determine_max_packet_size

#!/bin/sh
#
# determines the maximum packet size of the current connection to a specific host
# tested on Arch Linux (bash/dash) and Gluon (busybox ash)
#
# based on script written by @pinguinpfleger (Kai Hauser) for Freifunk Suedpfalz
# https://github.com/freifunk-suedpfalz/check_icmp_packetsize
# license: GPLv3
#

set -u  # treat unset variables as an error

# ---------- configuration ----------
checkMinPacketSize=1232
checkMaxPacketSize=1500

# ---------- argument parsing ----------

if [ "$#" -eq 0 ]; then
    echo "Usage: $(basename "$0") [-v] <host-or-ip>" >&2
    exit 1
fi

_verbose=0
_host=""

if [ "$1" = "-v" ]; then
    _verbose=1
    if [ "$#" -lt 2 ]; then
        echo "Usage: $(basename "$0") [-v] <host-or-ip>" >&2
        exit 1
    fi
    _host=$2
else
    _host=$1
fi

# ---------- helpers (all take arguments, no globals) ----------

is_ipv4() {
    printf '%s\n' "$1" | grep -Eq '^[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+$'
}

is_ipv6() {
    printf '%s\n' "$1" | grep -Eq '^[0-9A-Fa-f]*:[0-9A-Fa-f:]*$'
}

# Portable DNS check. Tries modern nslookup querytype first, falls back to
# generic nslookup + grep for BusyBox/minimal systems.
has_dns_record() {
    __host="$1"
    __rtype="$2"

    # Attempt 1: GNU nslookup / recent BusyBox (-querytype)
    if ! __out=$(nslookup -querytype="$__rtype" "$__host" 2>/dev/null); then
        :
    elif printf '%s\n' "$__out" | grep -A1 "Name:" | grep -q "Address:"; then
        return 0
    fi

    # Attempt 2: alternate syntax (-type)
    if ! __out=$(nslookup -type="$__rtype" "$__host" 2>/dev/null); then
        :
    elif printf '%s\n' "$__out" | grep -A1 "Name:" | grep -q "Address:"; then
        return 0
    fi

    # Fallback: generic nslookup (older BusyBox). A valid record section contains
    # a "Name:" line followed by an "Address:" line. NXDOMAIN output has no "Name:".
    if ! __out=$(nslookup "$__host" 2>/dev/null); then
        return 1
    fi
    # shellcheck disable=SC3057
    case "$__rtype" in
        A|AAAA)
            printf '%s\n' "$__out" | grep -q "^Name:"
            ;;
        *)
            return 1
            ;;
    esac
}

# ---------- general DNS sanity check ----------
# Some corporate resolvers return SERVFAIL for AAAA even when A works.
# Only fail if NEITHER A nor AAAA resolves.
if ! is_ipv4 "$_host" && ! is_ipv6 "$_host"; then
    if ! has_dns_record "$_host" "A" && ! has_dns_record "$_host" "AAAA"; then
        echo "Error: Name resolution of '$_host' failed entirely." >&2
        exit 20
    fi
fi

# ---------- Gluon environment checks ----------

if [ -e /lib/gluon/release ]; then
    if ! batctl o 2>/dev/null | grep -q mesh-vpn; then
        echo "This Gluon node has no VPN connection." >&2
        exit 3
    fi

    if ! [ -x /bin/opkg ]; then
        echo "This Gluon node doesn't have opkg. The necessary advanced ping utilities can't be installed." >&2
        exit 4
    fi

    for _pkg in iputils-ping iputils-ping6; do
        if ! opkg list-installed 2>/dev/null | grep -q "$_pkg"; then
            echo "This Gluon node doesn't have the necessary advanced ping utility yet. Please install it first:" >&2
            echo "> opkg update && opkg install iputils-ping iputils-ping6" >&2
            exit 5
        fi
    done
fi

# ---------- ping command selection ----------

if ping 2>&1 | grep -q interval; then
    pingparams="-q -c 2 -W 1 -M do -i 0.2 -s"
else
    pingparams="-q -c 2 -W 1 -M do -s"
fi

ping4="ping"
if command -v ping6 >/dev/null 2>&1; then
    ping6="ping6"
else
    ping6="ping -6"
    ping4="ping -4"
fi

# ---------- safe ping execution (POSIX/ash compatible) ----------
# Instead of storing commands in a string and relying on unquoted expansion,
# we build positional parameters with "set --" and execute via "$@".
# This avoids SC2086 word-splitting issues while staying ash-compatible.

_p4addr=""
_p6addr=""

ping_host() {
    __proto="$1"
    __size="$2"
    __target="$3"
    __verb="$4"

    if [ "$__proto" = "IPv4" ]; then
        if [ -n "$_p4addr" ]; then
            set -- "$ping4" -I "$_p4addr" $pingparams "$__size" "$__target"
        else
            set -- "$ping4" $pingparams "$__size" "$__target"
        fi
    else
        if [ -n "$_p6addr" ]; then
            set -- "$ping6" -I "$_p6addr" $pingparams "$__size" "$__target"
        else
            set -- "$ping6" $pingparams "$__size" "$__target"
        fi
    fi

    if [ "$__verb" -eq 1 ]; then
        "$@"
    else
        "$@" >/dev/null 2>&1
    fi
}

# ---------- core function ----------

mtu_check() {
    _proto="$1"

    # 1. Literal / DNS checks
    if [ "$_proto" = "IPv4" ]; then
        if is_ipv6 "$_host"; then return 10; fi
        if ! is_ipv4 "$_host" && ! has_dns_record "$_host" "A"; then
            echo "IPv4: No A record found for '$_host'."
            return 20
        fi
    else
        if is_ipv4 "$_host"; then return 10; fi
        if ! is_ipv6 "$_host" && ! has_dns_record "$_host" "AAAA"; then
            echo "IPv6: No AAAA record found for '$_host'."
            return 20
        fi
    fi

    # 2. Route Logic
    if [ "$_proto" = "IPv6" ]; then
        if ! ip -6 route show default 2>/dev/null | grep -q "^default"; then
            echo "IPv6: No default route available." >&2
            return 10
        fi
        if [ -e /lib/gluon/release ] && ! ip -6 route show default 2>/dev/null | grep -q "br-wan"; then
            echo "IPv6: Default route not on br-wan (Gluon), skipping." >&2
            return 10
        fi
    fi

    # 3. Bind to br-wan IPs on Gluon
    _p4addr=""
    _p6addr=""
    if [ -e /lib/gluon/release ]; then
        _p4addr=$(ip -4 addr show dev br-wan scope global 2>/dev/null | grep inet | head -1 | awk '{print $2}' | cut -d/ -f1)
        _p6addr=$(ip -6 addr show dev br-wan scope global 2>/dev/null | grep inet6 | grep -v "inet6 fd" | head -1 | awk '{print $2}' | cut -d/ -f1)
    fi

    [ "$_verbose" -eq 1 ] && echo "--- Checking $_proto for $_host ---"

    # 4. Initial Connectivity Check
    if ! ping_host "$_proto" "$checkMinPacketSize" "$_host" 0; then
        echo "$_proto: Smallest packet size $checkMinPacketSize is not usable (Host unreachable or ICMP blocked)."
        return 30
    fi

    # 5. Max Size Check
    if ping_host "$_proto" "$checkMaxPacketSize" "$_host" 0; then
        echo "$_proto: Packet size: $checkMaxPacketSize"
        return 0
    fi

    # 6. Binary Search
    _low=$checkMinPacketSize
    _high=$checkMaxPacketSize
    # FIX from v3: initialize with the KNOWN GOOD minimum
    _best=$checkMinPacketSize

    while [ $((_high - _low)) -gt 1 ]; do
        _mid=$(( (_low + _high) / 2 ))
        [ "$_verbose" -eq 1 ] && echo "Testing size: $_mid"

        if ping_host "$_proto" "$_mid" "$_host" 0; then
            _low=$_mid
            _best=$_mid
        else
            _high=$_mid
        fi
    done

    # _best is the max ping -s payload. +8 = ICMP echo header.
    # On-wire total = payload + ICMP(8) + IPv4(20) / IPv6(40).
    _icmp_total=$((_best + 8))
    if [ "$_proto" = "IPv4" ]; then
        _wire_total=$((_icmp_total + 20))
    else
        _wire_total=$((_icmp_total + 40))
    fi
    echo "$_proto: ping -s payload: $_best (ICMP: $_icmp_total, on-wire: $_wire_total)"
    return 0
}

# ---------- run ----------

mtu_check "IPv4"
_rc4=$?

mtu_check "IPv6"
_rc6=$?

# Exit with the highest error code if both protocols fail.
# "Skipped" (10, 20) counts as failure for this purpose.
if [ "$_rc4" -ne 0 ] && [ "$_rc6" -ne 0 ]; then
    if [ "$_rc4" -ge "$_rc6" ]; then
        exit "$_rc4"
    else
        exit "$_rc6"
    fi
fi
exit 0