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Re: nmap: [REGRESSION 5.00-3 -> 6.00-0.3] -sP fails with "nexthost: failed to determine route to X.X.X.X"
From: Timo Juhani Lindfors <timo.lindfors () iki fi>
Date: Tue, 06 Aug 2013 10:30:26 +0300

David Fifield <david () bamsoftware com> writes:
I haven't been able to reproduce it yet, but I haven't been on a
(simulated) network with more than 1024 live hosts.

I did not have 1024 hosts in my network either. Even an empty network
was enough to reproduce the issue here.

the proximate cause of the the problem you encountered. A good quick
test is to edit libnetutil/netutil.cc to disable route_dst_netlink and
enable route_dst_generic, and see if the problem persists.

That doesn't seem to help.

Another thing to try: The --route-dst option makes Nmap make a routing
decision, without sending pings or any other traffic. Try something like
      sudo nmap --route-dst 10.7.24.1
and see if it increases the number of ARP entries in the cache. If so,
we will have the problem localized closely.

It does:

$ for i in $(seq 20 30); do sudo nmap --route-dst 10.7.24.$i > /dev/null 2>&1; ip ntable show dev eth0 name arp_cache | 
grep refcnt; done
    refcnt 17 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 18 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 19 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 20 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 21 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 22 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 23 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 24 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 25 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 26 reachable 26920 base_reachable 30000 retrans 1000 
    refcnt 27 reachable 26920 base_reachable 30000 retrans 1000 
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