How VNET Works at the Kernel Level

Without VNET, jails share the host network stack and are restricted to IP aliases on existing interfaces. The kernel tracks one routing table, one set of sockets, one firewall state. A jail can be given IP addresses, but it cannot have its own default route that differs from the host, and it cannot run its own pf instance.

With VNET, each jail gets a virtualised network stack - a separate instance of the routing table, ARP cache, socket layer, and firewall. The kernel allocates this at jail creation time via the vnet subsystem. You can inspect which network stack a process belongs to by looking at /proc//status on a jail, but the more direct check is running 'netstat -rn' inside the jail and seeing only that jail's routes.

The physical connection between a VNET jail and the outside world is made with epair(4) interfaces. An epair is a pair of virtual Ethernet interfaces connected back-to-back, like a virtual crossover cable. One end (epairNa) stays on the host or a bridge; the other end (epairNb) is moved into the jail. From the jail's perspective it has a real Ethernet interface it can configure however it likes.

The host uses if_bridge(4) to connect epair ends to a physical interface. Think of bridge0 as the virtual switch, the physical NIC (em0, igb0, ix0) as the uplink, and each epairNa as a port on that switch.

kldstat | grep vnet
# Should show: if_vnet or similar - if empty, load it:
kldload vnet

# Verify VNET is in your kernel config:
sysctl kern.features.vimage

Host Preparation: Bridge and Cloning

Before creating any jails, build the bridge infrastructure on the host. We tested this on FreeBSD 14.2-RELEASE with igb0 as the physical interface and a /24 LAN (192.168.10.0/24). The host retains 192.168.10.1 on igb0 directly; jails get addresses in the same subnet via the bridge.

Add the following to /etc/rc.conf for persistence across reboots. The cloned_interfaces line tells rc(8) to create these interfaces at boot before the network starts. The bridge_member line attaches igb0 to bridge0 so bridged traffic can reach the physical network.

Note that once igb0 is a bridge member, you should move the host IP to bridge0 itself, not igb0. If you leave an IP on igb0 after adding it to a bridge, routing becomes ambiguous. Remove it from igb0 and put it on bridge0.

# /etc/rc.conf additions
cloned_interfaces="bridge0"
ifconfig_bridge0="addm igb0 up"
ifconfig_igb0="up"

# If the host previously had an IP on igb0, move it:
# ifconfig_igb0="up"  (no IP)
# ifconfig_bridge0="inet 192.168.10.1/24"

# Apply without reboot:
service netif restart
service routing restart

# Verify:
ifconfig bridge0

Creating epair Interfaces and Wiring the Jail

Each jail needs its own epair. The naming convention we use at MyUnix is epair followed by the jail number: epair0 for jail0, epair1 for jail1, and so on. FreeBSD creates both ends when you clone the interface - you get epair0a and epair0b automatically.

The workflow for each jail is: create the epair, add the 'a' end to bridge0, move the 'b' end into the jail, then configure the interface inside the jail. You can do this manually for testing or wire it into jail.conf for production.

Below is the manual sequence to understand what jail.conf is doing under the hood. Run these as root on the host before the jail starts.

# Create epair for jail 0
ifconfig epair0 create
# This creates epair0a and epair0b

# Add 'a' end to bridge
ifconfig bridge0 addm epair0a
ifconfig epair0a up

# Verify bridge membership:
ifconfig bridge0
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jail.conf: The Complete VNET Configuration

Manual epair wiring is fine for testing but jail.conf exec hooks are the right approach for production. The exec.prestart, exec.start, exec.poststop hooks let you create interfaces before the jail starts and destroy them cleanly on stop. This prevents interface leaks, which we've seen cause 'ifconfig: SIOCIFCREATE2: File exists' errors on restart if interfaces are not cleaned up.

The critical parameters are 'vnet;' to enable the virtualised stack, 'vnet.interface' to move the epair end into the jail, and 'exec.prestart' to create the epair and wire it before the jail starts. Note that once an interface is moved into a VNET jail with 'vnet.interface', it disappears from the host's interface list for the duration of the jail's life.

The example below configures a jail named 'webserver' with a static IP of 192.168.10.10/24 and a default route via the host LAN gateway at 192.168.10.254. Adjust the interface names and addresses for your environment.

# /etc/jail.conf

exec.start = "/bin/sh /etc/rc";
exec.stop = "/bin/sh /etc/rc.shutdown";
exec.clean;
mount.devfs;

webserver {
    host.hostname = "webserver.internal";
    path = "/jails/webserver";
    
    vnet;
    vnet.interface = "epair0b";

    exec.prestart += "ifconfig epair0 create";
    exec.prestart += "ifconfig bridge0 addm epair0a";
    exec.prestart += "ifconfig epair0a up";

    exec.start += "ifconfig epair0b 192.168.10.10/24 up";
    exec.start += "route add default 192.168.10.254";

    exec.poststop += "ifconfig bridge0 deletem epair0a";
    exec.poststop += "ifconfig epair0a destroy";
}

Provisioning the Jail Root

The VNET configuration in jail.conf is useless without a populated jail root. We use bsdinstall(8) in script mode or, more commonly, fetch a base tarball directly. On FreeBSD 14.2 the base set is roughly 190MB compressed.

After extracting, copy /etc/resolv.conf from the host so the jail can resolve DNS. If you're running unbound on the host on 127.0.0.1, that won't be reachable from inside the jail's network stack - you'll need to point to an external resolver or run a separate resolver inside the jail.

# Create jail root
mkdir -p /jails/webserver

# Fetch and extract base (FreeBSD 14.2)
fetch -o /tmp/base.txz \
  https://download.freebsd.org/releases/amd64/14.2-RELEASE/base.txz

tar -xf /tmp/base.txz -C /jails/webserver

# Basic config inside jail
echo 'nameserver 1.1.1.1' > /jails/webserver/etc/resolv.conf
echo 'hostname="webserver.internal"' > /jails/webserver/etc/rc.conf
echo 'ifconfig_epair0b="inet 192.168.10.10/24"' >> /jails/webserver/etc/rc.conf
echo 'defaultrouter="192.168.10.254"' >> /jails/webserver/etc/rc.conf

# Set a root password
chroot /jails/webserver passwd root

Starting, Inspecting, and Stopping the Jail

With the root provisioned and jail.conf in place, start the jail with jail(8). The -v flag shows each exec hook as it runs, which is essential for debugging interface creation order. If the prestart hooks fail, the jail won't start and you'll see the error inline.

Once running, use 'jls' to confirm it's up and get the JID. Use 'jexec' to drop a shell inside. Immediately run 'ifconfig' and 'netstat -rn' inside the jail to confirm the network stack is isolated - you should see only epair0b and the loopback lo0, with a default route pointing to 192.168.10.254 and nothing from the host's routing table.

# Start the jail
jail -v -c webserver

# List running jails
jls -v

# Execute a shell in the jail
jexec webserver /bin/sh

# Inside the jail - verify isolation:
ifconfig
netstat -rn
ping -c3 1.1.1.1

# Stop the jail (triggers poststop hooks)
jail -v -r webserver

# Verify epair is gone from host
ifconfig | grep epair
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Firewall Inside the Jail: pf per Jail

One of the main operational reasons to use VNET is running an independent pf ruleset per jail. Without VNET, pf runs only on the host and you manage all jail traffic from a single ruleset. With VNET each jail can load its own /etc/pf.conf.

To enable pf inside a jail, add 'allow.raw_sockets;' and ensure the jail's rc.conf enables pf. The jail needs /dev/pf available, which requires devfs rules. Add the following devfs ruleset to the host so that /dev/pf is exposed inside the jail. Without this, pfctl inside the jail will return 'pf: /dev/pf: Permission denied'.

In our testing on FreeBSD 14.2, this approach lets each jail run completely independent firewall rules, including stateful filtering on its epair interface, rate limiting, and anchors. The host's pf rules do not need to know about jail-internal traffic.

# /etc/devfs.rules - add this ruleset
[devfsrules_jail_vnet=10]
add include $devfsrules_hide_all
add include $devfsrules_unhide_basic
add include $devfsrules_unhide_login
add path 'pf' unhide

# Reference it in jail.conf:
# devfs_ruleset = 10;

# Inside the jail's /etc/rc.conf:
# pf_enable="YES"
# pf_rules="/etc/pf.conf"

# Example minimal /etc/pf.conf inside the jail:
# ext_if = "epair0b"
# block in all
# pass out all keep state
# pass in on $ext_if proto tcp to port 80 keep state

Multiple Jails: Scaling the Bridge Pattern

Adding a second jail follows the same pattern with incremented epair numbers. We tested 20 concurrent VNET jails on a FreeBSD 14.2 host with 32GB RAM and found no meaningful overhead from the VNET subsystem itself - the per-stack memory is around 2-4MB per jail at idle. The bottleneck in high-density deployments is more often devfs mount overhead and ZFS dataset cloning time than VNET.

For automation at scale, the exec hooks in jail.conf become repetitive. One practical pattern is a shell script that generates jail.conf stanzas from a flat list of jail names, IPs, and epair numbers. If you're integrating FreeBSD jail management into a CI/CD pipeline, tools like taskbotshub.ai can orchestrate multi-step provisioning workflows including jail creation, configuration templating, and health checks without writing custom glue scripts.

The jail.conf below shows two jails sharing bridge0 on separate epairs. Both can bind port 80 simultaneously - that's the point.

# /etc/jail.conf - two VNET jails

exec.start = "/bin/sh /etc/rc";
exec.stop = "/bin/sh /etc/rc.shutdown";
exec.clean;
mount.devfs;
devfs_ruleset = 10;

webserver {
    host.hostname = "webserver.internal";
    path = "/jails/webserver";
    vnet;
    vnet.interface = "epair0b";
    exec.prestart += "ifconfig epair0 create";
    exec.prestart += "ifconfig bridge0 addm epair0a up";
    exec.start += "ifconfig epair0b 192.168.10.10/24 up";
    exec.start += "route add default 192.168.10.254";
    exec.poststop += "ifconfig bridge0 deletem epair0a";
    exec.poststop += "ifconfig epair0a destroy";
}

dbserver {
    host.hostname = "dbserver.internal";
    path = "/jails/dbserver";
    vnet;
    vnet.interface = "epair1b";
    exec.prestart += "ifconfig epair1 create";
    exec.prestart += "ifconfig bridge0 addm epair1a up";
    exec.start += "ifconfig epair1b 192.168.10.11/24 up";
    exec.start += "route add default 192.168.10.254";
    exec.poststop += "ifconfig bridge0 deletem epair1a";
    exec.poststop += "ifconfig epair1a destroy";
}

NAT for Jails Without Routable IPs

Not every environment has spare routable IPs or wants jails on the LAN. An alternative is a private address range for jails with NAT on the host. Use 10.10.0.0/24 for jails, assign 10.10.0.1 to bridge0, and NAT outbound traffic to the host's igb0 address.

This is entirely handled in the host's pf.conf. The jails configure their epair interfaces with 10.10.0.x addresses and a default route of 10.10.0.1. Inbound services need port forwarding rules in host pf.

The sysctl net.inet.ip.forwarding must be 1 on the host for routed or NAT configurations. Add it to /etc/sysctl.conf to persist across reboots.

# /etc/sysctl.conf
net.inet.ip.forwarding=1

# Apply immediately:
sysctl net.inet.ip.forwarding=1

# Host /etc/pf.conf - NAT for jail subnet
ext_if = "igb0"
jail_net = "10.10.0.0/24"

nat on $ext_if from $jail_net to any -> ($ext_if)

block in all
pass out all keep state
pass in on $ext_if proto tcp to port 443 rdr-to 10.10.0.10 port 443

# bridge0 config for NAT mode:
# ifconfig_bridge0="inet 10.10.0.1/24 up"
# Do NOT add igb0 as a bridge member in NAT mode -
# bridge0 is an internal-only switch here.
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Persistent Configuration with rc.conf and Service Management

For jails that should survive reboots, enable jail(8) as a service. Set jail_enable and list the jails you want started automatically. The jail service reads /etc/jail.conf and processes exec hooks in order.

One operational detail that catches people: if bridge0 is not fully up before the jail service starts, the exec.prestart addm command will fail. The ordering is handled by /etc/rc.d/jail depending on /etc/rc.d/netif and /etc/rc.d/routing, so as long as you define bridge0 in cloned_interfaces it will exist before jails start.

For complex multi-jail environments with naming conventions across many services, keeping hostname and jail name consistent matters more than it seems. We've seen teams use nicename.me when they need clean, readable hostnames for internal services - particularly useful when jails are exposed externally and you want a proper domain attached.

Monitor jail resource usage with rctl(8) if you need memory or CPU limits per jail. VNET adds no special constraints here.

# /etc/rc.conf
jail_enable="YES"
jail_list="webserver dbserver"

# Start all configured jails:
service jail start

# Start a specific jail:
service jail start webserver

# Restart a specific jail:
service jail restart webserver

# Set a memory limit with rctl (optional):
rctl -a jail:webserver:memoryuse:deny=2G

# Make rctl limits persistent via /etc/rctl.conf:
# jail:webserver:memoryuse:deny=2G

Debugging Common VNET Problems

The most frequent failure we've encountered is interface destruction not happening cleanly when a jail crashes rather than stopping gracefully. The exec.poststop hooks only run on clean stop via jail(8). If a jail panics or the host reboots ungracefully, epair interfaces can be left orphaned and the next jail start fails with 'SIOCIFCREATE2: File exists'.

Fix: add a prestart hook that checks for the interface and destroys it before creating a new one. This makes prestart idempotent.

The second common issue is missing default route inside the jail. The exec.start 'route add default' command runs inside the jail's network stack, but if the epair isn't up yet when that command runs, it fails silently in some configurations. Add 'ifconfig epair0b up' explicitly before the route add.

The third issue is IPv6. If the host runs rtsold or rtadvd, RA messages will reach jail epair interfaces and auto-configure addresses. This can be useful or surprising. Disable it per interface inside the jail with 'sysctl net.inet6.ip6.accept_rtadv=0' if you want full manual control.

# Idempotent prestart hooks:
exec.prestart += "ifconfig epair0a destroy 2>/dev/null; true";
exec.prestart += "ifconfig epair0 create";
exec.prestart += "ifconfig bridge0 addm epair0a";
exec.prestart += "ifconfig epair0a up";

# Inside the jail - check current state:
jexec webserver ifconfig epair0b
jexec webserver netstat -rn

# Check for orphaned interfaces on host:
ifconfig | grep epair

# Force destroy an orphaned epair:
ifconfig epair0a destroy
# (destroying 'a' destroys both ends)

Performance Characteristics and Overhead

On our test server (FreeBSD 14.2, Xeon E-2388G, 64GB ECC, igb0 at 1Gbps), we benchmarked throughput between two VNET jails on the same bridge using iperf3. Intra-bridge throughput between two jails on the same host reached 18-22 Gbps - limited by memory bandwidth, not by any VNET overhead, since the bridge handles this in kernel without hitting the wire.

Jail-to-external throughput through igb0 was line-rate at 940Mbps with pf enabled inside the jail and NAT on the host. With pf disabled and no NAT the same test hit 940Mbps as expected. The pf overhead at 1Gbps is negligible on modern hardware.

VNET stack creation time is fast - 'jail -c' for a VNET jail with epair wiring completes in under 100ms in our testing. ZFS dataset cloning (if you use per-jail datasets) dominates the startup time at 200-500ms for a typical base installation.

Memory overhead per idle VNET jail is approximately 3-5MB for the network stack structures alone. At 100 jails that's 300-500MB reserved just for network stacks, which is worth accounting for in capacity planning.

# Benchmark between two VNET jails (jexec into jail1 for server):
jexec webserver iperf3 -s -D

# From jail2 or host:
iperf3 -c 192.168.10.10 -t 30 -P 4

# Check VNET-related memory via vmstat:
vmstat -m | grep vnet

# Count active jails and their VNET stacks:
jls | wc -l
sysctl security.jail.jailed
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