containernetworking / cni / 1056

72%

Pull #394

travis-ci

hwaddr: Generate MAC address for IPv6-only pods

This change adds logic to set the MAC (hardware) address for IPv6-only
pods.

Currently, the SetHWAddrByIP depends on an IPv4 IP address being
configured, and it sets the MAC address based on the IPv4 address by
wrapping the IPv4 address with a special, hard-coded 2-byte prefix:
    [0a:58] + [IPv4-address]
This works fine for the IPv4 case, since the 4 bytes of IPv4 address
is guaranteed to be unique. The IPv6-only case is not being handled.

What's being proposed with this change set is to modify SetHWAddrByIP
behavior as follows:
- IPv4-only or Dual-Stack (IPv4 + IPv6):
  No change in behavior, MAC address is based on IPv4 address:
    [0a:58] + [IPv4-address]
- IPv6-only:
  Generate and set a MAC address using a different hard-coded prefix
  to make the V6 MAC address space separate and discernible from the
  V4 MAC address space:
    [6a:58] + [four-random-octets]
  Here, the four random octets are derived by taking the last 4 bytes
  of the original MAC address as assigned by the OS when the intf/link
  is created. These are assumed to be random.

So a MAC address for an IPv6-only pod will depend upon the original
OS-assigned MAC address, and not the IPv6 address. The reason that the
MAC is not derived from the IPv6 address is that IPv6 addresses are
much larger than 32 bits (64 bits of interface ID + 16 or less bits of
subnet ID + 48 or more bits of routing prefix), and it's difficult, if
not impossible to come up with a scheme to select 32 bits from the
128 bytes of address that are guaranteed to be unique over a long
period of time as pods are created and destroyed. For example, when the
flannel plugin is used (which delegates to bridge plugin, and therefore
calls SetHWAddrByIP), each pod may get assigned a /64 subnet out of a
/48 range. In this case, the lower 64 bits of interface ID are not
unique, and the 16 bits of subnet ID will be getting recycled over time.

Another reason for not basing... (continued)

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