full-disclosure - DDoS on the 16th - Fail if no DNS resolution?

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From: nick at virus-l.demon.co.uk (Nick FitzGerald)
Subject: DDoS on the 16th - Fail if no DNS resolution?

"Chris Eagle" <cseagle@...shift.com> wrote:

> Yes I have looked at the code, no it doesn't exit. It simply directs the SYN
> packet at 255.255.255.255, i.e. it ignores the fact that the lookup failed
> and returned -1 and uses -1 as the destination IP.

Hmmmmm -- that's odd...

The MSDN/SDK dox for gethostbyname says this about return values:

   If no error occurs, gethostbyname returns a pointer to the HOSTENT
   structure described above. Otherwise, it returns a NULL pointer and
   a specific error number can be retrieved by calling WSAGetLastError.

And in my disasm the worm writer seems to have taken this into account:

.text:00401EA4                 call    gethostbyname
.text:00401EA9                 mov     esi, eax
.text:00401EAB                 or      esi, esi
.text:00401EAD                 jnz     short loc_401EB4
.text:00401EAF                 or      eax, 0FFFFFFFFh
.text:00401EB2                 jmp     short loc_401EBD
.text:00401EB4 loc_401EB4:
.text:00401EB4                 mov     eax, [esi+0Ch]
.text:00401EB7                 mov     eax, [eax]
.text:00401EB9                 mov     edi, [eax]
.text:00401EBB 
.text:00401EBB loc_401EBB:
.text:00401EBB                 mov     eax, edi
.text:00401EBD 
.text:00401EBD loc_401EBD:
.text:00401EBD                 pop     edi
.text:00401EBE                 pop     esi
.text:00401EBF                 pop     ebp
.text:00401EC0                 retn

...by testing for a null return and OR'ing FFFFFFFFh with EAX if it was 
null.  Thus it is the worm's checking and post-processing of the 
gethostbyname result that returns "-1" (or the equivalent of 
255.255.255.255) from the "get DoS victim IP" sub-routine.

-- 
Nick FitzGerald
Computer Virus Consulting Ltd.
Ph/FAX: +64 3 3529854