Inconsistent limits for address generation via `-g`, `--generator` using either range or CIDR

[Erik Auerswald]

When using fping -g with a range, the number of addresses that may be generated is limited (via the MAX_GENERATE constant in src/fping.c):

$ fping -q --stats -g 127.0.0.1 127.1.255.254
./src/fping: -g parameter generates too many addresses

But this limit is not applied when using CIDR notation:

$ ipcalc 127.0.0.0/15 | grep 'HostM[ai][nx]'
HostMin:      127.0.0.1
HostMax:      127.1.255.254
$ fping -q --stats -g 127.0.0.0/15

  131070 targets
  131070 alive
       0 unreachable
       0 unknown addresses

       0 timeouts (waiting for response)
  131070 ICMP Echos sent
  131070 ICMP Echo Replies received
       0 other ICMP received

 0.005 ms (min round trip time)
 0.033 ms (avg round trip time)
 0.166 ms (max round trip time)
     1328.103 sec (elapsed real time)

I have mentioned this in a comment on issue #59, because I think that applying a consistent limit to address generation is a prerequisite for expanding -g functionality to include IPv6, but I'd prefer to track this issue independently.

I can refresh my respective patches that have been part of the IPv6 generator pull request #254 and submit them as a new pull request. Just let me know if this desired.

[Erik Auerswald]

To collect relevant information in one place:

• The restrictions for address generation are not documented (neither man page nor --help output mention them).

• There is also an inconsistency (off-by-one) in the comment describing the constant used to limit address generation when using start and end addresses:

  $ git grep -n MAX_GENERATE
  src/fping.c:142:#define MAX_GENERATE 100000 /* maximum number of hosts that -g can generate */
  src/fping.c:1342:    if (end_long > start_long + MAX_GENERATE) {
  $ ./src/fping -q -s -g 127.0.0.1 127.1.134.162
  ./src/fping: -g parameter generates too many addresses
  $ ./src/fping -q -s -g 127.0.0.1 127.1.134.161
  
    100001 targets
    100001 alive
         0 unreachable
         0 unknown addresses
  
         0 timeouts (waiting for response)
    100001 ICMP Echos sent
    100001 ICMP Echo Replies received
         0 other ICMP received
  
   0.006 ms (min round trip time)
   0.032 ms (avg round trip time)
   0.321 ms (max round trip time)
       1012.501 sec (elapsed real time)
  
  

[Erik Auerswald]

Another point to consider: Using --generate with a large CIDR subnet can result in problems, e.g., using too much memory:

$ ./src/fping -q -s -g 127.0.0.0/8
Killed
$ dmesg -t | grep -E '^(oom-kill|Out of memory)'
oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/user.slice/user-1000.slice/[e-mail],task=fping,pid=10558,uid=1000
Out of memory: Killed process 10558 (fping) total-vm:5346240kB, anon-rss:5343568kB, file-rss:172kB, shmem-rss:0kB, UID:1000 pgtables:10504kB oom_score_adj:0

Therefore I'd say that having a limit is useful.