[RFC] ip: introduce IFA_F_DHCP flag

[RFC] ip: introduce IFA_F_DHCP flag

[Girish Moodalbail]

This flag identifies that the address was obtained through DHCP.

Today there is no easy way to find out whether an address on an
interface is DHCP controlled or is static. Either you will need to
grep for 'dhclient' process (or something else in case one is using a
different DHCP client) or if you are using NetworkManager (or some
such), then you will need to query through their interface to find out
if an address is DHCP or not.

This flag will be set by the DHCP clients in the userspace when it
brings up the DHCP address on an interface. For example: ISC DHCP
client (aka dhclient) today brings up the address on an interface by
running ip-address(8) command (in dhclient-script). This command can
be extended to include 'dhcp' keyword in its 'add' or 'replace'
subcommand. Once this flag is set, the show subcommand can display the
keyword 'dhcp' against the address to indicate that the address was
obtained through DHCP.

This flag can also be set and obtained programmatically using
AF_NETLINK. Besides providing observability, this flag will be useful
for applications that need to prevent/allow certain settings on
addresses based on whether they are DHCP or not.

Signed-off-by: Girish Moodalbail <girish.moodalbail@oracle.com>
---
 include/uapi/linux/if_addr.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/uapi/linux/if_addr.h b/include/uapi/linux/if_addr.h
index 4318ab1..61aa8f8 100644
--- a/include/uapi/linux/if_addr.h
+++ b/include/uapi/linux/if_addr.h
@@ -52,6 +52,7 @@ enum {
 #define IFA_F_NOPREFIXROUTE	0x200
 #define IFA_F_MCAUTOJOIN	0x400
 #define IFA_F_STABLE_PRIVACY	0x800
+#define IFA_F_DHCP		0x1000
 
 struct ifa_cacheinfo {
 	__u32	ifa_prefered;
-- 
1.8.3.1

Re: [RFC] ip: introduce IFA_F_DHCP flag

[David Miller]

From: Girish Moodalbail <girish.moodalbail@oracle.com>
Date: Wed, 18 Oct 2017 11:16:46 -0700

> This flag identifies that the address was obtained through DHCP.
> 
> Today there is no easy way to find out whether an address on an
> interface is DHCP controlled or is static. Either you will need to
> grep for 'dhclient' process (or something else in case one is using a
> different DHCP client) or if you are using NetworkManager (or some
> such), then you will need to query through their interface to find out
> if an address is DHCP or not.
> 
> This flag will be set by the DHCP clients in the userspace when it
> brings up the DHCP address on an interface. For example: ISC DHCP
> client (aka dhclient) today brings up the address on an interface by
> running ip-address(8) command (in dhclient-script). This command can
> be extended to include 'dhcp' keyword in its 'add' or 'replace'
> subcommand. Once this flag is set, the show subcommand can display the
> keyword 'dhcp' against the address to indicate that the address was
> obtained through DHCP.
> 
> This flag can also be set and obtained programmatically using
> AF_NETLINK. Besides providing observability, this flag will be useful
> for applications that need to prevent/allow certain settings on
> addresses based on whether they are DHCP or not.
> 
> Signed-off-by: Girish Moodalbail <girish.moodalbail@oracle.com>

Well, two things.

if nobody has asked for this for 25+ years, I have serious doubts that
there is something fundamental you cannot do without this piece of
information.

Second, a lack of this flag being set won't mean "is static", it will
mean "might be STATIC, might be DHCP" and frankly that is completely
useless.  It is this way because you cannot have every dhcp agent on
every Linux system changed immediately.

I think it will give a false sense of accuracy, even on tightly
controlled sets of userspace components.

I'm not applying this, sorry.