Classic and Lightweight Tunnels

Classic and Lightweight Tunnels

Synopsis #

This chapter covers lightweight, in-base tunneling mechanisms for pragmatic overlays and migrations: gif(4) (generic IP-in-IP), gre(4) (Generic Routing Encapsulation), and etherip(4) (Layer-2 Ethernet over IP). These interfaces interoperate well with non-OpenBSD devices and are suitable when cryptography is not required, when you must carry non-IP protocols (GRE), or when you must extend L2 domains (EtherIP). Boot-time configuration uses hostname.if(5); runtime management uses ifconfig(8). Filtering and allowances live in pf.conf(5) and are applied with pfctl(8).

Use these mechanisms for temporary migrations, quick overlays between sites, lab environments, or when you need simple encapsulation without the control-plane complexity of a full VPN.

Design Considerations #

  • Encapsulation choice.
    • gif: IP-in-IP only (no L2); smallest header; good for routing IP subnets between known endpoints.
    • gre: Flexible payload (commonly IP); widely supported on routers and appliances.
    • etherip: Bridges Ethernet frames across an IP path; use sparingly and avoid L2 loops.
  • Security. These mechanisms provide no encryption or integrity. Use them only on trusted or controlled underlays, or combine with pf(4) filtering. For cryptographic tunnels, prefer IKEv2 or wg(4) as covered in the previous chapter.
  • Forwarding and routing. Enable IPv4/IPv6 forwarding when routing subnets through the tunnel. Add explicit routes for remote prefixes.
  • MTU and fragmentation. Encapsulation reduces effective MTU. Set a conservative MTU initially and refine after measuring path MTU.
  • Symmetry and addressing. Tunnels are point-to-point. Configure the correct local/remote order at both ends.
  • Filtering. Permit the relevant IP protocols on the underlay: IP protocol 4 (gif), 47 (GRE), and 97 (EtherIP).
  • Operational scope. Prefer routed overlays (gif/gre) over L2 extension. Use bridge(4) with EtherIP only when you must extend broadcast domains.

Configuration #

Assume two sites:

  • Site A WAN: 198.51.100.10
  • Site B WAN: 203.0.113.10

LANs:

  • A LAN: 10.0.0.0/24
  • B LAN: 10.20.0.0/24

Common system settings (both sites) #

Enable L3 forwarding so routed prefixes traverse the tunnel. Persist with sysctl.conf(5).

# printf '%s\n' \
    'net.inet.ip.forwarding=1' \
    'net.inet6.ip6.forwarding=1' >> /etc/sysctl.conf
  # Persist IPv4/IPv6 forwarding

# sysctl net.inet.ip.forwarding=1 net.inet6.ip6.forwarding=1
  # Apply immediately

Allow encapsulation protocols on the WAN in PF. Adjust the interface and policy as required.

## /etc/pf.conf — underlay allowances for lightweight tunnels

set skip on lo
wan = "em0"   # adjust to your WAN interface

block all
pass out on $wan keep state

# Encapsulation protocols (permit from known peer for tighter policy)
pass in on $wan proto { 4, 47, 97 } from 203.0.113.10 to ($wan) keep state
pass out on $wan proto { 4, 47, 97 } to 203.0.113.10 keep state

Reload with pfctl(8).

# pfctl -f /etc/pf.conf

A. gif(4): IP-in-IP routed overlay #

Create gif0 at both sites, assign a point-to-point address pair, and route the remote LAN via the tunnel.

Site A #

# /etc/hostname.gif0
tunnel 198.51.100.10 203.0.113.10
inet 10.254.0.1 255.255.255.252 10.254.0.2
mtu 1400
up

# sh /etc/netstart gif0
  # Activate the tunnel
# route add -net 10.20.0.0/24 10.254.0.2
  # Route B LAN via the tunnel

Site B #

# /etc/hostname.gif0
tunnel 203.0.113.10 198.51.100.10
inet 10.254.0.2 255.255.255.252 10.254.0.1
mtu 1400
up

# sh /etc/netstart gif0
# route add -net 10.0.0.0/24 10.254.0.1

B. gre(4): Generic Routing Encapsulation #

GRE is commonly required when the peer expects GRE specifically (for example, a router or cloud gateway). Configuration is analogous to gif.

Site A #

# /etc/hostname.gre0
tunnel 198.51.100.10 203.0.113.10
inet 10.254.1.1 255.255.255.252 10.254.1.2
mtu 1400
up

# sh /etc/netstart gre0
# route add -net 10.20.0.0/24 10.254.1.2

Site B #

# /etc/hostname.gre0
tunnel 203.0.113.10 198.51.100.10
inet 10.254.1.2 255.255.255.252 10.254.1.1
mtu 1400
up

# sh /etc/netstart gre0
# route add -net 10.0.0.0/24 10.254.1.1

Note: If the far-end requires GRE keys or additional attributes, configure them per gre(4) using ifconfig gre0 ... options supported by that implementation.

C. etherip(4): Layer-2 extension over IP #

EtherIP transports Ethernet frames between sites. Use it only when routed overlays are not viable (for example, when you must carry non-IP broadcasts or a VLAN intact). Avoid loops and keep broadcast domains constrained.

Site A #

# /etc/hostname.etherip0
tunnel 198.51.100.10 203.0.113.10
up

Create a bridge to join the local LAN interface (em1) to the tunnel. Bridge parameters are managed by bridge(4).

# /etc/hostname.bridge0
add em1
add etherip0
up

Activate:

# sh /etc/netstart etherip0 bridge0

Site B #

Mirror the configuration with reversed tunnel endpoints.

# /etc/hostname.etherip0
tunnel 203.0.113.10 198.51.100.10
up

# /etc/hostname.bridge0
add em1
add etherip0
up

# sh /etc/netstart etherip0 bridge0

Caution: L2 extension spans broadcast and failure domains. Ensure there is exactly one active path between the bridged segments. If you must introduce redundancy, design and test loop prevention per bridge(4) before production.

Verification #

$ ifconfig gif0
  # Expect "UP" and the correct tunnel src/dst and P2P addresses
$ ifconfig gre0
$ ifconfig etherip0
$ ifconfig bridge0
  # For EtherIP, ensure both members (em1, etherip0) are in the bridge
  • Routing and reachability with route(8) and ping(8):
$ netstat -rn | egrep '10\.254\.|10\.0\.0\.|10\.20\.0\.'
  # Confirm tunnel /30 and remote LAN routes
$ ping -n -c 3 10.20.0.1
  # From Site A LAN host to Site B LAN host (gif/gre)

# tcpdump -ni em0 proto 4
  # gif (IP-in-IP) packets on the WAN
# tcpdump -ni em0 proto 47
  # GRE packets on the WAN
# tcpdump -ni em0 proto 97
  # EtherIP frames on the WAN

$ pfctl -vvsr | egrep 'proto (4|47|97)'
  # Verify allowances are loaded and counters increment under test

Troubleshooting #

  • No tunnel traffic on the WAN. Check that PF permits the correct protocol (4, 47, or 97) to and from the peer address. Use tcpdump on the WAN to confirm encapsulated packets.
  • Encapsulation up, but no routing. Ensure net.inet.ip.forwarding=1 and that static routes to the remote LAN point at the tunnel neighbor. Confirm with netstat -rn.
  • Asymmetric or intermittent reachability. Verify both ends use matching local/remote tunnel endpoints and the correct /30 addressing. Asymmetry commonly results from reversed arguments.
  • Fragmentation or stalls. Lower the tunnel MTU (for example, to 1400) and retest. Measure PMTU after stability and increase cautiously.
  • Broadcast storms over EtherIP. EtherIP extends the broadcast domain. Confirm there is only one L2 path and avoid bridging additional WAN links without loop prevention.
  • Interoperability with non-OpenBSD peers. For GRE, check peer-specific requirements (keys, keepalives, or DF handling) and adjust ifconfig gre0 ... options accordingly.

See Also #