[SOLVED] Small script to check if what the user entered is an ip or not

robertjinx · 11-28-2010, 03:32 PM

Hello, does anyone know how to do a small script that checks if what the users entered is an ip address or not? Also possible to do the same but instead of an ip address is a netmask.

For example the user entered: 192.168.0.1 so its true this is an ip address, if it doesn't look like an ip address then is false

And second example, the user entered: 255.255.255.0 so the script returns true, and if its not a netmask the script returns false.

If anyone knows how to do this, I will really appreciate the help.

Thank you!

Guttorm · 11-28-2010, 03:52 PM

Code:

egrep "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}"

But this allows illegal addresses as well, as 999.999.999.999

robertjinx · 11-28-2010, 04:27 PM

What about if the ip is 10.10.234.10?

Noway2 · 11-29-2010, 04:05 AM

The technique to use, which Guttorm was pointing to, is called regular expressions. Regular expressions are a pattern matching syntax that uses symbols to represent logical pattern constructs. For example, in the sample provided in the earlier post, "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}", the [] are called character classes where you match on the range of characters in the block. This is followed the {1,3} which means that there must be at least 1 of these and no more than 3. the \. means that this should then be followed by a literal . character, which is escaped with the \ because it is normally a wild card that matches a single character.

Putting this all together then and repeating the three times says to match a set of three, 1 to 3 digit numbers with periods in between. As was pointed out, though, this will match numbers like 999.999.999.999, which are not valid. A similar pattern could be used with a netmask.

The problem with any type of pattern matching like this is that it gets really tricky to be selective, as the 999 example shows and this results in more complex expressions, which can include things like conditionals. What you can do is use the expression to match and verify the form of the input and then use your application code to verify the range and validity of the data.

You didn't say what your application is programmed in, but many recent programming languages, such as PHP and PERL, have support for regular expressions as do utilities such as egrep, sed, awk, etc.

robertjinx · 11-29-2010, 05:09 AM

I found a small function which works quite well:

Code:

valid_ip()
{
    local  ip=$1
    local  stat=1

    if [[ $ip =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
        OIFS=$IFS
        IFS='.'
        ip=($ip)
        IFS=$OIFS
        [[ ${ip[0]} -le 255 && ${ip[1]} -le 255 \
            && ${ip[2]} -le 255 && ${ip[3]} -le 255 ]]
        stat=$?
    fi
    return $stat
}

catkin · 11-29-2010, 07:06 AM

Nice looking function and thanks for sharing it.

Two concerns for completeness, though:

The function validates broadcast and network addresses as IP addresses. OK?
ISTR that IP address octets, normally given in decimal, can also be given in octal in which case they must begin with 0. If true this means that a valid octet with decimal value 64 to 255 would have four octal digits.. Maybe you don't want to support IP addresses expressed in octal. If that is so, the validating regex becomes ^([1-9][0-9]{1,2})|(0)\. and so on (assuming you don't want to accept octets of 00 and 000 which you might because they are valid if perverse).

In the OP you asked for netmask validation too:

Code:

validate_netmask()
{
    local netmask=$1
    local netmask_binary
    local octet
    local stat

    if [[ $netmask =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
        stat=0
        for ((i=0; i<4; i++))
        do
            octet=${netmask%%.*}
            netmask=${netmask#*.}
            [[ $octet -gt 255 ]] && { stat=1; break; }
            netmask_binary=$netmask_binary$( echo "obase=2; $octet" | bc )
            [[ $netmask_binary =~ 01 ]] && { stat=1; break; }
        done
    else
        stat=1
    fi
    return $stat
}

robertjinx · 11-30-2010, 01:44 AM

Hi, thx for the netmask function, it really helps.

Thanks for all the help and ideas