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Blog Posts in January 2007

Cisco IOS web server with no enable password

IOS has (yet another) nice "underdocumented" feature - if you don't have an enable password/secret configured on your router, you can access IOS HTTP(S) server (assuming it's enabled with the ip http server command) without any authentication whatsoever. Of course you'd never do that in a production environment, but it's nice to know you can always configure the router from a web browser if needed (see also the discussion on default passwords with Cisco SDM).
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Default username on Cisco routers

I get a lot of hits via Google from people searching for a default username on Cisco router. It's ages-old news, but there is no default username. Period. If you have to get access to a router and cannot remember the password(s), the only thing left is the password recovery mechanism ... although even that can be disabled with the no service password-recovery configuration command.

There are, however, a few things you can do if you want to relax the access to your router in a lab environment (never do it in a production network):
  • If you configure no enable password, you can switch to enable mode without supplying a password
  • If you want to telnet to a router without supplying a password, configure no login on the vty lines.
  • If you want to be in privilege mode immediately after accessing the router, configure privilege level on the console or vty lines.
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Firewalls kill TCP performance when faced with out-of-order packets

In my discussion of per-packet versus per-destination load sharing, I've relied on the "accepted wisdom" that out-of-order TCP packets reduce session performance (as a side note, out-of-order UDP packets are a true performance killer; just try running NFS with out-of-order packets).

Today I've discovered another huge show-stopper: stateful firewalls (read: almost everything in use today) might just drop out-of-order packets, resulting in TCP timeouts and retransmissions (and repeated timeouts will totally wreck the session throughput). Here's how Cisco devices handle this problem:
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VTY access-class accepts extended and named access lists

You could limit terminal access to a router with an access-class in line configuration command for a very long time (since, at least, IOS release 10.0). However, the access-class command only accepted standard access-lists, allowing you to restrict access solely based on source IP addresses. In the meantime, this feature quietly got upgraded to support extended access lists. In the IOS release 12.4, the command even accepts (undocumented !) named access lists.

These new features give you the ability to implement interesting policies, for example:
  • Telnet access is only allowed from the network management station.
  • SSH access is allowed from anywhere within internal network

You can also use the extended access list logging functionality, making it possible to log every connection attempt to the router.

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Disable the "more" prompt

If you want to disable the Cisco IOS more ... prompt (for example, when listing router's configuration with the show running command), set the terminal screen length to zero with the terminal length 0 exec-mode command. To change the terminal lenght permanently, use the length lines line configuration command, for example:
line console
length 0
line vty 0 4
length 0
Note: this article is part of You've asked for it series.
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Cisco IOS Login Enhancements

Cisco has in IOS release 12.3(4)T (integrated into 12.4) finally introduced features (long available in Unix and Windows) that slow down dictionary attacks on a router. On top of logging of login failures, you can also slow down the login process by delaying the router response after a login failure with the login delay seconds command.

On top of that, the you can configure the router to enter quiet mode after several login failures have been detected in specified timeframe with the login block-for seconds attempts tries within seconds configuration command.
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CEF punted packets

The packets that cannot be CEF-switched in a box with CEF switching enabled are punted to the next switching level (fast switching or process switching). The incoming packets can be punted for a number of reasons, for example:
  • If the destination is reachable over an interface that cannot use CEF-switching due to a feature not supported by CEF (for example, X.25 link), the packet has to be fast- or process-switched.

These destinations are easily discovered by inspecting the punt adjacencies).

  • All packets destined for the router itself are process switched (thus punted).
  • If the router needs to reply back to the source with an ICMP packet (redirect, unreachable ...), the reply can be generated only in the process-switching path.
  • All packets with the IP options are punted to process switching.
  • Fragments that have to be processed by the router are also process-switched.
You can inspect the amount of punted packets with the show cef not-cef-switched command.

This article is part of You've asked for it series.

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CEF punt adjancency

In "border cases" you might find interesting CEF adjacencies in your CEF adjacency table (displayed with show ip cef adjacency). Most common one is the glean adjacency used for directly connected routes (this adjacency type is a placeholder that indicates the router it should perform the ARP table lookup and send the packet to directly connected neighbor). Discard, Drop, Noroute and Null adjacencies are obvious, the "weird" one is the Punt adjacency, which indicates that the router cannot CEF-switch the packet toward the destination (due to a feature being used that is not yet supported by CEF), thus the packet is punted to the next switching method (fast switching and ultimately process switching).

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Reload the router from an interim privilege level

While you wouldn't usually want non-privileged user to reload a Cisco IOS-based router, you might also not appreciate the need to give the network operator level-15 access (which includes configuration privileges) just to reload the box. The solution is the privilege configuration command. To lower the privilege level of the reload command, configure privilege exec level desired-level reload.
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Per-port CEF load sharing

In designs with very low number of IP hosts, no per-destination load-sharing algorithm will work adequately. Consider, for example, an extranet design where a large number of IP hosts are NAT-ed to a single IP address which then accesses a single remote server.


In this design, all the traffic flows between a single pair of IP addresses, making per-destination load-sharing unusable.

Cisco has addressed this problem in IOS release 12.4(11)T with per-port CEF load sharing, which extends the CEF hashing function to include source and/or destination TCP or UDP port.
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Enhanced OSPF adjacency logging

The log-adjacency-changes OSPF configuration command got improved (IOS documentation claims it's happened in release 12.1) with the detail command that logs every step of OSPF adjacency establishment (sample printout below). Great troubleshooting tool :)

%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from DOWN to INIT, Received Hello
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from INIT to 2WAY, 2-Way Received
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from 2WAY to EXSTART, AdjOK?
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from EXSTART to EXCHANGE, Negotiation Done
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from EXCHANGE to LOADING, Exchange Done
%OSPF-5-ADJCHG: Process 1, Nbr 172.16.0.21 on Serial0/0/0.100 from LOADING to FULL, Loading Done
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Disable console logging

Large amount of logging output (most often produced in a debugging process) sent to a router's console can significantly increase the router's CPU load and even stop the box from forwarding packets (high-end routers with distributed forwarding architecture are obviously an exception, but even they can lose routing adjacencies). The reason is very simple - console interrupt is one of the highest-priority interrupts on the router (otherwise you wouldn't be able to get a response to the BREAK key on a hung box).

Update January 9th 2007: The router does not check if a user is logged into the console port or a device (for example, a terminal) is attached to it; if console logging is enabled, messages are always sent to the console port (causing CPU load).

To stop the console logging, use the no logging console global configuration command (highly recommended for routers that are not usually accessed through the console port) or you might want to limit the amount of messages sent to the console with the logging console level configuration command (for example, logging console notifications).

Note: this article is part of You've asked for it series.
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"You've asked for it" series

Analyzing Google query strings that brought visitors to my blog (StatCounter is an excellent free tool to do this job), I usually find interesting (often repeating) queries that are not yet answered in my blog. Obviously there are not too many good answers on other web sites, otherwise Google users would probably not click on a hit on the second or third page (where my blog usually appears for more generic queries).

So, to help my fellow networking engineers, I've decided to start a series of "You've asked for it" articles answering the questions that brought many of you to my site in the first place (and, don't forget, you can always send me an interesting question with the Send a message link on my bio page.
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Remove timestamps from syslog messages

The ability to replace router uptime with date and time in the logging messages with the service timestamps log datetime command was present in IOS for a long time, but I was always annoyed at timestamps when collecting syslog messages for demonstration purposes. The command to turn them off has also been available "forever", but was too obvious for me to try out ... the no service timestamps log command.
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