The localhostAuthBypass test uses the get_ipaddr method from jstests/libs/host_ipaddr.js to obtain the host's ipv4 address. get_ipaddr works on Unix systems by calling /sbin/ifconfig and performing some text processing on the output. An example of this output is as follows:

inet addr:10.4.102.211 Bcast:10.4.255.255 Mask:255.255.0.0

get_ipaddr greps for all ifconfig lines containing the string "inet", filters out all lines containing "127.0.0.1" and then removes the first occurrence of the string "addr:". The address is found using a regular expression which returns all characters after the string "inet " until whitespace is encountered.

This process can fail if ifconfig returns a result which, when grepped, looks like:

inet6 addr: ::1/128 Scope:Host
inet addr:10.4.102.211 Bcast:10.4.255.255 Mask:255.255.0.0
inet6 addr: fe80::3e15:c2ff:fedc:4994/64 Scope:Link

When ifconfig returns an IPv6 address before an IPv4 address the "addr:" string will be removed from the leading IPv6 entry, which is subsequently ignored by the regular expression. This results in get_ipaddr returning an IPv4 address prefixed by "addr:", which is invalid.