Autora (auto-RA)

autora is a small Linux IPv6 Router Advertisement daemon.

It watches selected network interfaces and advertises enough IPv6 information for hosts on those links to configure an address with SLAAC and install a default route through the local link-local router address. It is intentionally narrow: it does not run DHCPv6, publish DNS settings, manage addresses, proxy neighbors, or install routes.

The daemon is designed for links where every served interface has one IPv6 policy and one routed /64. A common use case is a host with one VM-facing interface per guest, but the same model can apply to any isolated L2 segment where the attached systems should receive the same Router Advertisement.

Status

This project was written by Codex-5.5 for a specific use case. It implements the planned control loop, polling-based interface discovery, fail-closed eligibility checks, RA packet construction, RS parsing, jittered scheduling, per-interface multicast rate limiting, and a Linux raw ICMPv6 socket path.

What it does

The daemon serves interfaces that match the configured regular expression and meet all of these requirements:

• IPv6 must be enabled.
• The interface is up, multicast-capable, and non-loopback.
• A link-local IPv6 address exists.
• Exactly one usable non-link-local /64 is present.
• /proc/sys/net/ipv6/conf/<ifname>/forwarding is 1.
• No other RA daemon should advertise on the same interface.

autora sends only multicast Router Advertisements. Solicited replies are also multicast, which avoids creating kernel neighbor-cache state for client-provided or spoofed source addresses.

Every Router Advertisement uses:

• ICMPv6 type 134, code 0;
• IPv6 hop limit 255;
• RA Current Hop Limit 0;
• Managed Address Configuration flag M = 0;
• Other Configuration flag O = 0;
• Router Lifetime from configuration, default 180s;
• Reachable Time 0;
• Retrans Timer 0;
• one Prefix Information Option for the selected /64;
• PIO Prefix Length 64;
• PIO Autonomous Address-Configuration flag A = 1;
• PIO On-Link flag L = 0;
• PIO Valid Lifetime from configuration, default 86400s;
• PIO Preferred Lifetime from configuration, default 14400s.

Quick Start

cargo run -- \
  --interface-regex '^lan[0-9]+$' \
  --min-ra-interval 20s \
  --max-ra-interval 60s \
  --router-lifetime 180s

The default interface regex is ^$, so no interface is served until you provide one. Use anchored regexes that match only interfaces you intend to advertise on.

Dry-run mode shows discovery, eligibility, scheduling, and packet construction without opening a raw socket:

cargo run -- --dry-run --interface-regex '^eth[0-9]+$'

Raw packet operation requires CAP_NET_RAW. For a built binary, run as root or grant the binary that capability through your service manager.

The main options also have environment variable equivalents:

• --interface-regex, AUTORA_INTERFACE_REGEX
• --min-ra-interval, AUTORA_MIN_RA_INTERVAL
• --max-ra-interval, AUTORA_MAX_RA_INTERVAL
• --router-lifetime, AUTORA_ROUTER_LIFETIME
• --pio-valid-lifetime, AUTORA_PIO_VALID_LIFETIME
• --pio-preferred-lifetime, AUTORA_PIO_PREFERRED_LIFETIME
• --forwarding-resync-interval, AUTORA_FORWARDING_RESYNC_INTERVAL
• --heartbeat-interval, AUTORA_HEARTBEAT_INTERVAL
• --log-level, AUTORA_LOG
• --dry-run, AUTORA_DRY_RUN

VM Example

For a routed-prefix-per-VM setup, give each guest its own L2-facing host interface, such as a tap or veth, and route one /64 to that interface. Then match only those interfaces:

cargo run -- \
  --interface-regex '^tap-vm-[0-9]+$' \
  --min-ra-interval 20s \
  --max-ra-interval 60s \
  --router-lifetime 180s

With the default packet settings, guests can form SLAAC addresses from the advertised /64 and install the host link-local address as their default router. The advertised prefix is intentionally not marked on-link.

Host Setup

For each served interface, assign one routed /64, bring the interface up, and enable per-interface IPv6 forwarding:

ip addr add 2001:db8:1200::1/64 dev tap-vm-1
ip link set tap-vm-1 up
sysctl -w net.ipv6.conf.tap-vm-1.forwarding=1

Make sure firewall rules allow ICMPv6 Router Solicitation, Router Advertisement, and Neighbor Discovery traffic on the served interfaces.

An example systemd unit is available at contrib/autora.service.

Debian Package

Build a local binary package with Debian tooling:

dpkg-buildpackage -us -uc -b

The package installs:

• /usr/bin/autora
• /lib/systemd/system/autora.service
• /etc/default/autora

The packaged service reads /etc/default/autora:

AUTORA_INTERFACE_REGEX=^tap-vm-[0-9]+$
AUTORA_MIN_RA_INTERVAL=20s
AUTORA_MAX_RA_INTERVAL=60s
AUTORA_ROUTER_LIFETIME=180s

After installing the package, edit /etc/default/autora with a restrictive interface regex, then enable the service:

systemctl enable --now autora.service

The Debian package version comes from the first entry in debian/changelog. For GitHub releases, .github/workflows/release.yml derives the version from the release tag, strips a leading v, rewrites the top changelog version, runs dpkg-buildpackage -us -uc -b, and uploads the generated .deb files to the release. For example, a release tag v0.2.0 produces Debian package version 0.2.0.

Notes

Router Solicitations are accepted only on eligible interfaces and only with IPv6 hop limit 255. Solicited replies are sent as multicast RAs to ff02::1 scoped to the receiving interface, rather than as unicast replies to client-provided source addresses. This avoids creating kernel neighbor-cache state for spoofed addresses. Solicited replies use a random delay from 0 through 500ms, unless an already scheduled multicast RA will happen earlier.

Multicast Router Advertisements are rate-limited per interface with at least 3s between sends.

Verification

Non-privileged checks:

cargo fmt --check
cargo test
cargo check

Packet-level verification should be done in a Linux namespace or VM test environment with CAP_NET_RAW and packet capture. Confirm that RAs are sent from the interface link-local address to ff02::1, hop limit is 255, the Prefix Information Option has A=1 and L=0, and the attached guest installs a default route without an on-link route for the advertised /64.

The namespace smoke test creates an isolated veth pair and checks both a non-matching regex case and a working advertisement case:

sudo tests/ns-smoke.sh