#!/bin/bash
# fips-dns-setup — Configure DNS routing for the .fips domain.
#
# Detects the system's DNS resolver and configures it to forward .fips
# queries to the FIPS DNS responder on 127.0.0.1:5354 (or [::1]:5354
# for the global drop-in backend).
#
# Backends (tried in order):
#   1. systemd dns-delegate (systemd >= 258, declarative drop-in)
#   2. systemd-resolved global drop-in via /etc/systemd/resolved.conf.d/
#      (preferred for systemd-resolved hosts — DNS=[::1]:5354 routes
#      via standard loopback with no interface scoping, sidestepping
#      the IPV6_PKTINFO ifindex behaviour that causes self-destined
#      traffic to fips0's address to be silently dropped by the
#      mesh-interface filter)
#   3. systemd-resolved via per-link resolvectl (legacy fallback —
#      historically failed silently due to the issue described above;
#      kept for hosts where the global drop-in is unsuitable)
#   4. dnsmasq (standalone)
#   5. NetworkManager with dnsmasq plugin
#   6. Warning with manual instructions

set -e

FIPS_DNS_PORT="5354"
FIPS_DNS_LOOPBACK_V6="::1"
FIPS_INTERFACE="fips0"

DNS_DELEGATE_DIR="/etc/systemd/dns-delegate.d"
DNS_DELEGATE_FILE="${DNS_DELEGATE_DIR}/fips.dns-delegate"
RESOLVED_DROPIN_DIR="/etc/systemd/resolved.conf.d"
RESOLVED_DROPIN_FILE="${RESOLVED_DROPIN_DIR}/fips.conf"
DNSMASQ_CONF="/etc/dnsmasq.d/fips.conf"
NM_DNSMASQ_CONF="/etc/NetworkManager/dnsmasq.d/fips.conf"

# Record which backend was configured for teardown.
STATE_DIR="/run/fips"
STATE_FILE="${STATE_DIR}/dns-backend"

log() { echo "fips-dns: $*"; }

save_backend() {
    mkdir -p "$STATE_DIR"
    echo "$1" > "$STATE_FILE"
}

# Wait for fips0 interface (up to 30s).
wait_for_interface() {
    for i in $(seq 1 30); do
        ip link show "$FIPS_INTERFACE" >/dev/null 2>&1 && return 0
        sleep 1
    done
    log "ERROR: $FIPS_INTERFACE interface not found after 30s"
    return 1
}

# Wait for fips0 to have a global IPv6 address assigned (up to 30s).
# Returns the address on stdout, or empty string if timeout.
wait_for_fips_addr() {
    for i in $(seq 1 30); do
        local addr
        addr=$(ip -6 -o addr show dev "$FIPS_INTERFACE" scope global 2>/dev/null \
               | awk '{print $4}' | cut -d/ -f1 | head -1)
        if [ -n "$addr" ]; then
            echo "$addr"
            return 0
        fi
        sleep 1
    done
    return 1
}

# Get systemd version number.
systemd_version() {
    systemctl --version 2>/dev/null | head -1 | grep -oE '[0-9]+' | head -1
}

# Check if a systemd service is active.
is_active() {
    systemctl is-active --quiet "$1" 2>/dev/null
}

# Backend 1: systemd dns-delegate (>= 258)
#
# Targets [::1]:5354 to match the daemon's default IPv6 loopback
# bind. v4-targeted DNS= lines do not reach a daemon bound on
# explicit `::1` (Linux v6 sockets bound to ::1 don't accept
# v4-mapped traffic).
try_dns_delegate() {
    local ver
    ver=$(systemd_version)
    [ -z "$ver" ] && return 1
    [ "$ver" -lt 258 ] && return 1
    is_active systemd-resolved.service || return 1

    log "Configuring via dns-delegate (systemd $ver)"
    mkdir -p "$DNS_DELEGATE_DIR"
    cat > "$DNS_DELEGATE_FILE" <<EOF
[Delegate]
DNS=[${FIPS_DNS_LOOPBACK_V6}]:${FIPS_DNS_PORT}
Domains=fips
EOF
    systemctl restart systemd-resolved
    save_backend "dns-delegate"
    return 0
}

# Backend 2: systemd-resolved global drop-in (preferred for systemd-resolved hosts)
#
# Writes /etc/systemd/resolved.conf.d/fips.conf with DNS=[::1]:5354 and
# Domains=~fips. systemd-resolved routes the .fips zone via standard
# loopback (no per-link interface scoping), which sidesteps two issues:
#
#   - Ubuntu 22 (systemd 249) interface-scoped routing of per-link DNS
#     servers, which would otherwise cause `resolvectl dns fips0
#     127.0.0.1:5354` to route through fips0 and fail.
#   - The IPV6_PKTINFO ifindex attribution behaviour where Linux reports
#     a packet to fips0's own IPv6 address as arriving on fips0 (despite
#     loopback delivery), which would otherwise be silently dropped by
#     the daemon's mesh-interface filter — see ISSUE-2026-0002 in the
#     project tracker.
#
# This backend is the recommended path on every systemd-resolved host
# that doesn't have native dns-delegate support (systemd >= 258).
try_global_drop_in() {
    is_active systemd-resolved.service || return 1

    log "Configuring via global resolved.conf.d drop-in (DNS=[${FIPS_DNS_LOOPBACK_V6}]:${FIPS_DNS_PORT})"
    mkdir -p "$RESOLVED_DROPIN_DIR"
    cat > "$RESOLVED_DROPIN_FILE" <<EOF
# Managed by fips-dns.service (fips-dns-setup, global-drop-in backend).
# Routes .fips queries to the fips daemon's local DNS responder.
[Resolve]
DNS=[${FIPS_DNS_LOOPBACK_V6}]:${FIPS_DNS_PORT}
Domains=~fips
EOF
    systemctl restart systemd-resolved \
        || log "WARNING: systemd-resolved restart failed (config written, may need manual reload)"
    save_backend "global-drop-in"
    return 0
}

# Backend 3: resolvectl (legacy fallback)
#
# systemd-resolved applies interface-scoped routing to per-link DNS servers.
# On Ubuntu 22 (systemd 249), configuring `resolvectl dns fips0 127.0.0.1:5354`
# causes resolved to route the DNS query through fips0 and fail (127.0.0.1 is
# not reachable via a TUN with only fd00::/8 routes). Using fips0's own global
# IPv6 address (locally delivered by the kernel) sidesteps that — but on hosts
# where the daemon binds wildcard `::`, IPV6_PKTINFO reports `ipi6_ifindex ==
# fips0` for those packets and the daemon's mesh-interface filter drops every
# query silently. The global-drop-in backend above avoids both issues; this
# resolvectl path is retained only as a fallback for environments where the
# global drop-in is unsuitable.
try_resolvectl() {
    command -v resolvectl >/dev/null 2>&1 || return 1
    is_active systemd-resolved.service || return 1

    local fips_addr
    fips_addr=$(wait_for_fips_addr)
    if [ -z "$fips_addr" ]; then
        log "ERROR: $FIPS_INTERFACE has no global IPv6 address after 30s"
        return 1
    fi

    log "Configuring via resolvectl (DNS=[${fips_addr}]:${FIPS_DNS_PORT})"
    resolvectl dns "$FIPS_INTERFACE" "[${fips_addr}]:${FIPS_DNS_PORT}"
    resolvectl domain "$FIPS_INTERFACE" "~fips"
    save_backend "resolvectl"
    return 0
}

# Backend 4: standalone dnsmasq
#
# dnsmasq's `server=/<domain>/<addr>#<port>` syntax accepts an IPv6
# literal directly (no brackets). Targets ::1#5354 to match the
# daemon's default IPv6 loopback bind.
try_dnsmasq() {
    is_active dnsmasq.service || return 1
    [ -d /etc/dnsmasq.d ] || return 1

    log "Configuring via dnsmasq"
    cat > "$DNSMASQ_CONF" <<EOF
# FIPS .fips domain forwarding (managed by fips-dns.service)
server=/fips/${FIPS_DNS_LOOPBACK_V6}#${FIPS_DNS_PORT}
EOF
    systemctl reload dnsmasq || log "WARNING: dnsmasq reload failed (config written, may need manual reload)"
    save_backend "dnsmasq"
    return 0
}

# Backend 5: NetworkManager with dnsmasq plugin
try_nm_dnsmasq() {
    is_active NetworkManager.service || return 1

    # Check if NM is configured to use dnsmasq
    local nm_dns
    nm_dns=$(grep -sh '^dns=' /etc/NetworkManager/NetworkManager.conf \
             /etc/NetworkManager/conf.d/*.conf 2>/dev/null | tail -1 | cut -d= -f2)
    [ "$nm_dns" = "dnsmasq" ] || return 1
    [ -d /etc/NetworkManager/dnsmasq.d ] || return 1

    log "Configuring via NetworkManager dnsmasq plugin"
    cat > "$NM_DNSMASQ_CONF" <<EOF
# FIPS .fips domain forwarding (managed by fips-dns.service)
server=/fips/${FIPS_DNS_LOOPBACK_V6}#${FIPS_DNS_PORT}
EOF
    nmcli general reload || log "WARNING: NetworkManager reload failed (config written, may need manual reload)"
    save_backend "nm-dnsmasq"
    return 0
}

# --- Main ---

wait_for_interface || exit 1

try_dns_delegate && exit 0
try_global_drop_in && exit 0
try_resolvectl && exit 0
try_dnsmasq && exit 0
try_nm_dnsmasq && exit 0

log "WARNING: No supported DNS resolver detected."
log "To resolve .fips domains, configure your DNS resolver to forward"
log "the .fips domain to [${FIPS_DNS_LOOPBACK_V6}]:${FIPS_DNS_PORT} (matches the daemon's default bind)."
log ""
log "Examples:"
log "  systemd-resolved: sudo apt install systemd-resolved"
log "  dnsmasq:          echo 'server=/fips/${FIPS_DNS_LOOPBACK_V6}#${FIPS_DNS_PORT}' | sudo tee /etc/dnsmasq.d/fips.conf"
save_backend "none"
exit 0
