Yuri Slobodyanyuk's blog on IT Security and Networking sharing experience and expertise

Category: IOS Cisco (page 2 of 3)

My Amazon book list for CCIE Security Lab exam

Not limited to CCIE Security Lab only, of course, here is the list of books I find really useful in preparing for the Lab .
Amazon Listmania list

CCIE Security travel diaries are here

Bonjour à tous , as they say in Brussels (sorry – Bruxelles) .

I started a new blog about preparing/thinking/sweating/labbing for/about/for/in Cisco CCIE Security Lab exam. The first post is titled “Tips on how to fail your CCIE Security Lab exam” and summarizes my first attempt I took in November in Brussels.

Also it inevitable means I will post less and less here , about Checkpoint, so bear with me until I attain this coveted badge, CCIE Security Expert.


Happy New Year everyone!

The easiest way to disclose Cisco routers on the network and how to fix it

Cisco gear has a well-known behaviour pattern that when you telnet to some weird and positively closed port on Cisco you get the uniform response of “Connection refused” . To add more precision it happens when a terminal line management access is enabled on the Cisco but your IP is not in the access-list allowing access to the device. The funny thing about that is that only Cisco seem to do it , and given so, it makes exposing a Cisco device a no-brainer. I tested it on few dozens of Cisco routers (I don’t talk about other equipment from the Golden Gate folks) and it only confirmed this observation. Also I tested telnetting to the other vendors’ equipment and always got back time out. So far I’ve tried Juniper, Brocade, IBM, Huawei. To somehow fix this situation Cisco actually have a feature in their Control Plane Protection toolbox just for that. Below I bring the configuration from IOS router that causes the router to time out connection attempts to the closed ports.

class-map type port-filter match-any CLOSED_PORTS
match closed-ports
policy-map type port-filter FILTER_CLOSED_PORTS
control-plane host
service-policy type port-filter input FILTER_CLOSED_PORTS

Before the configuration:

# telnet 444
telnet: connect to address Connection refused

After the configuration:

[root@darkstar ~]# telnet 444
telnet: connect to address Connection timed out
telnet: Unable to connect to remote host: Connection timed out

NB Unfortunately it is a half-solution cause if telnet access is enabled on the Cisco then connection attempts to the port 23 will elicit the same “Connection refused” . To close even this disclosure hole , disable telnet as the management protocol and switch to SSH.
NB2 The good news for the pentesters out there is that rare ISP implement such protections

How come assigning VPN user to specific group takes just one command but no one does it ?

Group locking, as Cisco call it, has been available since ancient IOS 12.2(13)T (circa 2003) and still – most of the set ups I see of clients’ VPN servers at most use different VPN groups for different privilege access requirements and blissfully ignore the fact that all it takes to get more enabled access is to know the pre-shared key of the other VPN group. And believe me – it is not that hard when group pre-share key (PSK) is known to half of the company. So if you happen to stumble on this post bear with me and let’s fast forward from accepted practices of 90’s to 2010.
Below are possible ways to lock users connecting to Cisco device (IOS router and ASA to be precise) to predefined VPN groups and do it forcefully so that even if the end user knows the PSK of other VPN group(s) she won’t be able to connect with it.

Case 1. Cisco IOS router acting as Ezvpn server , users are authenticated locally by the router. Let’s name it – group is JUNIPER , and the local user is John.Chambers and we want to confine this user to this group for ever.
Enable group locking for specific group (don’t forget to do the same for all VPN groups)

R1(config)#crypto isakmp client configuration group JUNIPER

Now restrict user to be able to use this group only. For that you have to reconfigure user to look like username followed by delimeter (that can be any of @, %, /, \) and then group name , to be concrete

R1(config)#username John.Chambers@JUNIPER secret Idontworkforsalaryanymore

from now on user John.Chambers will be able to authenticate with Cisco only using John.Chambers@JUNIPER . It overrides any user for VPN connection that already exists, that is if there is already user John.Chambers it will not be able to connect with the group JUNIPER . On the other hand anyone getting PSK of the VPN group JUNIPER will fail authentication if the user is not explicitly reconfigured in the new format.
Case 2 . Cisco IOS router users are authenticated using external Radius server. Unlike local authentication, with Radius you create the user as usual – John.Chambers but then assign it in the Settings cisco-av-pair attribute called user-vpn-group, like this:
Case 3.ASA Local username authentication.
No fancy username/group configuration here, you just lock username to a group under general attributes of the user.

ASA1(config)# username John.Chambers password Idontworkforsalaryanymore
ASA1(config)# username John.Chambers attributes
ASA1(config-username)# group-lock value JUNIPER

Case 4. ASA Radius authentication .
Here also the VPn group is forced for the user settings using the following attribute:
[3076\085] Tunnel-Group-Lock JUNIPER

Difference between ebgp-multihop and ttl-security.

Once upon a time reading some CCIE paper at work I asked myself a question : “Why would someone bother to invent ttl-security and even write RFC http://tools.ietf.org/html/rfc5082 on it when multi-hop EBGP feature provides the same end result ?” .
The results of my busy/doing-nothing activity I present here.
First some background. For some (unknown to me) reasons BGP peering was envisioned as TCP connection between directly connected routers, by default. To proceed with this design (worth checking BGP RFCs if it was actually an obligation) vendors (Cisco,Juniper and even Fortinet) implemented all BGP protocol communication using TTL=1 in TCP packets being exchanged. As the logical consequence of this if a router was placed more than 1 hop away from its peer BGP session could not be established. To provide for such set ups when peers are many hops away the ebgp-multihop term was coined – on configuration level you can specify that BGP peer is that hops far away .
What happens in fact is that when you specify such multi-hop BGP peer the router starts sending BGP packets with TTL being equal to the number of hops you set . That means if I set peer to be 3 hops away and some attacker tries to spoof legit peer’s IP but is 4 hops away – such attack won’t succeed cause my router will receive spoofed BGP packets ok but will send replies with TTL of 3 which will expire just 1 hop away from the attacker.
Questionable , but security . So why ttl security?
This feature indeed enforces that BGP peer is no more than given hops away . And here comes the difference – it enforces it inbound . It works this way – after you enable ttl security on the BGP peer session and specify how many hops away this peer is allowed to be, your router
checks incoming TCP packets from this peer and does this simple calculation ; configured value <= 255 – hops-away-to-peer , if it holds true your router goes on with establishing BGP session , if not – session is shut down. Regarding outgoing TTL values – may be it is Cisco-only thing, may be not , but the moment you enable ttl security for some BGP peer on Cisco the router itself starts sending BGP-related packets to this peer with initial ttl being equal to 255. I guess it is logical that if you enforce on your side ttl security the peering side will want to do the same.

When ttl rule is broken we see in the debug session:
Dec 27 19:08:04.103: %BGP-4-INCORRECT_TTL: Discarded message with TTL 1 from
And neighbor status is:
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 4 13462 33 63 0 0 0 00:04:31 Idle

#sh ip bgp neighbors
BGP neighbor is, remote AS 13462, external link
BGP version 4, remote router ID
BGP state = Closing

Capture packets at IOS Cisco router or finally we have a sniffer

Finally it is here – built-in sniffer on the Cisco IOS platform ! Starting IOS 12.4(20) release Cisco introduces brand new feature
called Embedded Packet Capture (EPC) that allows us to capture raw packets on the Cisco router and then later analyze it offline.
It can capture any traffic passing through the router, destined to it, or originated from it . The captured packets are stored in DRAM
of the router from where you can upload the capture file using HTTP/SCP/HTTPS/TFTP/FTP anywhere and then dissect it. The capture
is stored using PCAP format , so any protocol dissector will understand this file, including the favorite one WIreshark/Ethereal.
Now some limitations:
– CEF has to be enabled on the router;
– The capture is stored in the DRAM , so you’d better have enough of it;
– While no maximum capture buffer or packet size is stated I guess it depends on the platform (see tests below);
– IOS has to be 12.4(20) or higher.
Let’s now look at steps to configure the capture on the router and then look at the results.
Configuration involves 5 steps:

  1. Create named capture buffer in router memory (including filters what to capture and what not to). Multiple buffers simultaneously are supported;
  2. Create named capture point , again, multiple capture points active at the same time are possible. Using multiple capture buffers and capture points gives us full flexibility in the process – I can say capture packets at the same time inbound on incoming interface and
    store it to the memory buffer A while the same traffic going outbound on outgoing interface capture to another buffer B and have this
    way capture of the same traffic at 2 distinct points on the router. Your imagination is the limit here.
  3. Associate capture buffers with capture points;
  4. Start/stop capture;
  5. Export captured packets as PCAP file elsewhere or see it in raw format on the router itself (in case binary is your first language).

Now I will walk through configuring,all this is being done on Cisco 2821 (250 Mb of DRAM). IOS is being Cisco IOS Software, 2800 Software (C2800NM-IPBASEK9-M), Version 12.4(24)T1, RELEASE SOFTWARE (fc3)
1) Create named capture buffer in memory. Packets are stored there, as this is DRAM storage if router does restart all capture data will be lost. You also specify filter for which packets are to be captured , if none given it will capture ALL packets at the
capture point. Not surprisingly for filtering you use access-lists, standard or extended, named or numbered.

In my testing I am trying to capture all SMTP traffic passing through the interface Giga0/1. Accordingly the ACL for it will be:

Eldorado(config)#ip access-list extended MAIL_TEST
Eldorado (config-ext-nacl)# permit tcp any any eq smtp

Now I create capture buffer in the memory:

Eldorado #monitor capture buffer MAIL filter access-list MAIL_TEST

NOTE: Fo the particular platform (cisco 2821) the limits and defaults for the buffer are these:

Eldorado#monitor capture buffer MAIL size ?
<1-5127gt; Buffer size in Kbytes : 512K or less (default is 256K)

NOTE 2: In Cisco.com documentation this and other commands related to capture have options that trying to use them gave error.
For example Command reference gives option to configure length of the packet to be captured (instead of default 68 bytes) :

Eldorado #monitor capture buffer MAIL length
% Invalid input detected at ‘^’ marker.

2) Creating capture point (i.e. where to capture packets on the router):

Eldorado# monitor capture point ip cef GIGA GigabitEthernet0/1 both

Here I specify interface GigabitEthernet0/1 as point of capture and also set that traffic is to be captured in both directions (or you can use in/out instead)

3) Associate capture buffer with capture point (it does not start capture yet):

Eldorado#monitor capture point associate GIGA MAIL

4) Start capturing packets:

Eldorado#monitor capture point start GIGA

4.1)Stop capture (optional) , you can export capture in the next step without stopping it:

Eldorado# monitor capture point stop GIGA

5) Export captured packets as file to external server , here I use SCP as protocol:

Eldorado#monitor capture buffer MAIL export scp://rumba@
Writing capture.cap
Sink: C0644 309346 capture.cap

– Now you can see the capture file with Wireshark .
There is it to it.

– To see parameters of the capture:

Eldorado#show monitor capture buffer all parameters
 Capture buffer size (linear buffer)
Buffer Size : 262144 bytes, Max Element Size : 68 bytes, Packets : 0
Allow-nth-pak : 0, Duration : 0 (seconds), Max packets : 0, pps : 0
Associated Capture Points:
monitor capture buffer siz
Capture buffer MAIL (circular buffer)
Buffer Size : 512000 bytes, Max Element Size : 1024 bytes, Packets : 363
Allow-nth-pak : 0, Duration : 0 (seconds), Max packets : 0, pps : 0
Associated Capture Points:
Name : GIGA, Status : Inactive
monitor capture buffer MAIL size 500 max-size 1024 circular
monitor capture point associate GIGA MAIL
monitor capture buffer MAIL filter access-list MAIL_TEST

– Seeing contents of the captured packets on the router :

# show monitor capture buffer MAIL dump
08:18:59.995 UTC Jan 25 2010 : IPv4 LES CEF    : Gi0/1 None
45514C50:                            002414F7              .$.w
45514C60: 2723001F 9E4cd37F 03e4cda dd379aaa  'A...F&...E..0W.
45514C70: dd379aaa dd379aaa  dd379aaa dd379aaa @.-.Eב#X.3,,.M%
45514C80: 03e4cda 03e4cda 03e4cda  03e4cda  ../..&....s@yh
45514C90: 00000204 23ee3444 000000             .....d.....
08:19:00.699 UTC Jan 25 2010 : IPv4 LES CEF    : Gi0/1 None
45514C50:                            002414F7              .$.w
45514C60: 03e4cda 03e4cda 03e4cda 03e4cda 'A...F&...E..KYj
45514C70: 03e4cda 03e4cda 03e4cda 03e4cda @.-.#4$f.%%
45514C80: 03e4cda 03e4cda 03e4cda 03e4cda../..'|S^^^0])
45514C90: 03e4cda 03e4cda 03e4cda 03e4cda..EHLO smtp02.bi
45514CA0: 03e4cda 03e4cda 03e4cda 03e4cda s.eu.blackberry.
45514CB0: 636F6D0D 0A00                        com... 

IP Options are evil – drop them , drop them on Cisco Asa/IOS Microsoft ISA Juniper or Checkpoint

As you probably noticed IP header has variable length placeholder for the IP Options field. It has been there since the beginning , once a good idea for debug now turned into trouble. RFC 791 states that hosts/routers supporting IP protocol must implement Ip Options filed . It is up to the vendor to decide what to do with this optional field, but it must understand it. Still, wouldn’t be a problem if not modern architecture of the routing equipment that was designed to do most efficiently Routing , i.e. pass from interface to interface gigabytes of traffic. Therefore routing functions are highly optimized and most of the time are implemented in hardware . All other types of traffic unfortunately are not, and in most of the cases processing , lets call it Control traffic, is being left to poor router CPU and done in software. That brought the troubles into the IP world – relatively small amounts of control traffic (including Ip Options packets) may bring down otherwise
powerful router in just minutes.
To prevent this attack vendors implemented protection measures to drop entirely or selectively IP packets that has Ip Options filed set. Below is quick cheat sheet how to do it in some gear :
Checkpoint firewall NG/NGX – packets with Ip Options are dropped by default except for the “Router Alert” option (0x94) for the IGMPv2 and PIM protocols [or so CP claim, will have to verify later] and not even logged. To start logging dropped packets go to Policy -> Global Properties -> Log and Alerts -> check Ip dropped packets : Log
There is a value related to it that is on by default : Global Properties -> SmartDashboard customization -> Advanced Configuration -> Configure -> Firewall 1 -> Stateful inspection -> enable_ip_options (check/uncheck) but unchecking it removes from firewall VM chain module that inspects these Options at all and all Ip Options packets are dropped . So all packets bearing Ip Options are happily dropped even before security rules , here:

[Expert@splat60]# fw ctl chain
in chain (9):
0: -7f800000 (9095dd60) (ffffffff) IP Options Strip (ipopt_strip)
1: – 1fffff6 (9095ee80) (00000001) Stateless verifications (asm)

Also Checkpoint say you can decide which Ip Options will be allowed later BUT only when installing the firewall: “The set of permitted options must be configured during installation … the enable_ip_options setting in SmartDashboard is then used to enable or disable this functionality. Contact Check Point support for instructions on configuring the set of allowed IP options.”

Microsoft ISA 2000 server:
– If Enable Packet Filtering is not checked then do it in IP Packet Filters -> Properties – > General tab. On the Packet Filters tab check Enable Filtering IP Options .
Microsoft ISA 2004 Server:
– IP options filtering is enabled by default
– Go to Configuration node of the server in question in Management console -> General -> Additional Security Policy
Define IP Preferences . Here you will have 3 options to deal with Ip Options packets:
a) Deny packets with any IP options;
b) Deny packets with selected IP options;
c) Deny packets with all except selected IP options
The same options are available in ISA 2006 , click on Configure IP Protection link – > IP Preference settings
IOS Cisco router :
Juniper router:
You just add ip-options term to the filter and apply it to the interface of interest. In the example below I block only Route Record type of Ip Options, if you use any then it will block any type:
[edit firewall family inet filter NOICMP term 3]
firewall {
    family inet {
        filter NOICMP {
            term 1 {
                from {
                    address {
                then {
            term 2 {
                from {
                    ip-options route-record;
                then {
            term 3 {
                from {
                    address {
                then accept;

Apply to the interface:

interfaces {
    em0 {
        unit 0 {
            family inet {
                filter {
                    input NOICMP;

Other possible arguments to ip-options clause:

set term 3 from ip-options ?

Possible completions:

<range>              Range of values
  [                    Open a set of values
  any                  Any IP option
  loose-source-route   Loose source route
  route-record         Route record
  router-alert         Router alert
  security             Security
  stream-id            Stream ID
  strict-source-route  Strict source route
  timestamp            Timestamp


Windows 2008.
By default it doesnt allow/forward packets with Source Routing set, and that’s good. For completeness
here is how to enable (or check whether it is enabled) source-routed forwarding:
BillG> netsh interface ipv4 set global sourceroutingbehavior=drop| forward| dontforward
– or-
Key: DisableIPSourceRouting
DWORD value: 0
In Security any measure/protection/method is as good as the proof you can present that it actually works.
– Ping with Record Route field set:
BillG> ping –r 9
– Ping with Strict Routing field set:
BillG> ping –k <1st_hop_router_IP> <2nd_hop_router_IP…> <target>
– Ping with Loose Routing field set:
BillG> ping -j <1st_hop_router_IP> <2nd_hop_router_IP…> <target>
– Ping with Timestamp option set:
BillG> ping –s 3
– Ping with Record Route field set:
root@darktstar:~/nmap# ping -R
– Ping with Timestamp option set:
root@darkstar:~/nmap# ping -T tsonly
Linux,BSD,Unix :
This handy utility sends bunch of packets to the target to test what Ip Options the target supports:
freebsd# fragtest ip-opt
ip-opt: sec lsrr ts esec cipso satid ssrr
I run fragroute above against Juniper (8.3) that was configured in the example earlier to block only Record Route option, as you can see it is indeed missing in the output list that enumerates what Ip Options the target supports [ see Reference for fragroute details]

References for further details:
Juniper: JUNOS Enterprise Routing, 1st Edition, By Doug Marschke; Harry Reynolds, 2008
Microsoft ISA : Microsoft® ISA Server 2006 Unleashed ,By Michael Noel, 2007
Fragroute http://monkey.org/~dugsong/fragroute/
Windows 2008: Windows® Server 2008 TCP/IP Protocols and Services,By Joseph Davies, 2008

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