How does snmpd process SNMP requests?

Question

I am new to the SNMP protocol. I want to know how snmpd processes SNMP requests. For example:

snmpget -v1 -c public localhost sysName

My understanding is it is implementing MIBs, e.g. SNMPv2-MIB, but is executing the uname -n command?

Answer 1

For the net-snmp snmpd mapping for the sysname OID see system_mib.c where it is sometimes indeed based on uname, see code snippet below:

#ifdef HAVE_GETHOSTNAME
   gethostname(sysName, sizeof(sysName));
#else
#ifdef HAVE_UNAME
   strlcpy(sysName, utsName.nodename, sizeof(sysName));
#else
#if defined (HAVE_EXECV) && !defined (mingw32)
   sprintf(extmp.command, "%s -n", UNAMEPROG);
  /*
   * setup defaults
   */
   extmp.type = EXECPROC;
   extmp.next = NULL;
   exec_command(&extmp);
   strlcpy(sysName, extmp.output, sizeof(sysName));
   if (strlen(sysName) >= 1)
      sysName[strlen(sysName) - 1] = 0; /* chomp new line */
#else
   strcpy(sysName, "unknown");
#endif /* HAVE_EXECV */
#endif /* HAVE_UNAME */
#endif /* HAVE_GETHOSTNAME */

To understand how snmpd internally works look at agent architecture page on net-snmp site. It is detailed and also explains how to extend net-snmp with new MIBs.