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Special Forums IP Networking Packet Length greater than MTU Post 302500819 by DGPickett on Tuesday 1st of March 2011 03:46:38 PM
Old 03-01-2011
Sometimes localhost, a software device, has a big MTU, like 8K, and it may apply for all local packets. Ethernet is 1500 total, and the payload is less IPV4 40 bytes plus 4 x any options. It seems like the app should be allowing frag if using non-tcp, and tcp is a stream, so the app has no business fussing with packet bits and lengths.

It may be PATH MTU testing, where the routing tables is extended with dynamic host route lines and the MTU of path to host. If ICMP says the packet does not fit, it will retry with a lower size, hence it does not want fragmentation, it wants PATH MTU information. Black holes are when firewalls block that ICMP informations, and the PATH MTU has to time out and assume it went into a black hole, and try a lower packet size to ensure a route exists.

Not Solaris, but you should see similar:
Code:
$ netstat -rn
Routing tables
Dest/Netmask          Gateway            Flags  Refs       Use  Interface  Pmtu
127.0.0.1             127.0.0.1          UH       0   64980021  lo0        4136
191.136.182.236       191.136.182.236    UH       0   23515341  lan7       4136
191.136.182.0         191.136.182.236    U        2          0  lan7       1500
127.0.0.0             127.0.0.1          U        0          0  lo0        4136
default               191.136.182.1      UG       0          0  lan7       1500
$

 

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IPTEST(1)						      General Commands Manual							 IPTEST(1)

NAME
iptest - automatically generate packets to test IP functionality SYNOPSIS
iptest [ -1234567 ] [ -d <device> ] [ -g <gateway> ] [ -m <MTU> ] [ -p <pointtest> ] [ -s <source> ] <destination> DESCRIPTION
iptest ... OPTIONS
-1 Run IP test group #1. This group of tests generates packets with the IP header fields set to invalid values given other packet characteristics. The point tests are: 1 (ip_hl < ip_len), 2 (ip_hl > ip_len), 3 (ip_v < 4), 4 (ip_v > 4), 5 (ip_len < packetsize, long packets), 6 (ip_len > packet size, short packets), 7 (Zero length fragments), 8 (packet > 64k after reassembly), 9 (IP offset with MSB set), 10 (ttl variations). -2 Run IP test group #2. This group of tests generates packets with the IP options constructed with invalid values given other packet characteristics. The point tests are: 1 (option length > packet length), 2 (option length = 0). -3 Run IP test group #3. This group of tests generates packets with the ICMP header fields set to non-standard values. The point tests are: 1 (ICMP types 0-31 & 255), 2 (type 3 & code 0 - 31), 3 (type 4 & code 0, 127, 128, 255), 4 (type 5 & code 0, 127, 128, 255), 5 (types 8-10,13-18 with codes 0, 127, 128 and 255), 6 (type 12 & code 0, 127, 128, 129, 255) and 7 (type 3 & codes 9-10, 13-14 and 17-18 - shortened packets). -4 Run IP test group #4. This group of tests generates packets with the UDP header fields set to non-standard values. The point tests are: 1 (UDP length > packet size), 2 (UDP length < packetsize), 3 (sport = 0, 1, 32767, 32768, 65535), 4 (dport = 0, 1, 32767, 32768, 65535) and 5 (sizeof(struct ip) <= MTU <= sizeof(struct udphdr) + sizeof(struct ip)). -5 Run IP test group #5. This group of tests generates packets with the TCP header fields set to non-standard values. The point tests are: 1 (TCP flags variations, all combinations), 2 (seq = 0, 0x7fffffff, 0x8000000, 0xa0000000, 0xffffffff), 3 (ack = 0, 0x7fffffff, 0x8000000, 0xa0000000, 0xffffffff), 4 (SYN packet with window of 0, 32768, 65535), 5 (set urgent pointer to 1, 0x7fff, 0x8000, 0xffff), 6 (data offset), 7 (sport = 0, 1, 32767, 32768, 65535) and 8 (dport = 0, 1, 32767, 32768, 65535). -6 Run IP test group #6. This test generates a large number of fragments in an attempt to exhaust the network buffers used for holding packets for later reassembly. WARNING: this may crash or cause serious performance degradation to the target host. -7 Run IP test group #7. This test generates 1024 random IP packets with only the IP version, checksum, length and IP offset field correct. -d <interface> Set the interface name to be the name supplied. -g <gateway> Specify the hostname of the gateway through which to route packets. This is required whenever the destination host isn't directly attached to the same network as the host from which you're sending. -m <MTU> Specify the MTU to be used when sending out packets. This option allows you to set a fake MTU, allowing the simulation of network interfaces with small MTU's without setting them so. -p <test> Run a... SEE ALSO
ipresend(1), ipsend(1), bpf(4), ipsend(5) DIAGNOSTICS
Only one of the numeric test options may be given when iptest is run. Needs to be run as root. BUGS
If you find any, please send email to me at darrenr@pobox.com IPTEST(1)
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