Re: [PATCH] conf: Separate local mode for each IP version, don't enable disabled IP version

[Paul Holzinger <pholzing@redhat.com>]

On 05/12/2025 01:51, Stefano Brivio wrote:
> This fixes an issue and introduces a feature:
>
> - in local mode, we accidentally ignored the -4 / --ipv4-only and
>    -6 / --ipv6-only command line options
>
> - if no template interface is available for a given IP version,
>    instead of disabling that IP version, use local mode, separately,
>    for it
>
> Link: https://bugs.passt.top/show_bug.cgi?id=129
> Link: https://bugs.passt.top/show_bug.cgi?id=128
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Tested-by: Paul Holzinger <pholzing@redhat.com>
> ---
>   conf.c  | 19 +++++++++++--------
>   passt.1 | 15 ++++++++-------
>   2 files changed, 19 insertions(+), 15 deletions(-)
>
> diff --git a/conf.c b/conf.c
> index 02a4b65..fdc19e8 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -1109,7 +1109,7 @@ static void conf_print(const struct ctx *c)
>   		info("Template interface: %s%s%s%s%s",
>   		     c->ifi4 > 0 ? if_indextoname(c->ifi4, ifn) : "",
>   		     c->ifi4 > 0 ? " (IPv4)" : "",
> -		     (c->ifi4 && c->ifi6) ? ", " : "",
> +		     (c->ifi4 > 0 && c->ifi6 > 0) ? ", " : "",
>   		     c->ifi6 > 0 ? if_indextoname(c->ifi6, ifn) : "",
>   		     c->ifi6 > 0 ? " (IPv6)" : "");
>   	}
> @@ -2001,20 +2001,23 @@ void conf(struct ctx *c, int argc, char **argv)
>   	    (*c->ip6.ifname_out && !c->ifi6))
>   		die("External interface not usable");
>   
> +	if (!c->ifi4 && !c->ifi6 && !*c->pasta_ifn) {
> +		strncpy(c->pasta_ifn, pasta_default_ifn,
> +			sizeof(c->pasta_ifn) - 1);
> +	}
>   
> -	if (!c->ifi4 && !c->ifi6) {
> -		info("No external interface as template, switch to local mode");
> +	if (!c->ifi4 && !v6_only) {
> +		info("IPv4: no external interface as template, use local mode");
>   
>   		conf_ip4_local(&c->ip4);
>   		c->ifi4 = -1;
> +	}
> +
> +	if (!c->ifi6 && !v4_only) {
> +		info("IPv6: no external interface as template, use local mode");
>   
>   		conf_ip6_local(&c->ip6);
>   		c->ifi6 = -1;
> -
> -		if (!*c->pasta_ifn) {
> -			strncpy(c->pasta_ifn, pasta_default_ifn,
> -				sizeof(c->pasta_ifn) - 1);
> -		}
>   	}
>   
>   	if (c->ifi4 && !no_map_gw &&
> diff --git a/passt.1 b/passt.1
> index 0c17454..db0d662 100644
> --- a/passt.1
> +++ b/passt.1
> @@ -166,8 +166,8 @@ By default, assigned IPv4 and IPv6 addresses are taken from the host interfaces
>   with the first default route, if any, for the corresponding IP version. If no
>   default routes are available and there is any interface with any route for a
>   given IP version, the first of these interfaces will be chosen instead. If no
> -such interface exists, the link-local address 169.254.2.1 is assigned for IPv4,
> -and no additional address will be assigned for IPv6.
> +such interface exists for a given IP version, the link-local address 169.254.2.1
> +is assigned for IPv4, and no additional address will be assigned for IPv6.
>   
>   .TP
>   .BR \-n ", " \-\-netmask " " \fImask
> @@ -194,9 +194,9 @@ first default route, if any, for the corresponding IP version. If the default
>   route is a multipath one, the gateway is the first nexthop router returned by
>   the kernel which has the highest weight in the set of paths. If no default
>   routes are available and there is just one interface with any route, that
> -interface will be chosen instead. If no such interface exists, the link-local
> -address 169.254.2.2 is used for IPv4, and the link-local address fe80::1 is used
> -for IPv6.
> +interface will be chosen instead. If no such interface exists for a given IP
> +version, the link-local address 169.254.2.2 is used for IPv4, and the link-local
> +address fe80::1 is used for IPv6.
>   
>   Note: these addresses are also used as source address for packets directed to
>   the guest or to the target namespace having a loopback or local source address,
> @@ -1117,8 +1117,9 @@ throughput of TCP connections.
>   .SS Local mode for disconnected setups
>   
>   If \fBpasst\fR and \fBpasta\fR fail to find a host interface with a configured
> -address, other than loopback addresses, they will, obviously, not attempt to
> -source addresses or routes from the host.
> +address for a given IP version, other than loopback addresses, they will,
> +obviously, not attempt to source addresses or routes from the host, for that
> +IP version.
>   
>   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. The

-- 
Paul Holzinger