MacOS DHCP with bootpd, static IPs in bootptab not working

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MacOS DHCP with bootpd, static IPs in bootptab not working

I am trying to configure my MacBookPro to server as a DHCP server. I have researched and been able to get bootpd working, however when I try to configure static IP addresses in the /etc/bootptab file it doesn't work.

I have my /etc/bootpd.plist file configured to dynamically assign IPs between 10.0.0.3 and 10.0.0.99 with a IP lease term of 1 month. In my case, the network interface I want DHCP running on is called en11. The file contents are:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>bootp_enabled</key>
    <false/>
    <key>detect_other_dhcp_server</key>
    <integer>1</integer>
    <key>dhcp_enabled</key>
    <array>
        <string>en11</string>
    </array>
    <key>reply_threshold_seconds</key>
    <integer>0</integer>
    <key>Subnets</key>
    <array>
        <dict>
            <key>allocate</key>
            <true/>
            <key>lease_max</key>
            <integer>2592000</integer>
            <key>lease_min</key>
            <integer>2592000</integer>
            <key>name</key>
            <string>10.0.0</string>
            <key>net_address</key>
            <string>10.0.0.0</string>
            <key>net_mask</key>
            <string>255.255.255.0</string>
            <key>net_range</key>
            <array>
                <string>10.0.0.3</string>
                <string>10.0.0.99</string>
            </array>
        </dict>
    </array>
</dict>
</plist>

I am able to start and stop the DHCP server with the following commands:

sudo /bin/launchctl load -w /System/Library/LaunchDaemons/bootps.plist

sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/bootps.plist

All of the above works perfectly. The last piece of the puzzle is having a device with a static IP address. To accomplish that, I created the /etc/bootptab file to map a static IP to a MAC address. My /etc/bootptab file are as follows:

%%
# hostname hwtype hwaddr ipaddr bootfile
kvm-ubuntu 1 00:0c:29:1f:ed:54 10.0.0.128

But whenever I test it, the kvm-ubuntu PC is assigned an IP address of 10.0.0.3 instead of 10.0.0.128.

I have tried looking at (and clearing) /var/db/dhcpd_leases file which hasn't helped. After clearing the file and re-testing the DHCP functionality, the file is re-created by bootpd with the following entry:

{
	name=kvm-ubuntu
	ip_address=10.0.0.3
	hw_address=1,0:c:29:1f:ed:54
	identifier=1,0:c:29:1f:ed:54
	lease=0x61809a32
}

Any ideas for troubleshooting or help configuring a static IP address with bootp DHCP server would be immensely appreciated!

Posted on Oct 2, 2021 6:50 PM

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Answer 1

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Junto26 Author

Level 1

10 points

Oct 4, 2021 10:29 PM in response to Junto26

I was able to solve this after a lot of trial and error. Bootpd does not give a lot of log information.

I had two problems.

My first was that the /etc/bootptab file was the wrong file format. I had copied a .plist file and edited the contents, however .plist files are actually a different document type called "property list." I created a new file with TextEdit (making sure not to save as a .txt file) and replaced the /etc/bootptab file with that.

My second issue was that in the /etc/bootptab file, there needs to be a space after the IP address. The bootfile column can be left blank, but it needed the space at the end.

Also, when bootpd does recognize the bootptab file and finds a correct entry that matches a MAC address requesting an IP address, it assigns the static IP but does not write an entry to the /var/db/dhcp_leases file. I actually had it working for a while and was monitoring the dhcp_leases file to confirm/deny if it worked. Because it didn't write the static IP leases into the /var/db/dhcp_leases file, I thought it was not working when it actually was.

My final configurations are:

/etc/bootpd.plist (which is a property list file format):

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>bootp_enabled</key>
        <false/>
    <key>detect_other_dhcp_server</key>
        <integer>1</integer>
    <key>dhcp_enabled</key>
        <array>
            <string>en11</string>
        </array>
    <key>reply_threshold_seconds</key>
        <integer>0</integer>
    <key>Subnets</key>
        <array>
            <dict>
                <key>allocate</key>
                    <true/>
                <key>lease_max</key>
                    <integer>86400</integer>
                <key>lease_min</key>
                    <integer>86400</integer>
                <key>name</key>
                    <string>10.0.0</string>
                <key>net_address</key>
                    <string>10.0.0.0</string>
                <key>net_mask</key>
                    <string>255.255.255.0</string>
                <key>net_range</key>
                    <array>
                        <string>10.0.0.3</string>
                        <string>10.0.0.250</string>
                    </array>
            </dict>
        </array>
    <key>use_server_config_for_dhcp_options</key>
        <true/>
</dict>
</plist>

/etc/bootptab file (which is a document file format)

%%
# machine entries have the following format:
#
# hostname      hwtype  hwaddr              ipaddr          bootfile
hostname001     1       1c:69:7a:a8:94:ba   10.0.0.104 
hostname002     1       00:0c:29:15:7c:6e   10.0.0.120 
hostname003     1       00:0c:29:56:d5:ec   10.0.0.121 
hostname004     1       00:0c:29:66:c0:ec   10.0.0.122 
hostname005     1       00:0c:29:bb:2c:70   10.0.0.123 
hostname006     1       00:0c:29:c5:6c:5f   10.0.0.124 
hostname007     1       00:15:5d:00:cc:06   10.0.0.125 
hostname008     1       00:0c:29:9c:c3:fc   10.0.0.126 
hostname009     1       00:0c:29:5d:8c:b7   10.0.0.127 
hostname010     1       00:0c:29:1f:ed:54   10.0.0.128 	
hostname011     1       52:54:00:40:ef:76   10.0.0.129 
hostname012     1       00:0c:29:d0:68:ae   10.0.0.130 
hostname013     1       00:0c:29:b4:54:7f   10.0.0.131 
hostname001     1       00:0c:29:f6:ad:e9   10.0.0.132 
hostname001     1       00:0c:29:13:07:6c   10.0.0.134 
hostname001     1       00:0c:29:30:c3:4b   10.0.0.135

To enable and disable the service from running:

sudo /bin/launchctl load -w /System/Library/LaunchDaemons/bootps.plist

sudo /bin/launchctl unload -w /System/Library/LaunchDaemons/bootps.plist

Troubleshooting tips for others:

1 - Bootpd doesn't give a lot of log information, but you can get some logs by typing this into Terminal:

sudo log stream --process bootpd --info --debug

2 - Know that bootpd only writes to /var/db/dhcp_leases when it assigns from a dynamic range. It does not write anything to that file if it uses a static IP listed in the /etc/bootptab file.

3 - I ran Wireshark on the interface and filtered for dhcp traffic which was helpful. That is how I realized the DHCP offers were actually working despite /var/db/dhcp_leases being empty.

4 - Include a space at the end of each entry in the /etc/bootptab file.

5 - the reply_threshold_seconds is best left at 0. I had increased it to 1 during troubleshooting. That caused an issue for 1 client for some reason - that client would issue a dhcp discovery broadcast and my Mac would not respond to it, I'm not sure why. Changing it back to 0 solved the issue for that client.

6 - You can type the following commands into Terminal to get a lot of technical information on bootpd and bootptab, however it took a while to digest many of the settings and how to manipulate them.

man bootpd

man bootptab

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