Udhcpc: Difference between revisions
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{{DISPLAYTITLE:udhcpc}} | |||
{{Main|Configure Networking}} | |||
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:<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" | {| cellpadding="15" border="1" class="wikitable" | ||
| Line 12: | 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 | ||
|} | |} | ||
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>}} | |||
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 == | |||
[https://udhcp.busybox.net/README.udhcpc Busybox udhcpc Readme] | |||
[[Category:Networking]] | |||
[[Category:Embedded Systems]] | |||
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
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
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.