> Here is a diff to add IPv6 support to ppp(4).
> With this diff, I can send and receive IPv6 packets over a PPP connection.

Look good to me, but...

> Index: if_ppp.c

> @@ -579,13 +583,9 @@ pppsioctl(struct ifnet *ifp, u_long cmd,
>  		break;
>  
>  	case SIOCSIFADDR:
> -		if (ifa->ifa_addr->sa_family != AF_INET)
> -			error = EAFNOSUPPORT;
>  		break;
>  
>  	case SIOCSIFDSTADDR:
> -		if (ifa->ifa_addr->sa_family != AF_INET)
> -			error = EAFNOSUPPORT;
>  		break;
>  
>  	case SIOCSIFMTU:

You probably want to nevertheless check sa_family and only allow
AF_INET, and AF_INET6 #ifdef INET6, as done e.g. in netinet/ip_carp.c.