dhcpcd static assignment required

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Setting the IP with dhcpcd

I have a setup where my system is in the regular home network connected to the router. The router assigns the IP to my device without issues.

I would like to use the same IP every time for my device. Either set it as static or as preferred. This is a fully controlled environment so there should be no problem getting the preferred IP from the DHCP.

Update: I can not change anything on the DHCP server side, I need to come to a solution from the client side. The server is ICS and can not be configured.

My question is, is this possible via dhcpcd ? Can I use a static IP with dhcpcd at all? Or even preferred? I can not avoid the use of dhcpcd because of some unrelated requirements.

Done so far

What I have tried so far is to add these lines to the /etc/dhcpcd.conf file:

This is what the manpages for dhcpcd.conf say:

static value Configures a static value. If you set ip_address then dhcpcd will not attempt to obtain a lease and just use the value for the address with an infinite lease time. Here is an example which configures a static address, routes and dns. interface eth0 static ip_address=192.168.0.10/24 static routers=192.168.0.1 static domain_name_servers=192.168.0.1

But the problem is that even when I do it like this, I only get to see my device using that IP for a short while, after which it seems to be lost and replaced with another IP most probably assigned via DHCP (it is next in the pool i guess).

Here is my complete dhcpcd.conf file contents:

Do I need to add some more options to let DHCP know that I don't need a new IP address or something? Or did I completely misunderstand the topic at hand?

I tried searching for more detailed help online, but either there is not much info on this subject, or more probably, I don't know enough about the topic to perform a valid search.

Any help is appreciated!

• 1 You assign reserved IP addresses in the DHCP server , not the client . –  Michael Hampton Jun 5, 2014 at 16:51
• 1 Hmm, i guess. The problem is that I can not change anything on the DHCP server. What can be done from the client side? (The server is a ICS DHCP server which I can not edit) I updated my question with this info.. –  bosnjak Jun 5, 2014 at 17:03
• You contact the person who does run the server. –  Michael Hampton Jun 5, 2014 at 17:10
• Lets just assume that I can not do anything on the server. The point is, is there anything that can be done on the client to achieve this? –  bosnjak Jun 5, 2014 at 17:36
• No. There is not. Asking the same thing over and over will not get you a different answer. –  Michael Hampton Jun 5, 2014 at 17:38

3 Answers 3

you need to do MAC reservation, like this:

• This must be done on the server side? I forgot to mention that I don't have the possibility to change anything on DHCP server side, because it is an ICS DHCP. Ill update that to my question. –  bosnjak Jun 5, 2014 at 17:00
• This is the way you do it in ISC DHCP. Though, you are still going to have to get the person who runs the server to do it for you. –  Michael Hampton Jun 5, 2014 at 17:11
• Its not an ISC server, rather the ICS (internet connection sharing) setup. Its the most basic fixed setup so I can't change anything on the server side. –  bosnjak Jun 5, 2014 at 17:15

If you are using dhcpcd (the client daemon, most people here are confusing it with DHCP and DHCPd which is different), then add the following text on the bottom of /etc/dhcpcd.conf

Of course remember to replace the IP info with your network details.

Are you trying to use as a static address one which falls inside the router's address pool? If so, there is no telling what your router will do. Check the router address pools from the GUI, and pick an address outside the pool.

You may check whether you still experience such phenomena simply by stopping network manager (I presume you use one), giving yourself an address,

without using dhcpcd. This has the advantage of isolating the problem: if it happens again, then it's your router's fault, otherwise it was an incorrect choice of static IP address.

• Even when I use the IP outside of the pool, the behaviour is strange - i get that IP for some time, but then I just dissapear from the LAN altogether.. I will try the manual way and report back. –  bosnjak Jun 5, 2014 at 17:38
• @Lawrence I forgot one command in the manual sequence, please take a look at the revised version. –  MariusMatutiae Jun 5, 2014 at 17:47

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Related articles

• Network configuration
• Wireless network configuration

dhcpcd is a DHCP and DHCPv6 client. It is currently the most feature-rich open source DHCP client; see the home page for the full list of features.

Installation

Install the dhcpcd package.

dhcpcd-ui AUR is a GTK frontend for the dhcpcd daemon, and optionally wpa_supplicant . It features a configuration dialogue and the ability to enter a pass phrase for wireless networks.

To start the daemon for all network interfaces, start/enable dhcpcd.service .

To start the daemon for a specific interface alone, start/enable the template unit dhcpcd@ interface .service , where interface can be found with Network configuration#Listing network interfaces .

Using the template unit is recommended; see #dhcpcd and systemd network interfaces for details. In either case, you will be assigned a dynamic IP address. To assign a static IP address, see #Static profile .

Configuration

The main configuration is done in /etc/dhcpcd.conf . See dhcpcd.conf(5) for details. Some of the frequently used options are highlighted below.

DHCP static route(s)

If you need to add a static route client-side, add it to /etc/dhcpcd.exit-hook . The example shows a new hook-script which adds a static route to a VPN subnet on 10.11.12.0/24 via a gateway machine at 192.168.192.5 :

You can add multiple routes to this file.

DHCP Client Identifier

The DHCP client may be uniquely identified in different ways by the server:

• hostname (or the hostname value sent by the client),
• MAC address of the network interface controller through which the connection is being made, linked to this is the third,
• Identity Association ID (IAID), which is an abstraction layer to differentiate different use-cases and/or interfaces on a single host,
• DHCP Unique Identifier (DUID).

For a further description, see RFC 3315 .

It depends on the DHCP-server configuration which options are optional or required to request a DHCP IP lease.

If the dhcpcd default configuration fails to obtain an IP, the following options are available to use in dhcpcd.conf :

• hostname sends the hostname set in /etc/hostname
• clientid sends the MAC address as identifier
• iaid <interface> derives the IAID to use for DHCP discovery. It has to be used in an interface block (started by interface <interface> , see [1] ), but more frequently the next option is used:
• duid triggers using a combination of DUID and IAID as identifier.

The DUID value is set in /var/lib/dhcpcd/duid . For efficient DHCP lease operation it is important that it is unique for the system and applies to all network interfaces alike, while the IAID represents an identifier for each of the systems' interfaces (see RFC 4361 ).

Care must be taken on a network running Dynamic DNS to ensure that all three IDs are unique. If duplicate DUID values are presented to the DNS server, e.g. in the case where a virtual machine has been cloned and the hostname and MAC have been made unique but the DUID has not been changed, then the result will be that as each client with the duplicated DUID requests a lease the server will remove the predecessor from the DNS record.

Static profile

Required settings are explained in Network configuration . These typically include the network interface name, IP address , router address , and name server .

Configure a static profile for dhcpcd in /etc/dhcpcd.conf , for example:

More complicated configurations are possible, for example combining with the arping option. See dhcpcd.conf(5) for details.

Fallback profile

It is possible to configure a static profile within dhcpcd and fall back to it when DHCP lease fails. This is useful particularly for headless machines , where the static profile can be used as "recovery" profile to ensure that it is always possible to connect to the machine.

The following example configures a static_eth0 profile with 192.168.1.23 as IP address, 192.168.1.1 as gateway and name server, and makes this profile fallback for interface eth0 .

dhcpcd executes all scripts found in /usr/lib/dhcpcd/dhcpcd-hooks/ in a lexical order. See dhcpcd.conf(5) and dhcpcd-run-hooks(8) for details.

• Each script can be disabled using the nohook option in dhcpcd.conf .
• The env option can be used to set an environment variable for all hooks. For example, you can force the hostname hook to always set the hostname with env force_hostname=YES .

10-wpa_supplicant

Enable this hook by creating a symbolic link, which ensures the current version is used, even after package updates:

The 10-wpa_supplicant hook, if enabled, automatically launches wpa_supplicant on wireless interfaces. It is started only if:

• no wpa_supplicant process is already listening on that interface.
• a wpa_supplicant configuration file exists. dhcpcd checks

by default, in that order, but a custom path can be set by adding env wpa_supplicant_conf= configuration_file_path into /etc/dhcpcd.conf .

If you manage wireless connections with wpa_supplicant itself, the hook may create unwanted connection events. For example, if you stop wpa_supplicant the hook may bring the interface up again. Also, if you use netctl-auto , wpa_supplicant is started automatically with /run/network/wpa_supplicant_ interface .conf for config, so starting it again from the hook is unnecessary and may result in boot-time parse errors of the /etc/wpa_supplicant/wpa_supplicant.conf file, which only contains dummy values in the default packaged version.

To disable the hook remove the symbolic link you added, or add nohook wpa_supplicant to dhcpcd.conf .

Tips and tricks

Speed up dhcp by disabling arp probing.

dhcpcd contains an implementation of a recommendation of the DHCP standard ( RFC 2131 ) to verify via ARP if the assigned IP is not used by something else. This is usually not needed in home networks, so it is possible to save about 5 seconds on every connect by disabling it:

This is equivalent to passing --noarp to dhcpcd , and disables the described ARP probing, speeding up connections to networks with DHCP.

Remove old DHCP lease

The file /var/lib/dhcpcd/ interface .lease , where interface is the name of the interface on which you have a lease, contains the actual DHCP lease reply sent by the DHCP server. For a wireless interface, the filename is /var/lib/dhcpcd/ interface - ssid .lease , where ssid is the name of the wireless network. It is used to determine the last lease from the server, and its mtime attribute is used to determine when it was issued. This last lease information is then used to request the same IP address previously held on a network, if it is available. If you do not want that, simply delete this file.

If the DHCP server still assigns the same IP address, this may happen because it is configured to keep the assignment stable and recognizes the requesting DHCP client id or DUID (see #DHCP Client Identifier ). You can test it by stopping dhcpcd and removing or renaming /var/lib/dhcpcd/duid . dhcpcd will generate a new one on next run.

Keep in mind that the DUID is intended as persistent machine identifier across reboots and interfaces. If you are transferring the system to new computer, preserving this file should make it appear as old one.

Different IPs when multi-booting

If you are dualbooting Arch and macOS or Windows and want each to receive different IP addresses, you can exert control about the IPs leased by specifying a different DUID in each operating system installation.

In Windows the DUID should be stored in the

registry key.

On macOS it is directly accessible in Network\adapter\dhcp preferences panel .

If you are using a dnsmasq DHCP server, the different DUIDs can be used in appropriate dhcp-host= rules in its configuration.

/etc/resolv.conf

If resolvconf is available, DNS information will be sent to it; otherwise, dhcpcd itself will write to /etc/resolv.conf .

/etc/resolv.conf overwriting can be stopped by disabling the hook /usr/lib/dhcpcd/dhcpcd-hooks/20-resolv.conf . Do so by adding the following to the last section of /etc/dhcpcd.conf :

Note that disabling this hook also disables dhcpcd's use of resolvconf in general.

Alternatively, you can create a file called /etc/resolv.conf.head containing your DNS servers. dhcpcd will prepend this file to the beginning of /etc/resolv.conf .

Or you can configure dhcpcd to use the same DNS servers every time. To do this, add the following line at the end of your /etc/dhcpcd.conf , where dns-server-ip-addressses is a space separated list of DNS IP addresses.

For example, to set it to Google's DNS servers:

Troubleshooting

If you are on a network with DHCPv4 that filters Client IDs based on MAC addresses, you may need to change the following line:

Else, you may not obtain a lease since the DHCP server may not read your DHCPv6-style Client ID correctly. See RFC 4361 for more information.

Check DHCP problem by releasing IP first

A problem may occur when DHCP gets a wrong IP assignment, such as when two routers are tied together through a VPN. The router that is connected through the VPN may be assigning IP address. To fix it, as root, release the IP address:

Then request a new one:

You may have to run those two commands many times.

Problems with noncompliant routers

For some (noncompliant) routers, you will not be able to connect properly unless you comment the line

in /etc/dhcpcd.conf . This should not cause issues unless you have multiple DHCP servers on your network (not typical); see this page for more information.

dhcpcd and systemd network interfaces

dhcpcd.service can be enabled without specifying an interface. This may, however, create a race condition at boot with systemd-udevd trying to apply a predictable network interface name:

To avoid this problem use denyinterfaces or allowinterfaces in dhcpcd.conf(5) to stop dhcpcd from binding to kernel names, for example:

It is also possible to enable dhcpcd on a per interface basis as described in #Running . The downside of the template unit is, however, that it does not support hot-plugging of a wired connection and will fail if the network cable is not connected. To work-around the failure, see #Timeout delay .

Timeout delay

If dhcpcd operates on a single interface and fails to obtain a lease after 30 seconds (for example when the server is not ready or the cable not plugged), it will exit with an error.

To have dhcpcd wait indefinitely for one-time, edit the unit and set the timeout option to 0 :

To have it wait indefinitely, let the unit restart after it exited:

Known issues

[email protected] causes slow startup.

By default the [email protected] waits to get an IP address before forking into the background via the -w flag for dhcpcd . If the unit is enabled, this may cause the boot to wait for an IP address before continuing. To fix this, create a drop-in file for the unit with the following:

See also FS#49685 .

• dhcpcd.conf(5)
• Pages or sections flagged with Template:Expansion
• Toggle limited content width

dhcpcd — a DHCP client

DESCRIPTION

dhcpcd is an implementation of the DHCP client specified in RFC 2131 . dhcpcd gets the host information (IP address, routes, etc) from a DHCP server and configures the network interface of the machine on which it is running. dhcpcd then runs the configuration script which writes DNS information to resolvconf(8) , if available, otherwise directly to /etc/resolv.conf . If the hostname is currently blank, (null) or localhost, or force_hostname is YES or TRUE or 1 then dhcpcd sets the hostname to the one supplied by the DHCP server. dhcpcd then daemonises and waits for the lease renewal time to lapse. It will then attempt to renew its lease and reconfigure if the new lease changes when the lease begins to expire or the DHCP server sends a message to renew early.

If any interface reports a working carrier then dhcpcd will try to obtain a lease before forking to the background, otherwise it will fork right away. This behaviour can be modified with the -b , --background and -w , --waitip options.

dhcpcd is also an implementation of the BOOTP client specified in RFC 951 .

dhcpcd is also an implementation of the IPv6 Router Solicitor as specified in RFC 4861 and RFC 6106 .

dhcpcd is also an implementation of the IPv6 Privacy Extensions to AutoConf as specified in RFC 4941 . This feature needs to be enabled in the kernel and dhcpcd will start using it.

dhcpcd is also an implementation of the DHCPv6 client as specified in RFC 3315 . By default, dhcpcd only starts DHCPv6 when instructed to do so by an IPV6 Router Advertisement. If no Identity Association is configured, then a Non-temporary Address is requested.

Local Link configuration

If dhcpcd failed to obtain a lease, it probes for a valid IPv4LL address (aka ZeroConf, aka APIPA). Once obtained it restarts the process of looking for a DHCP server to get a proper address.

When using IPv4LL, dhcpcd nearly always succeeds and returns an exit code of 0. In the rare case it fails, it normally means that there is a reverse ARP proxy installed which always defeats IPv4LL probing. To disable this behaviour, you can use the -L , --noipv4ll option.

Multiple interfaces

If a list of interfaces are given on the command line, then dhcpcd only works with those interfaces, otherwise dhcpcd discovers available Ethernet interfaces that can be configured. When dhcpcd is not limited to one interface on the command line, it is running in Manager mode. The dhcpcd-ui project expects dhcpcd to be running this way.

If a single interface is given then dhcpcd only works for that interface and runs as a separate instance to other dhcpcd processes. The -w , --waitip option is enabled in this instance to maintain compatibility with older versions. Using a single interface, optionally further limited to an address protocol, also affects the -k , -N , -n and -x options, where the same interface and any address protocol will need to be specified, as a lack of an interface will imply Manager mode which this is not. To force starting in Manager mode with only one interface, the -M , --manager option can be used.

Interfaces are preferred by carrier, DHCP lease/IPv4LL and then lowest metric. For systems that support route metrics, each route will be tagged with the metric, otherwise dhcpcd changes the routes to use the interface with the same route and the lowest metric. See options below for controlling which interfaces we allow and deny through the use of patterns.

Non-ethernet interfaces and some virtual ethernet interfaces such as TAP and bridge are ignored by default, as is the FireWire interface. To work with these devices they either need to be specified on the command line, be listed in --allowinterfaces or have an interface directive in /etc/dhcpcd.conf .

Hooking into events

dhcpcd runs /usr/lib/dhcpcd/dhcpcd-run-hooks , or the script specified by the -c , --script option. This script runs each script found in /usr/lib/dhcpcd/dhcpcd-hooks in a lexical order. The default installation supplies the scripts 01-test , 20-resolv.conf and 30-hostname . You can disable each script by using the -C , --nohook option. See dhcpcd-run-hooks(8) for details on how these scripts work. dhcpcd currently ignores the exit code of the script.

More scripts are supplied in /usr/share/dhcpcd/hooks and need to be copied to /usr/lib/dhcpcd/dhcpcd-hooks if you intend to use them. For example, you could install 29-lookup-hostname so that dhcpcd can lookup the hostname of the IP address in DNS if no hostname is given by the lease and one is not already set.

Fine tuning

You can fine-tune the behaviour of dhcpcd with the following options:

Here is an example which configures a static address, routes and DNS.

You cannot presently set static DHCPv6 values. Use the -e , --env option instead.

Set the vendor option 01 with an IP address.

Restricting behaviour

dhcpcd will try to do as much as it can by default. However, there are sometimes situations where you don't want the things to be configured exactly how the DHCP server wants. Here are some options that deal with turning these bits off.

Note that when dhcpcd is restricted to a single interface then the interface also needs to be specified when asking dhcpcd to exit using the commandline. If the protocol is restricted as well then the protocol needs to be included with the exit instruction.

So to stop dhcpcd from touching your DNS settings you would do:-

3RDPARTY LINK MANAGEMENT

Some interfaces require configuration by 3rd parties, such as PPP or VPN. When an interface configuration in dhcpcd is marked as STATIC or INFORM without an address then dhcpcd will monitor the interface until an address is added or removed from it and act accordingly. For point to point interfaces (like PPP), a default route to its destination is automatically added to the configuration. If the point to point interface is configured for INFORM, then dhcpcd unicasts INFORM to the destination, otherwise it defaults to STATIC.

dhcpcd requires a Berkeley Packet Filter, or BPF device on BSD based systems and a Linux Socket Filter, or LPF device on Linux based systems for all IPv4 configuration.

If restricting dhcpcd to a single interface and optionally address family via the command-line then all further calls to dhcpcd to rebind, reconfigure or exit need to include the same restrictive flags so that dhcpcd knows which process to signal.

Some DHCP servers implement ClientID filtering. If dhcpcd is replacing an in-use DHCP client then you might need to adjust the clientid option dhcpcd sends to match. If using a DUID in place of the ClientID, edit /var/lib/dhcpcd/duid accordingly.

fnmatch(3) , if_nametoindex(3) , dhcpcd.conf(5) , resolv.conf(5) , dhcpcd-run-hooks(8) , resolvconf(8)

RFC 951, RFC 1534, RFC 2104, RFC 2131, RFC 2132, RFC 2563, RFC 2855, RFC 3004, RFC 3118, RFC 3203, RFC 3315, RFC 3361, RFC 3633, RFC 3396, RFC 3397, RFC 3442, RFC 3495, RFC 3925, RFC 3927, RFC 4039, RFC 4075, RFC 4242, RFC 4361, RFC 4390, RFC 4702, RFC 4074, RFC 4861, RFC 4833, RFC 4941, RFC 5227, RFC 5942, RFC 5969, RFC 6106, RFC 6334, RFC 6355, RFC 6603, RFC 6704, RFC 7217, RFC 7550, RFC 7844.

Roy Marples < [email protected] >

Please report them to https://roy.marples.name/projects/dhcpcd

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• Topics: Active | Unanswered
• »  Newbie Corner
• »  dhcpcd.conf with fallback to static IP

#1 2021-09-11 01:10:37

Dhcpcd.conf with fallback to static ip.

I am trying to setup DHCP with a fallback to a static ip address on a wired port.

For starters, a) the situation: I'm connecting to my friend's SUSE machine with an ethernet cable, and his side is configured properly with a static IP (i.e. he can directly plug into a zedboard which also has a static IP and the two machines talk as expected) aa) I'm running native arch linux b) if I manually run

"ip addr" shows the address has been applied and the link is up, and I can ping, ssh, et cetera into my friend's machine.

c) "$ sudo systemctl start [email protected]" works as expected when I plug the ethernet cable into a router running a dhcp server d) I have verified that I have no other network daemons enabled/started/running e) I've replaced the really long "enp..." nic name from systemd with eth0 herein for simplicity

Now for the broken part:

I am using the default "dhcpcd.conf", with these next few lines added to the end of it:

After I "sudo systemctl start [email protected]" (while connected to my friend's machine), I get the following output from "systemctl status [email protected]":

and "ip addr" also shows the nic has not received an ip address.

I've tried adding various forms of "static routers=169.254.8.1" among other things, but haven't been able to get the service to work. Any tips on where I could look for more clues as to the issue? Is it required I add a static route in the exit-hook? It doesn't seem like that should be required, from my understanding.

#2 2021-09-11 04:54:34

Re: dhcpcd.conf with fallback to static ip.

please output your entire /etc/dhcpcd.conf

#3 2021-09-11 05:06:16

/etc/dhcpcd.conf

edit: comment out "static routers" to reflect the state the config was in when the output printed in the first post was generated.

Last edited by bbus (2021-09-11 05:08:15)

#4 2021-09-11 06:08:13

With almost identical dhcpcd.conf, in my own testbed setup, I disabled dhcpcd.service and enable/start [email protected] and it works as expected. I notices in your systemctl status [email protected] there were neither "DHCP lease" nor "selected profile"

What is output from your

Last edited by solskog (2021-09-11 07:30:27)

#5 2021-09-12 00:40:04

Note, I've using a different adapter today, so it is using a different enp* compared to yesterday. Further, I don't have access to my friend's machine, so I was only able to connect my primary laptop to an old one I have around the house. The second machine is also running Arch, and I put the identical /etc/dhcpcd.conf on it, with the only thing being altered is the name of the nic.

I just rebooted again, so it was fresh. I powered off my wireless card, and stopped iwd.service. I then plugged in the cable, waited a few seconds, then ran "start [email protected]":

Is it required to have the "static routers=<169.254.8.1>" line? Without it:

That time "dhcpcd -4" seemed to assign a static ip, but "ip addr" reports that it still doesn't have that IP.

edit: realized at some point this morning I had added a bogus line by accident in "/etc/dhcpcd.conf". I also realized that changes via "sudoedit" don't take effect until after I exit (is this right?). In my testing yesterday, I was ":w" but not exiting, So most of my tests yesterday probably were never running on different versions that I thought I was. Either way, the info above was created with the following conf, I "cat"ed it this time, after exiting "sudoedit" and this is the direct copy, unaltered,that the above results were genereated with:

edit 2: just realized you wanted the raw "status dhcpcd.service" so here is the output of this boot from journalctl:

Last edited by bbus (2021-09-12 01:06:47)

#6 2021-09-12 02:44:16

Is it required to have the "static routers=<169.254.8.1>" line?

No I don't have a router defined for Link-local address range. Your dhcpcd.conf is correct.

Sep 10 14:39:01 myhostname dhcpcd[1237977]: ps_inet_listenin: Cannot assign requested address Sep 10 14:39:01 myhostname dhcpcd[1237977]: ps_root_recvmsg: Cannot assign requested address

These two lines indicate something prevent the adapter been assigned. So I suspect the problem is NOT your dhcpcd service. Are you still be able to manually add ip address to this adapter? And you are positive no other network services running e.g: systemd-networkd, networkManager etc?

Last edited by solskog (2021-09-12 03:17:52)

#7 2021-09-14 04:30:14

I haven't yet looked extensively into those error messages, beyond the first few google results. But I did some more testing today, and when I replace "169.254.8.201" with "192.168.1.201" everything just works.

Here is the journalctl from a fresh boot:

Is there something about dhcpcd that blocks any static assignment outside of 192.168/16? Do I understand from your comment that maybe it has something to do with the hardware of my adapter?

#8 2021-09-14 05:50:08

It shouldn't matter which address you assigned, they all should work. I guess you use a USB to RJ45 adapter? It's all right, I have tested with both these adapters and real network adapters, all works. No, the problem seems software/configuration related. You can try with archiso install image with disabled systemd-networkd and install/configure dhcpcd on it. Or try fresh install arch into an external USB3.0 drive and test from there instead.

Last edited by solskog (2021-09-14 09:48:20)

#9 2021-09-15 07:54:09

I created a new archiso usb (from the stock, Sept 01 image), and booted from it. I ran "systemctl stop systemd-networkd.socket", "systemctl stop systemd-networkd.service", and "systemctl stop systemd-networkd-wait-online.service". I added the fallback in "/etc/dhcpcd.conf" for 192.168.1.201/24 and started "dhcpcd@<nic>.service" and it worked. So I stopped the "dhcpcd@<nic>.service", flushed the ip addresses and ip routes ("ip addr flush dev <nic>" and "ip route flush dev <nic>") and changed the "dhcpcd.conf" to use the fallback 169.254.8.201/24, and again, the dhcpcd template unit failed, just as before.

I did all of this attached to a different adapter (completely different brand), to be sure it isn't something wonky with my original adapter. All the errors show the same as above, i.e. "ps_inet_listenin" and "ps_root_recvmesg".

This is very strange. Seems like it shouldn't be this difficult to assign whatever IP I want as a static fallback.

Should this be moved to "networking, server, and protection"?

#10 2021-09-15 08:50:34

* Did you try "noipv6"? * Can you enforce the static IP w/o the fallback? * How many NICs are there actually in that system, are others in the 192.168.1.0/24 subnet? Are there bridges?

* How many NICs are in your friends system? Is there a rogue dhcp server in his subnet?

How to upload text · How to boot w/o GUI · Disable Windows Fast-Start! · Fix your xinitrc

#11 2021-09-15 20:55:43

No, my friend's laptop wasn't connected to any other network on a different nic. To avoid any potential doubt, I have now made two archiso usb drives. I booted two separate towers (both the same model, hp elitedesk 800 g4, with 8th-gen, i7 processors) on the archiso. These towers do not have any wireless nic, and only have one (intel) ethernet nic, each.

Upon booting both of them, I ran the following, and got almost identical results from both.

The "dhcpcd.conf" file I used for the results was the stock archiso version with certain things attached at the end, as before. For space, I will only delineate the final lines that I added:

For the results above, I used:

I also tried setting the static ip, without any fallback, both with and without including the "noipv6{rs}" options. I would always boot fresh into the archiso, to ensure no configuration got in the way of the next. Each time I booted, I would always run those first three commands separately (condensed here) "systemctl stop systemd-networkd{.socket,.service,-wait-online.service}" I got very similar results each time I try to use 169.254.x.x/24.

With the above described configurations, there is no possible way for there to be a rogue dhcp server on any subnet of the machines.

On each of these configurations, I always got the same two errors.

#12 2021-09-15 21:52:23

#13 2021-09-15 22:03:21

Sounds good, thanks for looking into it. After all that testing, I hope it still doesn't turn out to be some oversight on my part. Should I make an issue on GitHub? I just found his mailing list from March announcing he has cancer, and he's looking for a new maintainer. I'd feel bad trying to contact him on Twitter or irc or something. Hope he's been able to enjoy what it sounds like may be his last bit of summer.

#14 2021-09-16 00:45:12

I hope it still doesn't turn out to be some oversight on my part.

So far my tests didn't have any issue to assign any address range as fallback. Let me reproduce this. Default dhcpcd.conf include noipv4ll, which prevent assignment of Link local address range.

man dhcpcd.conf noipv4ll Don't attempt to obtain an IPv4LL address if we failed to get one via DHCP.See RFC 3927.

But what really makes it to work is this option "noarp".

man dhcpcd.conf noarp   Don't send any ARP requests.  This also disables IPv4LL.

Last edited by solskog (2021-09-16 04:12:25)

#15 2021-09-16 20:19:22

Confirmed, that did work. Now I know that the "LL" stands for link local. I should have known that. So I tried it with both, (a) replacing "noipv4ll" with "noarp" and (b) including both options. They both work, as the documentation suggests that "noarp" supersedes "noipv4ll".

My question is, if "noarp" "also disables IPv4LL" how am I allowed to acquire a link local address? And another, what is ARP doing that blocks it from working when I comment out "noipv4ll" and do not include "noarp"?

#16 2021-09-16 22:32:32

This looks like to me as an "undocumented behavior".

Last edited by solskog (2021-09-16 22:43:05)

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Static and dynamic IP address configurations: DHCP deployment

%t min read | by Damon Garn

In my Static and dynamic IP address configurations for DHCP article, I discussed the pros and cons of static versus dynamic IP address allocation. Typically, sysadmins will manually configure servers and network devices (routers, switches, firewalls, etc.) with static IP address configurations. These addresses don’t change (unless the administrator changes them), which is important for making services easy to find on the network.

With dynamic IP configurations, client devices lease an IP configuration from a Dynamic Host Configuration Protocol (DHCP) server. This server is configured with a pool of available IPs and other settings. Clients contact the server and temporarily borrow an IP address configuration.

In this article, I demonstrate how to configure DHCP on a Linux server.

[ You might also like:  Using systemd features to secure services ]

Manage the DHCP service

First, install the DHCP service on your selected Linux box. This box should have a static IP address. DHCP is a very lightweight service, so feel free to co-locate other services such as name resolution on the same device.

Note : By using the -y option, yum will automatically install any dependencies necessary.

Configure a DHCP scope

Next, edit the DHCP configuration file to set the scope. However, before this step, you should make certain you understand the addressing scheme in your network segment. In my courses, I recommend establishing the entire range of addresses, then identifying the static IPs within the range. Next, determine the remaining IPs that are available for DHCP clients to lease. The following information details this process.

How many static IP addresses?

Figure out how many servers, routers, switches, printers, and other network devices will require static IP addresses. Add some additional addresses to this group to account for network growth (it seems like we’re always deploying more print devices).

What are the static and dynamic IP address ranges?

Set the range of static IPs in a distinct group. I like to use the front of the available address range. For example, in a simple Class C network of 192.168.2.0/24, I might set aside 192.168.2.1 through 192.168.2.50 for static IPs. If that’s true, you may assume I have about 30 devices that merit static IP addresses, and I have left about twenty addresses to grow into. Therefore, the available address space for DHCP is 192.168.2.51 through 192.168.2.254 (remember, 192.168.2.255 is the subnet broadcast address).

This screenshot from the part one article is a reminder:

Note : Some administrators include the static IPs in the scope and then manually mark them as excluded or unavailable to the DHCP service for leasing. I’m not a fan of this approach. I prefer that the DHCP not even be aware of the addresses that are statically assigned.

What is the router’s IP address?

Document the router’s IP address because this will be the default gateway value. Administrators tend to choose either the first or the last address in the static range. In my case, I’d configure the router’s IP address as 192.168.2.1/24, so the default gateway value in DHCP is 192.168.2.1.

Where are the name servers?

Name resolution is a critical network service. You should configure clients for at least two DNS name servers for fault tolerance. When set manually, this configuration is in the /etc/resolv.conf file.

Note that the DNS name servers don’t have to be on the same subnet as the DNS clients.

Lease duration

In the next section, I’ll go over the lease generation process whereby clients receive their IP address configurations. For now, suffice it to say that the IP address configuration is temporary. Two values are configured on the DHCP server to govern this lease time:

default-lease-time - How long the lease is valid before renewal attempts begin.

max-lease-time - The point at which the IP address configuration is no longer valid and the client is no longer considered a lease-holder.

Configure the DHCP server

Now that you understand the IP address assignments in the subnet, you can configure the DHCP scope. The scope is the range of available IP addresses, as well as options such as default gateway. There is good documentation here .

Create the DHCP scope

Begin by editing the dhcp.conf configuration file (you’ll need root privileges to do so). I prefer Vim :

Next, add the values you identified in the previous section. Here is a subnet declaration (scope):

Remember, that spelling counts and typos can cause you a lot of trouble. Check your entries carefully. A mistake in this file can prevent many workstations from having valid network identities.

Reserved IP addresses

It is possible to reserve an IP address for a specific host. This is not the same thing as a statically-assigned IP address. Static IP addresses are configured manually, directly on the client. Reserved IP addresses are leased from the DHCP server, but the given client will always receive the same IP address. The DHCP service identifies the client by MAC address, as seen below.

Start the DHCP service

Start and enable the DHCP service. RHEL 7 and 8 rely on systemd to manage services, so you’ll type the following commands:

See this article I wrote for a summary on successfully deploying services.

Don’t forget to open the DHCP port in the firewall:

Explore the DORA process

Now that the DHCP server is configured, here is the lease generation process. This is a four-step process, and I like to point out that it is entirely initiated and managed by the client, not the server. DHCP is a very passive network service.

The process is:

• Acknowledge

Which spells the acronym DORA .

• The client broadcasts a DHCPDiscover message on the subnet, which the DHCP server hears.
• The DHCP server broadcasts a DHCPOffer on the subnet, which the client hears.
• The client broadcasts a DHCPRequest message, formally requesting the use of the IP address configuration.
• The DHCP server broadcasts a DHCPAck message that confirms the lease.

The lease must be renewed periodically, based on the DHCP Lease Time setting. This is particularly important in today’s networks that often contain many transient devices such as laptops, tablets, and phones. The lease renewal process is steps three and four. Many client devices, especially desktops, will maintain their IP address settings for a very long time, renewing the configuration over and over.

Updating the IP address configuration

You may need to obtain a new IP address configuration with updated settings. This can be an important part of network troubleshooting.

Manually generate a new lease with nmcli

You can manually force the lease generation process by using the nmcli command. You must know the connection name and then down and up the card.

Manually force lease generation with dhclient

You can also use the dhclient command to generate a new DHCP lease manually. Here are the commands:

dhclient -r to release it

dhclient (no option) to lease a new one

dhclient -r eth0 for specific NIC

Note : use -v for verbose output

Remember, if the client’s IP address is 169.254.x.x, it could not lease an IP address from the DHCP server.

Other DHCP considerations

There are many ways to customize DHCP to suit your needs. This article only covers the most common options. Two settings to keep in mind are lease times and dealing with routers.

Managing lease times

There is a good trick to be aware of. Use short lease durations on networks with many portable devices or virtual machines that come and go quickly from the network. These short leases will allow IP addresses to be recycled regularly. Use longer durations on unchanging networks (such as a subnet containing mostly desktop computers). In theory, the longer durations reduced network traffic by requiring fewer renewals, but on today’s networks, that traffic is inconsequential.

Routers and DHCP

There is one other aspect of DHCP design to consider. The DORA process covered above occurs entirely by broadcast. Routers, as a general rule, are configured to stop broadcasts. That’s just part of what they do. There are three approaches you can take to managing this problem:

• Place a DHCP server on each subnet (no routers between the DHCP server and its clients).
• Place a DHCP relay agent on each subnet that sends DHCP lease generation traffic via unicast to the DHCP server on a different subnet.
• Use RFC 1542-compliant routers, which can be configured to recognize and pass DHCP broadcast traffic.

[ Getting started with containers? Check out this free course. Deploying containerized applications: A technical overview . ]

DHCP is a simple service but an absolutely critical one. Understanding the lease generation process helps with network troubleshooting. Proper planning and tracking are essential to ensuring you don’t permit duplicate IP address problems to enter your network environment.

Damon Garn owns Cogspinner Coaction, LLC, a technical writing, editing, and IT project company based in Colorado Springs, CO. Damon authored many CompTIA Official Instructor and Student Guides (Linux+, Cloud+, Cloud Essentials+, Server+) and developed a broad library of interactive, scored labs. He regularly contributes to Enable Sysadmin, SearchNetworking, and CompTIA article repositories. Damon has 20 years of experience as a technical trainer covering Linux, Windows Server, and security content. He is a former sysadmin for US Figure Skating. He lives in Colorado Springs with his family and is a writer, musician, and amateur genealogist. More about me

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• Configuring a DHCP server static IP address

For certain types of servers, you must assign a static IP address and subnet mask during or after Setup. These servers include DHCP servers, DNS servers, WINS servers, and any server providing access to users on the Internet. It is also recommended that you assign a static IP address and subnet mask for each domain controller. If a computer has more than one network adapter, you must assign a separate IP address for each adapter.

Click Start , click Control Panel , click Network and Internet , click Network and Sharing Center and then click Change Adapter Settings .

Right-click the connection to which you want to add a static IP address and then click Properties .

Acknowledge the UAC dialog and then double-click Internet Protocol Version 4 (TCP/IP/IPv4).

Click Use the following IP address , and do one of the following:

• For a local area connection, in IP address, Subnet mask , and Default gateway , type the IP address, subnet mask, and default gateway addresses.
• For all other connections, in IP address , type the IP address.

Click Use the following DNS server addresses .

In Preferred DNS server and Alternate DNS server , type the primary and secondary DNS server addresses.

To configure advanced static IPv4 address settings for a local area connection, click Advanced .

Acknowledge the UAC dialog and then double-click Internet Protocol Version 4 (TCP/IP/IPv6).

Click Use the following IPv6 address , and then do one of the following:

• For a local area connection, in IPv6 address, Subnet prefix length , and Default gateway , type the IP address, subnet prefix length, and default gateway address.
• For all other connections, in IPv6 address , type the IP address.

To configure advanced static IPv6 address settings for a local area connection, click Advanced .

Additional Resources

For a list of Help topics providing related information, see Configuring DHCP Server Role Settings .

For updated detailed IT pro information about DHCP and TCP/IP, see the Windows Server 2008 documentation on the Microsoft TechNet Web site.

Table Of Contents

• DHCP Overview
• Installing the DHCP Server Role
• More about WINS server settings
• Integrating DHCP with DNS
• More about adding scopes
• More about authorizing DHCP servers in AD DS
• More about DHCPv6 stateless mode
• DHCP network interface card bindings
• Configuring Split Scopes
• Configuring Name Protection
• Configuring Link Layer Filtering
• Configuring a DHCP Scope
• Configuring a DHCP Multicast Scope
• Configuring a DHCP Superscope
• More About Predefined DHCP Options
• Increase fault tolerance by splitting DHCP scopes
• Eliminate manual updates of DNS records by configuring dynamic update and secure dynamic update
• Allow remote administration of DHCP servers by configuring Windows Firewall ports
• Prevent rogue DHCP servers on your network by authorizing DHCP servers in AD DS
• Enforce network access policies for client health by configuring DHCP with Network Access Protection
• Automate management of devices that have static IP addresses by creating DHCP reservations
• Support multiple subnets with one DHCP server by configuring DHCP relay agents
• Avoid reconfiguring DHCP on a new server by migrating an existing DHCP infrastructure
• Balance the load on your DHCP servers by using the 80/20 rule for scopes
• Centralize management of two or more DHCP servers as a single system by clustering DHCP servers
• More About DHCP Backup and Restore
• More About Exporting and Importing the DHCP Database
• More About DHCP Security Groups
• Distribute DHCP Leases Based on MAC Address
• Prevent Name Squatting with Name Protection
• More About DHCP Audit and Event Logging

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Combine static and dynamic setup in dhcpd.conf

I'm trying to migrate from a situation with static ip addresses and a dhcp server on a router to one new dhcp server.

My idea was to add the current static ip addresses as static ip addresses in the dhcp server, so I can still manage them from that server, and to move the current dhcp leases to static ones as well, as these ip addresses are more or less used as static ones too, being referred to in hosts.allow etc on some machines for example.

I do however also still need dynamic addresses, and I'd like to know if I can for example define that my pool of dynamic addresses is from 192.168.1.2 to 192.168.1.200, but have some exlusions in that range for the static addresses.

What would my dhcpd.conf have to look like to achieve that, can I just define the pool and define seperate static addresses and will the dhcpd take those out of the pool automatically?

I can't test this setup as this is a live network with a working dhcp on a router already, that's why I have to ask it up front.

• dhcp-server

I think you're getting your terminology confused sorry, let me try to clarify this for you.

Static addresses are just that, absolutely static, they're set on the individual host machines and have nothing to do with DHCP at all - as such they're not reliant on DHCP existing and don't have leases at all.

Fully dynamic addresses are given out from a DHCP server as part of a scope, do have leases and are succeptable to the various problems that DHCP brings such as what happens when the DHCP server falls over, what happens if/when someone switches on another DHCP server etc.

DHCP Reservations are almost identical to fully dynamic addresses with the exception that specified hosts always get the same IP address but are no different in the face of the problems listed above.

So getting to your question, yes you can define a dynamic scope and reserved IPs on one server but you have to manually define both sets and yes this does mean that you could misconfigure the server by having overlapped reserved addresses and dynamic scoped addresses - basically just don't do that by being careful and using something like a spreadsheet to keep you records and plans up to date.

• Ok, this helps a lot! Too bad that the scope of dynamic addresses and reserved addresses can't overlap.. –  datadevil Jan 10, 2012 at 12:13
• It's far from too bad, the point is you don't want to hand out already-reserved IP addressed to dynamic clients, otherwise you'd have two machines trying to use the same IP. What you can do is, as an example, have a dynamic scope of say 192.168.1.1-254 then put your rserved IPs at the top of that rang (starting at .254 and moving down) then reducing the dynamic scope to end before the reserved IPs start. Does that make sense? –  Chopper3 Jan 10, 2012 at 12:27
• yeah it makes sense, but I'd like it to be flexible that my pool is like you say e.g. 192.168.1.1-254, but upon parsing the config file and finding reservations it'd take those out of the pool automatically, thus avoiding the double ip address problem. –  datadevil Jan 11, 2012 at 12:49
• I suppose you could script something like that with lots of greps and seds but it'd be fragile as hell. –  Chopper3 Jan 11, 2012 at 13:32
• yeah, not going in that direction –  datadevil Jan 17, 2012 at 15:46

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static ip address assignment #164

seamusdemora commented Jan 12, 2023 • edited

• 👍 1 reaction

maciej-napora commented Jan 12, 2023 • edited

Sorry, something went wrong.

rsmarples commented Jan 12, 2023

• 👍 2 reactions

Which means the addr (dst) is in class C and netmask will be 255.255.255.0 (CIDR = 24)

As for the man page document provides an example for IP address with CIDR specified after /. So one might wonder what happens if no CIDR is provided. But I think this thread here would clarify that.

Many thanks!

• 😄 1 reaction

seamusdemora commented Jan 13, 2023 • edited

Rsmarples commented jan 13, 2023, seamusdemora commented jan 14, 2023, rsmarples commented feb 9, 2023, seamusdemora commented feb 9, 2023, rsmarples commented feb 10, 2023, seamusdemora commented feb 10, 2023.

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