How to check current MTU value

ifconfig

to view the current MTU. Example (just searching for MTU):

ifconfig| grep -i MTU
      UP BROADCAST MULTICAST  MTU:1500  Metric:1
      UP LOOPBACK RUNNING  MTU:65536  Metric:1
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

Alternative method:

ip addr | grep mtu

For example:

 ip addr | grep mtu
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1
2: enp0s31f6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1300 qdisc fq_codel state UP group default qlen 1000
3: wlp4s0: <BROADCAST,MULTICAST> mtu 1500 qdisc mq state DOWN group default qlen 1000
4: veth0@if5: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1400 qdisc noqueue state UP group default qlen 1000

You can read the value from the /sys kernel file system:

cat /sys/class/net/<interface>/mtu

Replace <interface> with the name of the interface to query.