Dummy Interfaces in VyOS

Dummy interfaces in VyOS are virtual interfaces that are always in the “up” state and are not tied to any physical hardware. They are used for a variety of technical tasks, including testing, routing, and serving as stable endpoints.

Overview

Dummy interfaces are used for:

Testing: Simulating interfaces without physical hardware
Stable endpoint: An always-available IP for routing protocols
Router ID: A stable identifier for OSPF/BGP (an alternative to loopback)
Blackhole routing: Dropping traffic through a dummy
Source address: A stable source IP for outbound connections
Development: Developing and debugging network configurations

Dummy vs Loopback

Characteristic	Dummy	Loopback
State	Always up	Always up
Use case	Testing, routing, blackhole	Router ID, management
Count	Multiple (dum0, dum1, …)	One (lo)
Standardization	Linux-specific	Universal (RFC 1122)
Recommendation	For temporary tasks	For production router ID

When to use Dummy:

Testing configurations
Temporary endpoints
Blackhole routing
Multiple virtual interfaces

When to use Loopback:

Router ID for OSPF/BGP
Management address
Production stable endpoint

Basic Configuration

Creating a Dummy Interface

# Create a dummy interface with an IP address
set interfaces dummy dum0 address 192.168.255.1/32

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Multiple Dummy Interfaces

# First dummy
set interfaces dummy dum0 address 192.168.255.1/32

# Second dummy
set interfaces dummy dum1 address 192.168.255.2/32

# Third dummy with a description
set interfaces dummy dum2 address 192.168.255.3/32
set interfaces dummy dum2 description 'Test interface for BGP'

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Multiple IP Addresses on a Dummy

# Dummy with several addresses
set interfaces dummy dum0 address 192.168.255.1/32
set interfaces dummy dum0 address 10.255.255.1/32
set interfaces dummy dum0 address 172.16.255.1/32

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IPv6 on a Dummy

# IPv4 + IPv6
set interfaces dummy dum0 address 192.168.255.1/32
set interfaces dummy dum0 address 2001:db8::1/128

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Advanced Configuration

Dummy for Router ID (OSPF/BGP)

# Dummy interface for the Router ID
set interfaces dummy dum0 address 10.255.255.1/32
set interfaces dummy dum0 description 'Router ID for OSPF/BGP'

# OSPF uses this address as its Router ID
set protocols ospf parameters router-id 10.255.255.1
set protocols ospf area 0 network 10.255.255.1/32

# BGP uses it as well
set protocols bgp local-as 65000
set protocols bgp parameters router-id 10.255.255.1

# Advertise into OSPF (optional)
set protocols ospf area 0 network 10.255.255.1/32

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Blackhole Routing with a Dummy

Dummy interfaces can be used for blackhole routing - dropping unwanted traffic.

# Create a dummy for blackhole
set interfaces dummy dum0 address 192.0.2.1/32
set interfaces dummy dum0 description 'Blackhole interface'

# Routes pointing to the dummy (traffic will be dropped)
set protocols static route 10.0.0.0/8 interface dum0
set protocols static route 172.16.0.0/12 interface dum0
set protocols static route 192.168.0.0/16 interface dum0

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Now all traffic to these subnets will be dropped (useful for DDoS protection or for routing bogon prefixes).

Source Address for Outbound Connections

# Dummy for a stable source address
set interfaces dummy dum0 address 203.0.113.100/32
set interfaces dummy dum0 description 'Stable source for outgoing connections'

# BGP uses the dummy as its source
set protocols bgp neighbor 198.51.100.1 remote-as 65001
set protocols bgp neighbor 198.51.100.1 update-source dum0

# Static route to make the dummy address reachable
set protocols ospf area 0 network 203.0.113.100/32

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Dummy for NAT Testing

# The dummy simulates an internal network
set interfaces dummy dum0 address 192.168.10.1/24
set interfaces dummy dum0 description 'Test internal network'

# NAT for the dummy interface
set nat source rule 100 outbound-interface name eth0
set nat source rule 100 source address 192.168.10.0/24
set nat source rule 100 translation address masquerade

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Dummy for Policy-Based Routing

# Dummy for PBR testing
set interfaces dummy dum0 address 192.168.100.1/24

# Policy route for traffic to the dummy
set policy route TEST-ROUTE rule 10 destination address 192.168.100.0/24
set policy route TEST-ROUTE rule 10 set table 100

# Apply to the interface
set interfaces ethernet eth1 policy route TEST-ROUTE

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Configuration Examples

Example 1: Router ID for OSPF/BGP

# Dummy interface for a stable Router ID
set interfaces dummy dum0 address 10.255.255.1/32
set interfaces dummy dum0 description 'Router ID'

# OSPF configuration
set protocols ospf parameters router-id 10.255.255.1
set protocols ospf area 0 network 10.0.0.0/24
set protocols ospf area 0 network 10.255.255.1/32

# BGP configuration
set protocols bgp local-as 65000
set protocols bgp parameters router-id 10.255.255.1
set protocols bgp neighbor 10.0.0.2 remote-as 65001
set protocols bgp neighbor 10.0.0.2 update-source dum0

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Example 2: Blackhole Routing for Bogon Prefixes

# Dummy for blackhole
set interfaces dummy dum0 address 192.0.2.1/32
set interfaces dummy dum0 description 'Blackhole for bogon prefixes'

# RFC 1918 private addresses (if they should not be routed to the internet)
set protocols static route 10.0.0.0/8 blackhole distance 254
set protocols static route 172.16.0.0/12 blackhole distance 254
set protocols static route 192.168.0.0/16 blackhole distance 254

# Other bogon prefixes
set protocols static route 0.0.0.0/8 blackhole distance 254
set protocols static route 127.0.0.0/8 blackhole distance 254
set protocols static route 169.254.0.0/16 blackhole distance 254
set protocols static route 224.0.0.0/4 blackhole distance 254
set protocols static route 240.0.0.0/4 blackhole distance 254

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Note: blackhole is more efficient than a route through a dummy, but a dummy can be used for specific cases.

Example 3: Test Environment with Multiple Dummies

# Simulating multiple networks for testing
set interfaces dummy dum0 address 192.168.10.1/24
set interfaces dummy dum0 description 'Test network VLAN 10'

set interfaces dummy dum1 address 192.168.20.1/24
set interfaces dummy dum1 description 'Test network VLAN 20'

set interfaces dummy dum2 address 192.168.30.1/24
set interfaces dummy dum2 description 'Test network VLAN 30'

# OSPF advertises all test networks
set protocols ospf area 0 network 192.168.10.0/24
set protocols ospf area 0 network 192.168.20.0/24
set protocols ospf area 0 network 192.168.30.0/24

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save

Example 4: Stable Source for BGP Peering

# Dummy for BGP peering
set interfaces dummy dum0 address 10.255.0.1/32
set interfaces dummy dum0 description 'BGP peering endpoint'

# Advertise the dummy into the IGP (OSPF)
set protocols ospf area 0 network 10.255.0.1/32

# BGP neighbors use the dummy as update-source
set protocols bgp local-as 65000
set protocols bgp parameters router-id 10.255.0.1

set protocols bgp neighbor 10.255.0.2 remote-as 65000
set protocols bgp neighbor 10.255.0.2 update-source dum0

set protocols bgp neighbor 10.255.0.3 remote-as 65000
set protocols bgp neighbor 10.255.0.3 update-source dum0

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Now BGP peering uses a stable IP that does not depend on the state of the physical interfaces.

Example 5: Dummy for VRF Testing

# VRF with a dummy interface
set vrf name TEST-VRF table 100

# Dummy in the VRF
set interfaces dummy dum0 vrf TEST-VRF
set interfaces dummy dum0 address 192.168.100.1/24
set interfaces dummy dum0 description 'Test interface in VRF'

# Static route in the VRF
set protocols vrf TEST-VRF static route 192.168.200.0/24 next-hop 192.168.100.254

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Example 6: Anycast Address on a Dummy

# Anycast address for a service (for example, DNS)
set interfaces dummy dum0 address 10.10.10.10/32
set interfaces dummy dum0 description 'Anycast DNS service'

# Advertise into OSPF
set protocols ospf area 0 network 10.10.10.10/32

# Or into BGP
set protocols bgp address-family ipv4-unicast network 10.10.10.10/32

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Now all routers with an identical configuration advertise 10.10.10.10, and clients will be directed to the nearest router.

Monitoring and Diagnostics

Viewing Dummy Interfaces

# Show all dummy interfaces
show interfaces dummy

# Detailed information
show interfaces dummy dum0

# Statistics (usually zero for a dummy)
show interfaces dummy dum0 statistics

Checking the State

# The dummy should always be up
show interfaces dummy dum0

# Output:
# dum0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 1500
#     inet 192.168.255.1/32

Checking Routing

# Check that the dummy address is in the routing table
show ip route

# Ping the dummy address (from the router itself)
ping 192.168.255.1

# Traceroute (to verify routing)
traceroute 192.168.255.1

Checking in Routing Protocols

# OSPF - the dummy in the database
show ip ospf database

# BGP - the dummy is advertised
show ip bgp
show ip bgp 192.168.255.1/32

# Neighbors use the dummy
show ip bgp neighbors

Logging

# Interface logs
show log | match dum0

# Kernel messages
dmesg | grep dum

Troubleshooting

Problem: The Dummy Interface Is Not Created

Diagnostics:

# Check the configuration
show configuration interfaces dummy

# Check the state
show interfaces dummy

Solution:

# Make sure the configuration is applied
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# Check the kernel module (usually loaded by default)
lsmod | grep dummy

# If it is not - load it (usually not required)
sudo modprobe dummy

Problem: The Dummy Address Is Unreachable from Other Devices

Cause: The dummy address is not advertised in a routing protocol.

Solution:

# Add the dummy to OSPF
set protocols ospf area 0 network 192.168.255.1/32

# Or to BGP
set protocols bgp address-family ipv4-unicast network 192.168.255.1/32

# Or a static route on the other routers
set protocols static route 192.168.255.1/32 next-hop 10.0.0.1

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Problem: BGP Neighbors Do Not Use the Dummy as Source

Diagnostics:

# Check the BGP neighbors
show ip bgp neighbors

# Check the source address in the configuration
show configuration protocols bgp neighbor

Solution:

# Explicitly specify update-source
set protocols bgp neighbor 10.0.0.2 update-source dum0

# Make sure the dummy address is reachable through the IGP
set protocols ospf area 0 network 192.168.255.1/32

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Problem: Blackhole Routing Does Not Work

Diagnostics:

# Check the routes
show ip route 10.0.0.0/8

# Traceroute to verify
traceroute 10.1.1.1

Solution:

Use blackhole instead of a dummy for efficient dropping:

# Instead of:
# set protocols static route 10.0.0.0/8 interface dum0

# Use:
set protocols static route 10.0.0.0/8 blackhole distance 254

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Problem: A Dummy in a VRF Does Not Work

Diagnostics:

# Check the VRF
show vrf TEST-VRF

# Check the interface in the VRF
show interfaces dummy dum0

Solution:

# Make sure the dummy is bound to the VRF
set interfaces dummy dum0 vrf TEST-VRF

# Check routing in the VRF
show ip route vrf TEST-VRF

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Best Practices

1. Use Loopback for the Production Router ID

For the production router ID, loopback is preferable:

# Better:
set interfaces loopback lo address 10.255.255.1/32
set protocols ospf parameters router-id 10.255.255.1

# Than:
set interfaces dummy dum0 address 10.255.255.1/32
set protocols ospf parameters router-id 10.255.255.1

2. Dummy for Temporary/Test Tasks

Use a dummy for testing and temporary configurations:

set interfaces dummy dum0 address 192.168.100.1/24
set interfaces dummy dum0 description 'TEST - Remove after testing'

3. Describe the Purpose

Always add a description:

set interfaces dummy dum0 description 'BGP peering endpoint'
set interfaces dummy dum1 description 'Test network simulation'

4. Use /32 for Endpoint Addresses

For Router IDs and stable endpoints, use /32:

set interfaces dummy dum0 address 10.255.0.1/32

5. Blackhole via the blackhole Keyword

For blackhole routing, use the built-in blackhole:

# Preferred:
set protocols static route 10.0.0.0/8 blackhole distance 254

# Instead of:
set protocols static route 10.0.0.0/8 interface dum0

6. Document Test Dummies

Document all dummy interfaces:

# /config/dummy-interfaces.txt
# dum0: BGP peering endpoint (10.255.0.1/32)
# dum1: Test VLAN 10 simulation (192.168.10.1/24)
# dum2: Anycast DNS service (10.10.10.10/32)

7. Remove Unused Dummies

Clean up old dummies from the configuration:

# Remove an unused dummy
delete interfaces dummy dum5
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8. Use Consistent Naming

Use a consistent naming scheme:

# dum0 - Router ID / BGP endpoint
# dum1-dum9 - Test networks
# dum10+ - Special purposes

9. Monitor Dummy Usage

Track which dummies are in use:

show interfaces dummy
show configuration interfaces dummy

10. Security

For dummies with real IPs, do not forget the firewall:

set firewall ipv4 name LOCAL_IN rule 100 action accept
set firewall ipv4 name LOCAL_IN rule 100 destination address 192.168.255.1
set firewall ipv4 name LOCAL_IN rule 100 protocol tcp
set firewall ipv4 name LOCAL_IN rule 100 destination port 179  # BGP

Useful Commands

# Show all dummy interfaces
show interfaces dummy

# Details of a specific dummy
show interfaces dummy dum0

# Configuration
show configuration interfaces dummy

# Create a dummy
set interfaces dummy dum0 address 192.168.255.1/32

# Add a description
set interfaces dummy dum0 description 'My description'

# Delete a dummy
delete interfaces dummy dum0

# Check the state
show interfaces dummy dum0

# Ping the dummy address
ping 192.168.255.1

# Check in the routing table
show ip route 192.168.255.1

# OSPF database
show ip ospf database | match 192.168.255.1

# BGP routes
show ip bgp 192.168.255.1/32

# Kernel interface details
ip addr show dum0

# Logs
show log | match dum

Real-World Use Cases

Anycast DNS with a Dummy

Several routers advertise the same IP for a DNS service:

Router 1:

set interfaces dummy dum0 address 10.10.10.10/32
set protocols ospf area 0 network 10.10.10.10/32

# The DNS server listens on this IP
# (configure the DNS server separately)

Router 2 (identical configuration):

set interfaces dummy dum0 address 10.10.10.10/32
set protocols ospf area 0 network 10.10.10.10/32

Clients are directed to the nearest router based on the OSPF metric.

Testing Firewall Rules

# The dummy simulates the protected network
set interfaces dummy dum0 address 192.168.50.1/24
set interfaces dummy dum0 description 'Firewall test network'

# Firewall rules for the dummy
set firewall ipv4 name DUMMY_IN default-action drop
set firewall ipv4 name DUMMY_IN rule 10 action accept
set firewall ipv4 name DUMMY_IN rule 10 state established
set firewall ipv4 name DUMMY_IN rule 10 state related

set interfaces dummy dum0 firewall in name DUMMY_IN

# Testing from another interface
# ping 192.168.50.1

Development and Debugging

# Multiple dummies for development
set interfaces dummy dum0 address 172.16.0.1/24
set interfaces dummy dum0 description 'Dev: Frontend network'

set interfaces dummy dum1 address 172.16.1.1/24
set interfaces dummy dum1 description 'Dev: Backend network'

set interfaces dummy dum2 address 172.16.2.1/24
set interfaces dummy dum2 description 'Dev: Database network'

# Now you can test routing between the "networks"
set protocols static route 172.16.1.0/24 next-hop 172.16.0.254

Conclusion

Dummy interfaces in VyOS are a versatile tool for a wide range of tasks, from testing to production routing. They provide stable, always-available interfaces that are not tied to physical hardware.

Key advantages of dummy interfaces:

Always in the “up” state
Do not require physical hardware
Flexible in use (Router ID, BGP endpoint, blackhole, testing)
Easy to configure
VRF support

Recommendations:

Use Loopback for the production Router ID
Use Dummy for testing and special tasks
Always add a description
Document the purpose of each dummy
Remove unused dummies
Use /32 for endpoint addresses
Prefer the blackhole keyword for blackhole routing

Dummy interfaces are a powerful tool in a network engineer’s arsenal, providing flexibility and reliability across a variety of scenarios, from development to production environments.

Reviewed by OpenNix LLC · Last updated on 29 July 2026

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