BusyBox

BusyBox is used for several system utilities in OpenWrt like ash shell, cp, ls, echo, ping and many others. It provides tiny replacements with fewer options for most of the utilities from GNU Core Utilities, GNU Inetutils and other essential tools like gzip.

All this utilities are compiled as “applets” into a single binary file /bin/busybox.

When an OpenWrt system tries to obtain an IP using DHCP on its WAN interface, it adds a Vendor-Class option that contains the Busybox version:

Vendor-Class Option 60, length 12: “udhcp 1.28.3”

This can be used to estimate which OpenWrt version is running on a router if you don't have access to the router itself.

OpenWrt version	Busybox version
7.06	1.4.2
7.07	1.4.2
7.09	1.4.2
8.09	1.11.2
8.09.1	1.11.2
8.09.2	1.11.2
10.03	1.15.3
10.03.1	1.15.3
12.09	1.19.4
17.01.0	1.25.1
17.01.1	1.25.1
17.01.2	1.25.1
17.01.3	1.25.1
17.01.4	1.25.1
17.01.5	1.25.1
17.01.6	1.25.1
17.01.7	1.25.1
18.06.0	1.28.3
18.06.1	1.28.3
18.06.2	1.28.4
18.06.3	1.28.4
18.06.4	1.28.4
18.06.5	1.28.4
19.07.0	1.30.1

BusyBox

Identify OpenWrt version through udhcp