Compile and use Dropbear on a '486 machine

Well, I was able to compile and run latest dropbear-2018.76 on an i486 machine running stock RedHat 6.0 "Hedwig" featuring:

• kernel Linux 2.2.5 released March 1999
• gcc version egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)
• libc-2.1.1
• fun fact: there were no ssh packages in RedHat 6.0 install CD
• very fun fact: I can now ssh to a ~24 years old machine sporting a ~20 years old Linux kernel and libc.

Steps:

• login as root, because the default RedHat 6 installation does not create an unprivileged user (if you want to access it via telnet you need first to create an user; then you can switch to root using su);
• wget --verbose http://matt.ucc.asn.au/dropbear/releases/dropbear-2018.76.tar.bz2
  (note the "http" instead of usual "https")
• bunzip2 dropbear-*.bz2
  tar xf dropbear-*.tar
  cd dropbear-2018.76
• vi ifndef_wrapper.sh
  (remove the "-E" flag from "sed" command)
• ./configure --prefix=/usr OLDCFLAGS=-O2
  (using "-O2" because "-Os" optimizes for size: this is an AMD 486 and I need speed over size;
  also ignore those "mkdir -v" warning; mkdir shipped in 1999 did not have verbose flag;will take more than 8 minutes)
• vi Makefile
  (remove the "-pie" from "LDFLAGS=" line; it was detected but the compiler does not like it)
• vi libtommath/makefile_include.mk
  (remove the "-Wextra" from the "CFLAGS +=" line and remove the "-Wsystem-headers -Wdeclaration-after-statement" in the following "CFLAGS +=" additional warnings line; these flags are not supported by egcs
• vi libtomcrypt/makefile_include.mk
  (in the "LTC_CFLAGS +=" lines, remove the "-Wextra" and the "-Wsystem-headers" and the "-Wdeclaration-after-statement")
• vi common-channel.c
  libc-2.1.1 does not define SHUT_* constants in its header files; just add at the top:
  #define SHUT_RD 0
  #define SHUT_WR 1
  #define SHUT_RDWR 2
• vi netio.c
egcs doesn't support variable declarations in the middle of a function; move these two lines just before TRACE(("enter dropbear_listen")):
u_int16_t *allocated_lport_p = NULL;
int allocated_lport = 0;
• vi cli-main.c
  same as above: move this declaration at the start of cli_main():
  pid_t proxy_cmd_pid = 0;
• make
  it will take less than 25 minutes if "-Os" (37 minutes if "-O2");
  note: ignore these compiler warnings:
  
  • "...tomcrypt_prng.h:76: warning: unnamed struct/union that defines no instances"
  • "...bignum.h:32: warning: `sentinel' attribute directive ignored"
• make install
  mkdir /etc/dropbear
  dropbearkey -t rsa -f /etc/dropbear/dropbear_rsa_host_key
  dropbearkey -t dss -f /etc/dropbear/dropbear_dss_host_key
  dropbearkey -t ecdsa -f /etc/dropbear/dropbear_ecdsa_host_key
  
• usage: you have to explicitly tell the ethernet interface IP address and port because that ancient libc will crash if defaulting to "all interfaces". This is the line I added to /etc/rc.d/rc.local file:
•  /usr/sbin/dropbear -p 192.168.1.17:22
  
• also added my ssh key into ~/ssh/.authorized_keys for a passwordless login
• note: if you want some Superb Security you should disable all the other services, including telnet

Bleeding edge security !! ('486 processor doesn't have Spectre/Meltdown/etc vulnerabilities).
Now I can ssh into a real '486. Wow!