MIB-II: Check for presence of /proc/net/if_inet6 file.

[jridky]

Recently, I've received complains from our customers, why is snmpd logging ERROR messages about failing opening of /proc/net/if_inet6 file on systems, where IPv6 support is intentionally disabled.

After closer look to the source code I've found, snmpd is just trying to open /proc/net/if_inet6, but when it fails, it doesn't check, whether the failure was due of lack of access rights or due of missing /proc/net/if_inet6 file.

I believe, following patch can be beneficial for users, which are using net-snmp package build with --enable-ipv6 option on systems without actual IPv6 capabilities and don't want to be bothered with error message about if_inet6 file open failure.

What is your opinion on this solution? Any comment is welcome.

diff -urNp a/agent/mibgroup/ip-mib/data_access/ipaddress_linux.c b/agent/mibgroup/ip-mib/data_access/ipaddress_linux.c
--- a/agent/mibgroup/ip-mib/data_access/ipaddress_linux.c	2020-09-29 14:08:09.742478965 +0200
+++ b/agent/mibgroup/ip-mib/data_access/ipaddress_linux.c	2020-10-01 14:20:25.575174851 +0200
@@ -19,6 +19,7 @@
 
 #include <errno.h>
 #include <sys/ioctl.h>
+#include <sys/stat.h>
 
 netsnmp_feature_require(prefix_info)
 netsnmp_feature_require(find_prefix_info)
@@ -234,7 +235,18 @@ _load_v6(netsnmp_container *container, i
 
 #define PROCFILE "/proc/net/if_inet6"
     if (!(in = fopen(PROCFILE, "r"))) {
-        NETSNMP_LOGONCE((LOG_ERR, "ipaddress_linux: could not open " PROCFILE));
+
+        /* 
+         * If PROCFILE exists, but isn't readable, file ERROR message.
+         * Otherwise log nothing, due of IPv6 support on this machine is
+         * intentionaly disabled/unavailable.
+         */
+
+        struct stat filestat;
+
+        if(stat(PROCFILE, &filestat) == 0){
+            NETSNMP_LOGONCE((LOG_ERR, "ipaddress_linux: could not open " PROCFILE));
+        }
         return -2;
     }

[jridky]

Or maybe, is there way how to set proper flag to load_flags variable e.g. from snmpd.conf file?

[jridky]

@bvanassche any suggestion?

[bvanassche]

A slightly modified patch has been applied. Please take a look.

[jridky]

Looks good. Thanks.