iproute: restrict hoplimit values to be in range [0; 255]

Technically, the range of possible hoplimit values are defined by IPv4
and IPv6 header formats. Both define the field to be eight bits in size,
which leads to a value range of [0;255]. Setting a packet's hoplimit
field to 0 though makes not much sense, as the next hop would
immediately drop the packet. Therefore Linux uses 0 as a special value
indicating to use the system's default hoplimit (configurable via
sysctl). In iproute, setting the hoplimit of a route to 0 is equivalent
to omitting the hoplimit parameter alltogether, so it is actually not
necessary to allow that value to be specified, but keep it anyway for
backwards compatibility.
Signed-off-by: Phil Sutter <phil@nwl.cc>

ip/iproute.c

@@ -931,7 +931,7 @@ static int iproute_modify(int cmd, unsigned flags, int argc, char **argv)
 				mxlock |= (1<<RTAX_HOPLIMIT);
 				NEXT_ARG();
 			}
-			if (get_unsigned(&hoplimit, *argv, 0))
+			if (get_unsigned(&hoplimit, *argv, 0) || hoplimit > 255)
 				invarg("\"hoplimit\" value is invalid\n", *argv);
 			rta_addattr32(mxrta, sizeof(mxbuf), RTAX_HOPLIMIT, hoplimit);
 		} else if (strcmp(*argv, "advmss") == 0) {