ICMP Fragmentation and Firewall

Following an installation of new firewalls, I was facing an issue of communication between a CMC (Central Management Console) and a Riverbed Steelhead.
In a normal way, the CMC open a SSH session to push rules to the Riverbed. But after the installation, it was not working anymore. I have investigated and found the root cause of this issue.

Packets between the CMC and the Riverbed are fragmented. Without firewalls, CMC and Riverbed use PMTU in order to discover the MTU small enough to traverse the entire path without fragmentation.
In a first step, Path MTU uses the option Don't Fragment (DF). When a packet is sent with this option, it cannot been fragmented. If a router with a smaller MTU receives this packet, it will drop it and send to the sender an ICMP Fragmentation Needed. This step is repeated until the source has reached the destination without fragmentation.

As you can see below, our problem come from this ICMP Fragmentation Needed packet.
When the steelhead sends a packet (with option Don't fragment) to the CMC, a router on the path sends a ICMP fragmentation Needed. However, the new firewall filters this packet and now the handshake failed.

Workaround:

• Allow ICMP option on the firewall.
• Reduce MTU on primary interface of the Riverbed.

QoS Traffic Policing (drop excess traffic)

Today, I was facing an issue with several users. These users were uploading big files on server (http). Unfortunately, they were using all available bandwidth. It's why, I have decided to police this specific traffic (any users to this server). With the following configuration, the bandwidth for users is limited at 3Mbps (configuration applied on a Layer 3 Switch):

• If the bandwidth exceeds 3Mbps, following packets are dropped:

access-list 100 permit tcp any 10.10.10.200 0.0.0.0 eq www

!

class-map match-all UserTraffic

match access-group 100

!

policy-map policeTraffic

class UserTraffic

    police 3000000 conform-action transmit  exceed-action drop

!

interface Vlan999

service-policy output policeTraffic

• Check statistics:

MYSWITCH#show policy-map  interface vlan 999

Vlan999

  Service-policy output: policeTraffic

    Class-map:UserTraffic (match-all)

      558663 packets, 827048161 bytes

      5 minute offered rate 3643000 bps, drop rate 645000 bps

      Match: access-group 100

      police:

          cir 3000000 bps, bc 93750 bytes

        conformed 460702 packets, 679305595 bytes; actions:

          transmit

        exceeded 97962 packets, 147744080 bytes; actions:

          drop

        conformed 2994000 bps, exceed 669000 bps

    Class-map: class-default (match-any)

      1626596 packets, 568490144 bytes

      5 minute offered rate 3555000 bps, drop rate 0 bps

      Match: any

BGP Decision Process

Nothing new in this post! It's just a reminder regarding the BGP process decision:

1. Weight (Bigger win, Cisco proprietary)
2. LOCAL_PREF (Bigger Win)
3. Locally injected routes (Locally injected win overiBGP/eBGP learned)
4. AS_PATH length (Smaller Win)
5. ORIGIN (code I win over E, E win over ? )
6. MED (Smaller Win)
7. Neighbor Type (eBGP win over iBGP)
8. IGP metric to NEXT_HOP (Smaller win)

Key Based SSH Authentication on a Linux Host using Putty

In order to be more secure, I have decided to use a key instead of a password authentication. I'm using putty to connect to my linux server. I have used the following steps in order to configure it:

• Generate a public and private key with PuTTYgen (click on Generate):

The key is generated by mouving randomly the mouse.

• Save the public and private key.
• The next step is to add the public key on the server:

mkdir ~/.ssh
chmod 700 ~/.ssh

• Paste the public key to the following file:

vim ~/.ssh/authorized_keys
chmod 600 ~/.ssh/authorized_keys

• Configure the putty session:
• Create a new session and save it:

• Select data and configure 'Auto-login' (usersame used by the created key):

• Configure the path to the private key:

After this, if you open the session, you will be directly prompted to the linux host.

OSPF Database Reminder in the same area

This is a reminder in order to interpret an ospf database on a Cisco router. I have created the following lab to test the 'show ip ospf datase':

In red, the router ID:

R3#show ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 1)

The list of all routers in the area:

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         129         0x80000002 0x0057AC 1
2.2.2.2         2.2.2.2         25          0x80000003 0x00536B 2
3.3.3.3         3.3.3.3         24          0x80000002 0x0016B6 2

All the network segments in the area and the DR for each segment. Here, 10.1.2.2 is the IP address of the DR on the segment and 2.2.2.2 is the router id:

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.2.2        2.2.2.2         103         0x80000001 0x0021F5
10.1.4.2        2.2.2.2         25          0x80000001 0x006F9D

Each subnet, even if there is no OSPF neighbor:

R1#show ip ospf database internal

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Stub Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Mask
10.1.2.255      1.1.1.1         355         0x0        0x006D69 /24
10.1.3.255      2.2.2.2         98          0x0        0x00448D /24
10.1.4.255      2.2.2.2         315         0x0        0x003997 /24
10.1.5.255      3.3.3.3         230         0x0        0x0010BB /24

CCIE Routing & Switching v5 !!

https://www.ciscolivemilan.com/connect/sessionDetail.ww?SESSION_ID=3480

WAIT and SEE!

Reflexive ACLs

Introduction:

By default, an ACL is not stateful. When an 'inside' client open a session to an 'outside' host you have to create 2 ACLs:
- Client to Server
- Server to Client
In our case, we would like open everything coming from the client side and going to the server side. We also deny each session coming from server side. With traditional ACLs it will be really difficult to realize this configuration. It's why, we will use reflexive ACLs. This kind of ACL record session open from the client side and open the necessary port and IP source for the traffic coming back from the specific server.

In our case, we are using a simple test infrastructure as can be seen in the following diagram:

The traffic coming from the client side and going to server side is authorized. Only the response to a session open by a client is authorized (handle dynamically by a recursive ACL). Any other traffic coming from the server side is denied.

Configuration:

In a first step, we create an ACL to reflect outgoing packets:

ip access-list extended OUTBOUND

  permit ip any any reflect MIRROR

!

interface FastEthernet0/1

  ip access-group OUTBOUND out

When the client initiates a session, a reflected ACL is created in MIRROR.

Router#show ip access-lists MIRROR

Reflexive IP access list MIRROR

  permit tcp host 10.1.3.10 eq telnet host 10.1.1.10 eq 27797 (27 matches) (time left 159)

In a second step, we create an ACL which uses the MIRROR entry to authorize the traffic.
ip access-list extended INBOUND

  evaluate MIRROR

!

interface FastEthernet0/1

  ip access-group INBOUND in

 

Results:

Connexion coming from the server side are denied:

SERVER#telnet 10.1.1.10

Trying 10.1.1.10 ...

% Destination unreachable; gateway or host down

Connexion coming from the client side are authorized:

client#telnet 10.1.3.10

 Trying 10.1.3.10 ... Open

 User Access Verification

 Password:

 SERVER>en

As you can see below, a 'reflexive' session is authorized from the server to the client:

Router#show ip access-lists

Extended IP access list INBOUND

  10 evaluate Mirror

Reflexive IP access list Mirror

  permit tcp host 10.1.3.10 eq telnet host 10.1.1.10 eq 27797 (27 matches) (time left 159)

Extended IP access list OUTBOUND

10 permit ip any any reflect Mirror (27 matches)
 

Remarks:

　

By default the timeout for a reflexive ACL is 300sec.