Udhcpc: Difference between revisions

Latest revision as of 14:56, 17 October 2024

This page documents the working of default DHCP client udhcpc from Busybox.

Configuration

The default DHCP client udhcpc from busybox gets invoked by the networking scripts /sbin/ifup and /sbin/ifdown when "dhcp" is added to the interface name in the file /etc/network/interfaces.

The default behavior of udhcpc is driven by the script /usr/share/udhcpc/default.script

Entries in /etc/network/interfaces for DHCP interfaces will drive the udhcpc command line. For example:

Contents of /etc/network/interfaces

auto eth0
iface eth0 inet dhcp
        hostname myhostname

will set these parameters on the command line:

udhcpc -i eth0 -x hostname:myhostname

The hostname will send the DHCP option to the server to tell the server the name of this client.

You can add arbitrary command line parameters to the udhcpc_opts setting in /etc/network/interfaces. See the example of udhcpc_opts in the main networking article.

The default configuration of udhcpc may be overridden by /etc/udhcpc/udhcpc.conf. Please see /usr/share/udhcpc/default.script for how the values are used.

udhcpc.conf accepts the following key:value pairs:

key	default value	possible values	meaning
RESOLV_CONF	/etc/resolv.conf	no ; NO ; -	do not overwrite or alternative path for resolv.conf
NO_DNS	-	<list of iface names>	Prevent overwriting of resolv.conf on a per-interface basis
NO_GATEWAY	-	<list of iface names>	List of interfaces where DHCP routes are ignored
IF_METRIC	-	<metric value>	offset value for routing metric
IF_PEER_DNS	yes	<anything but yes>	please use RESOLV_CONF or NO_DNS instead

Example /etc/udhcpc/udhcpc.conf:

Contents of /etc/udhcpc/udhcpc.conf

RESOLV_CONF="no" # Prevents overwriting of /etc/resolv.conf

Custom scripts can be added as /etc/udhcpc/pre-* and /etc/udhcpc/post-* to be run before/after deconfig/renew/bound DHCP events. The custom scripts must be set as executable by root, e.g. chmod 744. Refer an example custom script

Troubleshooting

Todo: The below section need testing and confirmation. Please help contributing.

Change MTU for ADSL

As an example, /etc/udhcpc/post-bound/mtu could contain, to change the interface MTU from the default (1500) to 1492, which is useful if on ADSL that uses PPPoE (which uses 8 bytes for its own header):

r=$(/sbin/ip route | grep ^default | head -n 1) # Needs {{pkg|iproute2}} package, rather than busybox's "ip", to change mtu /sbin/ip route replace $r mtu 1492

I needed to restart my firewall (which replaces the iptables script from Alpine) when the client binds to a new IP, so I added the following line in the function bound(), right after 'resolvconf':

# rc-service iptables reload

The reload drops all firewall rules, re-acquires the Internal and external IPs, and re-writes the rules. I'm sure there is a better way.

@@ Line 1: / Line 1: @@
-{{Expand|Please help us expand this article by contributing.}}
+{{DISPLAYTITLE:udhcpc}}
+{{Main|Configure Networking}}
+This page documents the working of default DHCP client <code>udhcpc</code> from Busybox.
-You may want to customize the behavior of the default DHCP client (udhcpc from busybox), which is called by /sbin/ifup by having "dhcp" in /etc/network/interfaces.
+== Configuration ==
-The default behavior is driven by the script /usr/share/udhcpc/default.script
+The default DHCP client <code>udhcpc</code> from busybox gets invoked by the networking scripts {{path|/sbin/ifup}} and {{path|/sbin/ifdown}} when "<code>dhcp</code>" is added to the interface name in the file {{path|/etc/network/interfaces}}.
-Its default configuration may be overwritten by /etc/udhcpc/udhcpc.conf
+The default behavior of udhcpc is driven by the script {{path|/usr/share/udhcpc/default.script}}
-Authorized key:value pairs are:
+Entries in {{path|/etc/network/interfaces}} for DHCP interfaces will drive the <code>udhcpc</code> command line. For example:<br>
+{{cat|/etc/network/interfaces|auto eth0
+iface eth0 inet dhcp
+        hostname myhostname
+}}
+will set these parameters on the command line:<br>
+{{cmd|udhcpc -i eth0 -x hostname:myhostname}}
+The hostname will send the DHCP option to the server to tell the server the name of this client.
+You can add arbitrary command line parameters to the <code>udhcpc_opts</code> setting in {{path|/etc/network/interfaces}}. See the example of <code>udhcpc_opts</code> in [[Configure Networking#IPv4 DHCP Configuration|the main networking article]].
+The default configuration of <code>udhcpc</code> may be overridden by {{path|/etc/udhcpc/udhcpc.conf}}. Please see {{path|/usr/share/udhcpc/default.script}} for how the values are used.
+{{path|udhcpc.conf}} accepts the following key:value pairs:
 {| cellpadding="15" border="1" class="wikitable"
@@ Line 14: / Line 33: @@
 | default value
 | possible values
+| meaning
+|-
+| RESOLV_CONF
+| {{path|/etc/resolv.conf}}
+| no ; NO ; -
+| do not overwrite or alternative path for {{path|resolv.conf}}
+|-
+| NO_DNS
+| -
+| <list of iface names>
+| Prevent overwriting of {{path|resolv.conf}} on a per-interface basis
 |-
 | NO_GATEWAY
 | -
 | <list of iface names>
+| List of interfaces where DHCP routes are ignored
 |-
 | IF_METRIC
 | -
 | <metric value>
+| offset value for routing metric
 |-
 | IF_PEER_DNS
 | yes
 | <anything but yes>
-|-
+| please use RESOLV_CONF or NO_DNS instead
-| RESOLV_CONF
-| /etc/resolv.conf
-| no ; NO ; -
-|-
-| NO_DNS
-| -
-| <list of iface names>
 |}
-Example /etc/udhcpc/udhcpc.conf:
+Example {{path|/etc/udhcpc/udhcpc.conf}}:
+{{cat|/etc/udhcpc/udhcpc.conf|RESOLV_CONF{{=}}"no" # Prevents overwriting of /etc/resolv.conf}}
+Custom scripts can be added as {{path|/etc/udhcpc/pre-*}} and {{path|/etc/udhcpc/post-*}} to be run before/after deconfig/renew/bound DHCP events. The custom scripts must be set as '''executable''' by root, e.g. <code>chmod 744</code>. Refer an example [[#Change MTU for ADSL|custom script]]
+== Troubleshooting ==
+{{Todo|The below section need testing and confirmation. Please help contributing.}}
+=== Change MTU for ADSL ===
+As an example, {{path|/etc/udhcpc/post-bound/mtu}} could contain, to change the interface MTU from the default (1500) to 1492, which is useful if [https://en.wikipedia.org/wiki/Maximum_transmission_unit on ADSL that uses PPPoE] (which uses 8 bytes for its own header):
+{{cmd|<nowiki>r=$(/sbin/ip route | grep ^default | head -n 1)
+# Needs {{pkg|iproute2}} package, rather than busybox's "ip", to change mtu
+/sbin/ip route replace $r mtu 1492
+</nowiki>}}
- RESOLV_CONF="no" # Prevents overwriting of /etc/resolv.conf
+I needed to restart my firewall (which replaces the <code>iptables</code> script from Alpine) when the client binds to a new IP, so I added the following line in the function <code>bound()</code>, right after '<code>resolvconf</code>':
-Custom scripts can be added as /etc/udhcpc/pre-* and /etc/udhcpc/post-* to be run before/after deconfig/renew/bound DHCP events - they must be marked as '''executable''' by root, e.g. chmod 744
+{{cmd|# rc-service iptables reload}}
-As an example, /etc/udhcpc/post-bound/mtu could contain, to change the interface MTU from the default (1500) to 1492, which is useful if [https://en.wikipedia.org/wiki/Maximum_transmission_unit on ADSL] (which uses 8 bytes):
+The reload drops all firewall rules, re-acquires the Internal and external IPs, and re-writes the rules. I'm sure there is a better way.
- r=$(/sbin/ip route | grep ^default | head -n 1)
+== See Also ==
- # Needs iproute2 package, rather than busybox's "ip", to change mtu
+[https://udhcp.busybox.net/README.udhcpc Busybox udhcpc Readme]
- /sbin/ip route replace $r mtu 1492
 [[Category:Networking]]
 [[Category:Embedded Systems]]