No luck? sad! The Problem with TCP Checksums

By Brandon James on 2020-12-30 Tags: Route/Switch, Security

It's simple to craft packets with identical checksums. TCP datagrams with UTF-8 encoded payloads 'No muck? rad!' and 'No luck? sad!' have identical checksums if the rest of the packet is identical. Changing the 'm' in muck to an 'l' and the 'r' in rad to a 's' results in a binary 1 being flipped to 0 and vice versa. Since these binary digits lie in the same position in different 16-bit words, the changes cancel each other out, resulting in the same checksum.

Practical Automation - mod_acl

By Brandon James on 2019-12-31 Tags: Route/Switch, Automation, Programming

Managing access lists is one of the more painful parts of being a network engineer. Once you've finishing working out what should or should not be allowed, you write the ACL and then paste it into all your devices. The minute you finish, the requirements change or the business lets you know what you just broke. In the future, SGTs and SDN promise to fix this problem, but you might not be there yet. `mod_acl` is a simple and fast way to manage ACLs.

Bootstrap Router Hash Function

By Brandon James on 2019-12-06 Tags: Route/Switch, Programming

RFC 7761 describes a hash function used to load balance multicast groups between Rendezvous Point (RP) candidates when you are using Bootstrap Router (BSR). The hash function isn't straight forward and I was unable to find a resource that described it in detail. One feature of the hash function is an adjustable hash mask length. I think it's important to understand what the function does and how adjusting the mask impacts RP selection. I do my best to describe it here and provide some resources to help you select the best hash mask length for your environment.

Locator/ID Separation Protocol - LISP

By Brandon James on 2019-09-04 Tags: Route/Switch

The Locator/ID Separation Protocol or LISP was originally designed to decrease the size of routing tables on the Internet. As the protocol matured it made it's way into the enterprise though solutions like Cisco Software Defined Access. In this article I provide a summary of the problem LISP solves and how LISP functions. The purpose of this article isn't to cover the protocol in it's entirety, but to build an awareness of how the protocol works.

Bitwise Operations and Subnetting

By Brandon James on 2019-07-29 Tags: Route/Switch, Programming

Back in September of 2016 I wrote a subnet calculator in C and then blogged about it. This entry is based on that old blog post.

Don't use FHRPs without Authentication

By Brandon James on 2019-02-17 Tags: Route/Switch, Security

You are most likely running a first hop redundancy protocol somewhere in your network. If you aren't routing at the access layer and your running a traditional redundant core (ie you aren't using a switch virtualization platform such as Cisco's VSS), one of those places is probably your user facing SVIs. If you aren't using encrypted authentication on your FHRP, you're putting your enterprise at risk.

Conf T via SNMP

By Brandon James on 2017-01-26 Tags: Route/Switch

Lately we've been hitting bugs that cause switches (specifically Cisco 4500-E's) to lose their SSH keys after reboots or crashes. Due to this we've changed our standard operating procedure for reboots to include enabling telnet. This workaround is great during planned outages, but it doesn't really help in the case of unplanned outages. I knew that it was possible to make configuration changes via SNMP, so today when I replaced a lab router and forgot to generate SSH keys, I took advantage of the situation and generated the keys via SNMP.