Assign IP based on if giaddr is even or odd

[Thor Simon]

You could easily enough do this with a funky “netmask” (the mask that picks out the “even Internet” from the “odd Internet” used to be a favorite puzzler question of mine) but ISC DHCP, like much other software these past 30 years, may not support that.

From: dhcp-users <dhcp-users-bounces at lists.isc.org> On Behalf Of Stephen Donovan
Sent: Wednesday, January 6, 2021 12:54 PM
To: dhcp-users at lists.isc.org
Subject: Assign IP based on if giaddr is even or odd

Hello Gurus

I am trying to deploy a series of routers that can use DHCP to provide an IP address to the neighbouring router and once the router is reachable management software will push configuration to it.

All links between the routers are /31 subnets and it is possible that each router could be configured in each direction, for example if router A is up, then it will offer an address to router B, however if router B is up before A then B will offer an address to A.

I tried the sample subnet shown below and everything works, if I use the full giaddr in the match statement, however if I try to simplify it to match the class if the giaddr is even or odd doesn't work. My attempt on using a regex to match even/odd is commented out.

Does anyone have any suggestions on how I could use a different match to hand out different addresses based on if the giaddr is even or odd? Did I make a fundamental mistake in my match statement. I'm willing to use the full match, but would like to simplify if possible.

Thanks in advance
Steve

#test subnet
subnet 11.12.13.44 netmask 255.255.255.254 {
log(debug, concat ("giaddr: ", binary-to-ascii(10,8, ".", packet(27,1)))) ;
class "cw" {
#match if (binary-to-ascii(10,8, ".", packet(27,1)) ~~
#"^\d*[02468]$");
match if (binary-to-ascii(10,8, ".", packet(24,4)) =
"11.12.13.44");
}

pool
{
allow members of "cw";
range 11.12.13.45 11.12.13.45;
option routers 11.12.13.44;
option subnet-mask 255.255.255.254;
}

pool
{
deny members of "cw";
range 11.12.13.44 11.12.13.44;
option routers 11.12.13.45;
option subnet-mask 255.255.255.254;
}

}
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