"Maximum Transmission Unit" (MTU) is the maximum number of bytes a frame or packet may have on a network (in several layers of the OSI model). Wrong MTU can lead to packet loss.

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HTTP webserver gateway timeout 504 only from one ISP

I have configured a nginx http webserver to serve a site on CentOS 6 Virtual machine. every things are works fine but when I connect to site from a special ISP, the site wait and after a while respond ...
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34 views

VLANs with mixed MTUs trunked over a LAG

I'm contemplating enabling Jumbo frames for a specific network/VLAN (a dedicated storage network). Ports are spread across two switches, linked together using a LAG, which is configured to carry all ...
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47 views

How to setup linux to retransmit TCP packets by reducing the packet size like OS X

On OS X I noticed that for packets which are not ACKed, the system retransmits them with reduced packet size. This can be seen in the image below. Is it possible to enable the same behavior in ...
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37 views

Linux host does not retransmit with a lower packet size in response to ICMPv6 Packet Too Big message

I'm seeing this in Wireshark when curling this URL: On the same network, a Windows host retransmits properly, but an Android host does not. On a different network (presumably appropriately ...
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1answer
44 views

Can I trust a TCP segment with a payload of 255 bytes will be delivered in a single Ethernet frame?

I'm writing some server software that upgrades the firmware on an electronic device. It's a tricky task, because currently these devices don't have enough memory to hold a full firmware before ...
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16 views

traceroute with MTU option: Different outputs when frag kicks in; what's the meaning?

I'm currently verifying a problem with the MTU on a route between "SITE_A" and "SITE_B" and have noticed that traceroute (more specifically, traceroute-2.0.20 on Fedora 22/23) seems to detect two ...
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2answers
114 views

Very bizarre Linux-to-Linux but not Windows-to-Linux packet loss over the Internet

I am right now getting additional grey hair fighting a phenomenon concerning packet loss between machines on the Internet. Check the diagram below. Note that whenever I use "SSH" I could use "HTTPS"; ...
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1answer
284 views

OpenVPN low performance. Do I have MTU problems? Dumps inside

I have problems with an OpenVPN tunnel which does not reach line speed. The gateway is a Debian Jessy virtual server hosted at OVH. The client is either my freebsd 10.2 homeserver (Intel I3 Ivy Bridge)...
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1answer
210 views

Forcing IP fragmentation and reassembly for some traffic thru a VPN tunnel

Is there a way to force IP packet fragmentation before they go into tun0 and then force reassemble them on the other side of tun device? I have some IPSec traffic that I can not control, and it wants ...
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1answer
196 views

Why can I receive ethernet frames bigger than my current MTU size?

I configured the MTU size to 1000 with ip link set eth0 mtu 1000 on link eth0. I disabled Generic receiving offloading (gro) with ethtool -K gro off (since this can lead to "false" frame length in ...
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1answer
52 views

Force Windows to send packets as soon as they arrive the nic

I have a network application running on Windows Server 2008 R2, that must send small packets in a very short time interval, each microsecond matters. My problem is that windows accumulates the packets ...
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23 views

Server Not Working With Jumbo MTUs

I am a network engineer & my server guy doesn't appear to be able to get the jumbo MTU's to work on his NIC team. We are using LACP for two 10gig interfaces between the switch. I am 100% certain ...
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647 views

How to prevent SSH freezes over an openvpn client to client connection

I have the exact same issue as the one described here, but I cannot request clarification from the author, since I am a new user and I cannot post a comment on that, so I am posting a new question (I ...
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1answer
308 views

Change Xenserver 6.5 MTU

How do I go about change the MTU setting for my management interface in xenserver 6.5? Right now its set to the default 1500 but I want to increase it to 9000. For my setup I have 2 nics on each host ...
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3answers
73 views

How to set permanent mtu size for ppp0

every time I connect to my VPN, I should run sudo ifconfig ppp0 mtu 1300 How could I make it permanent? I'm using Ubuntu 14.04
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1answer
123 views

Linux MTU and UDP

Can someone please explain this behavior to me. I have a few VMS (centos) running on a cloud provider. The interface is set to the standard 1500 MTU pining with large ICMP packets work fine: # ...
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69 views

What should happen when receiving an ICMP Fragmentation Needed message?

In general, and in particular on a Solaris 10 machine ... See, we are having an issue in our network. I handle some reverse proxy instances on a Solaris 10 server that is behind a load balancer, and ...
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1answer
134 views

VPN problems: MTU and Don't-Fragment

I am setting up a VPN on Ubuntu 14.04LTS which uses IPSEC and performs NAT for clients. It is basically working - I can connect to it from a clinet (Android device) and browse (for example) www....
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1answer
119 views

Does TCP's MSS hide a mismatched intra-LAN MTU?

I'm trying to understand how mismatched MTUs behave in a network. Suppose two hosts are able to talk to each other through an Ethernet-based network. The network and host A are configured for jumbo ...
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1answer
2k views

Can't SSH to any servers

It's for a day that I can't login through SSH to my servers. Using both Putty and MobaXterm. MobaXterm ends with: debug1: sending SSH2_MSG_KEX_ECDH_INIT debug1: expecting SSH2_MSG_KEX_ECDH_REPLY Read ...
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1answer
215 views

Fragmented DNS Packet

The DNS response against my domain is 1228 bytes long and I need the packet to get fragmented in 3 packets. To achieve this I have set the MTU at name server (bind9 running on the machine) to be 500. ...
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1answer
133 views

Linux route cache seems to load outdated settings

On SLES 11 SP3 (Kernel 3.0.101-0.46) I see the following route behavior: When I set a route specific MTU once via ip route change ... mtu lock 1500, everything works as expected (setting an MTU of ...
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2answers
288 views

ADSL router happily accepts IPv6 packets of 2K in size

As I understand IPv6 fragmentation, the routers do not perform fragmentation, only the end-to-end nodes do. And when any router along the path receives a packet larger than the MTU of the link to its ...
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2answers
2k views

MTU on server accidentally set to 0, can't ssh into it?

My server had some problem with the ssh and I couldn't upload files of size > 10 KB, as scp would hang during the copy. I found a solution for this problem here, and I was changing the MTU, when I ...
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157 views

Can't mount read-write iSCSI with MTU=9000

I'm attempting to connect my desktop (running debian 7) to a QNAP NAS (running QTS 4.1.1) using iSCSI with an MTU=9000. The network infrastructure consists of a cat5e crossover cable between the ...
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1answer
421 views

IPsec transport mode and MTU

[Similar to Right way to set the MTU of an IPsec Client (Linux/Racoon), but different in that there is no router on the responder side] I have a setup where machines in a local network need to talk ...
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1answer
2k views

Set MTU of interface via powershell

I'm trying to set the MTU for a physical interface programmatically on Windows 7: PS> (Get-WmiObject -Class Win32_NetworkAdapterConfiguration | Where { $_.Description -match '^Red Hat.*#2' }) ...
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1answer
774 views

CentOS 7 MTU setting not works

I am using CentOS 7 to test some network performance. The interface setting is normal like that : saberin@localhost ~]$ ifconfig enp0s25: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500 ...
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1answer
184 views

POST of MTU + ~798 bytes gets lost

I have a very weird issue with certain packets not arriving on the destination host. It happens when we transmit a POST that is somewhat larger than the MTU. We can reproduce it with this script: #!/...
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1answer
585 views

Does setting MTU on logical interfaces affect physical interfaces

I've been using a combination of interface bond-, vlan- and bridge-interfaces to provide redundancy and different logical network layers to xen domU's. This setup is working well however i'm a bit ...
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1answer
688 views

Debian don't forward through L2TPv3 more than mtu l2tp virtual interface

R3--R1--Internet--R90 R3: ip addr 10.123.0.3 R1: sudo ip l2tp add tunnel tunnel_id 699 peer_tunnel_id 699 encap udp local 5.254.174.8 remote 5.254.174.90 udp_sport 699 udp_dport 699 sudo ip l2tp add ...
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OpenVPN: How to mitigate path MTU issues on a per-client basis?

We have dozens of embedded devices installed at customers, all calling home to our OpenVPN service. That works fine in general, but a few of our customers have severe path MTU issues. Our influence on ...
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1answer
431 views

VMWare - Jumbo Frames over iSCSI to a NAS4Free array

I am having problems getting Jumbo Frames working over my iSCSI connection. Here is what I try. 4 1GbE connections on 4 different subnets on my NAS4Free array for iSCSI, MTU set at 9000 2 redunant ...
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1answer
323 views

windows update error 80072ee2 caused by linux router, possible mtu issue? [closed]

For a few days now, the Windows 7 workstations on my network have been unable to connect to Windows Update. They are getting the infamous error 80072ee2. They are connected to a simple SOHO switch. ...
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2answers
9k views

Permanently set MTU on Ubuntu 14.04 machine within an Amazon VPC

I have some Ubuntu 14.04 machines running in an Amazon VPC. When they start, they have an MTU of 9001, but this appears to cause some problems when communicating with other servers which have an MTU ...
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2answers
2k views

MTU between Juniper and Cisco, cause of packet loss?

This office has a network consisting of Cisco Catalyst and Juniper EX L3 switch. Cisco Router and Juniper L3 switch share a OSPF area in between them. Hosts on Juniper side are running FTP servers ...
2
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1answer
2k views

Full MTU not utilized with jumbo frames enabled

I've been testing to see if I can get any benefit from enabling jumbo frames. I've set up two identical Dell R210 servers with Xeon quad core E3122 CPUs, 8G of RAM and Broadcom NetXtreme II BCM5716 ...
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2answers
233 views

CentOS ssh connection stalls after server bandwidth upgrade

I have a few linux servers inside a customer that recently MASSIVELY increased his bandwidth - from about 0.8/5mbps to 25mpbs/100mbps... (yes 100mpbs!) this is very good, but ever since my WinSCP/...
2
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1answer
909 views

MTU on mobile data networks

I found various lists of MTUs for different links, but they never contain UMTS, LTE, EDGE, HSPA, GPRS. What are the MTUs of the above mentioned technologies?
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2answers
4k views

XenServer 6.2 changing mtu

So I've looked around for a way to change the MTU of interfaces on XenServer 6.2, but can't find anything that works for me ... One method spoke about changing the ifcnfg-intX files in the /etc/...
5
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1answer
6k views

Relationship between MTU and NFS rsize/wsize options

I'm trying to understand networking settings related to NFS and various buffer sizes (and there are quite a few). I'm running wireshark and inspecting the TCP packets arriving at the NFS server. ...
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1answer
2k views

Cisco IPSEC VPN Slow Speeds

We have a site to site IPSEC VPN, both endpoints are Cisco PIX 515e's. The links at both end are 100mb, however speeds over the VPN (logged using jperf) are at most 4mb. Obviously this represents a ...
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157 views

Severe performance issues sharing openvpn client connection to LAN clients virtualbox

I am trying to set up an internal network in Virtualbox in order to share an openvpn client tunnel with multiple VMs acting as clients that should be completely isolated from everything except for the ...
2
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1answer
148 views

Maximum MTU size

Thought one of the issues I'm experiencing with the following question is due to MTU rightfully so. ESXi 5 VM Putty session hangs, vSphere client timing out However, when I tried testing the ...
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2answers
5k views

How do I view cached PMTU under Linux (and all other OSs)

When I ping a remote site with the DF bit set and a packet size that is too big for my router the first ICMP "fragmentation required" message is sent from the router. After that the message comes from ...
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2answers
2k views

SFTP fails on listing directories with large number of files

I try to connect to an sftp server (vsftpd) on Centos server through port 22. there is no problem in connection and initial directory listing; there is also no problem if I click on a directory with ...
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1answer
134 views

How to maximize mtu size for SunOS 5.10 Generic_147440-26 sun4v sparc sun4v [closed]

Despite installing and enabling the 10G fiber card, data copy or transfer is still happening on a 1G rate.
3
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1answer
389 views

IPv6: using a link mtu greater than the mtu of the default route

When configuring an IPv6 LAN it may be desirable to use router advertisements to announce a link MTU of either the standard 1500 bytes or something larger to enable usage of jumbo frames, perhaps 9000....
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1answer
173 views

How do I force a specific MTU for only certain TCP ports?

Background I have a set of embedded hardware deployed in the field. These remote machines connect back to my servers at AWS running Ubuntu and I use the iptables mangle chain to lower the MTU to 500 ...
2
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3answers
870 views

disadvantages MTU over 1500

What disadvantages have to set the MTU to the maximum possible value in ALL network adapters on a network (including virtual switches vmware esx)? Why do not the producers as a default? I do not ...