On Thu, Jul 23, 2026 at 01:26:38AM +0200, Stefano Brivio wrote:
> When I implemented local mode in 14b84a7f077e ("treewide: Introduce
> 'local mode' for disconnected setups"), I didn't consider the
> possibility that, also in that case, the user might want to override
> addresses, default gateway or netmask, even though I expressly
> mentioned this in the man page:
> 
>   In this case, **unless configured otherwise**, they will assign the
>   IPv4 link-local address 169.254.2.1 to the guest or target
>   namespace, and no IPv6 address.
> 
> Fix this by checking if an address, gateway, or netmask length was
> explicitly set by the user, before overriding them with the default
> parameters for local mode.
> 
> This might lead to invalid configurations where we won't be able to
> set the default gateway passed by the user, but we print a warning
> message, and we assume users know what they're doing in that case.
> 
> Link: https://bugs.passt.top/show_bug.cgi?id=217
> Fixes: 14b84a7f077e ("treewide: Introduce 'local mode' for disconnected setups")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
>  conf.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index 0fcba5c..7908123 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -435,9 +435,16 @@ static unsigned int conf_ip4(unsigned int ifi, struct ip4_ctx *ip4)
>   */
>  static void conf_ip4_local(struct ip4_ctx *ip4)
>  {
> -	ip4->addr_seen = ip4->addr = IP4_LL_GUEST_ADDR;
> -	ip4->our_tap_addr = ip4->guest_gw = IP4_LL_GUEST_GW;
> -	ip4->prefix_len = IP4_LL_PREFIX_LEN;
> +	if (IN4_IS_ADDR_UNSPECIFIED(&ip4->addr))
> +		ip4->addr = IP4_LL_GUEST_ADDR;
> +	ip4->addr_seen = ip4->addr;
> +
> +	if (IN4_IS_ADDR_UNSPECIFIED(&ip4->guest_gw))
> +		ip4->guest_gw = IP4_LL_GUEST_GW;
> +	ip4->our_tap_addr = ip4->guest_gw;
> +
> +	if (!ip4->prefix_len)
> +		ip4->prefix_len = IP4_LL_PREFIX_LEN;
>  
>  	ip4->no_copy_addrs = ip4->no_copy_routes = true;
>  }
> @@ -497,7 +504,13 @@ static unsigned int conf_ip6(unsigned int ifi, struct ip6_ctx *ip6)
>   */
>  static void conf_ip6_local(struct ip6_ctx *ip6)
>  {
> -	ip6->our_tap_ll = ip6->guest_gw = IP6_LL_GUEST_GW;
> +	if (IN6_IS_ADDR_UNSPECIFIED(&ip6->guest_gw))
> +		ip6->guest_gw = IP6_LL_GUEST_GW;
> +
> +	if (IN6_IS_ADDR_LINKLOCAL(&ip6->guest_gw))
> +		ip6->our_tap_ll = ip6->guest_gw;
> +	else
> +		ip6->our_tap_ll = IP6_LL_GUEST_GW;
>  
>  	ip6->no_copy_addrs = ip6->no_copy_routes = true;
>  }
> -- 
> 2.43.0
> 

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson