tcpdump: print-snmp.c recognize notfound conditions

[Martijn van Duren]

Right now tcpdump prints noSuchObject/noSuchInstance/endOfMibView as
[P/x/GetRequest]/[P/x/GetNextRequest]/[P/x/GetResponse]. This is because
tcpdump doesn't treat CONTEXT for what it is: context dependent.

I'm not going to untangle this entire mess, but this diff at least gives
us a better output:
martijn$ doas tcpdump -s1500 -ilo0 port snmp                  
tcpdump: listening on lo0, link-type LOOP
11:33:04.381363 localhost.40238 > localhost.snmp: GetRequest(27) 
system.sysContact
11:33:04.381476 localhost.snmp > localhost.40238: GetResponse(27) 
system.sysContact=[P/x/GetRequest]
11:33:06.564163 localhost.5317 > localhost.snmp: GetRequest(29) 
system.sysContact.0.1
11:33:06.564278 localhost.snmp > localhost.5317: GetResponse(29) 
system.sysContact.0.1=[P/x/GetNextRequest]
^C
4 packets received by filter
0 packets dropped by kernel
martijn$ make -j4 && doas ./obj/tcpdump -s1500 -ilo0 port snmp
tcpdump: listening on lo0, link-type LOOP
11:33:11.814521 localhost.19859 > localhost.snmp: GetRequest(27) 
system.sysContact
11:33:11.814655 localhost.snmp > localhost.19859: GetResponse(27) 
system.sysContact=noSuchObject
11:33:12.346477 localhost.2383 > localhost.snmp: GetRequest(29) 
system.sysContact.0.1
11:33:12.346584 localhost.snmp > localhost.2383: GetResponse(29) 
system.sysContact.0.1=noSuchInstance

OK?

martijn@

Index: print-snmp.c
===================================================================
RCS file: /cvs/src/usr.sbin/tcpdump/print-snmp.c,v
retrieving revision 1.26
diff -u -p -r1.26 print-snmp.c
--- print-snmp.c        23 Oct 2021 09:42:14 -0000      1.26
+++ print-snmp.c        23 Oct 2021 10:35:15 -0000
@@ -134,6 +134,15 @@ char *Context[] = {
 #define REPORT 8
 };
 
+char *ContextVarbind[] = {
+       "noSuchObject",
+#define NOSUCHOBJECT 0
+       "noSuchInstance",
+#define NOSUCHINSTANCE 1
+       "endOfMibView"
+#define ENDOFMIBVIEW 2
+};
+
 /*
  * Private ASN.1 types
  * The Internet SMI does not specify any
@@ -196,7 +205,7 @@ char *GenericTrap[] = {
 struct {
        char    *name;
        char    **Id;
-           int numIDs;
+       int     numIDs;
     } Class[] = {
        defineCLASS(Universal),
 #define        UNIVERSAL       0
@@ -315,6 +324,7 @@ struct be {
 #define BE_INETADDR    8
 #define BE_PDU         9
 #define BE_UNS64       10
+#define BE_VB          11
 };
 
 
@@ -576,6 +586,22 @@ asn1_parse(const u_char *p, u_int len, s
                        }
                        break;
 
+               case CONTEXT:
+                       switch (id) {
+                       case NOSUCHOBJECT:
+                       case NOSUCHINSTANCE:
+                       case ENDOFMIBVIEW:
+                               elem->type = BE_VB;
+                               elem->data.raw = NULL;
+                               break;
+                       default:
+                               elem->type = BE_OCTET;
+                               elem->data.raw = (caddr_t)p;
+                               printf("[P/C/%d]", id);
+                               break;
+                       }
+                       break;
+
                default:
                        elem->type = BE_OCTET;
                        elem->data.raw = (caddr_t)p;
@@ -723,6 +749,11 @@ asn1_print(struct be *elem)
        case BE_PDU:
                printf("%s(%u)",
                        Class[CONTEXT].Id[elem->id], elem->asnlen);
+               break;
+       case BE_VB:
+               if (elem->id > sizeof(ContextVarbind)/sizeof(ContextVarbind[0]))
+                       break;
+               printf("%s", ContextVarbind[elem->id]);
                break;
        case BE_ANY:
                printf("[BE_ANY!?]");