dhcpcd-discuss

Hi Michal,

that's neither a contradiction nor a bug in dhcpcd, but fully conformant 
behavior. You quote from the context of RAs(!), and thus prefix lengths 
transmitted as part of RAs in PIOs only apply in this context, and this 
means SLAAC in particular (for A=1 in a PIO). Prefix lengths do not 
carry over to DHCPv6, and thus dhcpcd correctly sets the individual 
address route to itself, always being an /128. It would be an error to 
deduce a prefix and prefix length for a leased IPv6 address from another 
RA PIO, as also stated in the RFC specs.

Best regards,
Harald


Am 16.03.2018 um 11:04 schrieb Michał Kępień:
> > You always need router advertisements to inform the nodes of the prefixes and their length to be used. These RAs also inform nodes whether these addresses covered by a prefix are considered to be on-link or not on-link
> Exactly.  But dhcpcd does not seem to be taking this information into
> account when creating the IPv6 address it subsequently assigns to an
> interface:
>
>    - dhcp6_addrequestedaddrs():
>
>          a = ipv6_newaddr(ifp, &ia->addr,
>                  /*
>                   * RFC 5942 Section 5
>                   * We cannot assume any prefix length, nor tie the
>                   * address to an existing one as it could expire
>                   * before the address.
>                   * As such we just give it a 128 prefix.
>                   */
>              ia->ia_type == D6_OPTION_IA_PD ? ia->prefix_len : 128,
>              IPV6_AF_REQUEST);
>
>    - dhcp6_findna():
>
>          /*
>           * RFC 5942 Section 5
>           * We cannot assume any prefix length, nor tie the
>           * address to an existing one as it could expire
>           * before the address.
>           * As such we just give it a 128 prefix.
>           */
>          a = ipv6_newaddr(ifp, &ia.addr, 128, IPV6_AF_ONLINK);
>
> I think this is an overinterpretation of RFC 5942 Section 5, which
> discusses the case of a client which "receives a Router Advertisement
> message _with no on-link prefix advertised_" (emphasis mine) and then
> _assumes_ the leased address belongs to a prefix which is on-link.  But
> if the leased address belongs to a prefix which was present in a
> previously received RA with the on-link flag set, no assumption will be
> made and the client should use prefix length information from the RA
> when configuring the address on an interface.  Meanwhile, dhcpcd always
> sets the prefix length of the address to 128 bits (unless a prefix
> delegation is involved, which is not the case discussed above).  I think
> this is not the right thing to do.