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Created attachment 1605727 [details]
Reproduce script.

Description of problem:

Cannot switch ipv6.method from ignore to ipv6.method manual with gateway

Version-Release number of selected component (if applicable):
NetworkManager-1.20.0-1.el8.x86_64

How reproducible:
100%

Steps to Reproduce:
1. sudo ./ipv6_bug.sh
2.
3.

Actual results:

Error: Connection activation failed: IP configuration could not be reserved (no available address, timeout, etc.)
Hint: use 'journalctl -xe NM_CONNECTION=6850b6a2-e8d0-4a02-aa37-76818b4d7636 + NM_DEVICE=dhcpcli' to get more details.

Expected results:
No failure.

Additional info:

If I remove the `ipv6.gateway 2001:db8:1::`, it works well.

Created attachment 1605728 [details]
System logs with NM trace enabled

The route can't be added because gateway address 2001:db8:1:: is already assigned to interface dhcpsrv, and kernel complains with:

  platform-linux: netlink: recvmsg: error message from kernel: Invalid argument (22) "Gateway can not be a local address" for request 50

You should put the two dhcp* interfaces in different namespaces to reproduce a valid scenario, or use a different gateway address.

For example, I use a script like this to test DHCPv4 and IPv6 autoconf locally without conflicts between the two veth interfaces:

#!/bin/sh

ip link add veth0 type veth peer name veth1
ip netns add ns1
ip link set veth1 netns ns1
ip link set veth0 up
ip -n ns1 link set veth1 up
ip -n ns1 address add dev veth1 172.25.1.1/24
ip -n ns1 address add dev veth1 fd01::1/64
ip netns exec ns1 dnsmasq --bind-interfaces --interface veth1 -d --dhcp-range=172.25.1.100,172.25.1.200,1m --dhcp-range=fd01::100,fd01::200,slaac --enable-ra -h

Hi Beniamino Galvani,

Thanks. Changing the gateway address fixed the problem.