Udhcpc: Difference between revisions
Prabuanand (talk | contribs) (Added reference to Main Networking article, headings and moved the content related to ADSL MTU to troubleshooting section.) |
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{{DISPLAYTITLE:udhcpc}} | {{DISPLAYTITLE:udhcpc}} | ||
{{Main|Configure Networking}} | {{Main|Configure Networking}} | ||
This page documents the working of default DHCP client <code>udhcpc</code> from Busybox. | This page documents the working of default DHCP client <code>udhcpc</code> from Busybox. | ||
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{{cat|/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 {{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 | 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 == | == Troubleshooting == | ||
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{{Todo|The below section need testing and confirmation. Please help contributing.}} | {{Todo|The below section need testing and confirmation. Please help contributing.}} | ||
=== Change | === 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): | 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): | ||
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.