Validating IP addresses (IPv4 and IPv6)

Question

Quite simply I'd like to verify that a string represents a valid IP address in a Bash script. I think like many others I've fallen into the pitfall of trying to do this with a regular expression; while this works well enough for IPv4, IPv6 is more complex (as it supports compression of zeroes) and it's just a complicated and not especially readable solution.

However, Unix and Linux clearly understand IP addresses well enough, so I'm wondering if there is a better way that I can validate IP addresses in bash? Preferably a reasonably common tool that can be used to do-so.

I'd like to avoid using other languages such as Python to do this, as I am in fact hoping to replace a current solution which is to use PHP's filter_var function. It works, but as with regular expressions, it's introducing a second language where I hope to avoid using one.

Lastly, if there's a solution that can also handle IP address ranges (such as 0.0.0.0-255.255.255.255 or 0.0.0.0/16 etc.) that'd be ideal, but I can handle these myself.

Answer 1

How about using ip command;

ip route get <ip-address>

If it's not valid, the return value would be 1.

(2022/9/17) To answer @MaXi32's comment, I quickly checked this on my Linux laptop without an internet connection. The ip command returns 1 for invalid IP and 2 for valid IP when the network is down.

For valid IPs:

$ ip route get 192.168.0.255 ; echo $?
RTNETLINK answers: Network is unreachable
2
$ ip route get fd00::192.168.0.255 ; echo $?
RTNETLINK answers: Network is unreachable
2

For invalid IPs:

$ ip route get 192.168.0.256 ; echo $?
Error: any valid prefix is expected rather than "192.168.0.256".
1
$ ip route get fd00::192.168.0.256 ; echo $?
Error: any valid prefix is expected rather than "fd00::192.168.0.256".
1

Version of ip command:

$ ip -V
ip utility, iproute2-5.9.0, libbpf 0.3.0

(2022/9/17#2) One can write a small program using inet_pton. I think that's even better. The following code from the example of inet_pton manpage tries to convert IPs from text to binary and raise error if the IPs are not valid:

   #include <arpa/inet.h>
   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>

   int
   main(int argc, char *argv[])
   {
       unsigned char buf[sizeof(struct in6_addr)];
       int domain, s;
       char str[INET6_ADDRSTRLEN];

       if (argc != 3) {
           fprintf(stderr, "Usage: %s {i4|i6|<num>} string\n", argv[0]);
           exit(EXIT_FAILURE);
       }

       domain = (strcmp(argv[1], "i4") == 0) ? AF_INET :
                (strcmp(argv[1], "i6") == 0) ? AF_INET6 : atoi(argv[1]);

       s = inet_pton(domain, argv[2], buf);
       if (s <= 0) {
           if (s == 0)
               fprintf(stderr, "Not in presentation format");
           else
               perror("inet_pton");
           exit(EXIT_FAILURE);
       }

       if (inet_ntop(domain, buf, str, INET6_ADDRSTRLEN) == NULL) {
           perror("inet_ntop");
           exit(EXIT_FAILURE);
       }

       printf("%s\n", str);

       exit(EXIT_SUCCESS);
   }

Answer 2

Based on the answer from @ktaka I don't recommend using ip route get to validate IP addresses unless your script depends heavily on the network connection. The reason is because, if your system does not have a good internet connection or is completely disconnected from the internet, then when you use ip route get, your script will always return error code (other than 0) even the IP is correctly defined.

Solution:

I looked up the manual link given by @Rolf Rander and I was confused why my ipcalc binary on Debian 11 does not have the -c option, and also my previous ipcalc binary does not support ipv6. So, I searched on Google the author username taken from the manual page with this keyword dcantrell ipcalc and I got the official link here:

https://gitlab.com/ipcalc/ipcalc

So the above is the official ipcalc that supports both IPv4 and IPv6 IP addresses. To install this, they recommend using the modern build with meson (but they also provided old make build if you don't want to use meson)

So, this is how I setup ipcalc on Debian 11:

Remove existing ipcalc (the existing version is not good)

apt-get -y remove ipcalc
rm -rf "/usr/bin/ipcalc"

Now install ipcalc from the official repo here:

apt -y install meson
git clone https://gitlab.com/ipcalc/ipcalc.git
cd ipcalc
meson setup build --buildtype=release
ninja -C build
cp build/ipcalc /usr/bin
apt -y remove meson

ipcalc --version

1.0.1

Now you can use this ipcalc to check both IPv4 and IPv6:

#!/bin/bash
IP="1.1.1.1"
if ipcalc -s -c $IP; then
  echo "valid IPv4 or IPv6"
else
  echo "Invalid IPv4 or Ipv6"
fi

The -s is to ignore error message, the -c is for checking without output

If you want to check specific IPv4 use -4 or use -6 to check IPv6. Example:

#!/bin/bash

# IP is a valid IPv6
IP="::1"

# But we want to check only IPv4

if ipcalc -s -4 -c $IP; then
  echo "valid IPv4"
else
  echo "Invalid IPv4"
fi

Output:

Invalid IPv4

I have been using sipcalc and ipv6calc long time ago and they can do both IPv4 and IPv6 checking, but I do like more on ipcalc because most of the output are simple and it has json output for other purposes.

EDIT

(23/10/2022):

Based on the latest comment from @ktaka, my main point was that when using ip route get without internet connection like below:

$ ip route get 192.168.0.255 ; echo $?
RTNETLINK answers: Network is unreachable
2

it is not going to return a valid IP if your system is behind firewall or you have a bad connection. The IP is valid, and it should return the success code of 0, instead of telling unnecessary things like unreachable network.

Answer 3

bash probably lacks direct access to the relevant inet_pton(3) system call, so you'll probably need to call something that does, e.g. sipcalc (which should be in the ports or packages tree for various unix). Higher level languages can probably also get at the inet_pton(3) call, or equivalent, but the details will be language specific, e.g. I've used NetAddr::IP for subnet related code in Perl.

Answer 4

Ipcalc can help you:

ipcalc -c <ip-address>

Answer 5

This will do the trick for ipv4

# check IPv4 syntax
if [[ "$ip" =~ ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$ ]]; then
    echo 'Valid IP'
else
    echo "Invalid IP: $ip" >&2
    exit 1
fi

I'm implementing ipv6 into the script I extracted this from anyway, once I have that done, I'll update this post.