Increase maximum -s value to 65507

Correct definitions: maximum theoretical IPv4 packet size and minimum
IPv4 header size (previously probably IPv6 size was used).

Because fping does not know ahead whether address is IPv4 or IPv6 assume
IPv4. Previously fping allowed only 65488, but real maximum for IPv4 on
Linux is 65507 (IPv6 would allow 65527, but fping unlike ping from
iputils does not try print errno when it fails on higher value, thus
follow the current approach and allow smaller value than reachable for
IPv6).

Update test affected by this change. While at this, test 65508 (one
above the limit) instead of 65509.

Signed-off-by: Petr Vorel <pvorel@suse.cz>

ci/test-03-forbidden.pl

@@ -38,11 +38,11 @@ END
 $cmd7->stdout_is_eq("");
 $cmd7->stderr_is_eq("fping: specify only one of c, l\n");
 
-# fping -b 65509
-my $cmd8 = Test::Command->new(cmd => "fping -b 65509 127.0.0.1");
+# fping -b 65508
+my $cmd8 = Test::Command->new(cmd => "fping -b 65508 127.0.0.1");
 $cmd8->exit_is_num(1);
 $cmd8->stdout_is_eq("");
-$cmd8->stderr_is_eq("fping: data size 65509 not valid, must be lower than 65488\n");
+$cmd8->stderr_is_eq("fping: data size 65508 not valid, must not be larger than 65507\n");
 
 # fping -B 0.9
 my $cmd9 = Test::Command->new(cmd => "fping -B 0.9 127.0.0.1");

src/fping.c

@@ -134,8 +134,8 @@ extern int h_errno;
 
 /*** Ping packet defines ***/
 
-#define MAX_IP_PACKET 65536 /* (theoretical) max IP packet size */
-#define SIZE_IP_HDR 40
+#define MAX_IP_PACKET 65535 /* (theoretical) max IPv4 packet size */
+#define SIZE_IP_HDR 20 /* min IPv4 header size */
 #define SIZE_ICMP_HDR 8 /* from ip_icmp.h */
 #define MAX_PING_DATA (MAX_IP_PACKET - SIZE_IP_HDR - SIZE_ICMP_HDR)