HOAB

Problem :

I built a network monitoring solution following this guides :

• https://www.technicallywizardry.com/building-your-own-router-raspberry-pi/
• https://www.technicallywizardry.com/raspberry-pi-network-monitor/

Truly, a great job.

But I has build the solution at home for another network, I would like that my raspberry start and monitor at boot. And I missed in the comments or text a few thing.

Solution :

I created services as described in the comment for prometheus, add another for the tcpdump. Don't forget prometheus working directory in this configuration (no specific user for prometheus).
 

[Unit]
Description=Prometheus
After=tcpdump.service

[Service]
User=pi
Group=pi
Type=simple
WorkingDirectory=/home/pi/prometheus/prometheus-2.23.0.linux-armv7
ExecStart=/home/pi/prometheus/prometheus-2.23.0.linux-armv7/prometheus \
    --config.file /home/pi/prometheus/prometheus.yml

[Install]
WantedBy=multi-user.target

/etc/systemd/system/tcpdump.service

[Unit]
Description=TCPDump service for traffic monitoring
After=network-online.target
[Service]
Type=idle
ExecStart=python3 /home/pi/network-traffic-metrics/network-traffic-metrics.py "(src net 192.168.10.0/24 and not dst net 192.168.10.0/24) or (dst net 192.168.10.0/24 and not src net 192.168.10.0/24)"
[Install]
WantedBy=default.target

But I had some network issues :

First, disable dhcpcd and install isc-dhcp-server

In my case, I keep dhcpcd as it mount the network interfaces eth0 & eth1. I also put a no gateway on eth0 (my lan part)

My dhcpcd.conf configuration for interfaces :

interface eth1
static ip_address=192.168.1.10
static routers=192.168.1.1
static domain_name_servers=1.1.1.1
static domain_search=1.1.1.1

interface eth0
static ip_address=192.168.10.1
static routers=192.168.10.1
static domain_name_servers=1.1.1.1
static domain_search=1.1.1.1
nogateway

But with this configuration, at boot :

• isc-dhcp-server and tcpdump

were not started because eth0 was not up or plugged. In my case, I could plug eth0 later.

So I took a while, but I found the network hook that works (forget all /etc/network thing, dhcpcd do not use it).

Create a file (not a directory...) called /etc/dhcpcd.exit-hook with

#!/bin/bash

if [ "$interface" = "eth0" & "$reason" = "STATIC" & "$if_up" = "true" ]
then
	systemctl start tcpdump
	systemctl start isc-dhcp-server.service
fi 

And all is starting when eth0 is going up.

Problem :

I had a problem, nearly simple : on an angular 11 application, font was not displayed on IE11 on customer site.

After a few tests :

• I reproduce it on my demo site
• I do not reproduce in dev mode

Solution :

Well, I search for a while, and here is a summary.

1. IE11 has a problem if header "Cache-Control: nostore" is set for fonts... Not easy to find but there is some reference here
2. On my dev, it's working, after a check, I'm not setting Cache-Control: nostore
3. On my demo site, I have this header, but I have a Nginx as a reverse proxy. Ok, that's why
4. On customer site, I do not have control on network elements. One must add it also

I made the assumption that if I set a Cache directive, intermediate server won't modify it.

It works on my demo site. I'll check next on my customer site.

To add cache control on springboot, thanks Baeldung !

        registry.addResourceHandler("/assets/**").addResourceLocations("classpath:/static/hpa-portal/assets/")
                .setCacheControl(CacheControl.maxAge(1, TimeUnit.DAYS));

Problem :

On Debian, do not update resolv.conf (DNS) when we have multiple DHCP network interfaces.

Solution :

A first link : Never update resolv.conf with DHCP client

But we don't want to never update, but sometimes update...

On Redhat families it's simple (see the previous link) : PEERDNS=NO on the right interfaces

On Debian families.... let's use the hook as suggested :

Create hook to avoid /etc/resolv.conf file update

You need to create /etc/dhcp3/dhclient-enter-hooks.d/nodnsupdate file under Debian / Ubuntu Linux:
# vi /etc/dhcp3/dhclient-enter-hooks.d/nodnsupdate
Append following code:

#!/bin/sh
make_resolv_conf()
{ : }

OK, but the hook prevent ALL interfaces to update resolv.conf, the idea :

1. in the hook test the interface name
2. if one authorized, call the original make_resolv_conf
3. otherwise to nothing

In bash it's not easy to have multiple function with the same name, but thanks stackoverlow !:

#!/bin/bash


# copies function named $1 to name $2
copy_function() {
    declare -F $1 > /dev/null || return 1
    eval "$(echo "${2}()"; declare -f ${1} | tail -n +2)"
}

# Import the original make_resolv_conf
# Normally useless, hooks are called after make_resolv_conf declaration
# . /sbin/dhclient-script

copy_function make_resolv_conf orignal_make_resolv_conf

make_resolv_conf() {
        if [ ${interface} = "auhtorizedInterface" ] ; then
                original_make_resolv_conf
        fi
}

Update :

The previous solution is not working...  declare is not known by sh/dash and the script is run by sh/dash. So the copy function is not possible.

Ideas :

• copy make_resolv_conf in this file under original_make_resolv_conf : it works, but ugly due to security patch not handled
• use 2 hooks : one enter : save resolv.conf, one on exit : restore resolv.conf if ${interface} is not authorized
• try to extract make_resolv_conf from /sbin/dhclient-script : not so easy...

Best solution, the two hooks, it's a pity :) I like the copy_functions :) :

# vi /etc/dhcp3/dhclient-enter-hooks.d/selectdns-enter

#!/bin/sh

cp /etc/resolv.conf /tmp/resolv.conf.${interface}

# vi /etc/dhcp3/dhclient-exit-hooks.d/selectdns-exit

#/bin/sh

if [ ${interface} = "auhtorizedInterface" ] ; then
       echo "${interface} not authorized"
       cp /tmp/resolv.conf.${interface} /etc/resolv.conf
fi