@george1421 So I know how it all works and what is DNSMASQ.
I tested this at home and everything was fine. When I tried to move it to the existing network at school, problems began to appear, which until now have not resolved. At first, after configuring FOG and DNSMASQ, clients could call the menu where they had options to choose , what I wrote about before.
Unfortunately today I checked the effect without any previous changes and I got the error as in the picture.

@george1421

1. The network is running a DHCP server and assigns IP addresses.
2. I have no rights or access to modify this server.
3. The FOG server is on the same subnet as the PXE boot clients

I checked everything and was configured correctly. The server still did not work as it should, so I decided to install FOG server again. Maybe I made some mistake when installing the server. I have one more question as to its installation to be 100 percent sure.
I did not want to set up a separate topic so I’m asking a question here. I found on the Wiki page information about how to install for the type of network. Which is not too much access to change anything because it is in the school server to which it does not have access.
All the computers in the room are connected to the switch, the switch itself on the router in the server room.
The network already exists for a year and is fully operational.

What type of installation would you like to do? [N] n

What is the IP address to be used by this FOG Server? [current address]192.168.4. 70 ( Static Address )

Would you like to setup a router address for the DHCP server? [Y/n] n

Would you like to setup a DNS address for the DHCP server and client boot image? [Y/n] n

Would you like to change the default network interface from eth0?
If you are not sure, select No. [y/N] n

Would you like to use the FOG server for dhcp service? [Y/n] Y

Thank you for your help.

@Tom-Elliott
In configuration after installing FOG server there was nothing to change. I’m pretty sure that FOG_WEB_ROOT is set to / fog / . Is it possible that the server itself has some options?
I have installed many times a server in my environment either virtual or physical, and somehow there was no problem with different options for FOG server side setting .

Probably you are right, because I noticed that it gets access to the GUI just as I enter - http://192.168.4.70/fog/
When I enter http://192.168.4.70 or http://192.168.4.70/ unfortunately, the GUI will not trigger.
Of course I do not know myself so well and I will check this option when I have access to the server.

@sanman
I see we have a similar problem. I also reinstalled the server, but the problem is still the same.
However, there may be someone who has an idea how to solve the problem. I recently used FOG server, installed it on my home network and everything worked as it should. When I moved my idea to the school network came the problem.

@Sebastian-Roth
Thank you for your hint. I will definitely check.
And when logs do not show anything.
@george1421 Are there any other suggestions that could still be checked on the server side?

@george1421 Yes, this is a statically set address

@george1421
I have had some time and I went to the room where the server was placed and I looked at the code you asked.
Everything looks good but maybe you better look it over. I enclose the whole code.

#!ipxe
set fog-ip 192.168.4.70
set fog-webroot fog
set boot-url http://${fog-ip}/${fog-webroot}
cpuid --ext 29 && set arch x86_64 || set arch i386
goto get_console
:console_set
colour --rgb 0x00567a 1 ||
colour --rgb 0x00567a 2 ||
colour --rgb 0x00567a 4 ||
cpair --foreground 7 --background 2 2 ||
goto MENU
:alt_console
cpair --background 0 1 ||
cpair --background 1 2 ||
goto MENU
:get_console
console --picture http://192.168.4.70/fog/service/ipxe/bg.png --left 100 --right 80 && goto console_set || goto alt_console
:MENU
menu
colour --rgb 0xff0000 0 ||
cpair --foreground 1 1 ||
cpair --foreground 0 3 ||
cpair --foreground 4 4 ||
item --gap Host is NOT registered!
item --gap -- -------------------------------------
item fog.local Boot from hard disk
item fog.memtest Run Memtest86+
item fog.reginput Perform Full Host Registration and Inventory
item fog.reg Quick Registration and Inventory
item fog.deployimage Deploy Image
item fog.multijoin Join Multicast Session
item fog.sysinfo Client System Information (Compatibility)
choose --default fog.local --timeout 3000 target && goto ${target}
:fog.local
sanboot --no-describe --drive 0x80 || goto MENU
:fog.memtest
kernel memdisk initrd=memtest.bin iso raw
initrd memtest.bin
boot || goto MENU
:fog.reginput
kernel bzImage32 loglevel=4 initrd=init_32.xz root=/dev/ram0 rw ramdisk_size=127000 web=192.168.4.70/fog/ consoleblank=0 rootfstype=ext4 storage=192.168.4.70:/images/ storageip=192.168.4.70 loglevel=4 mode=manreg
imgfetch init_32.xz
boot || goto MENU
:fog.reg
kernel bzImage32 loglevel=4 initrd=init_32.xz root=/dev/ram0 rw ramdisk_size=127000 web=192.168.4.70/fog/ consoleblank=0 rootfstype=ext4 storage=192.168.4.70:/images/ storageip=192.168.4.70 loglevel=4 mode=autoreg
imgfetch init_32.xz
boot || goto MENU
:fog.deployimage
login
params
param mac0 ${net0/mac}
param arch ${arch}
param username ${username}
param password ${password}
param qihost 1
isset ${net1/mac} && param mac1 ${net1/mac} || goto bootme
isset ${net2/mac} && param mac2 ${net2/mac} || goto bootme
:fog.multijoin
login
params
param mac0 ${net0/mac}
param arch ${arch}
param username ${username}
param password ${password}
param sessionJoin 1
isset ${net1/mac} && param mac1 ${net1/mac} || goto bootme
isset ${net2/mac} && param mac2 ${net2/mac} || goto bootme
:fog.sysinfo
kernel bzImage32 loglevel=4 initrd=init_32.xz root=/dev/ram0 rw ramdisk_size=127000 web=192.168.4.70/fog/ consoleblank=0 rootfstype=ext4 storage=192.168.4.70:/images/ storageip=192.168.4.70 loglevel=4 mode=sysinfo
imgfetch init_32.xz
boot || goto MENU
:bootme
chain -ar http://192.168.4.70/fog/service/ipxe/boot.php##params ||
goto MENU
autoboot

George also surprised me that something is wrong because everything was created on the new system and properly configured to work.
Unfortunately, I do not have access to the room where the server was created.I did not know how to check the result, so thank you for the hint.
This week ( Friday or Thursday ) I will try to check the effect you have proposed and thank you for your help because I myself would not advise it.

Thank you for your help George.
The server is set to static address before installation and after installation was not changed.
All devices located in the room are on the same subnet as the Fog server.
Below I paste the source code ( Static address of the server : 192.168.4.70 )

# Don't function as a DNS server:
port=0

# Log lots of extra information about DHCP transactions.
log-dhcp

# Dnsmasq can also function as a TFTP server. You may uninstall
# tftpd-hpa if you like, and uncomment the next line:
# enable-tftp

# Set the root directory for files available via FTP.
tftp-root=/tftpboot

# The boot filename, Server name, Server Ip Address
dhcp-boot=undionly.kpxe,,192.168.4.70

# rootpath option, for NFS
#dhcp-option=17,/images

# kill multicast
#dhcp-option=vendor:PXEClient,6,2b

# Disable re-use of the DHCP servername and filename fields as extra
# option space. That's to avoid confusing some old or broken DHCP clients.
dhcp-no-override

# PXE menu.  The first part is the text displayed to the user.  The second is the timeout, in seconds.
pxe-prompt="Press F8 for boot menu", 3

# The known types are x86PC, PC98, IA64_EFI, Alpha, Arc_x86,
# Intel_Lean_Client, IA32_EFI, BC_EFI, Xscale_EFI and X86-64_EFI
# This option is first and will be the default if there is no input from the user.
pxe-service=X86PC, "Boot from network", undionly

# A boot service type of 0 is special, and will abort the
# net boot procedure and continue booting from local media.
#pxe-service=X86PC, "Boot from local hard disk", 0

# If an integer boot service type, rather than a basename is given, then the
# PXE client will search for a suitable boot service for that type on the
# network. This search may be done by multicast or broadcast, or direct to a
# server if its IP address is provided.
# pxe-service=x86PC, "Install windows from RIS server", 1

# This range(s) is for the public interface, where dnsmasq functions
# as a proxy DHCP server providing boot information but no IP leases.
# Any ip in the subnet will do, so you may just put your server NIC ip here.
# Since dnsmasq is not providing true DHCP services, you do not want it
# handing out IP addresses.  Just put your servers IP address for the interface
# that is connected to the network on which the FOG clients exist.
# If this setting is incorrect, the dnsmasq may not start, rendering
# your proxyDHCP ineffective.
dhcp-range=192.168.4.70,proxy

# This range(s) is for the private network on 2-NIC servers,
# where dnsmasq functions as a normal DHCP server, providing IP leases.
# dhcp-range=192.168.4.0,192.168.4.255,8h

# For static client IPs, and only for the private subnets,
# you may put entries like this:
# dhcp-host=00:20:e0:3b:13:af,10.160.31.111,client111,infinite```