What are the cons of dhcp snooping, dynamic arp inspection and ip guard?

I read about a couple of layer 2 protections against dhcp starving, mac and ip spoofing- ip guard, dhcp snooping and dynamic arp inspection.

Are there any cons or vulnerabilies which enable bypassing them or are they safe to use?

I have noticed that each record in the dhcp binding table has a time to live and the table itself has a limited size, but I dont see how can it help the attack.

DHCP Interception

When a new client is trying to get a new IP from the DHCP server, it broadcasts the discover message. And also DHCP broadcasts the offer. And they use the transaction ID to communicate without the IP address. Would it be possible for another client in the same network to send a DHCP request with the same transaction ID to block the other client from getting an IP address?

Security of using DHCP and non-stanard ports for medical devices

I recently came across this comment written in a journal article.

“Lastly, medical apparatus are expected to use Dynamic Host Configuration Protocol (DHCP) for the allocation of their IP addresses and even worst [sic] use non-standard ports for their communication creating an unstable environment of information that is hard to address”

Can some explain why DHCP and non-standard ports are considered so risky?

Bug al iniciar DHCP en Ubuntu Desktop 18.04 / Bug when starting dhcp in Ubuntu 18.04

soy estudiante de Administración en Sistemas Informáticos en Red (ASIR). Estoy realizando la actividad para configurar el servidor dhcp en Ubuntu 18.04. Al comprobar su estado me resalta un mensaje diciendo que tengo un bug.

Hi. Im a computer student. I have to do an activity to set up a dhcp server on Ubuntu 18.04. When I check it condition, in the terminal appears a message say that I have a bug.

If you think you have received this message due to a bug rather than a configuration issue please read the section on submitting bugs on either our web page at www.isc.org or in the README file before submitting a bug. These pages explain the proper process and the information we find helpful for debugging.. exiting.

Contenido del archivo dhcpd.conf/ content of dhcpd.conf:

# dhcpd.conf # # Sample configuration file for ISC dhcpd # # Attention: If /etc/ltsp/dhcpd.conf exists, that will be used as # configuration file instead of this file. #  # option definitions common to all supported networks... option domain-name "example.org"; option domain-name-servers ns1.example.org, ns2.example.org;  default-lease-time 600; max-lease-time 7200;  # The ddns-updates-style parameter controls whether or not the server will # attempt to do a DNS update when a lease is confirmed. We default to the # behavior of the version 2 packages ('none', since DHCP v2 didn't # have support for DDNS.) ddns-update-style none;  # If this DHCP server is the official DHCP server for the local # network, the authoritative directive should be uncommented. authoritative;  # Use this to send dhcp log messages to a different log file (you also # have to hack syslog.conf to complete the redirection). #log-facility local7;  # No service will be given on this subnet, but declaring it helps the  # DHCP server to understand the network topology.  #subnet netmask { #}  # This is a very basic subnet declaration.  #subnet netmask { #  range; #  option routers rtr-239-0-1.example.org, rtr-239-0-2.example.org; #}  # This declaration allows BOOTP clients to get dynamic addresses, # which we don't really recommend.  #subnet netmask { #  range dynamic-bootp; #  option broadcast-address; #  option routers rtr-239-32-1.example.org; #}  # A slightly different configuration for an internal subnet. subnet netmask {   range;   option domain-name-servers;   option domain-name-servers ;   option subnet-mask;   option routers;   option broadcast-address;   default-lease-time 600;   max-lease-time 3600; }  # Hosts which require special configuration options can be listed in # host statements.   If no address is specified, the address will be # allocated dynamically (if possible), but the host-specific information # will still come from the host declaration.  #host passacaglia { #  hardware ethernet 0:0:c0:5d:bd:95; #  filename "vmunix.passacaglia"; #  server-name "toccata.example.com"; #}  # Fixed IP addresses can also be specified for hosts.   These addresses # should not also be listed as being available for dynamic assignment. # Hosts for which fixed IP addresses have been specified can boot using # BOOTP or DHCP.   Hosts for which no fixed address is specified can only # be booted with DHCP, unless there is an address range on the subnet # to which a BOOTP client is connected which has the dynamic-bootp flag # set. #host fantasia { #  hardware ethernet 08:00:07:26:c0:a5; #  fixed-address fantasia.example.com; #}  # You can declare a class of clients and then do address allocation # based on that.   The example below shows a case where all clients # in a certain class get addresses on the 10.17.224/24 subnet, and all # other clients get addresses on the 10.0.29/24 subnet.  #class "foo" { #  match if substring (option vendor-class-identifier, 0, 4) = "SUNW"; #}  #shared-network 224-29 { #  subnet netmask { #    option routers rtr-224.example.org; #  } #  subnet netmask { #    option routers rtr-29.example.org; #  } #  pool { #    allow members of "foo"; #    range; #  } #  pool { #    deny members of "foo"; #    range; #  } #} 

Contenido de netplan / Content of netplan:

# Let NetworkManager manage all devices on this system network:   version: 2   renderer: NetworkManager   ethernets:     enp0s3:       addresses: []       gateway4:       nameservers:         addresses: [] 

Gracias por la ayuda. Thanks for support

Is it possible for DHCP in one network interface to assign a local IP address that overlaps with the subnet of another interface?

iface eth0 inet dhcp  iface eth1 inet static   address   netmask 

Is it possible for a host to connect to eth0 and get assigned an address that is part of eth1‘s subnet domain (anything from to

What averse consequences will this have in practice and is the expected (mis-)behavior deterministic?

Is there something that can be done to prevent this while leaving the static and dhcp methods unchanged? I couldn’t find anything on the man page.

Restrict DHCP lease range using NetworkManager “Shared to other computers”

My computer has wireless internet access and I successfully share its connection using the built-in “Shared to other computers” feature of Network Manager:

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However, I’m only serving a single host (via cross-over cable) and because DHCP is used, the IP address of my host is indeterminate. I can’t save ssh or VNC connections because the route will change without warning.

  • Can I restrict the DHCP range of shared connections to a single address?

My downstream host is Untouchable – it’s outside my scope to change its network configuration. What I need is more control over how Network Manager launches dnsmasq to support the internet connection sharing. It seems to do so entirely with command line parameters:

lar@rpi-lgr13-0199:~$   ps ax | grep dnsmasq   817 ?        S      0:00 /usr/sbin/dnsmasq --conf-file --no-hosts --keep-in-foreground --bind-interfaces --except-interface=lo --clear-on-reload --strict-order --listen-address= --dhcp-range=,,60m --dhcp-option=option:router, --dhcp-lease-max=50 --pid-file=/var/run/nm-dnsmasq-enxb827eb017088.pid --conf-dir=/etc/NetworkManager/dnsmasq-shared.d  1013 ?        S      0:00 /usr/sbin/dnsmasq --no-resolv --keep-in-foreground --no-hosts --bind-interfaces --pid-file=/var/run/NetworkManager/dnsmasq.pid --listen-address= --cache-size=0 --conf-file=/dev/null --proxy-dnssec --enable-dbus=org.freedesktop.NetworkManager.dnsmasq --conf-dir=/etc/NetworkManager/dnsmasq.d  6987 pts/2    S+     0:00 grep --color=auto dnsmasq 

Server 18.04 netplan, multiple NICs / gateways, and routing (DHCP)

My apologies if this is dumb or redundant, but I have not found an accurate or complete answer to this question.

I have a fully current 18.04 server with multiple NICs across different networks. I have DHCP reservations set up for each adapter on its respective network, so I don’t need or particularly want to configure static IPs / gateways for each adapter, but I do need for each adapter to respond to traffic through the correct gateway because (for instance) I do have traffic coming to a specific adapter on that server across an openVPN connection (so coming from a different subnet that is not duplicated anywhere else in the system). As I have things configured now, each adapter responds correctly within it’s own subnet, so I know I have connectivity, but I need something more.

The problem is that all traffic from the server goes out through one gateway, so return traffic does not come back from the IP it is expected from (so is ignored). I’ve tried to use the information in the NetPlan man pages, but cannot find a clear answer for my configuration needs. My working 01-network-manager-all.yaml looks like this:

network:   version: 2   renderer: networkd   ethernets:     eno1:       dhcp4: yes       dhcp-identifier: mac       dhcp4-overrides:         route-metric: 100     eno2:       dhcp4: yes       dhcp-identifier: mac       dhcp4-overrides:         route-metric: 100     enp3s0f0:       dhcp4: yes       dhcp-identifier: mac       dhcp4-overrides:         route-metric: 100     enp3s0f1:       dhcp4: yes       dhcp-identifier: mac       dhcp4-overrides:         route-metric: 10 

This obviously pushes all non-adapter-local traffic out through the enp3s0f1 adapter, which isn’t what I want. If I change the route-metric on that adapter to 100, I don’t reliably get traffic out through any adapter. What I need are concrete examples of multiple adapters on DHCP with distinct subnets / routes, where traffic to each adapter goes out through its respective gateway. Can anyone help?

Missing gateway from DHCP lease

My computer has a gigabit Ethernet connection that is wired to a router configured as a DHCP server.

When plugging the cable, the computer gets its IP address and the netmask right, but do not assigns a gateway at all.

In syslog, I can see NetworkManager reading the right gateway from the DHCP response (and I checked with dhcpdump too), yet it does not keep it as the gateway, nor does it appear in the Gateway column of route -n output.

What could be happening here?