Port 113 IDENT Requests - How to Disable it on Nagios








Need support to disable Port 113 IDENT Requests in Nagios?

This guide will help you.


The Ident Protocol (Identification Protocol, Ident), is an Internet protocol that helps identify the user of a particular TCP connection. One popular daemon program for providing the ident service is identd.

Auth/Ident servers that usually run on the local user's machine often end up with open port 113 and listen for incoming connections and queries from remote machines.

Generally, these querying machines provide a local and remote “port pair” describing some other already-existing connection between the machines. 

As a result, The user’s “ident” server is tasked with looking up and returning the connection's “USER ID” and perhaps additional information, such as an email address, full name.

Here at Ibmi Media, as part of our Server Management Services, we regularly help our Customers to fix issues regarding port 113 IDENT requests.


What triggers Port 113 IDENT Requests ?

Generally, we can see port 113 return requests in any of the following:

1. From Nagios XI server to the originating host when submitting NSCA passive results.

2. While checking NRPE services.

3.  On the firewall logs.


Common causes for Port 113 IDENT Requests issue is given below:

1. This is usually seen when we are running an NRPE check through XINETD with USERID included on the log_on_success or log_on_failure options in remote hosts /etc/xinetd.d/nrpe file.

2. It could also be because we are submitting passive results to the XI server through NSCA (which is running under XINETD) /etc/xinetd.d/nsca with the same options.


We must keep in mind that the USERID option requires an IDENT request to port 113 on the originating server to determine the USERID, that is why we can see it.


How to Disable Port 113 IDENT Requests ?

Then remove the USERID option from the log_on_failure AND log_on_success to stop the IDENT from occurring.

The file we need to change depends on:

1.  NRPE on remote host

/etc/xinetd.d/nrpe

2.  NSCA on Nagios XI server

/etc/xinetd.d/nsca

We can either comment this line out or remove it completely:

# default: on
# description: NSCA (Nagios Service Check Acceptor)
service nsca
{
flags = REUSE
socket_type = stream
wait = no
user = nagios
group = nagios
server = /usr/local/nagios/bin/nsca
server_args = -c /usr/local/nagios/etc/nsca.cfg --inetd
log_on_failure += USERID
disable = no
only_from = 127.0.0.1
}

Once the changes are made, we need to restart the xinetd service using either of the commands below:

RHEL 7+|CentOS 7+|Oracle Linux 7+|Debian|Ubuntu 16/18/20

systemctl restart xinetd.service


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Conclusion

This article covers how to disable Port 113 IDENT Requests on Nagios. 

You are seeing port 113 return requests either from your Nagios XI server (when submitting NSCA passive results) to the originating host OR you are seeing port 113 return requests when checking NRPE services).

You will see this behavior on your firewall logs as you will most likely not have a firewall rule for port 113.

This is usually because you are running an NRPE check through XINETD with USERID included on the log_on_success or log_on_failure options in your remote hosts /etc/xinetd.d/nrpe file.

OR this could be because you are submitting passive results to the XI server through NSCA (which is running under XINETD) /etc/xinetd.d/nsca with the same options as above.


To disable Port 113 IDENT Requests:

1. Then remove the USERID option from the log_on_failure AND log_on_success to stop the IDENT from occurring. The file you need to change depends on:

i. NRPE on remote host

/etc/xinetd.d/nrpe

ii. NSCA on Nagios XI server

/etc/xinetd.d/nsca

2. After making the changes you need to restart the xinetd service using one of the commands below:

RHEL 7+ | CentOS 7+ | Oracle Linux 7+ | Debian | Ubuntu 16/18/20

$ systemctl restart xinetd.service


What is filter ident port 113?

Filter IDENT(port 113) (Enabled) IDENT allows hosts to query the device, and thus discover information about the host.

On the VPN Passthrough screen, you can configure the router to transparently pass IPSec, PPPoE, and PPTP traffic from internal hosts to external resources.


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