CentOS 7 DHCP Client - How to use “classless-static-routes” (“code 121”)?

Question

According to this answer...

https://unix.stackexchange.com/a/457577/61742

... I can use the following configuration to use pushed routes from my DHCP server in my DHCP client by placing in the /etc/dhclient.conf or /etc/dhcp3/dhclient.conf file the following configuration...

option classless-static-routes code 121 = array of { ip-address, ip-address };

But in CentOS 7 this file does not exist, but the following files...

[root@localhost ~]# ps -eaf | grep dhcli
root       780   650  0 15:02 ?        00:00:00 /sbin/dhclient -d -q -sf /usr/libexec/nm-dhcp-helper -pf /var/run/dhclient-enp0s8.pid -lf /var/lib/NetworkManager/dhclient-00cb8299-feb9-55b6-a378-3fdc720e0bc6-enp0s8.lease -cf /var/lib/NetworkManager/dhclient-enp0s8.conf enp0s8
root       783   650  0 15:02 ?        00:00:00 /sbin/dhclient -d -q -sf /usr/libexec/nm-dhcp-helper -pf /var/run/dhclient-enp0s17.pid -lf /var/lib/NetworkManager/dhclient-8512e951-6012-c639-73b1-5b4d7b469f7f-enp0s17.lease -cf /var/lib/NetworkManager/dhclient-enp0s17.conf enp0s17
root      2218  1152  0 15:36 pts/0    00:00:00 grep --color=auto dhcli

NOTE: Note the value for the "-cf" parameter (files: /var/lib/NetworkManager/dhclient-enp0s8.conf and /var/lib/NetworkManager/dhclient-enp0s17.conf).

My question is: What is the correct way to use "classless-static-routes" (code 121) on a CentOS 7 DHCP client?

Thanks!

---

PLUS:

This is the configuration of my ISC KEA DHCP (DHCPv4)...

cat /usr/local/etc/kea/kea-dhcp4.conf

[...]
"option-def": [{
        "name": "rfc3442-classless-static-routes",
        "code": 121,
        "space": "dhcp4",
        "type": "record",
        "record-types": "uint8,uint8,uint8,ipv4-address"
    }
],
"option-data": [{
        "name": "rfc3442-classless-static-routes",
        "data": "10,1,4, 10.1.6.4"
}]
[...]

The goal is that requests to 10.1.4.0/24 be routed to ip 10.1.6.4/32 in my DHCP clients.

NOTE: Is the same thing I would get with the command ip route add 10.1.4.0/24 dev enp0s8 via 10.1.6.4 in each my DHCP clients.

Answer 1

Shameless copying from this answer:

If you wish to do this via DHCP, ensure that your have properly configured your DHCP client per the RedHat Knowledgebase article in here

To ensure that RFC3442-standard classless static routes provided by a DHCP server are processed correctly when using NetworkManager, the following lines should be placed into /etc/dhclient.conf or, if using per-interface DHCP options, /etc/dhclient-<ifname>.conf:

option rfc3442-classless-static-routes code 121 = array of unsigned integer 8; 
option ms-classless-static-routes code 249 = array of unsigned integer 8; 
also request rfc3442-classless-static-routes;
also request ms-classless-static-routes;

These lines will ensure that RFC3442 classless static routes are requested from the DHCP server, and that they are properly processed by NetworkManager.

Answer 2

In fact this answer is a complement to @Isaac's answer and some pitfalls - may seem a bit obvious, but not to me - on the issue presented.

First of all, although there is some pattern within the presented universe, it is necessary to understand a correlation between what is configured in DHCP Server and what is expected in DHCP Client. Practically, parameters such as "option-def" -> "name" can vary between "rfc3442-classless-static-routes" and "classless-static-routes", "option-def" -> "record-types" between "uint8, uint8, uint8, ipv4-address" and "uint8, uint8, uint8, uint8, uint8, uint8, uint8, uint8", and so on... That is, it depends on what the DHCP Client expects from the "other side" basically.

In the case of CentOS 7 what is pointed out in the thread https://unix.stackexchange.com/a/459963/61742 is already configured by default, then it is only necessary to configure in ISC KEA DHCP (DHCPv4) (my case) what CentOS 7 waits from the "other side".

Finally, the configuration looks like this...

cat /usr/local/etc/kea/kea-dhcp4.conf

[...]
"option-def": [{
        "name": "rfc3442-classless-static-routes",
        "code": 121,
        "space": "dhcp4",
        "type": "record",
        "array": true,
        "record-types": "uint8,uint8,uint8,uint8,uint8,uint8,uint8,uint8"
    }
],
"option-data": [{
        "name": "rfc3442-classless-static-routes",
        "data": "24,10,1,4,10,1,6,4"
}]
[...]

... since CentOS 7 is configured as...

cat /var/lib/NetworkManager/dhclient-enp0s8.conf

[...]
option rfc3442-classless-static-routes code 121 = array of unsigned integer 8; 
also request rfc3442-classless-static-routes;
[...]

---

SOME CLARIFICATIONS:

"record-types" is a mask, that is, what is in each position of the array informed in "date" that as expected in CentOS 7 is an "array of unsigned integer 8" (uint8).

"data" works according to the following example: if I want the following routing 10.1.4.0/24 via 10.1.6.4 then I will inform "24,10,1,4,10,1,6,4". Note that the zero in 10.1.4.0/24 should be omitted since it is a mask for 3 octets (/24).

TIP: The "option-data" parameter can be placed in multiple locations in "subnet4" (file "/usr/local/etc/kea/kea-dhcp4.conf") and this will depend on how you want to distribute your configuration for routing.

Especial thanks to @Isaac , @Rui F Ribeiro and @A.B !