0017413: Impossibile to use /31 address

ID	Project	Category	View Status	Date Submitted	Last Update

0017413	CentOS-8	NetworkManager	public	2020-05-28 20:34	2020-05-28 20:34

Reporter	dlimadaum
Priority	high	Severity	block	Reproducibility	always
Status	new	Resolution	open
Product Version	8.1.1911
Target Version		Fixed in Version

Summary	0017413: Impossibile to use /31 address
Description	Migrating some servers from 7 to 8 I discovered a problem using /31 address on network interface. Same configuration of the interface but it not start up, and logging in via console and trying to star the interface via ifup it stuck for a while and then exit with error. "Connection activation failed: IP configuration could not be reserved (no available address, timeout, etc). Tried also with the network-script and disabling the management of NetworkManager result in an error "Error adding default gateway <ip-of-the-gateway> for <interface-name> It seems that the script set the broadcast address but in a /31 configuration no broadcast and no network address exist...so when it try to add the default route the gateway is the broadcast address... If I configure the interface via ip command, it work fine without errors, because it not specify a broadcast address in the interface so the ip route add work without issue: Interface configured with ifup: 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000 link/ether 7a:f1:2e:3c:d0:da brd ff:ff:ff:ff:ff:ff inet 46.20.84.242/31 brd 46.20.84.243 scope global eth0 valid_lft forever preferred_lft forever inet6 fe80::78f1:2eff:fe3c:d0da/64 scope link valid_lft forever preferred_lft forever Inteface configured with ip command: 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000 link/ether 7a:f1:2e:3c:d0:da brd ff:ff:ff:ff:ff:ff inet 46.20.84.242/31 scope global eth0 valid_lft forever preferred_lft forever inet6 fe80::78f1:2eff:fe3c:d0da/64 scope link valid_lft forever preferred_lft forever I think that also NetworkManager make the same mistake. The strange thing is that this configuration is working fine on a Centos7... The only way to put a patch is to configure the interface with an external script at boot and disable completely NetworkManager and network-script.
Steps To Reproduce	Configure a /31 address on the interface. Ifup interface-name
Additional Information	To try with the ip command: ip link set <interface-name> up ip add add <ip-address>/31 dev eth0 ip route add default via <gateway> dev eth0
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