Running pyfingerd#

Once installed, you can directly run pyfingerd with its default options through the following command, as root:

python3 -m pyfingerd

By default, this will run a finger server on TCP port 79 for both Internet protocols (IPv4 and IPv6) if available, returning native information if implemented or dummy information (no users) if not.

Setting up your finger client#

If you haven’t already, you should get a client for testing your finger server. It is recommended to use the original, finger client from BSD, by using one of the following packages:

  • Debian and derivatives: run sudo apt-get install finger.

  • Fedora and derivatives: install dnf install finger.

  • Archlinux and derivatives: install netkit-bsd-finger or netkit-bsd-finger-ipv6 (both AUR packages) using your favourite pacman frontend.

Once installed and your server is running, you should be able to run finger on the command-line. The default syntax doesn’t query any server; the syntaxes are the following:

# List connected users.
finger @localhost

# List connected users in the 'long' format.
finger -l @localhost

# Search for a given user.
finger user@localhost

# Ask for request forwarding.

Other options exist; for example, the Castor browser allows finger URLs for querying finger servers.

Finger being a simple protocol, you can also use any raw TCP utility for making requests; Socat is a renown one. Example commands using Socat are the following:

# List connected users.
echo | socat - tcp:localhost:79

# List connected users in the 'long' format.
echo '/W' | socat - tcp:localhost:79

# Search for a given user.
echo 'user' | socat - tcp:localhost:79

# Ask for request forwarding
echo '' | socat - tcp:localhost:79

If you choose this option, you might want to familiarize yourself with RFC 1288, since you’re doing the requests yourself.

Using a different port#

Due to historical reasons, on UNIX-like systems, by default, TCP ports below 1024 are considered “privileged ports”, which means a program needs to be run as root (uid 0) to bind that port. Although on Linux, this is configurable (see ip_unprivileged_port_start), it is rarely done in practice.

However, running a network server program in root is considered a bad security practice. Usually, application servers are run on a custom unprivileged port (usually 3000, 5000 or 8000 in my experience) and a load balancer, usually Apache or nginx for HTTP and HTTPS services, redirects traffic to that custom port.

Although pyfingerd seems harmless, it is recommended to run it as an unprivileged user that only can access the required data, usually session information from the host in its default configuration. In order for it to still be able to listen to requests and answer on TCP port 79, one possibility is to attribute a custom port to it, such as 3999, and redirect all inbound traffic on port 79 to that custom port using iptables. This can be done by appending a rule to the NAT table using a command such as the following:

iptables -t nat -A OUTPUT -p tcp [-s <source ip>] [-d <destination ip>] --dport 79 -j DNAT --to '<ip:port>'

For example, if the pyfingerd server is running on port 3999, we can use both these commands to redirect traffic to that server:

iptables -t nat -A OUTPUT -p tcp -d --dport 79 -j DNAT --to
ip6tables -t nat -A OUTPUT -p tcp -d ::1 --dport 79 -j DNAT --to '[::1]:3999'

Or, if you only want to accept request from localhost, you can also make use of the -s option to only accept traffic from localhost:

iptables -t nat -A OUTPUT -p tcp -s -d --dport 79 -j DNAT --to
ip6tables -t nat -A OUTPUT -p tcp -s ::1 -d ::1 --dport 79 -j DNAT --to '[::1]:3999'

However, you must think of listening on port 3999 on the command-line, by using the following command-line option:

python3 -m pyfingerd -b localhost:3999
# OR
BIND=localhost:3999 python3 -m pyfingerd

See Command line reference for more information.

Setting the hostname#

By default, pyfingerd uses the hostname localhost when answering to requests. Say you want the server to answer with the EXAMPLE.ORG hostname. You can do so using the following command-line option:

python3 -m pyfingerd -H
# OR python3 -m pyfingerd