacy TC in Go:

import (
    "github.com/​cilium/​ebpf/​link"
)

// For kernels < 6.6, use AttachTC instead of AttachTCX
tcLink, err := link.AttachTC(link.TCOptions{
    Program:   prog,
    Attach:    ebpf.AttachTCIngress,  // or ebpf.AttachTCEgress
    Interface: iface.Index,
})

Network Namespace Management

Create namespace:

# Create namespace
ip netns add test-ns

# Execute command in namespace
ip netns exec test-ns ip link show

# Delete namespace
ip netns del test-ns

Go code for namespace operations:

import (
    "runtime"
    "github.com/​vishvananda/​netns"
)

// Get current namespace
origns, err := netns.Get()
if err != nil {
    return err
}
defer origns.Close()

// Get target namespace
ns, err := netns.GetFromName("test-ns")
if err != nil {
    return err
}
defer ns.Close()

// Switch to namespace
runtime.LockOSThread()
defer runtime.UnlockOSThread()

if err := netns.Set(ns); err != nil {
    return err
}

// Do work in namespace...

// Restore original namespace
if err := netns.Set(origns); err != nil {
    return err
}

Virtual Interface Management

veth pairs (traditional):

# Create veth pair
ip link add veth0 type veth peer name veth1

# Move peer to namespace
ip link set veth1 netns test-ns

# Configure
ip addr add 10.0.0.1/​24 dev veth0
ip link set veth0 up

ip netns exec test-ns ip addr add 10.0.0.2/​24 dev veth1
ip netns exec test-ns ip link set veth1 up

Bridge setup:

# Create bridge
ip link add br0 type bridge

# Add interfaces to bridge
ip link set veth0 master br0

# Configure bridge
ip addr add 192.168.1.1/​24 dev br0
ip link set br0 up

Complete CNF Testing Setup Example

Scenario: CNF between two namespaces

#!/​bin/​bash

# Create namespaces
ip netns add ns1
ip netns add ns2

# Create netkit pair (ns1 <-> host)
ip link add dev netkit0 type netkit mode l3 peer name netkit0-peer peer_mode l3
ip link set netkit0-peer netns ns1

# Create netkit pair (host <-> ns2)
ip link add dev netkit1 type netkit mode l3 peer name netkit1-peer peer_mode l3
ip link set netkit1-peer netns ns2

# Configure host interfaces
ip addr add 10.0.1.1/​24 dev netkit0
ip addr add 10.0.2.1/​24 dev netkit1
ip link set netkit0 up
ip link set netkit1 up

# Configure ns1
ip netns exec ns1 ip addr add 10.0.1.2/​24 dev netkit0-peer
ip netns exec ns1 ip link set netkit0-peer up
ip netns exec ns1 ip link set lo up
ip netns exec ns1 ip route add default via 10.0.1.1

# Configure ns2
ip netns exec ns2 ip addr add 10.0.2.2/​24 dev netkit1-peer
ip netns exec ns2 ip link set netkit1-peer up
ip netns exec ns2 ip link set lo up
ip netns exec ns2 ip route add default via 10.0.2.1

# Enable forwarding on host
sysctl -w net.ipv4.ip_forward=1

# Now attach eBPF programs to netkit0 and netkit1
# Programs can inspect/​modify packets flowing between ns1 and ns2

Corresponding Go application:

package main

import (
    "log"
    "net"
    "os"
    "os/​signal"
    "syscall"

    "github.com/​cilium/​ebpf"
    "github.com/​cilium/​ebpf/​link"
)

func main() {
    // Load eBPF objects
    spec, err := LoadMyProgram()
    if err != nil {
        log.Fatalf("loading spec: %v", err)
    }

    objs := &MyProgramObjects{}
    if err := spec.LoadAndAssign(objs, nil); err != nil {
        log.Fatalf("loading objects: %v", err)
    }
    defer objs.Close()

    // Attach to netkit0 (primary and peer)
    iface0, _ := net.InterfaceByName("netkit0")

    link0Primary, err := link.AttachNetkit(link.NetkitOptions{
        Program:   objs.NetkitPrimary,
        Interface: iface0.Index,
        Attach:    ebpf.AttachNetkitPrimary,
    })
    if err != nil {
        log.Fatalf("attaching to netkit0 primary: %v", err)
    }
    defer link0Primary.Close()

    link0Peer, err := link.AttachNetkit(link.NetkitOptions{
        Program:   objs.NetkitPeer,
        Interface: iface0.Index,
        Attach:    ebpf.AttachNetkitPeer,
    })
    if err != nil {
        log.Fatalf("attaching to netkit0 peer: %v", err)
    }
    defer link0Peer.Close()

    // Attach to netkit1
    iface1, _ := net.InterfaceByName("netkit1")

    link1Primary, err := link.AttachNetkit(link.NetkitOptions{
        Program:   objs.NetkitPrimary,
        Interface: iface1.Index,
        Attach:    ebpf.AttachNetkitPrimary,
    })
    if err != nil {
        log.Fatalf("attaching to netkit1 primary: %v", err)
    }
    defer link1Primary.Close()

    log.Println("CNF is running, processing packets between ns1 and ns2...")

    // Wait for signal
    sig := make(chan os.Signal, 1)
    signal.Notify(sig, syscall.SIGINT, syscall.SIGTERM)
    <-sig

    log.Println("Shutting down...")
}

Routing Configuration

Add routes:

# Add route to specific network
ip route add 10.1.0.0/​24 via 10.0.0.2 dev veth0

# Add default route
ip route add default via 10.0.0.1

# In namespace
ip netns exec test-ns ip route add default via 10.0.0.1

Enable IP forwarding:

# Temporary
sysctl -w net.ipv4.ip_forward=1
sysctl -w net.ipv6.conf.all.forwarding=1

# Permanent (add to /​etc/​sysctl.conf)
echo "net.ipv4.ip_forward=1" >> /​etc/​sysctl.conf
sysctl -p

Debugging and Monitoring

View netkit devices:

ip link show type netkit

View tcx attachments:

# Using bpftool (kernel 6.6+)
bpftool net show dev eth0

# Legacy TC
tc filter show dev eth0 ingress

Monitor packets:

# tcpdump on interface
tcpdump -i netkit0 -n

# In namespace
ip netns exec test-ns tcpdump -i netkit0-peer -n

# View eBPF trace logs
tc exec bpf dbg  # Legacy TC
# or
bpftool prog tracelog  # Modern

Test connectivity:

# Ping between namespaces
ip netns exec ns1 ping 10.0.2.2

# netcat for TCP/​UDP testing
ip netns exec ns2 nc -l 8080
ip netns exec ns1 nc 10.0.2.2 8080

Cleanup

Remove netkit devices:

ip link del dev netkit0
ip link del dev netkit1

Remove namespaces:

ip netns del ns1
ip netns del ns2

Detach tcx programs (automatic on link.Close() in Go):

# Manual detach with bpftool
bpftool net detach tcx dev eth0 ingress

Best Practices

1. Use netkit over veth for eBPF-programmable paths
2. Use tcx over legacy TC on kernel 6.6+
3. Always defer link.Close() to ensure cleanup
4. Enable IP forwarding when routing between interfaces
5. Use network namespaces for isolation in testing
6. Monitor with bpftool for debugging attachments
7. Check kernel version before using netkit/​tcx features
8. Use L3 mode for netkit when Ethernet headers not needed

Kernel Version Requirements

• netkit: Linux 6.6+
• tcx: Linux 6.6+
• Legacy TC with eBPF: Linux 4.1+
• XDP: Linux 4.8+

Common Patterns

Pattern 1: Simple netkit CNF

[ns1] <--netkit--> [CNF with eBPF] <--netkit--> [ns2]

Pattern 2: Service mesh sidecar

[Pod netns] <--netkit--> [Sidecar CNF] <--tcx--> [Host]

Pattern 3: Chain of CNFs

[Source] <--netkit--> [CNF1] <--netkit--> [CNF2] <--netkit--> [Dest]

Pattern 4: Traffic mirroring

                    ┌──> [Monitor CNF]
                    │
[Traffic] <--tcx--> [Mirror CNF]
                    │
                    └──> [Forward]

Source-Based Policy Routing with eBPF

eBPF enables sophisticated routing decisions that go beyond traditional routing tables, solving problems like asymmetric routing and enabling dynamic network policies.

The Asymmetric Routing Problem

Common Scenario:

Client (10.0.2.2)
  ↓
CNF Router (10.0.2.1 / 10.0.1.1)
  ↓
Server (10.0.1.2)
  - Has default gateway: 192.168.0.1
  - Has virtual IP: 192.168.100.5 on loopback

The Issue:

1. Client sends packet to server's virtual IP (192.168.100.5)
2. CNF forwards it correctly to server ✅
3. Server receives packet, but routing table says "use default gateway"
4. Reply goes to 192.168.0.1 instead of back through CNF ❌
5. Asymmetric routing breaks the connection

eBPF Solution

Instead of modifying routing tables, attach an eBPF program to the server's egress that redirects based on source IP address:

Key Components:

1. Map-based policy: Source IP → {interface, next_hop}
2. bpf_redirect_neigh: Neighbor-aware packet redirection
3. Egress attachment: Process outgoing packets

Simplified eBPF Program:

// Look up source IP in policy map
struct route_policy *policy = bpf_map_lookup_elem(&policy_map, &src_ip);

if (policy) {
    // Redirect via specified interface/​next-hop
    struct bpf_redir_neigh nh = {
        .nh_family = AF_INET,
        .ipv4_nh = policy->next_hop,
    };
    return bpf_redirect_neigh(policy->interface_id, &nh, sizeof(nh), 0);
}

return TC_ACT_OK;  // No policy, use normal routing

Integration with netkit and tcx

Attach to netkit device (egress side):

// Create netkit pair
ip link add dev netkit0 type netkit mode l3 peer name netkit0-peer

// Attach eBPF router to primary (server-facing) interface
iface, _ := net.InterfaceByName("netkit0")
link, err := link.AttachNetkit(link.NetkitOptions{
    Program:   objs.PolicyRouter,
    Interface: iface.Index,
    Attach:    ebpf.AttachNetkitPrimary,
})

Attach using tcx (kernel 6.6+):

// Attach to egress for source-based routing
iface, _ := net.InterfaceByName("eth0")
link, err := link.AttachTCX(link.TCXOptions{
    Program:   objs.PolicyRouter,
    Attach:    ebpf.AttachTCXEgress,  // Important: egress!
    Interface: iface.Index,
})

Use Cases for CNFs

1. Virtual IP / Anycast Handling

Server has virtual IP on loopback, needs special routing for replies:

// Policy: Packets FROM virtual IP → route via specific interface
addPolicy(map, RoutingPolicy{
    SourceIP:  net.ParseIP("192.168.100.5"),
    Interface: "eth1",
    NextHop:   net.ParseIP("10.0.1.1"),
})

2. Multi-Homing / Multiple ISPs

Route different customers through different ISPs:

// Customer A → ISP 1
addPolicy(map, RoutingPolicy{
    SourceIP:  net.ParseIP("10.10.1.0"),  // Customer A subnet
    Interface: "isp1",
    NextHop:   net.ParseIP("203.0.113.1"),
})

// Customer B → ISP 2
addPolicy(map, RoutingPolicy{
    SourceIP:  net.ParseIP("10.10.2.0"),  // Customer B subnet
    Interface: "isp2",
    NextHop:   net.ParseIP("198.51.100.1"),
})

3. Service Mesh Sidecar Routing

Pod with multiple interfaces, route based on source IP:

// Service traffic → service network
addPolicy(map, RoutingPolicy{
    SourceIP:  net.ParseIP("10.96.0.10"),  // Service IP
    Interface: "net1",
    NextHop:   net.ParseIP("10.96.0.1"),
})

// Management traffic → management network
addPolicy(map, RoutingPolicy{
    SourceIP:  net.ParseIP("192.168.1.10"),  // Management IP
    Interface: "net0",
    NextHop:   net.ParseIP("192.168.1.1"),
})

4. VPN / Tunnel Endpoint

Traffic from tunnel IPs → tunnel interface:

addPolicy(map, RoutingPolicy{
    SourceIP:  net.ParseIP("172.16.0.0"),  // VPN subnet
    Interface: "tun0",
    NextHop:   net.ParseIP("172.16.0.1"),
})

Complete Example: CNF Router with Source-Based Routing

Network Setup:

# Create netkit pair between namespaces
ip netns add client
ip netns add server

ip link add dev veth0 type netkit mode l3 peer name veth0-peer peer_mode l3
ip link set veth0-peer netns client
ip link set veth1-peer netns server

# Configure server
ip netns exec server ip addr add 192.168.100.5/​32 dev lo
ip netns exec server ip addr add 10.0.1.2/​24 dev veth1-peer
ip netns exec server ip link set veth1-peer up

# Configure host (CNF router)
ip addr add 10.0.1.1/​24 dev veth0
ip addr add 10.0.2.1/​24 dev veth1
ip link set veth0 up
ip link set veth1 up

sysctl -w net.ipv4.ip_forward=1

CNF Application:

package main

import (
    "log"
    "net"

    "github.com/​cilium/​ebpf/​link"
)

func main() {
    // Load eBPF program with routing policies
    spec, _ := LoadPolicyRouter()
    objs := &PolicyRouterObjects{}
    spec.LoadAndAssign(objs, nil)
    defer objs.Close()

    // Attach to server-facing interface (egress)
    iface, _ := net.InterfaceByName("veth0")
    l, err := link.AttachTCX(link.TCXOptions{
        Program:   objs.PolicyRouter,
        Attach:    ebpf.AttachTCXEgress,
        Interface: iface.Index,
    })
    if err != nil {
        log.Fatal(err)
    }
    defer l.Close()

    // Configure policy: virtual IP traffic via veth1
    addPolicy(objs.PolicyRoutesV4, RoutingPolicy{
        SourceIP:  net.ParseIP("192.168.100.5"),
        Interface: "veth1",
        NextHop:   net.ParseIP("10.0.2.1"),
    })

    log.Println("CNF router running with source-based routing...")

    // Wait for signal...
}

Why This Matters for Cloud Native

Traditional Solutions:

• Require modifying routing tables on every host
• Need root access to change routing
• Complex iproute2 rules and policy routing
• Difficult to manage at scale

eBPF CNF Solution:

• ✅ Per-packet programmable routing
• ✅ No system routing table changes
• ✅ Dynamic policy updates (just update map)
• ✅ Works in containers without privilege escalation
• ✅ Can override normal routing completely
• ✅ Integrates with netkit/​tcx for modern networking

Advanced Patterns

Load Balancing:

// Round-robin or hash-based backend selection
__u32 backend_idx = bpf_get_prandom_u32() % num_backends;
struct backend *backend = bpf_map_lookup_elem(&backends, &backend_idx);

Conditional Routing:

// Route based on source IP + destination port
struct policy_key {
    __u32 src_ip;
    __u16 dst_port;
    __u8 protocol;
};

Policy Chaining:

// Try source-based, fall back to destination-based, then default
policy = src_policy ?? dst_policy ?? default_policy;

Testing Source-Based Routing

Setup Test:

# Terminal 1: Start CNF router
go run .

# Terminal 2: Test from client namespace
ip netns exec client nc 192.168.100.5 8080

Verify with tcpdump:

# Watch traffic on CNF interfaces
tcpdump -i veth0 -n icmp or tcp port 8080
tcpdump -i veth1 -n icmp or tcp port 8080

Check eBPF stats:

bpftool map dump name policy_routes_v4
bpftool map dump name stats

Best Practices

1. Attach to egress for source-based routing (not ingress)
2. Use network byte order for IP addresses in maps
3. Validate policies before inserting (check interface exists)
4. Monitor statistics to verify policies are being applied
5. Use netkit or tcx for better eBPF integration
6. Test both directions (client→server and server→client)
7. Handle policy lookup failures gracefully (fall back to normal routing)
8. Log policy changes for debugging
9. Consider IPv6 if running dual-stack
10. Document routing policies in configuration management

Further Reading

• See ebpf-packet-redirect skill for detailed redirection patterns
• See ebpf-map-handler skill for policy map management
• See ebpf-test-harness skill for multi-container testing scenarios