Interfaces cannot be enumerated on pure IPv6 Linux #2416
Description
Interfaces cannot be enumerated on a Linux system (Ubuntu 18.04 tested) if it completely lacks IPv4 addresses (including the loopback):
$ ./nmap --iflist
Starting Nmap 7.92SVN ( https://nmap.org ) at 2021-12-18 19:06 MST
INTERFACES: NONE FOUND(!)
ROUTES: NONE FOUND(!)
$
As expected, a privileged scan will fail:
$ ./nmap -6 ::1
Starting Nmap 7.92SVN ( https://nmap.org ) at 2021-12-18 19:08 MST
route_dst_netlink: can't find interface "lo"
$
Adding a single IPv4 address to any one of the interfaces is an effective workaround:
$ ip a add 169.254.11.22/16 dev ens38
$ nmap --iflist
Starting Nmap 7.92SVN ( https://nmap.org ) at 2021-12-18 19:11 MST
************************INTERFACES************************
DEV (SHORT) IP/MASK TYPE UP MTU MAC
ens38 (ens38) 169.254.11.22/16 ethernet up 1500 00:0C:29:D5:7E:57
ens38 (ens38) fe80::4eec:c4e9:392b:bb7d/64 ethernet up 1500 00:0C:29:D5:7E:57
ens33 (ens33) (none)/0 ethernet up 1500 00:0C:29:D5:7E:4D
lo (lo) (none)/0 loopback up 65536
lo (lo) ::1/128 loopback up 65536
**************************ROUTES**************************
DST/MASK DEV METRIC GATEWAY
169.254.0.0/16 ens38 0
::1/128 lo 0
fe80::4eec:c4e9:392b:bb7d/128 ens38 0
::1/128 lo 256
fe80::/64 ens38 101
fe80::/64 ens38 256
ff00::/8 ens38 256
$ nmap -6 ::1
Starting Nmap 7.92SVN ( https://nmap.org ) at 2021-12-18 19:11 MST
Nmap scan report for ip6-localhost (::1)
Host is up (0.0000040s latency).
Not shown: 999 closed tcp ports (reset)
PORT STATE SERVICE
631/tcp open ipp
Nmap done: 1 IP address (1 host up) scanned in 0.17 seconds
$
The issue can be traced to a SIOCGIFCONF ioctl call in libdnet:
nmap/libdnet-stripped/src/intf.c
Line 917 in 88c7e9d
Specifically, the call succeeds but returns a zero-length array of ifreq structures, which is not altogether surprising. Quoting from the manpage for netdevice:
Return a list of interface (transport layer) addresses. This currently means only addresses of the AF_INET (IPv4) family for compatibility.
The zero-length array is rejected later in the code:
nmap/libdnet-stripped/src/intf.c
Lines 696 to 699 in 88c7e9d
I have not investigated what the best course of remediation could be.