How to Compile Kitty SSH Client from Source Code

I used Putty most of the time. Once I discovered Kitty, I haven’t looked back as it is same as Putty but on steroids. In the world of secure shell (SSH) clients, Kitty stands out as a feature-rich and highly customizable option. While precompiled versions are readily available for different operating systems, I wanted to see how Kitty is compiled.

In this blog post, I will walk you through the process of compiling Kitty SSH client from source code.

Prerequisites

Before diving into the compilation process, ensure that you have the necessary tools and libraries installed on your system. I will use Ubuntu Linux virtual machine and will use docker image which comes with all necessary packages required for compiling.

Update the Installation

It is always a good approach to install latest updates

sudo apt-get update
sudo apt-get upgrade

Output:

Hit:1 http://us-central1.gce.archive.ubuntu.com/ubuntu jammy InRelease
Get:2 http://us-central1.gce.archive.ubuntu.com/ubuntu jammy-updates InRelease [119 kB]
Get:3 http://us-central1.gce.archive.ubuntu.com/ubuntu jammy-backports InRelease [108 kB]
Hit:4 http://security.ubuntu.com/ubuntu jammy-security InRelease
Fetched 226 kB in 0s (523 kB/s)
Reading package lists… Done

Install Docker

We will use Docker to download image which has all the compiling tools required

sudo apt-get install docker.io -y

Output:

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
The following package was automatically installed and is no longer required:
  libnuma1
Use 'sudo apt autoremove' to remove it.
The following additional packages will be installed:
  bridge-utils containerd dns-root-data dnsmasq-base pigz runc ubuntu-fan
Suggested packages:
  ifupdown aufs-tools cgroupfs-mount | cgroup-lite debootstrap docker-doc rinse zfs-fuse
  | zfsutils
The following NEW packages will be installed:
  bridge-utils containerd dns-root-data dnsmasq-base docker.io pigz runc ubuntu-fan
0 upgraded, 8 newly installed, 0 to remove and 0 not upgraded.
Need to get 72.4 MB of archives.
After this operation, 287 MB of additional disk space will be used.
Get:1 http://us-central1.gce.archive.ubuntu.com/ubuntu jammy/universe amd64 pigz amd64 2.6-1 [63.6 kB]
... (truncated for length) ...

Create Working Directory

We will create directory which will be mounted inside the container to save the compiled binaries

mkdir builds && ls -l

Output:

total 4
drwxrwxr-x 2 wazeem wazeem 4096 Jul  1 12:36 builds

Download Docker Image

Download the docker image by cyd01 which has necessary components for compilation

sudo docker pull cyd01/cross-gcc

Output:

Using default tag: latest
latest: Pulling from cyd01/cross-gcc
d99de3d304f0: Pull complete 
Digest: sha256:76e39c62686d0accda705b9f4b53bda43a08b15fbf1ceba54dc7620b347ed61a
Status: Downloaded newer image for cyd01/cross-gcc:latest
docker.io/cyd01/cross-gcc:latest

Run the Container

Now we will use a container from this image for code compilation. This command will drop us in container shell

sudo docker run --rm -it -v $(pwd)/builds:/builds cyd01/cross-gcc

Output:

user@cross-gcc:/sources$

Download Source Code

Pull the source code from Github repository

git clone https://github.com/cyd01/KiTTY.git

Output:

Cloning into 'KiTTY'...
remote: Enumerating objects: 4049, done.
remote: Counting objects: 100% (914/914), done.
remote: Compressing objects: 100% (281/281), done.
remote: Total 4049 (delta 641), reused 823 (delta 611), pack-reused 3135
Receiving objects: 100% (4049/4049), 10.02 MiB | 23.91 MiB/s, done.
Resolving deltas: 100% (2573/2573), done.

Modify the Code

Now we can make changes to code before compilation. For this demo, I will change the ssh client version. Using sed or vi, I am going to change ssh client version from “PuTTY-KiTTY” to “libssh_0.7.4“. Also make sure there is not spaces in the version name after “\0”

vi KiTTY/0.76b_My_PuTTY/utils/version.c

Save the changes and verify

cat KiTTY/0.76b_My_PuTTY/version.c |grep libssh

Output:

const char sshver[] = "libssh_0.7.4\0" ;

Compile the Code

Now, we will compile the windows binaries

cd KiTTY/0.76b_My_PuTTY/windows
make -f MAKEFILE.MINGW cross

Output:

sed -i ‘s#^typedef enum _WTS_VIRTUAL_CLASS #// typedef enum _WTS_VIRTUAL_CLASS #’ window.c
sed -i ‘s/0/999/’ version_major.txt
make -e TOOLPATH=/usr/bin/i686-w64-mingw32- -f MAKEFILE.MINGW putty.exe plink.exe pscp.exe psftp.exe pageant.exe puttygen.exe
… (truncated long compilation output) …
Packed 2 files.
mv putty.exe /builds/kitty.exe
mv putty_portable.exe /builds/kitty_portable.exe
mv putty_nocompress.exe /builds/kitty_nocompress.exe
mv plink.exe /builds/klink.exe
mv pscp.exe /builds/kscp.exe
mv psftp.exe /builds/ksftp.exe
mv pageant.exe /builds/kageant.exe
mv puttygen.exe /builds/kittygen.exe
rm *.o

Gather the Binary file

At the completion of compilation, we will have binary files in our build directory

exit
ls -l

Output:

total 6992
-rwxr-xr-x 1 wazeem wazeem  450560 Jul  1 13:43 kageant.exe
-rwxr-xr-x 1 wazeem wazeem  800256 Jul  1 13:42 kitty.exe
-rwxr-xr-x 1 wazeem wazeem 2028032 Jul  1 13:43 kitty_nocompress.exe
-rwxr-xr-x 1 wazeem wazeem  800256 Jul  1 13:43 kitty_portable.exe
-rwxr-xr-x 1 wazeem wazeem  539648 Jul  1 13:43 kittygen.exe
-rwxr-xr-x 1 wazeem wazeem  843776 Jul  1 13:43 klink.exe
-rwxr-xr-x 1 wazeem wazeem  839168 Jul  1 13:43 kscp.exe
-rwxr-xr-x 1 wazeem wazeem  845824 Jul  1 13:43 ksftp.exe

Verify the Changes

In the end, we will connect to ssh server using our compiled client and original client from vendor site to verify the modification using tcpdump/wireshark.

Tcpdump Output from Original Client

Tcpdump Output from Compiled Client

Download the Compiled Version

Here is the link to compiled version if you want to use. I highly recommend to use the official versions.

Download Kitty