network: don't fail the whole vif setup if IPv6 is disabled

Detect if IPv6 is disabled in the kernel (like it is in Whonix Gateway)
and skip setting IPv6 in that case. Otherwise 'ip' call would fail and
since the script is with 'set -e', it would interrupt setting IPv4 too.
Log error message in that case anyway.

Fixes QubesOS/qubes-issues#5110

network/vif-route-qubes

@@ -100,12 +100,15 @@ case "$command" in
         ipcmd='add'
         iptables_cmd='-I PREROUTING 1'
         cmdprefix=''
+        ipv6_disabled=$(cat /proc/sys/net/ipv6/conf/"${vif}"/disable_ipv6 || echo 1)
         ;;
     offline)
         do_without_error ifdown "${vif}"
         ipcmd='del'
         iptables_cmd='-D PREROUTING'
         cmdprefix='do_without_error'
+        # cleanup IPv6 config even if _now_ it is disabled
+        ipv6_disabled=0
         ;;
 esac
 
@@ -115,11 +118,16 @@ domid=${domid/.*/}
 #  32752 is max XID aka domid
 metric=$(( 32752 - domid ))
 
+
 if [ "${ip}" ]; then
     # If we've been given a list of IP addresses, then add routes from dom0 to
     # the guest using those addresses.
     for addr in ${ip};
     do
+        if [[ "$addr" = *:* ]] && [[ "$ipv6_disabled" = 1 ]]; then
+            log error "Cannot set IPv6 route to ${addr}, IPv6 disabled in the kernel"
+            continue
+        fi
         ${cmdprefix} ip route "${ipcmd}" "${addr}" dev "${vif}" metric "$metric"
         if [[ "$addr" = *:* ]]; then
             ipt=ip6tables-restore
@@ -137,7 +145,7 @@ if [ "${ip}" ]; then
             ${cmdprefix} ip6tables-restore --noflush $ipt_arg
     fi
     ${cmdprefix} ip addr "${ipcmd}" "${back_ip}/32" dev "${vif}"
-    if [ "${back_ip6}" ] && [[ "${back_ip6}" != "fe80:"* ]]; then
+    if [ "${back_ip6}" ] && [[ "${back_ip6}" != "fe80:"* ]] && [[ "$ipv6_disabled" != 1 ]]; then
         ${cmdprefix} ip addr "${ipcmd}" "${back_ip6}/128" dev "${vif}"
     fi
 else