Don't auto-set broadcast unless subnet larger than /31

Since [vishvananda#248](vishvananda#248), adding an address automatically sets the broadcast if the broadcast address was not specified. This is undesirable when adding an IP with a prefixlen of /31 or /32. (Additional details in the issues linked below.)

This changes the behavior so that the broadcast is only automatically set if the prefixlen is /30 or larger.

Issue reported in:

- vishvananda#329
- vishvananda#471

See also:

- [RFC 3021](http://tools.ietf.org/html/rfc3021)

Alternatives to this PR:

A. vishvananda#472 - Adds `AddrAddWithoutCalculatedBroadcast`.
B. jjastrze-ovh@9a85a61 - Breaking change to make auto-setting the broadcast address an opt-in feature.
C. already works - Suppress setting the broadcast when addr's broadcast address is set to `0.0.0.0`. (This works today, but I'm not sure the behavior can be relied upon as a public API.)

Author: brandt

addr_linux.go

@@ -15,39 +15,62 @@ import (
 const IFA_FLAGS = 0x8
 
 // AddrAdd will add an IP address to a link device.
+//
 // Equivalent to: `ip addr add $addr dev $link`
+//
+// If `addr` is an IPv4 address and the broadcast address is not given, it
+// will be automatically computed based on the IP mask if /30 or larger.
 func AddrAdd(link Link, addr *Addr) error {
 	return pkgHandle.AddrAdd(link, addr)
 }
 
 // AddrAdd will add an IP address to a link device.
+//
 // Equivalent to: `ip addr add $addr dev $link`
+//
+// If `addr` is an IPv4 address and the broadcast address is not given, it
+// will be automatically computed based on the IP mask if /30 or larger.
 func (h *Handle) AddrAdd(link Link, addr *Addr) error {
 	req := h.newNetlinkRequest(unix.RTM_NEWADDR, unix.NLM_F_CREATE|unix.NLM_F_EXCL|unix.NLM_F_ACK)
 	return h.addrHandle(link, addr, req)
 }
 
 // AddrReplace will replace (or, if not present, add) an IP address on a link device.
+//
 // Equivalent to: `ip addr replace $addr dev $link`
+//
+// If `addr` is an IPv4 address and the broadcast address is not given, it
+// will be automatically computed based on the IP mask if /30 or larger.
 func AddrReplace(link Link, addr *Addr) error {
 	return pkgHandle.AddrReplace(link, addr)
 }
 
 // AddrReplace will replace (or, if not present, add) an IP address on a link device.
+//
 // Equivalent to: `ip addr replace $addr dev $link`
+//
+// If `addr` is an IPv4 address and the broadcast address is not given, it
+// will be automatically computed based on the IP mask if /30 or larger.
 func (h *Handle) AddrReplace(link Link, addr *Addr) error {
 	req := h.newNetlinkRequest(unix.RTM_NEWADDR, unix.NLM_F_CREATE|unix.NLM_F_REPLACE|unix.NLM_F_ACK)
 	return h.addrHandle(link, addr, req)
 }
 
 // AddrDel will delete an IP address from a link device.
+//
 // Equivalent to: `ip addr del $addr dev $link`
+//
+// If `addr` is an IPv4 address and the broadcast address is not given, it
+// will be automatically computed based on the IP mask if /30 or larger.
 func AddrDel(link Link, addr *Addr) error {
 	return pkgHandle.AddrDel(link, addr)
 }
 
 // AddrDel will delete an IP address from a link device.
 // Equivalent to: `ip addr del $addr dev $link`
+//
+// If `addr` is an IPv4 address and the broadcast address is not given, it
+// will be automatically computed based on the IP mask if /30 or larger.
 func (h *Handle) AddrDel(link Link, addr *Addr) error {
 	req := h.newNetlinkRequest(unix.RTM_DELADDR, unix.NLM_F_ACK)
 	return h.addrHandle(link, addr, req)
@@ -108,14 +131,20 @@ func (h *Handle) addrHandle(link Link, addr *Addr, req *nl.NetlinkRequest) error
 	}
 
 	if family == FAMILY_V4 {
-		if addr.Broadcast == nil {
+		// Automatically set the broadcast address if it is unset and the
+		// subnet is large enough to sensibly have one (/30 or larger).
+		// See: RFC 3021
+		if addr.Broadcast == nil && prefixlen < 31 {
 			calcBroadcast := make(net.IP, masklen/8)
 			for i := range localAddrData {
 				calcBroadcast[i] = localAddrData[i] | ^mask[i]
 			}
 			addr.Broadcast = calcBroadcast
 		}
-		req.AddData(nl.NewRtAttr(unix.IFA_BROADCAST, addr.Broadcast))
+
+		if addr.Broadcast != nil {
+			req.AddData(nl.NewRtAttr(unix.IFA_BROADCAST, addr.Broadcast))
+		}
 
 		if addr.Label != "" {
 			labelData := nl.NewRtAttr(unix.IFA_LABEL, nl.ZeroTerminated(addr.Label))