[ipv4:PATCH] Allow userspace to specify primary or secondary ip on interface

[Vincent Li]

Allow userspace code to use flag IFA_F_SECONDARY to specify an ip address to
be primary or secondary ip on an interface.

the current behavior is when an IP is added to an interface, the primary
or secondary attributes is depending on the order of ip added to the interface
the first IP will be primary and second, third...or alias IP will be secondary
if the IP subnet matches.

this patch add the flexiblity to allow user to specify an argument 'primary' 
or 'secondary' (use 'ip addr add ip/mask primary|secondary dev ethX ' from 
iproute2 for example) to specify an IP address to be  primary or secondary.

the reason for this patch is that we have a multi blade cluster platform 
sharing 'floating management ip' and also that each blade has its own 
management ip on the management interface, so whichever blade in the
cluster becomes primary blade, the 'floating mangaement ip' follows it,
and we want any of our traffic originated from the primary blade source from
the 'floating management ip' for consistency. but in this case, since the local
blade management ip is always the primary ip on the mangaement interface and 
'floating management ip' is always secondary, kernel always choose the primary
ip as source ip address. thus we would like to add the flexibility in kernel to
allow us to specify which ip to be primary or secondary.

Signed-off-by: Vincent Li <vincent.mc.li@xxxxxxxxx>
---
 net/ipv4/devinet.c |    8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index a1b5bcb..5a7764e 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -440,8 +440,9 @@ static int __inet_insert_ifa(struct in_ifaddr *ifa, struct nlmsghdr *nlh,
 		return 0;
 	}
 
-	ifa->ifa_flags &= ~IFA_F_SECONDARY;
 	last_primary = &in_dev->ifa_list;
+	if(*last_primary == NULL)
+		ifa->ifa_flags &= ~IFA_F_SECONDARY;
 
 	for (ifap = &in_dev->ifa_list; (ifa1 = *ifap) != NULL;
 	     ifap = &ifa1->ifa_next) {
@@ -458,7 +459,10 @@ static int __inet_insert_ifa(struct in_ifaddr *ifa, struct nlmsghdr *nlh,
 				inet_free_ifa(ifa);
 				return -EINVAL;
 			}
-			ifa->ifa_flags |= IFA_F_SECONDARY;
+			if (!(ifa->ifa_flags & IFA_F_SECONDARY))
+				ifa1->ifa_flags |= IFA_F_SECONDARY;
+			else
+				ifa->ifa_flags |= IFA_F_SECONDARY;
 		}
 	}
 
-- 
1.7.9.5

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