After a juniper router improve we began having MTU issues. I am now not in a position to ship the router a 1500 byte ICMP packet (on a direct connected subnet with vlan) and get a reply. If I alter the MTU in my linux server to 1504, I do get a reply however with an additional 4 bytes. I should not want to regulate my mtu as a result of Linux routinely provides 4 bytes to the interface MTU beneath the covers whenever you add a vlan. So, I by no means wanted this config earlier than, nor do I would like it in different elements of the community, nevertheless it’s a thriller to me the place these further 4 bytes are coming from.
Right here is tcpdump dump and wireshark of the reply after I improve the MTU by 4 bytes:
14:35:55.754432 00:54:00:2f:4d:49 > 00:00:5e:00:01:47, ethertype
802.1Q (0x8100), size **1518**: vlan 10, p 0, ethertype IPv4
(0x0800), (tos 0x0, ttl 128, id 10523, offset 0, flags [DF], proto
ICMP (1), size 1500)
172.77.0.236 > 172.77.0.1: ICMP echo request, id 1, seq 35753, size 1480
14:35:55.754586 d8:53:9a:de:75:f0 > 00:54:00:2f:4d:49, ethertype
802.1Q (0x8100), size **1522**: vlan 10, p 0, ethertype IPv4
(0x0800), (tos 0x0, ttl 64, id 10523, offset 0, flags [DF], proto
ICMP (1), size 1500)
172.77.0.1 > 172.77.0.236: ICMP echo reply, id 1, seq 35753, size 1480
You possibly can see that the packet leaving linux is 1518 bytes (payload+6 byte src+6 byte dst +2 byte size + 4 byte vlan) however after I get the response it is 1522 bytes. After I open wireshark and look, I discover that I am getting a further 4 bytes (00 00 00 00) added to the tip of the packet:
0000 00 54 00 5c f7 00 d8 53 9a de 01 f0 81 00 00 0a .T....S.......G
0010 08 00 45 00 05 dc 13 cc 40 00 40 01 c8 3f ac 4d ..E.....@.@..?..
0020 00 01 ac 4d 00 ec 00 00 30 00 00 01 18 47 61 62 ........0....Gab
0030 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 cdefghijklmnopqr
0040 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b stuvwabcdefghijk
0050 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 lmnopqrstuvwabcd
0060 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 efghijklmnopqrst
0070 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d uvwabcdefghijklm
0080 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 nopqrstuvwabcdef
0090 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 ghijklmnopqrstuv
00a0 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f wabcdefghijklmno
00b0 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 pqrstuvwabcdefgh
00c0 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 ijklmnopqrstuvwa
00d0 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 bcdefghijklmnopq
00e0 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a rstuvwabcdefghij
00f0 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 klmnopqrstuvwabc
0100 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 defghijklmnopqrs
0110 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c tuvwabcdefghijkl
0120 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 mnopqrstuvwabcde
0130 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 fghijklmnopqrstu
0140 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e vwabcdefghijklmn
0150 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 opqrstuvwabcdefg
0160 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 hijklmnopqrstuvw
0170 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 abcdefghijklmnop
0180 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 qrstuvwabcdefghi
0190 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 jklmnopqrstuvwab
01a0 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 cdefghijklmnopqr
01b0 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b stuvwabcdefghijk
01c0 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 lmnopqrstuvwabcd
01d0 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 efghijklmnopqrst
01e0 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d uvwabcdefghijklm
01f0 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 nopqrstuvwabcdef
0200 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 ghijklmnopqrstuv
0210 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f wabcdefghijklmno
0220 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 pqrstuvwabcdefgh
0230 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 ijklmnopqrstuvwa
0240 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 bcdefghijklmnopq
0250 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a rstuvwabcdefghij
0260 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 klmnopqrstuvwabc
0270 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 defghijklmnopqrs
0280 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c tuvwabcdefghijkl
0290 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 mnopqrstuvwabcde
02a0 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 fghijklmnopqrstu
02b0 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e vwabcdefghijklmn
02c0 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 opqrstuvwabcdefg
02d0 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 hijklmnopqrstuvw
02e0 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 abcdefghijklmnop
02f0 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 qrstuvwabcdefghi
0300 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 jklmnopqrstuvwab
0310 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 cdefghijklmnopqr
0320 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b stuvwabcdefghijk
0330 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 lmnopqrstuvwabcd
0340 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 efghijklmnopqrst
0350 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d uvwabcdefghijklm
0360 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 nopqrstuvwabcdef
0370 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 ghijklmnopqrstuv
0380 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f wabcdefghijklmno
0390 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 pqrstuvwabcdefgh
03a0 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 ijklmnopqrstuvwa
03b0 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 bcdefghijklmnopq
03c0 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a rstuvwabcdefghij
03d0 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 klmnopqrstuvwabc
03e0 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 defghijklmnopqrs
03f0 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c tuvwabcdefghijkl
0400 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 mnopqrstuvwabcde
0410 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 fghijklmnopqrstu
0420 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e vwabcdefghijklmn
0430 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 opqrstuvwabcdefg
0440 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 hijklmnopqrstuvw
0450 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 abcdefghijklmnop
0460 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 qrstuvwabcdefghi
0470 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 jklmnopqrstuvwab
0480 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 cdefghijklmnopqr
0490 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b stuvwabcdefghijk
04a0 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 lmnopqrstuvwabcd
04b0 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 efghijklmnopqrst
04c0 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d uvwabcdefghijklm
04d0 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 nopqrstuvwabcdef
04e0 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 ghijklmnopqrstuv
04f0 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f wabcdefghijklmno
0500 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 pqrstuvwabcdefgh
0510 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 ijklmnopqrstuvwa
0520 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 bcdefghijklmnopq
0530 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 6a rstuvwabcdefghij
0540 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 klmnopqrstuvwabc
0550 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 defghijklmnopqrs
0560 74 75 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c tuvwabcdefghijkl
0570 6d 6e 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 mnopqrstuvwabcde
0580 66 67 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 fghijklmnopqrstu
0590 76 77 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e vwabcdefghijklmn
05a0 6f 70 71 72 73 74 75 76 77 61 62 63 64 65 66 67 opqrstuvwabcdefg
05b0 68 69 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 hijklmnopqrstuvw
05c0 61 62 63 64 65 66 67 68 69 6a 6b 6c 6d 6e 6f 70 abcdefghijklmnop
05d0 71 72 73 74 75 76 77 61 62 63 64 65 66 67 68 69 qrstuvwabcdefghi
05e0 6a 6b 6c 6d 6e 6f 70 71 72 73 74 75 76 77 00 00 jklmnopqrstuvw..
05f0 00 00 ..
What are these 4 bytes on the finish? And why did my juniper begin sending them? What do I do about it? Simply change the MTU all over the place in order that the body exhibits up and hope my hosts ignore it?
Edit (further particulars)
Host is a juniper SRX1500 operating 24.2R1-S2.5.
SRX has the MTU of the bodily interface set to 2000 however the irb interface with the ip handle I am pinging set to 1500.
That should not be a difficulty, however possibly the juniper is padding the body attributable to a bug?