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{{DISPLAYTITLE:udhcpc}}This page documents the working of default DHCP client <code>udhcpc</code> from [[Busybox]]. | |||
== Configuration == | |||
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}}. | |||
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 {{cat|/etc/network/interfaces|auto eth0 | |||
iface eth0 inet dhcp | |||
hostname myhostname | |||
}} | |||
will set these parameters on the command line: {{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. | |||
By default, the inbuilt Busybox DHCP client [[udhcpc]] requests a static set of options from the DHCP server. If you need to extend this set, you can do so by setting some additional command line options for the DHCP client, via the <code>udhcpc_opts</code> in your interface configuration. The following example requests <code>domain-search</code> option: | |||
{{cat|/etc/network/interfaces|... | |||
auto eth0 | |||
iface eth0 inet dhcp | |||
udhcpc_opts -O search}} | |||
For a complete list of command line options for udhcpc, see [https://busybox.net/downloads/BusyBox.html#udhcpc this document]. | |||
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" | {| cellpadding="15" border="1" class="wikitable" | ||
| Line 12: | Line 32: | ||
| 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 | ||
|} | |} | ||
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.}} | |||
=== 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>}} | |||
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>': | |||
{{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. | |||
== See also == | |||
* [[Configure Networking|Main Networking page]] | |||
* [https://udhcp.busybox.net/README.udhcpc Busybox udhcpc Readme] | |||
[[Category:Networking]] | |||
Latest revision as of 08:08, 18 July 2025
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
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.
By default, the inbuilt Busybox DHCP client udhcpc requests a static set of options from the DHCP server. If you need to extend this set, you can do so by setting some additional command line options for the DHCP client, via the udhcpc_opts in your interface configuration. The following example requests domain-search option:
Contents of /etc/network/interfaces
For a complete list of command line options for udhcpc, see this document.
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
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
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.