Re: [B.A.T.M.A.N.] [PATCHv3 2/6] batman-adv: speed up dat by snooping received ip traffic

["Linus Lüssing" <linus.luessing@c0d3.blue>]

On Thu, May 19, 2016 at 10:30:49PM +0200, Linus Lüssing wrote:
> > Not sure whether it is necessary, or whether there is a check
> > somewhere later within DAT. But should we exclude some
> > iphdr->saddr or ethhdr->h_source addresses? For instance a
> > DHCPDISCOVER usually has a zero-ip address.
> 
> And speaking of DHCP, do you (or anyone else) know, whether a
> dhcp-server (or its kernel) sends an ARP request before sending
> a unicast DHCPOFFER? Or do dhcp-servers usually craft DHCPOFFERs
> in userspace within their daemon including the ethernet header?
> 
> If the latter is the case, maybe we could/should dat-snoop the
> ethernet+IP destination of such DHCPOFFERs in interface_rx(),
> too?

Did a quick try in VMs with two standard Debian unstable machines
using isc-dhcp-client and -server. Also enabled "ping-check false"
on the dhcp-server.

Then I do not see any ARP messages, I just see four DHCP packets (see
below). And "ip neigh" stays empty.

So it seems that at least the isc-dhcp-server can assemble raw
packets, including the ethernet frame (*).


So the DHCPOFFER's ethernet and IP destinations on a
batadv_interface_tx() (sorry, wrongly wrote _rx() above) would
give us an even earlier opportunity to feed DAT from.

Would that make sense to do so?



(*): Though looking at the code of isc-dhcpd, there seem to be
compile-time options for two more, different send_packet() functions
which do not always do that.


-----

tcpdump: listening on eth1, link-type EN10MB (Ethernet), capture size 65535 bytes
22:32:03.379281 02:04:64:a4:39:e2 (oui Unknown) > Broadcast, ethertype IPv4 (0x0800), length 342: (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 02:04:64:a4:39:e2 (oui Unknown), length 300, xid 0x17feeb29, Flags [none]
          Client-Ethernet-Address 02:04:64:a4:39:e2 (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Discover
            Hostname Option 12, length 12: "Linus-Debian"
            Parameter-Request Option 55, length 13:
              Subnet-Mask, BR, Time-Zone, Default-Gateway
              Domain-Name, Domain-Name-Server, Option 119, Hostname
              Netbios-Name-Server, Netbios-Scope, MTU, Classless-Static-Route
              NTP
22:32:03.385454 02:04:64:a4:39:c3 (oui Unknown) > 02:04:64:a4:39:e2 (oui Unknown), ethertype IPv4 (0x0800), length 342: (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    192.168.123.1.bootps > 192.168.123.50.bootpc: BOOTP/DHCP, Reply, length 300, xid 0x17feeb29, Flags [none]
          Your-IP 192.168.123.50
          Client-Ethernet-Address 02:04:64:a4:39:e2 (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Offer
            Server-ID Option 54, length 4: 192.168.123.1
            Lease-Time Option 51, length 4: 600
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.123.1
            Domain-Name-Server Option 6, length 4: 192.168.123.1
22:32:03.386571 02:04:64:a4:39:e2 (oui Unknown) > Broadcast, ethertype IPv4 (0x0800), length 342: (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from 02:04:64:a4:39:e2 (oui Unknown), length 300, xid 0x17feeb29, Flags [none]
          Client-Ethernet-Address 02:04:64:a4:39:e2 (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: Request
            Server-ID Option 54, length 4: 192.168.123.1
            Requested-IP Option 50, length 4: 192.168.123.50
            Hostname Option 12, length 12: "Linus-Debian"
            Parameter-Request Option 55, length 13:
              Subnet-Mask, BR, Time-Zone, Default-Gateway
              Domain-Name, Domain-Name-Server, Option 119, Hostname
              Netbios-Name-Server, Netbios-Scope, MTU, Classless-Static-Route
              NTP
22:32:03.398683 02:04:64:a4:39:c3 (oui Unknown) > 02:04:64:a4:39:e2 (oui Unknown), ethertype IPv4 (0x0800), length 342: (tos 0x10, ttl 128, id 0, offset 0, flags [none], proto UDP (17), length 328)
    192.168.123.1.bootps > 192.168.123.50.bootpc: BOOTP/DHCP, Reply, length 300, xid 0x17feeb29, Flags [none]
          Your-IP 192.168.123.50
          Client-Ethernet-Address 02:04:64:a4:39:e2 (oui Unknown)
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 192.168.123.1
            Lease-Time Option 51, length 4: 600
            Subnet-Mask Option 1, length 4: 255.255.255.0
            Default-Gateway Option 3, length 4: 192.168.123.1
            Domain-Name-Server Option 6, length 4: 192.168.123.1
-----