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sshd: use exponential fail2ban backoff instead of fixed week-long bans #515

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Merged
mulatta merged 2 commits into from
May 31, 2026
Merged

sshd: use exponential fail2ban backoff instead of fixed week-long bans #515

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a7602ad
sshd: use exponential fail2ban backoff instead of fixed week-long bans
mulatta May 31, 2026
8b16337
sshd: stop the aggressive fail2ban jail from banning legit sessions
mulatta May 31, 2026
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35 changes: 29 additions & 6 deletions modules/sshd/default.nix
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Original file line number Diff line number Diff line change
Expand Up @@ -26,8 +26,9 @@ let
fail2ban = {
maxRetry = 3;
findTime = 600;
banTime = 86400;
aggressiveBanTime = 604800;
# Exponential backoff: ban doubles per re-offense from base to cap.
baseBanTime = 300;
maxBanTime = 604800;
};

rateLimiting = {
Expand Down Expand Up @@ -109,6 +110,16 @@ in
enable = true;
maxretry = fail2ban.maxRetry;

# overalljails counts offenses per-IP across jails, not per-filter.
bantime-increment = {
enable = true;
maxtime = toString fail2ban.maxBanTime;
rndtime = "60"; # jitter to avoid synchronized ban expiry
overalljails = true;
};

# Only ignoreIP truly whitelists: it is checked before banning, whereas the
# iptables ACCEPT sits below the f2b jump chain and cannot override a ban.
ignoreIP = [
"127.0.0.1/8"
"::1/128"
Expand All @@ -124,25 +135,37 @@ in
filter = "sshd";
maxretry = fail2ban.maxRetry;
findtime = fail2ban.findTime;
bantime = fail2ban.banTime;
bantime = fail2ban.baseBanTime;
backend = "systemd";
};
};

# maxretry 3 (not 1): one preauth disconnect from a legit client must not
# mean an instant ban. Backoff handles repeat abusers.
sshd-aggressive = {
settings = {
enabled = true;
inherit (ssh) port;
filter = "sshd[mode=aggressive]";
maxretry = 1;
findtime = fail2ban.banTime;
bantime = fail2ban.aggressiveBanTime;
maxretry = fail2ban.maxRetry;
findtime = fail2ban.findTime;
bantime = fail2ban.baseBanTime;
backend = "systemd";
};
};
};
};

# Spare clients that reach the auth phase then drop (e.g. an unconfirmed
# Secretive/Touch-ID prompt) from the aggressive jail. Anonymous floods lack
# the "authenticating user" qualifier, so they are still caught.
environment.etc."fail2ban/filter.d/sshd.local" = lib.mkIf isBastion {
text = ''
[Definition]
ignoreregex = ^(?:Connection closed|Disconnected) by authenticating user \S+ <HOST> port \d+(?: \[preauth\])?\s*$
'';
};

# ========== firewall ==========
# Bastion: SSH exposed to internet with rate limiting
# Non-bastion: SSH only via WireGuard (wg-admin)
Expand Down
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