upstream interfaces in dhcrelay

Bradshaw, James jbradsha at
Mon Apr 20 19:55:30 UTC 2009


According to the man page for dhcrelay (version 4.1.0)

-u [address%]ifname
              Specifies the ''upper'' network interface for DHCPv6 relay mode:
              the interface to which queries  from  clients  and  other  relay
              agents  should  be  forwarded.   At  least one -u option must be
              included in the command line when running in DHCPv6  mode.   The
              interface  name ifname is a mandatory parameter. The destination
              unicast or multicast address can be specified  by  address%;  if
              not  specified,  the  relay  agent  will  forward  to the DHCPv6
              All_DHCP_Relay_Agents_and_Servers multicast address.

If appears that relayed packets are routed normally by the ip stack (i.e. the egress interface is determined solely by the destination address) and do not necessarily egress through the specified interface. I am using linux Can someone please answer either or both of the following questions:

1)      Why should a user have to specify the egress interface (obviously the server address is required) for relayed messages-shouldn't such messages just be routed normally?

2)      Is the linux behavior correct given that the egress interface is specified (as it is in send_packet6)?  Shouldn't the sendmsg() return with an EUNREACH error rather than ignore the control message and use the routing tables?

Thanks very much.

Ted Bradshaw

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