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Initial release
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#!/bin/sh
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# PID 1 of a session VM: firewalls egress to the public internet (or only the
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# proxy, or only the WireGuard server and its tunnel), then runs knotchy-agent as
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# the knotchy user with no capabilities, so nothing in the browser can undo it.
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# Fails closed: a missing or malformed setting stops the VM. Needs NET_ADMIN.
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# Env: KNOTCHY_ROUTE (direct|proxy|wireguard), KNOTCHY_BLOCK_CIDRS, KNOTCHY_PROXY_ADDR,
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# KNOTCHY_PROXY_URL*, KNOTCHY_WG_SETCONF*, KNOTCHY_WG_ENDPOINT, KNOTCHY_WG_ADDRESS,
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# KNOTCHY_WG_DNS, KNOTCHY_WG_MTU. *Secrets, normally in /run/knotchy-secrets/route.
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set -eu
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fail() {
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echo "knotchy-init: $*" >&2
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exit 1
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}
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blocked="0.0.0.0/8, 10.0.0.0/8, 100.64.0.0/10, 127.0.0.0/8, 169.254.0.0/16,
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172.16.0.0/12, 192.0.0.0/24, 192.0.2.0/24, 192.88.99.0/24, 192.168.0.0/16,
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198.18.0.0/15, 198.51.100.0/24, 203.0.113.0/24, 224.0.0.0/4, 240.0.0.0/4"
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is_ipv4() {
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case "$1" in *[!0-9.]* | "" | *..* | .* | *.) return 1 ;; esac
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old_ifs=$IFS; IFS=.
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# shellcheck disable=SC2086 # splitting on dots is the point
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set -- $1
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IFS=$old_ifs
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[ $# -eq 4 ] || return 1
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for octet in "$@"; do
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[ "${#octet}" -le 3 ] && [ "$octet" -le 255 ] || return 1
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done
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}
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is_port() {
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case "$1" in *[!0-9]* | "") return 1 ;; esac
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[ "${#1}" -le 5 ] && [ "$1" -ge 1 ] && [ "$1" -le 65535 ]
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}
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is_cidr() {
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case "$1" in */*) ;; *) return 1 ;; esac
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prefix=${1##*/}
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case "$prefix" in *[!0-9]* | "") return 1 ;; esac
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is_ipv4 "${1%/*}" && [ "${#prefix}" -le 2 ] && [ "$prefix" -le 32 ]
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}
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set -f
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for cidr in ${KNOTCHY_BLOCK_CIDRS:-}; do
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case "$cidr" in
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*[!0-9./]* | "") fail "malformed CIDR '$cidr' in KNOTCHY_BLOCK_CIDRS" ;;
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*) blocked="$blocked, $cidr" ;;
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esac
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done
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route=${KNOTCHY_ROUTE:-direct}
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case $route in
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direct | proxy | wireguard) ;;
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*) fail "unknown KNOTCHY_ROUTE '$route'" ;;
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esac
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# Secrets come on a share because `container` keeps -e values on disk. Delete
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# the file (on the Mac too) and unmount before anything else runs.
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proxy_url=${KNOTCHY_PROXY_URL:-}
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wg_setconf=${KNOTCHY_WG_SETCONF:-}
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secrets=/run/knotchy-secrets
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if [ -e "$secrets/route" ]; then
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while IFS= read -r line; do
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case "$line" in
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KNOTCHY_PROXY_URL=*) proxy_url=${line#*=} ;;
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KNOTCHY_WG_SETCONF=*) wg_setconf=${line#*=} ;;
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esac
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done < "$secrets/route"
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rm -f "$secrets/route" || fail "could not delete the secrets file"
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fi
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if grep -q " $secrets " /proc/mounts; then
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umount "$secrets" || fail "could not unmount the secrets share"
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fi
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case $route in
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direct)
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[ -z "${KNOTCHY_PROXY_ADDR:-}${KNOTCHY_WG_ENDPOINT:-}$proxy_url$wg_setconf" ] ||
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fail "proxy or VPN settings given for a direct route"
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egress="ip daddr @blocked4 reject with icmpx type admin-prohibited" ;;
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proxy)
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proxy_addr=${KNOTCHY_PROXY_ADDR:-}
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proxy_ip=${proxy_addr%:*}
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proxy_port=${proxy_addr##*:}
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is_ipv4 "$proxy_ip" && is_port "$proxy_port" || fail "malformed KNOTCHY_PROXY_ADDR"
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case "$proxy_url" in
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http://*@"$proxy_addr" | http://"$proxy_addr" | socks5://*@"$proxy_addr" | socks5://"$proxy_addr") ;;
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*) fail "malformed or missing KNOTCHY_PROXY_URL" ;;
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esac
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egress="ip daddr $proxy_ip tcp dport $proxy_port accept
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reject with icmpx type admin-prohibited" ;;
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wireguard)
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[ -n "$wg_setconf" ] || fail "KNOTCHY_WG_SETCONF is missing"
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wg_endpoint=${KNOTCHY_WG_ENDPOINT:-}
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wg_ip=${wg_endpoint%:*}
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wg_port=${wg_endpoint##*:}
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is_ipv4 "$wg_ip" && is_port "$wg_port" || fail "malformed KNOTCHY_WG_ENDPOINT"
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[ -n "${KNOTCHY_WG_ADDRESS:-}" ] || fail "KNOTCHY_WG_ADDRESS is empty"
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for cidr in $KNOTCHY_WG_ADDRESS; do is_cidr "$cidr" || fail "malformed KNOTCHY_WG_ADDRESS"; done
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wg_dns=${KNOTCHY_WG_DNS:-1.1.1.1}
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[ -n "$wg_dns" ] || wg_dns=1.1.1.1
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for ns in $wg_dns; do is_ipv4 "$ns" || fail "malformed KNOTCHY_WG_DNS"; done
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wg_mtu=${KNOTCHY_WG_MTU:-1420}
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case "$wg_mtu" in *[!0-9]* | "") fail "malformed KNOTCHY_WG_MTU" ;; esac
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[ "${#wg_mtu}" -le 4 ] && [ "$wg_mtu" -ge 576 ] || fail "malformed KNOTCHY_WG_MTU"
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wg_dns_set=$(echo $wg_dns | sed 's/ /, /g')
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# DNS only to the VPN's (often private) resolvers; outside the tunnel, only wireguard-go.
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egress="oifname \"wg0\" ip daddr { $wg_dns_set } meta l4proto { tcp, udp } th dport 53 accept
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oifname \"wg0\" ip daddr @blocked4 reject with icmpx type admin-prohibited
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oifname \"wg0\" accept
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ip daddr $wg_ip udp dport $wg_port meta skuid 0 accept
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reject with icmpx type admin-prohibited" ;;
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esac
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# No DNS exception in the firewall: a --dns resolver in a blocked range can't answer.
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dns=$(awk '$1 == "nameserver" && $2 ~ /^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$/ {
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printf "%s%s", sep, $2; sep = ", " }' /etc/resolv.conf)
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bad_ns=$(awk '$1 == "nameserver" && $2 ~ /^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$/ {
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split($2, o, "."); a = o[1] + 0; b = o[2] + 0
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if (a == 10 || a == 127 || (a == 169 && b == 254) || (a == 172 && b >= 16 && b <= 31) ||
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(a == 192 && b == 168) || (a == 100 && b >= 64 && b <= 127)) { print $2; exit }
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}' /etc/resolv.conf)
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[ -z "$bad_ns" ] || fail "resolver $bad_ns is in a blocked range; it cannot be reached through the egress firewall"
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nft -f - <<EOF || fail "could not apply the egress firewall"
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flush ruleset
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table inet knotchy {
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set blocked4 {
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type ipv4_addr
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flags interval
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auto-merge
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elements = { $blocked }
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}
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chain output {
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type filter hook output priority filter; policy accept;
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oifname "lo" accept
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meta nfproto ipv6 reject with icmpx type admin-prohibited
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$egress
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}
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chain input {
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type filter hook input priority filter; policy drop;
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iifname "lo" accept
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ct state established,related accept
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}
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chain forward {
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type filter hook forward priority filter; policy drop;
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}
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}
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EOF
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echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6 2>/dev/null || true
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nft list table inet knotchy >/dev/null 2>&1 || fail "egress firewall is not in place"
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case $route in
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direct)
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echo "knotchy-init: egress locked to the public internet${dns:+ (DNS via $dns)}" ;;
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proxy)
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echo "knotchy-init: egress locked to the proxy at $KNOTCHY_PROXY_ADDR"
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export http_proxy="$proxy_url" https_proxy="$proxy_url" \
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HTTP_PROXY="$proxy_url" HTTPS_PROXY="$proxy_url" no_proxy="" ;;
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wireguard)
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# The firewall is already in place, so a half-built tunnel leaks nothing.
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old_umask=$(umask)
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umask 077
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printf '%s' "$wg_setconf" | base64 -d > /run/knotchy-wg.conf 2>/dev/null ||
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fail "malformed KNOTCHY_WG_SETCONF"
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# Without SYS_ADMIN, which init only needed to unmount the secrets share.
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WG_I_PREFER_BUGGY_USERSPACE_TO_POLISHED_KMOD=1 setpriv --bounding-set -sys_admin \
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wireguard-go wg0 >/dev/null 2>&1 ||
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fail "could not start wireguard-go"
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wg setconf wg0 /run/knotchy-wg.conf || fail "the WireGuard settings were rejected"
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rm -f /run/knotchy-wg.conf
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umask "$old_umask"
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for cidr in $KNOTCHY_WG_ADDRESS; do
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ip -4 address add "$cidr" dev wg0 || fail "could not give the tunnel its address"
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done
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ip link set dev wg0 mtu "$wg_mtu" up || fail "could not bring the tunnel up"
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via=$(ip -4 route get "$wg_ip" | awk '{ for (i = 1; i < NF; i++) if ($i == "via") { print $(i + 1); exit } }')
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dev=$(ip -4 route get "$wg_ip" | awk '{ for (i = 1; i < NF; i++) if ($i == "dev") { print $(i + 1); exit } }')
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[ -n "$dev" ] || fail "no route to the VPN server"
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ip -4 route add "$wg_ip/32" ${via:+via "$via"} dev "$dev" || fail "could not route to the VPN server"
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ip -4 route replace default dev wg0 || fail "could not route through the tunnel"
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printf 'nameserver %s\n' $wg_dns > /etc/resolv.conf || fail "could not set the VPN's DNS"
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echo "knotchy-init: egress locked to the WireGuard tunnel via $KNOTCHY_WG_ENDPOINT" ;;
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esac
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# The browser has no business knowing the Mac's networks or the VPN's keys.
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unset KNOTCHY_ROUTE KNOTCHY_BLOCK_CIDRS KNOTCHY_PROXY_ADDR KNOTCHY_PROXY_URL KNOTCHY_WG_SETCONF \
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KNOTCHY_WG_ENDPOINT KNOTCHY_WG_ADDRESS KNOTCHY_WG_DNS KNOTCHY_WG_MTU
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export HOME=/home/knotchy USER=knotchy LOGNAME=knotchy
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exec setpriv --reuid=knotchy --regid=knotchy --init-groups \
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--inh-caps=-all --bounding-set=-all --no-new-privs \
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/usr/local/bin/knotchy-agent
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