get IP4 network configuration from /proc/net

networking.sh

#!/bin/bash

# get network configuration for from /proc/net
# original: <https://stackoverflow.com/a/14725655/5147619>

# WARNING: works only for first adapter listed and only for IP4

# NOTE: ip functions set variables instead of returning to STDOUT

hex_to_int() {
  printf -v $1 "%d\n" 0x${2:6:2}${2:4:2}${2:2:2}${2:0:2}
}

int_to_ip() {
  local var=$1 ip
  shift
  for ip ;do
      printf -v $var "%s%s.%s.%s.%s\n" "${!var}" $(($ip>>24)) \
        $(($ip>>16&255)) $(($ip>>8&255)) $(($ip&255))
  done
}

mask_len() {
  local i
  for ((i=0; i<32 && ( 1 & $2 >> (31-i) ) ;i++));do :;done
  printf -v $1 "%d" $i
}

while read -a line ;do
  if [ ${line[2]} == "00000000" ] && [ ${line[7]} != "00000000" ] ;then
    hex_to_int net_int  ${line[1]}
    hex_to_int mask_int ${line[7]}
    if [ $((net_int&mask_int)) == $net_int ] ;then
      for connections in /proc/net/{tcp,udp} ;do
        while IFS=': \t\n' read -a conn ;do
          if [[ ${conn[1]} =~ ^[0-9a-fA-F]*$ ]] ;then
            hex_to_int ip_int ${conn[1]}
            [ $((ip_int&mask_int)) == $net_int ] && break 3
          fi
        done < $connections
      done
    fi
  elif [ ${line[1]} == "00000000" ] && [ ${line[7]} == "00000000" ] ;then
    hex_to_int gateway_int ${line[2]}
  fi
done < /proc/net/route

mask_len mask_bits $mask_int
int_to_ip address_line $ip_int $net_int $gateway_int $mask_int

if [[ -z $PRINT ]] ;then
  echo $line
  echo -n "$address_line"
else
  printf -v outForm '%-12s: %%s\\n' \
         Interface Address Network Gateway Netmask Masklen
  printf "$outForm" $line $address_line $mask_bits\ bits
fi