How to use shell to derive an IPv6 address from a MAC address?

Question

We know that we can use the MAC address to create an interface identifier, e.g. for a link-local IPv6 address which should be unique in the Network.

The image shows the way to do this:

My questions are:

• How can I create an IPv6 address from a MAC using awk or sed?
• OR is there any command that gives me the link-local IPv6 address for a specific MAC (something like that createIPv6 myMAC)?

Answer 1

If you want to create a whole IPv6 address from a MAC (and a given prefix), you could use the excellent ipv6calc tool by Peter Bieringer.

The following command creates a link-local IPv6 address (fe80:: prefix) from a MAC address:

$ ipv6calc --action prefixmac2ipv6 --in prefix+mac --out ipv6addr fe80:: 00:21:5b:f7:25:1b
fe80::221:5bff:fef7:251b

You can leave most of the options away and let the command guess what to do:

$ ipv6calc --in prefix+mac fe80:: 00:21:5b:f7:25:1b
No action type specified, try autodetection...found type: prefixmac2ipv6
fe80::221:5bff:fef7:251b

For Debian distros, ipv6calc is in the main repository.

Answer 2

From the IPv6 Wikipedia entry a more textual description:

A 64-bit interface identifier is most commonly derived from its 48-bit MAC address.

A MAC address 00:0C:29:0C:47:D5 is turned into a 64-bit EUI-64 by inserting FF:FE in the middle: 00:0C:29:FF:FE:0C:47:D5.

So replacing the third : with :FF:FE: should do the trick:

echo  00:0C:29:0C:47:D5 | sed s/:/:FF:FE:/3
00:0C:29:FF:FE:0C:47:D5

No idea if that syntax is specific to GNU sed.

---

Work in progress:

Convert that to bits:

for HEX in $(tr ":" " " <<< 00:0C:29:FF:FE:0C:47:D5) 
  do 
    printf "%08d " $(bc <<< "ibase=16;obase=2;$HEX") 
  done

should result in the bits 00000000 00001100 00101001 11111111 11111110 00001100 01000111 11010101 leaving only the flipping of bit number 7.

Answer 3

#! /usr/bin/env python
import sys
n=[int(x, 16) for x in sys.argv[1].split(":")]
print "fe80::%02x%02x:%02xff:fe%02x:%02x%02x" % tuple([n[0]^2]+n[1:])

Answer 4

You can create a bash function (and place it in your ~/.bashrc) that uses IFS to split the MAC address into 6 colon-separated groups and assembles them. You'll also need to flip the 7th most significant bit, i.e. bit 1 of the first byte:

mac_to_ipv6_ll() {
    IFS=':'; set $1; unset IFS
    echo "fe80::$(printf %02x $((0x$1 ^ 2)))$2:${3}ff:fe$4:$5$6"
}

Usage example:

$ mac_to_ipv6_ll 12:34:56:78:90:12
fe80::1034:56ff:fe78:9012