Your monitoring team reports that large HTTPS file transfers keep failing after traffic is routed through a site-to-site IPSec VPN. A packet capture shows the server sending 1500-byte Ethernet frames while the VPN gateway returns ICMP "fragmentation needed" messages. Before changing any interface settings, you need to discover the largest packet that can traverse the entire path without fragmentation from the Linux server. Which command will let you verify that maximum unfragmented size?
ethtool -S eth0 | grep drop to look for oversized-frame counters
ping -c 4 -M do -s 1472 203.0.113.10 and adjust the -s value downward until the echoes succeed
ip link set dev eth0 mtu 1400 && ping -c 4 203.0.113.10 to see if packets pass afterward
ss -lnt | awk '{print $2}' to view MSS values on listening sockets
The ping command with the options -M do (set DF flag, do not allow fragmentation) and -s (payload length) is the standard way to probe Path-MTU from a Linux host. Starting with a 1472-byte payload (1500-byte MTU minus 20-byte IP and 8-byte ICMP headers) and decreasing the size until you receive replies tells you the exact maximum size that can pass the VPN tunnel. The other choices either modify the local interface MTU (which masks, not measures, the problem), display driver statistics, or list TCP listeners-none of which determines the path MTU between two endpoints.
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What is the purpose of the DF (Don't Fragment) flag in the ping command?
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CompTIA Linux+ XK0-006 (V8)
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