leasedump

CLI tool for dumping dhcpcd .lease files.

Background

Life is not easy for those of us who set nohook resolv.conf in dhcpcd.conf, especially when accessing a gated wifi (i.e. hotels and airlines).

On my system, dhcpcd places the raw DHCP lease replies in /var/lib/dhcpcd/[if]-[ssid].lease{,6} (emphasis on raw).

So far, I've only found a few not-so-great methods of determining the router's DNS servers and subsequently determined that it would be easier to simply parse the .lease files directly. Is there a CLI tool for this? Apparently not.

dhcpdump and dhcprobe come close, but don't allow one to parse a DHCP message file directly.

leasedump repurposes some code from dhcpdump to parse DHCP messages. It does no network I/O and links to nothing other than libc, so you can mostly rest easy about running it as root (/var/lib/dhcpcd requires root access on my system). If you're extra paranoid, you can copy the .lease file and give it regular user permissions first.

Furthermore, unlike dhcpdump/dhcprobe, leasedump also supports DHCPv6 messages.

Usage

$ leasedump /var/lib/dhcpcd/wlan0-MySSID.lease

Output

    OP: 2 (BOOTPREPLY)
 HTYPE: 1 (Ethernet)
  HLEN: 6
  HOPS: 0
   XID: fe3d0f32
  SECS: 0
 FLAGS: 0
CIADDR: 0.0.0.0
YIADDR: 192.168.0.144
SIADDR: 0.0.0.0
GIADDR: 0.0.0.0
CHADDR: 12:14:25:11:78:e1:00:00:00:00:00:00:00:00:00:00
 SNAME: .
 FNAME: .
OPTION:  53 (  1) DHCP message type         5 (DHCPACK)
OPTION:  54 (  4) DHCP Server identifier    192.168.0.1
OPTION:  51 (  4) IP address leasetime      172800 (2d)
OPTION:   1 (  4) Subnet mask               255.255.255.0
OPTION:   3 (  4) Router                    192.168.0.1
OPTION:   6 (  4) DNS server                192.168.0.2
OPTION:  58 (  4) Renewal Time T1           86400 (24h)
OPTION:  59 (  4) Rebinding Time T2         151200 (1d18h)

Alternatives

These are the (messy) alternatives I've found so far:

dhcpcd:

# Requests a new lease, may try to run the daemon again?
$ dhcpcd -o domain_name_servers -T | grep name_servers

dhclient (no longer maintained):

# Requests a new lease and starts a daemon, probably not what you want.
$ dhclient wlan0
$ cat /var/lib/dhclient/dhclient.leases
$ killall dhclient

nmap:

# Scans for dhcp servers. Requires root.
$ nmap --script broadcast-dhcp-discover

dhcpdump:

# Uses pcap to watch for and dump dhcp messages. Requires root.
$ dhcpdump wlan0

dhcprobe:

# Requests a lease from specified server directly and dumps configuration.
$ dhcprobe -v -s 172.20.205.1

Contribution and License Agreement

If you contribute code to this project, you are implicitly allowing your code to be distributed under the MIT license. You are also implicitly verifying that all code is your original work. </legalese>

License

Parts of this software are based on dhcpdump:

See LICENSE for more info.

chjj / leasedump