Possible problem with IP validation in the installer

General
1

Of topic:

I just searched through the code of the installer and found a bug (i think).

The IP check you mentioned is there (valid_ip) but the problem i see is this:

if [[ ${ip} =~ [1]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}$ ]]; then

This checks if the IP has the numbers 0-9 in it which can go up to 3 digits. The problem is that IP's can only go up to 255.255.255.255, this check would let a IP such as 999.999.999.999 count as valid.

The code i mean.

Please correct me if i'm wrong.

  1. 0-9 ↩︎

2

You're not wrong, it's mostly for formatting of the numbers, it's rather difficult to actually check for a valid IP (IPv4), very difficult for v6 in hex notation.

3 
# Test an IP address for validity:
# Usage:
#      valid_ip IP_ADDRESS
#      if [[ $? -eq 0 ]]; then echo good; else echo bad; fi
#   OR
#      if valid_ip IP_ADDRESS; then echo good; else echo bad; fi
#
function 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
}

http://www.linuxjournal.com/content/validating-ip-address-bash-script

We could also try to dig something from the provided IP. If there is no result - well then that IP might be valid in principle, but still unusable?

4

I found a solution that works.

if [[ $PIHOLE_DNS_1 =~ .*:.* ]]; then
        dns1=IPv6
else
        dns1=IPv4
fi

if [[ $PIHOLE_DNS_2 =~ .*:.*  ]]; then
        dns2=IPv6
else
        dns2=IPv4
fi

lookupDNS1="$(nslookup -retry=1 -timeout=5 google.de $PIHOLE_DNS_1)"

if [[ $lookupDNS1 == *";; connection timed out; no servers could be reached"* ]]; then
        echo "Your first DNS server ($dns1) isn't working!"
else
        echo "Your first DNS server ($dns1) is working!"
fi

lookupDNS2="$(nslookup -retry=1 -timeout=5 google.de $PIHOLE_DNS_2)"

if [[ $lookupDNS2 == *";; connection timed out; no servers could be reached"* ]]; then
        echo "Your second DNS server ($dns2) isn't working!"
else
        echo "Your second DNS server ($dns2) is working!"
fi

This can also be used to see if a IPv6 connection is avaliable.

ipv6test="$(nslookup -retry=1 -timeout=5 google.de 2001:4860:4860::8888)"

if [[ $ipv6test == *";; connection timed out; no servers could be reached"*  ]]; then
        echo "You don't have a IPv6 connection!"
else
        echo "You have a Ipv6 connection!"
fi
5

Okay, this approach needs some more protection about injection of bad code through the entered variables. What is the advantage of using nslookup? We know that dig is always available.

6

Could you give me a example of bad code that could be injected? I'm not sure how this can be used to do something bad.

I don't think that there is a advantage of using nslookup, i just had the idea to use it.

7

You are root at this point. Not tested, just thinking aloud:

PIHOLE_DNS_1=";rm -rf /etc/*"

would evaluate to:

nslookup -retry=1 -timeout=5 google.de ;rm -rf /etc/*

i.e.

nslookup -retry=1 -timeout=5 google.de
rm -rf /etc/*
1 Like