Same netmask or /32 for secondary IP on Linux
- by derobert
There appear to be (at least) two ways to add a secondary IP address to an interface on Linux. By secondary, I mean that it'll accept traffic to the IP address, and responses to connections made to that IP will use it as a source, but any traffic the box originates (e.g., an outgoing TCP connection) will not use the secondary address.
Both ways start with adding the primary address, e.g., ip addr add 172.16.8.10/24 dev lan. Then I can add the secondary address with either a netmask of /24 (matching the primary) or /32.
If I add it with a /24, it gets flagged secondary, so will not be used as the source of outgoing packets, but that leaves a risk of the two addresses being added in the wrong order by mistake. If I add it with /32, wrong order can't happen, but it doesn't get flagged as secondary, and I'm not sure what the bad effects of that may be.
So, I'm wondering, which approach is least likely to break? (If it matters, the main service on this machine is MySQL, but it also runs NFSv3. I'm adding a second machine as a warm standby, and hope to switch between them by changing which owns the secondary IP.)
