ClusterLabs/resource-agents — Issue #52: `findif` doesn't properly detect interfaces
Author: b-a-t
Created: 2012-01-31 15:48:41+00:00
State: open

I was a bit surprised that despite description of IPAddr[2] just and IP address never was enough for configuration and I had to supply both netmask and NIC name to get things working properly. But a lot of tutorials said that's how it should be done, so I accepted it until got a configuration where NICs had different names on two machines in a cluster and migration of IP didn't work.

I never could get such a configuration to work in Debian squeeze, but on Debian lenny it worked. After digging into the code and configuration it came out that on lenny 'cat /proc/net/route' gives routing table like:

Iface Destination Gateway Flags RefCnt Use Metric Mask MTU Window IRTT
eth0 006BD35F 00000000 0001 0 0 0 E0FFFFFF 0 0 0
eth1 0000000A 00000000 0001 0 0 0 00FFFFFF 0 0 0
eth2 0001000A 00000000 0001 0 0 0 00FFFFFF 0 0 0
eth0 00000000 1E6BD35F 0003 0 0 0 00000000 0 0 0

When on squeeze the order of the information is different:

Iface Destination Gateway Flags RefCnt Use Metric Mask MTU Window IRTT
bond0 00000000 9E77D35F 0003 0 0 0 00000000 0 0 0
eth1 0000000A 00000000 0001 0 0 0 00FFFFFF 0 0 0
* FA00000A 00000000 0205 0 0 0 FFFFFFFF 0 0 0
bond0 8077D35F 00000000 0001 0 0 0 E0FFFFFF 0 0 0

Basically, with the newer kernels the default route 0/0 comes as a first line and matches any supplied IP address/mask combination, giving interface of the default route as a match. It may be suitable result in some cases, but just incidently. Also, the code tries to take route metric into account and rejects anything that is below already found one, although 'man route' says:

Metric The 'distance' to the target (usually counted in hops). It is not used by recent kernels, but may be needed by routing daemons.

And in most cases it's value 0 for all the entries. But current code just picks the first matching line with such a metric rejecting any following better routes with the same metric (0).

So, my proposal is to make this check more relaxed in the first place and consider equal metrics suitable as well. Still in this case default route possibly would be the best choice, so I add additional constrain - pick most specific matching route(with narrower netmask).

Here is the small patch:

```patch
--- findif.c.orig 2010-04-14 13:57:20.000000000 +0200
+++ findif.c 2012-01-27 22:06:14.000000000 +0100
@@ -171,11 +171,13 @@
long metric = LONG_MAX;
long best_metric = LONG_MAX;
int rc = 0;
-
+
char buf[2048];
char interface[MAXSTR];
FILE *routefd = NULL;

+ *best_netmask = 0;
+
if ((routefd = fopen(PROCROUTE, "r")) == NULL) {
snprintf(errmsg, errmsglen
, "Cannot open %s for reading"
@@ -199,17 +201,18 @@
, PROCROUTE, buf);
rc = -1; goto out;
}
- if ( (in->s_addr&mask) == (in_addr_t)(dest&mask)
- && metric < best_metric) {
- best_metric = metric;
- *best_netmask = mask;
- strncpy(best_if, interface, best_iflen);
+ if ((in->s_addr&mask) == (in_addr_t)(dest&mask)) {
+ if(metric <= best_metric && mask >= *best_netmask) {
+ best_metric = metric;
+ *best_netmask = mask;
+ strncpy(best_if, interface, best_iflen);
+ }
}
}

if (best_metric == LONG_MAX) {
snprintf(errmsg, errmsglen, "No route to %s\n", address);
- rc = 1;
+ rc = 1;
}

out:
```

Testing both on lenny and squeeze gave consistent right results :)

Still, this and current code doesn't take into account, for example case when 'reject' route present in the routing table:

* FA00000A 00000000 0205 0 0 0 FFFFFFFF 0 0 0

If desired IP happen to fall into the rejected range an '*' interface returned as a result. Well, it can easily be verified in the code, as well as on a higher level.

Regards,
Timur.