mx

#!/bin/sh
help() { cat <</help
Look up and parse MX record (or else A record; see RFC821) for given host(s)
Usge: mx [OPTIONS] host [host...]

  -0, --as-is     Do not clean up arguments. Assume hosts are valid.
  -d, --dns=HOST  Use HOST as the DNS server

Part of net-scripts: https://github.com/adamhotep/net-scripts
mx 0.6.20230923.1 Copyright 2010+ by Adam Katz, GPLv2+
/help
exit
}

# Defaults
as_is=

# in case column doesn't exist, create a dummy for it
if ! command -v column >/dev/null 2>&1; then column() { cat; }; fi

die() { echo "$*" >&2; exit 2; }  # complain to STDERR and exit with error
needs_arg() { if [ -z "$OPTARG" ]; then die "No arg for --$OPT option"; fi; }
while getopts 0d:hvV-: OPT; do
  if [ "$OPT" = - ]; then # long opt https://stackoverflow.com/a/28466267/519360
    OPT="${OPTARG%%=*}" OPTARG="${OPTARG#$OPT}" OPTARG="${OPTARG#=}"
  fi
  case $OPT in
    ( 0 | as-is )	as_is="-0" ;;
    ( d | dns* )	needs_arg; dns="$OPTARG" ;;
    ( h* )		help ;;
    ( [Vv] | vers* )	help |tail -n2 ;;
    ( \? )		die "Try --help" >&2 ;;
    ( ??* )		die "Illegal option --$OPTARG ... Try --help" ;;
  esac
done
shift $((OPTIND-1))

# very basic sanity check on the DNS server being legit (and okay to not quote)
case $dns in ( *[!A-Za-z0-9.-]* | [.-]* | *[.-][.-]* | *- )
  die "Invalid DNS server '$dns'" ;;
esac

for host in "$@"; do

  if [ "$host" != "$1" ]; then echo ""; fi	# space betwen hosts
  if [ $# -gt 1 ]; then echo "$host:"; fi	# identify each host if 2+ hosts

  if [ -z "$as_is" ]; then			# cleanup
    host="${host##*[<@]}"			# remove username if present
    while true; do	# peel off bad chars from start
      hh="${host#[!a-zA-Z0-9_.-]}"	# [^...] now fails(!) using older [!...]
      if [ "$hh" = "$host" ]; then break; fi
      host="$hh"
    done
    host="${host%%[!a-zA-Z0-9_.-]*}"		# remove trailing garbage
  fi

  host -t MX -- "$host" $dns |awk -v host="$host" -v dns="$dns" '
    function dns_notes() {
      return (NF==0 || /^(Using domain server|Name|Address|Aliases):/)
    }
    dns_notes() { next }
    NF == 7 && $2$3$4$5 == "mailishandledby" {
      pref[$NF] = $6
      next
    }

    # RFC5321 5.1 says we fail over to the host itself if there is no MX:
    #     If an empty list of MXs is returned, the address is
    #     treated as if it was associated with an implicit MX RR,
    #     with a preference of 0, pointing to that host.
    $3$4$5 == "noMXrecord" { pref[host] = 0 }
    { print > "/dev/stderr" }
    #/\(NXDOMAIN\)/ { retval = 1 }	# not usable thanks to sort filter

    END {

      for (mx in pref) {
        cmd = "host -- " mx " " dns
        while ( (cmd | getline) > 0) {
          if (dns_notes() || $2 $3 $4 $5 == "mailishandledby") continue
          if (index($NF,"(NXDOMAIN)")) {
            $NF = "NXDOMAIN"
            $1 = $2
          }
          printf "%4d  %-15s  %s\n", pref[mx], $NF, $1
        }
        close (cmd)
      }

    }
  ' |sort -k1n,1 -k3b,3 -k2n,2 |column -t

done