Putty — Recommended Default Settings for a Network Engineer

Putty is is a great piece of soft­ware. Because it works and it’s free I find it installed on most cor­por­ate desktops as the default soft­ware for ter­minal cli­ent on Windows.¹. Not many people seems to real­ise that Putty is highly cus­tom­is­able and has a num­ber of fea­tures that will improve your work­ing environment.

Set the Rows & Scrollback Lines

By default Putty only keeps the last 200 lines of text dis­played on your screen in the scroll­back buf­fer. For a typ­ical net­work engin­eer, this isn’t enough to show the con­fig­ur­a­tion, must less any decent amount of text. Typically I would set the default to be 20000 lines. And since most screens allows more than 25 rows, I would increase the Window Rows to forty. If you change the Windows Rows to forty, then you should also change your device ter­minal set­tings to use forty lines as well. See IOS — Setting the Terminal Windows Length for a good-​​ish descrip­tion of this.

Note that every Putty ses­sion will alloc­ate a block of memory suf­fi­cient to hold those lines in a buf­fer. Making it too large may impact your com­puter by redu­cing the amount of memory available.

Setting the SCROLLBACK to 20000 lines

Using Font Aliasing

The choice to use Antialiased or aliased fonts is a bit com­plic­ated, but its a ser­i­ous decision affect­ing how easy on the eyes ad how quickly you can read the screen. Less read­ing errors res­ults in bet­ter work performance.

The best choice here is ClearType.

Cleartype Font Aliasing

Window Names/​Title

It’s com­mon for me to have more than ten putty ses­sions open at a single time. With that many win­dows you need a way to tell them apart, thus, some names on the box would be use­ful in identi­fy­ing. them.

Window Title

And the res­ult will look some­thing like this:

Title of the Window for Putty

Keepalives and enabling Nagle’s Algorithm

Nagle’s Algorithm and Keepalives

Nagles Algorithm

Nagle’s algorithm is par­tic­u­larly use­ful when attempt­ing to trans­fer a lot of inform­a­tion in the ter­minal. For example, when log­ging at high speed or past­ing large con­fig­ur­a­tions to your con­sole (there are many other cri­teria that affect this, but this is one of them).

Wikipedia — http://en.wikipedia.org/wiki/Nagle’s_algorithm

Nagle’s algorithm, named after John Nagle, is a means of improv­ing the effi­ciency of TCP/​IP net­works by redu­cing the num­ber of pack­ets that need to be sent over the net­work.
Nagle’s doc­u­ment, Congestion Control in IP/​TCP Internetworks (RFC896) describes what he called the ‘small packet prob­lem’, where an applic­a­tion repeatedly emits data in small chunks, fre­quently only 1 byte in size. Since TCP pack­ets have a 40 byte header (20 bytes for TCP, 20 bytes for IPv4), this res­ults in a 41 byte packet for 1 byte of use­ful inform­a­tion, a huge over­head. This situ­ation often occurs in Telnet ses­sions, where most keypresses gen­er­ate a single byte of data which is trans­mit­ted imme­di­ately. Worse, over slow links, many such pack­ets can be in transit at the same time, poten­tially lead­ing to con­ges­tion collapse.

Keepalives

Keepalives are most use­ful when run­ning ter­minal ses­sions through fire­walls. If a telnet/​ssh ter­minal doesn’t keep traffic flow­ing then it will mark the ses­sion for timeout. Once the ses­sion times out you will need to recon­nect and go through the login pro­ced­ure again.

SSH Options

Lets looks at some of the SSH Connection Options and which items you should think about.

SSH Connections Options

SSH Version

Putty has a default that selects SSH Version 2 but will fall­back to SSH Version 1 if needed. This is the best choice for defaults and recom­mend that you do NOT change this.

Note that early ver­sions of IOS did not sup­port Version 2 (SSHv2 was intro­duced in June 2007 in IOS Version 12.1.19E, SSHv1 was Sep 2005 around IOS Version 12.1.3T ), and only Version 1 and some ver­sions of Putty did not fall back grace­fully. For these cases, you would need to con­fig­ure the ver­sion for that ses­sion only, not as a default for your global default.

Encryption Algorithms

It’s gen­er­ally accep­ted that DES is insec­ure (for prac­tical pur­poses). However, if you have a Cisco device that doesn’t have an Strong Encryption license (it is free, all you have to do is sup­ply the serial num­ber and fill out the form on Cisco’s web site to meet the US gov­ern­ment require­ments that says you aren’t a ter­ror­ist — www​.cisco​.com/​go/license) then you might want to set this for a single ses­sion. I wouldn’t set this as a global default.

SSH Compression

SSH Secure Shell uses GNU ZLIB (LZ77) for com­pres­sion. The “zlib” com­pres­sion is described in RFC 1950 and in RFC 1951.

The cli­ent can request a com­pres­sion level with a num­ber after +C (from +C1 to +C9). In this case, the com­pres­sion level is between the levels reques­ted by the cli­ent and offered by the server. For example, if the server offers level 6 (the default) and the cli­ent asks for level 1, level 2 is used.

Compression is worth using if your con­nec­tion is slow (for example a modem con­nec­tion). The effi­ciency of the com­pres­sion depends on the type of the file, and var­ies widely. It is close to 0% for already com­pressed files like zip and often 50% or even more for text files.((http://​www​.ssh​.com/​s​u​p​p​o​r​t​/​d​o​c​u​m​e​n​t​a​t​i​o​n​/​o​n​l​i​n​e​/​s​s​h​/​a​d​m​i​n​g​u​i​d​e​/​3​2​/​C​o​m​p​r​e​ssion.html))

As far as I can tell, Cisco IOS does not sup­port com­pres­sion. I believe that it wouldn’t make any dif­fer­ence since trans­fer­ring large volumes of data is not the nor­mal con­di­tion when con­nect­ing to ter­minal sessions.

Can be safely left alone.

Serial Settings

Putty did not always have serial cap­ab­il­ity. If it doesn’t show then you need to down­load a later ver­sion and get it installed. Putty Home Page.

Putty Serial Console settings

Since 9600,N,8,1 is the default serial con­fig­ur­a­tion for Cisco equip­ment, I would not change this. I have seen some com­pan­ies who have a stand­ard con­fig­ur­a­tion for IOS devices that set the serial port bit rate to 38400, how­ever, I would still not change the global default.

Saving the Global Default

This post cov­ers sav­ing these default set­tings for every new con­nec­tion that you make with putty.

Wrapup

That’s enough for this post I think, I have some more things that I think are worth doc­u­ment­ing, but I will do them later on. You can see all the posts by check­ing the Console Mastery series but­ton at the top of the page, which lists all the post in the Console Mastery series.

If you have any tips or tricks for putty, either get in con­tact via the Contact Form or drop me an email myether­e­al­mind -£$- gmail dot com.

Footnotes

1. After all, Microsoft doesn’t sup­ply any SSH in Windows or even tel­net cap­ab­il­ity in Vista [back]

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