Udhcpc: Difference between revisions

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{{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 (<code>udhcpc</code> from busybox), which is called by <code>/sbin/ifup</code> by having "<code>dhcp</code>" in <code>/etc/network/interfaces</code>.
== Configuration ==


The default behavior is driven by the script <code>/usr/share/udhcpc/default.script</code>
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}}.


Entries in <code>/etc/network/interfaces</code> for DHCP interfaces will drive the <code>udhcpc</code> command line. For example:<br>
The default behavior of udhcpc is driven by the script {{path|/usr/share/udhcpc/default.script}}
 
Entries in {{path|/etc/network/interfaces}} for DHCP interfaces will drive the <code>udhcpc</code> command line. For example:<br>
   
   
<pre>auto eth0
{{cat|/etc/network/interfaces|auto eth0
iface eth0 inet dhcp
iface eth0 inet dhcp
         hostname myhostname</pre>
         hostname myhostname
}}


will set these parameters on the command line:<br>
will set these parameters on the command line:<br>


<pre>-i eth0 -x hostname:myhostname</pre>
{{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.
The hostname will send the DHCP option to the server to tell the server the name of this client.


The documentation for <code>udhcpc</code> can be found in [https://udhcp.busybox.net/README.udhcpc the busybox udhcpc readme file]<br>
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]].


Its default configuration may be overwritten by <code>/etc/udhcpc/udhcpc.conf</code>
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.


Authorized key:value pairs are:
{{path|udhcpc.conf}} accepts the following key:value pairs:


{| cellpadding="15" border="1" class="wikitable"
{| cellpadding="15" border="1" class="wikitable"
Line 28: Line 33:
| default value
| default value
| possible values
| 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
| NO_GATEWAY
| -
| -
| <list of iface names>
| <list of iface names>
| List of interfaces where DHCP routes are ignored
|-
|-
| IF_METRIC
| IF_METRIC
| -
| -
| <metric value>
| <metric value>
| offset value for routing metric
|-
|-
| IF_PEER_DNS
| IF_PEER_DNS
| yes
| yes
| <anything but 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 <code>/etc/udhcpc/udhcpc.conf</code>:
Example {{path|/etc/udhcpc/udhcpc.conf}}:


RESOLV_CONF="no" # Prevents overwriting of /etc/resolv.conf
{{cat|/etc/udhcpc/udhcpc.conf|RESOLV_CONF{{=}}"no" # Prevents overwriting of /etc/resolv.conf}}


Custom scripts can be added as <code>/etc/udhcpc/pre-*</code> and <code>/etc/udhcpc/post-*</code> to be run before/after deconfig/renew/bound DHCP events. They must be set as '''executable''' by root, e.g. <code>chmod 744</code>
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]]


As an example, <code>/etc/udhcpc/post-bound/mtu</code> 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):
== Troubleshooting ==


r=$(/sbin/ip route | grep ^default | head -n 1)
{{Todo|The below section need testing and confirmation. Please help contributing.}}
# Needs iproute2 package, rather than busybox's "ip", to change mtu
 
/sbin/ip route replace $r mtu 1492
=== 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>}}


<b>Hack alert:</b>
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>':
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>':


  /etc/init.d/iptables reload
{{cmd|# 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.
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.
== See Also ==
[https://udhcp.busybox.net/README.udhcpc Busybox udhcpc Readme]


[[Category:Networking]]
[[Category:Networking]]
[[Category:Embedded Systems]]
[[Category:Embedded Systems]]

Latest revision as of 14:56, 17 October 2024


Main article: Configure Networking

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.

See Also

Busybox udhcpc Readme

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