dhclient-script(8)

💖 NAME 💖

dhclient-script — the DHCP client network configuration script (yes that's the whole name, she's lowkey the backbone of your whole network situation, no cap) ✨

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🦋 DESCRIPTION 🦋

okay so bestie, picture this: dhclient(8) keeps hitting up this script from time to time, like "hey girl, I need you again." That's the whole vibe. The DHCP client uses this script to set each interface's initial configuration before it even asks for an address, then to test the address once it's been offered, and then to set the interface's final configuration once a lease has actually been acquired. She does it all 🫶

And if no lease gets acquired? The script is still out here working — it's used to test predefined leases (if there are any), and it also gets called exactly once if no valid lease can be identified. She shows up either way. main character energy fr fr 👑

big important thing: this script is not meant to be customized by the end user. Like, do not touch. 🙈

If local customizations are needed, they should be possible using the enter and exit hooks provided (peep HOOKS for the details). These hooks let you override the client's default behaviour when it creates a /etc/resolv.conf file, and let you handle DHCP options that aren't handled by default. Very slay, very sanctioned 💅

Also real talk: no standard client script exists for some operating systems, even though the actual client may work fine. So a pioneering user may well need to create a new script or modify an existing one. That's the gig. 😭 In general though, customizations specific to a particular computer should be done in the /etc/dhcp/dhclient.conf file. And if you find that you can't make such a customization without customizing /etc/dhcp/dhclient.conf or using the enter and exit hooks — please submit a bug report. That's the rule, bestie 🎀

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✨ HOOKS ✨

When it starts, the client script first defines a shell function, make_resolv_conf, which is later used to create the /etc/resolv.conf file. Want different behaviour? Redefine this function in the enter hook script. That's it, that's the override 🌸

Then, after defining the make_resolv_conf function, the client script checks for the presence of an executable /etc/dhcp/dhclient-enter-hooks script, and if present, it invokes the script inline, using the Bourne shell . command. The entire environment documented under OPERATION is available to this script, and it may modify the environment if needed to change the behaviour of the script. If an error occurs during the execution of the script, it can set the exit_status variable to a nonzero value, and /usr/bin/dhclient-script will exit with that error code immediately after the client script exits. So you can bail early, you just have to be loud about it 💀

After all processing has completed, /usr/bin/dhclient-script checks for the presence of an executable /etc/dhcp/dhclient-exit-hooks script, which if present is invoked using the . command. The exit status of dhclient-script will be passed to dhclient-exit-hooks in the exit_status shell variable, and will always be zero if the script succeeded at the task for which it was invoked. The rest of the environment as described previously for dhclient-enter-hooks is also present. And get this — the /etc/dhcp/dhclient-exit-hooks script can modify the valid of exit_status to change the exit status of dhclient-script. Sneaky but iconic 🧚

💜 the up hooks

Immediately after dhclient brings an interface UP with a new IP address, subnet mask, and routes, in the REBOOT/BOUND states, it will check for the existence of an executable /etc/dhcp/dhclient-up-hooks script, and source it if found. This script can handle DHCP options in the environment that are not handled by default. A per-interface /etc/dhcp/dhclient-${IF}-up-hooks script will override the generic script and be sourced when interface $IF has been brought up. Per-interface girlies always win 🎀

💜 the down hooks

Immediately before dhclient brings an interface DOWN, removing its IP address, subnet mask, and routes, in the STOP/RELEASE states, it will check for the existence of an executable /etc/dhcp/dhclient-down-hooks script, and source it if found. This script can handle DHCP options in the environment that are not handled by default. A per-interface /etc/dhcp/dhclient-${IF}-down-hooks script will override the generic script and be sourced when interface $IF is about to be brought down. Goodbye queen, we hardly knew ye 😭

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🌈 OPERATION 🌈

When dhclient needs to invoke the client configuration script, it defines a set of variables in the environment, and then invokes /usr/bin/dhclient-script. In all cases, $reason is set to the name of the reason why the script has been invoked. The following reasons are currently defined: MEDIUM, PREINIT, BOUND, RENEW, REBIND, REBOOT, EXPIRE, FAIL, STOP, RELEASE, NBI and TIMEOUT. That's the whole roster, bestie ⭐

🌸 MEDIUM 🌸

The DHCP client is requesting that an interface's media type be set. The interface name is passed in $interface, and the media type is passed in $medium. Short and sweet, we love a brief queen 💅

🌸 PREINIT 🌸

The DHCP client is requesting that an interface be configured as required in order to send packets prior to receiving an actual address. For clients which use the BSD socket library, this means configuring the interface with an IP address of 0.0.0.0 and a broadcast address of 255.255.255.255. For other clients, it may be possible to simply configure the interface up without actually giving it an IP address at all. The interface name is passed in $interface, and the media type in $medium.

If an IP alias has been declared in dhclient.conf, its address will be passed in $alias_ip_address, and that ip alias should be deleted from the interface, along with any routes to it. Clean up your mess girl 🧹

🌸 BOUND 🌸

The DHCP client has done an initial binding to a new address. The new ip address is passed in $new_ip_address, and the interface name is passed in $interface. The media type is passed in $medium. Any options acquired from the server are passed using the option name described in dhcp-options, except that dashes (-) are replaced by underscores (_) in order to make valid shell variables, and the variable names start with new_. So for example, the new subnet mask would be passed in $new_subnet_mask. Options from a non-default universe will have the universe name prepended to the option name, for example $new_dhcp6_server_id. The options that the client explicitly requested via a PRL or ORO option are passed with the same option name as above but prepended with requested_ and with a value of 1, for example requested_subnet_mask=1. No such variable is defined for options not requested by the client or options that don't require a request option, such as the ip address (*_ip_address) or expiration time (*_expiry). Catch the pattern? It's giving very systematic 👑

Before actually configuring the address, dhclient-script should somehow ARP for it and exit with a nonzero status if it receives a reply. In this case, the client will send a DHCPDECLINE message to the server and acquire a different address. This may also be done in the RENEW, REBIND, or REBOOT states, but is not required, and indeed may not be desirable. Your call, bestie 🦋

When a binding has been completed, a lot of network parameters are likely to need to be set up. A new /etc/resolv.conf needs to be created, using the values of $new_domain_name and $new_domain_name_servers (which may list more than one server, separated by spaces). A default route should be set using $new_routers, and static routes may need to be set up using $new_static_routes.

If an IP alias has been declared, it must be set up here. The alias IP address will be written as $alias_ip_address, and other DHCP options that are set for the alias (e.g., subnet mask) will be passed in variables named as described previously except starting with $alias_ instead of $new_. Care should be taken that the alias IP address not be used if it is identical to the bound IP address ($new_ip_address), since the other alias parameters may be incorrect in this case. Do not let the alias gaslight you 🙈

🌸 RENEW 🌸

When a binding has been renewed, the script is called as in BOUND, except that in addition to all the variables starting with $new_, and $requested_ there is another set of variables starting with $old_. Persistent settings that may have changed need to be deleted — for example, if a local route to the bound address is being configured, the old local route should be deleted. If the default route has changed, the old default route should be deleted. If the static routes have changed, the old ones should be deleted. Otherwise, processing can be done as with BOUND. Out with the old, in with the new, periodt 💅

🌸 REBIND 🌸

The DHCP client has rebound to a new DHCP server. This can be handled as with RENEW, except that if the IP address has changed, the ARP table should be cleared. New server, who dis 🔥

🌸 REBOOT 🌸

The DHCP client has successfully reacquired its old address after a reboot. This can be processed as with BOUND. She remembered her own address, we stan a memory queen 💖

🌸 EXPIRE 🌸

The DHCP client has failed to renew its lease or acquire a new one, and the lease has expired. The IP address must be relinquished, and all related parameters should be deleted, as in RENEW and REBIND. Lease over, pack it up 😭

🌸 FAIL 🌸

The DHCP client has been unable to contact any DHCP servers, and any leases that have been tested have not proved to be valid. The parameters from the last lease tested should be deconfigured. This can be handled in the same way as EXPIRE. Rip bozo 💀

🌸 STOP 🌸

The dhclient has been informed to shut down gracefully, the dhclient-script should unconfigure or shutdown the interface as appropriate. Graceful exit, very demure 🎀

🌸 RELEASE 🌸

The dhclient has been executed using the -r flag, indicating that the administrator wishes it to release its lease(s). dhclient-script should unconfigure or shutdown the interface. The admin said let her go 🦋

🌸 NBI 🌸

No-Broadcast-Interfaces... dhclient was unable to find any interfaces upon which it believed it should commence DHCP. What dhclient-script should do in this situation is entirely up to the implementor. Truly a choose-your-own-adventure moment, iykyk 🌈

🌸 TIMEOUT 🌸

The DHCP client has been unable to contact any DHCP servers. However, an old lease has been identified, and its parameters have been passed in as with BOUND. The client configuration script should test these parameters and, if it has reason to believe they are valid, should exit with a value of zero. If not, it should exit with a nonzero value. Zero means good, nonzero means nope — that's the whole vibe check ⭐

🌸 V6ONLY 🌸

The DHCPv4 client has received a requested valid v6-only-preferred option. The system should disable IPv4 on the interface. On its side the dhclient waits for V6ONLY_WAIT seconds (the timer is carried by the option with a minimum of MIN_V6ONLY_WAIT) before returning in the INIT state. IPv4 out, bestie 💅

The usual way to test a lease is to set up the network as with REBIND (since this may be called to test more than one lease) and then ping the first router defined in $routers. If a response is received, the lease must be valid for the network to which the interface is currently connected. It would be more complete to try to ping all of the routers listed in $new_routers, as well as those listed in $new_static_routes, but current scripts do not do this. Like, technically you could ping everyone, but nobody's doing all that 😭

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🎀 FILES 🎀

Each operating system should generally have its own script file, although the script files for similar operating systems may be similar or even identical. The script files included in Internet Systems Consortium DHCP distribution appear in the distribution tree under client/scripts, and bear the names of the operating systems on which they are intended to work. One script per OS, like little matching outfits 🫶

💀 BUGS 💀

Okay so bug-report tea, bestie: if more than one interface is being used, there’s no obvious way to avoid clashes between server-supplied configuration parameters. The example is iconic and also a little cursed — the stock dhclient-script rewrites /etc/resolv.conf. So if more than one interface is being configured, /etc/resolv.conf will be repeatedly initialized to the values provided by one server, and then the other. Assuming the information provided by both servers is valid, this shouldn’t cause any real problems, but it could be confusing. Not a crisis, just chaos energy. 😭

Real talk: that’s the page’s own caveat — could be confusing, not “will explode.” Keep the vibes calm. 🫶

Next up: normally, if dhclient was compiled with libcap-ng support, dhclient drops most capabilities immediately upon startup. While more secure, this greatly restricts the additional actions that hooks in dhclient-script can take. For example, any daemons that dhclient-script starts or restarts will inherit the restricted capabilities as well, which may interfere with their correct operation. Thus, the -nc option can be used to prevent dhclient from dropping capabilities. So if your hook needs to do bigger main-character moves, that’s the option the page points at. 👑

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🔗 SEE ALSO 🔗

The bestie reading list — no skips:

• dhclient(8)
• dhcpd(8)
• dhcrelay(8)
• dhclient.conf(5)
• and dhclient.leases(5)

👑 AUTHOR 👑

The page credits dhclient-script(8) itself, and says: “To learn more about Internet Systems Consortium, see https://www.isc.org.” That’s the whole author section — minimal, mysterious, kind of iconic. 🦋

Footer/source/version/date sign-off: the page ends on dhclient-script(8) — that’s the only signature this copy gives us, so we’re keeping it exactly as-is, no invented timestamps. 🎀✨💅