0x000 open environment
Download the file to a local, view the file format for the 64-bit ELF, xxd command is to convert the file to view the hexadecimal format, the contents of the documents found there is a significant upx, upx compressed program is a shell, so the first the file shelling, shelling using upx -d command to flag_upx.
0x001 source code analysis
Careful that you will find that there is a comment "# 0x6c2070 <flag>", then you know :( "x / s", to view the contents of the address, s can be omitted, on behalf of the string)
This is the flag up!